compression ratio vs. octane rating

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If I have a 10.3:1 compression ratio, then what is the maximum octane fuel my car could handle? Does anybody know of a website with a chart?

posted by  whicky1978

That highly depends on the motor...

posted by  67Coronet383

What if it's a DOHC engine?

posted by  whicky1978

I think you mean minimum. That depends on alot more than just static compression. What motor? It could be anywhere from 87 to 93. Is the head alluminum?

posted by  rudypoochris

I know the minimum is 87 AKI (R+M/2 method). Is it possible to have too much octane in gasoline for a car? I drive a 2005 Hyundai Elantra GSL.

Edit: I drive a 2001. My wife drives a 2005 PT cruiser, premium gas only gave her car 2 more miles per gallon.

posted by  whicky1978

Yes sir. If you adjust timing to a certain extent you can harness the benefits of higher octane fuel, but pretty much only if you adjust for it. If you put 91 in a car meant to run 87 you won't be doing too much harm, maybe clogging some cats, foulding the chambers, and losing power, but ti wouldn't be catastrophic. Alot of engines are quite high strung though, especially modern ones. If you put 87 in a 91 modern car typically it will pull the timing which reduces power. If it doesn't have a knock sensor though, or if it is broken, then it won't notice and you will have detonation. Depending on severity of the detonation you can junk the motor, and you will be running alot less power.

posted by  rudypoochris

That sounds goood. I mostly use premium because of the extra fuel economy it gives me. I average 34-37 mpg. I'm do for a new timing belt, if they adjust the belt for higher octane, will I see better fuel economy or more power or both, or would it be the same because the car adjust according to the knock sensor?

posted by  whicky1978

What the f*ck is wrong with you you simple minded moron? We have already played this silly ass game with you. No matter what you think, running gasoline with a higher octane rating than your car requires WILL NOT INCREASE FUEL ECONOMY! Your Hyundai is optimized to run on 87 octane. Using higher octane gas is at best a waste of money and at worst can actually cause you to lower your fuel mileage and cause damage to your catalytic convertor.

I don't care what you think. This has been tested over and over ad infinitum by manufacturers, oil companies, private research firms, car magazines, etc. Unless you want us to believe the you are the one and only person on the face of the earth that sees decreased consumption when increasing octane in an engine that isn't designed for it, then just STFU... Moron.

posted by  vwhobo

What about higher octane being used to clean the fuel system, valves, injectors, etc.etc. from time-to-time.

posted by  newyorker

Octane DOES NOT clean anything. Octane in layman’s terms is a measure of resistance to detonation. Too high of an octane may actually burn too cool and create/increase carbon deposits in the combustion chamber, exhaust ports (including valves), exhaust and damage the cat.

Additive packages added to gasoline at the refinery, specifically detergents, can clean or keep clean fuel system and engine mechanical components. All gasoline sold in the USA has a minimum required amount of detergent by law. For most cars under most circumstances this is plenty of "cleaning". If you're concerned, run a bottle of Lucas fuel treatment through the engine (follow the directions) every oil change.

The biggest problem with gas in the US is not what comes from the refineries. It's the cleanliness of the tanker it's transported in and most importantly the underground tanks it's stored in at the gas station. Avoid the high dollar stations that move gas slowly. Purchase at the places that move it the fastest, which are usually the cheapest places in town. In many cases the gas is exactly the same gas from exactly the same truck, just different names and prices on the pump.

Also keep your tank as full as is reasonably possible at all times. The lower the the fuel level, the more space there is for condensation to form in the tank. Up north where you live, the cold of winter and the humid summers will fill (not literally) a gas tank with water. Same thing in the deep south where I live. Humidity, humidity, humidity. Even in the arid southwest, condensation will accumulate in an empty(ish) tank. Eight ounces of denatured alcohol every three months will take care of it for the most part and help keep your tank clean.

True story. A week or so ago the fuel pump on my '95 Dakota finally crapped out. Twelve years old and 240k+ miles on the original pump and tank, never been dropped, and when I drained it I let the gas and water seperate, just to see. Guess what? It had absolutely zero water and essentially no sediment. It's amazing what being cheap enough to do just a little preventative maintenance will do.

Edit: I knew after typing that much I'd forget something. Try not to fill your tank at a station within about 12 hours of their tanks being filled. As you can imagine having a tanker dump it's load causes some turbulence in the underground tank. This serves to stir up all the sh*t down there and if you fill up too soon, some of will get into your tank. The 12 hour cool off period gives it a chance to settle.

posted by  vwhobo

Thanks for the clarification. I also heard that if you run the tank too low, and there are any contaminants in the tank, they can "clog" the pump...not sure if this is true either.

As far as what you said with octane, there are certain fuel system cleaners out there with octane booster...wouldnt these be giving the same effect as higher octane gas, but at a more expeditious rate?? I usually try to keep my tank at above 1/2 level, but lately ive been neglecting it a bit.

posted by  newyorker

I've heard the same thing for years myself. Whenever I do, I ask the person making the statement this;

"Any contaminant that may be in your gas tank, be it water, rust, dirt or some other type of sediment, is heavier than the gasoline. That means it'll always be at the bottom of the tank... The same place the fuel pick-up is located. Based on these facts, specifically that the junk will always be on the bottom and the pick-up is at the bottom, how will running low on gas cause you to pick-up any more junk?"

I'm always given a "deer in the headlights" look followed by them stammering something about that's what their father/mother/brother/sister/next door neighbor/mechanic/bag boy at the grocery store/palm reader/homosexual lover/second grade teacher/etc told them, so it must be true. Well it's not.

That being said, a car that has an electric fuel pump will see increased wear and shorter service life of the fuel pump if you make it a habit of running the tank way low on fuel. The only thing that cools and lubricates that little motor that spins about a bazillion RPM and keeps it alive is the fuel being pumped through it and in the case of an in-tank pump, the fuel it's submerged in.

Fuel system cleaner with octane booster. I can't recommend them. Most every engine I've been in, whether just replacing plugs or a full rebuild that has been run with any form of octane booster has a fair amount of deposits left behind because of the booster. That being said, most people who run octane booster probably do so to excess and that's probably part of the cause. On the other hand, if you follow the directions, you'll only use it once a month, so what's the point? My call? Stay away and invest your money in mutual funds.

posted by  vwhobo

While that qoute sounds true, I dont get one thing. You say it will be on the bottom of the tank, but routine driving does see some bumps, and that causes the gasoline to splash around right? Wouldnt that cause all of the crap at the bottom to be constantly floating around? Or are gas tanks not just tanks, but have some kind of dividers in them like oil pans?

Last question. Is it true that higher viscosity motor oil is better for higher milage motors. I use 5w-20, but at 115k some have told me to go to 5w-30 to be a bit kinder to the engine. I already use synthetic, but would the swich be of any benefit??

EDIT: also, does a magnetic oil drain plug hold any real benefit compared to a non-magnetic??

posted by  newyorker

You won't get better economy with premium in a regular engine. The only way you would would be if the O2 sensors were compensating and leaning out the mixture. In this case you probably wouldn't mess up the cats and yes you would get better economy... but you would lose horsepower. Horsepower and economy oppose eachother. I doubt this is happening though.

posted by  rudypoochris

1. Yes, there is no doubt in my mind that driving may indeed cause some of the swill at the bottom of your tank to move around. But unless you're driving the Baja 1000 I believe it would be minimal. Also keep in mind that no matter what, gravity is still eventually going to take it back to the lowest part of the tank, which also just happens to be where the pick-up is. How much crap has settled there after sitting overnight waiting for the first start of the morning?

2. Many modern fuel tanks have a sump area, much like your oil pan example, to prevent fuel from sloshing away from the pick-up during acceleration, deceleration and cornering. Depending on the design they can also help, but not entirely prevent the pump from picking up debris.

3. The reason modern engine use lower viscosity oils are two-fold. One, is for lower lubricating system operating losses especially when the engine is cold. This can help achieved the all important higher EPA fuel mileage ratings. Two, because modern engines have much tighter operating tolerances than engines of old, it allows the pressurized oil to reach all parts of the engine sooner after start-up, especially when cold. Notice a trend? I personally recommend using somewhat higher viscosity oil as the engine wears, 100k being my transition point, and also in places that see extended periods of hot weather (it's 102F here right now). Knowing what little I do about your car, I would recommend 10W-40 in the summer and 5W-30 in the winter.

4. I think a magnetic drain plug is one of those things that may not be real helpful, but they sure can't hurt. Plus they're cheap and cause no extra work to install. I don't have magnetic plugs in any of my stuff unless they came that way from the factory. I do however recommend them to all my customers that need a drain plug replaced because... Well because I make a little more money on them.

Really? Are you sure? Horsepower and economy oppose each other? If I have two engines of the same design and displacement, engine A with a VE of 90% that produces 100 hp and engine B with a VE of 50% that produces 90 hp, which one uses less fuel at maximum power? If you answered engine A you're correct, but it makes less power, so... Care to explain the discrepancy?

Think, type, submit.

posted by  vwhobo

Yes. It takes fuel to make power. The more power you make, the more fuel will be consumed. I wasn't talking about different engines. I was refering to the same engine. Hence VE would be the same. Thus if you are making less HP on the same engine for whatever reason, you are using less fuel. Unless it is leaving unburned.

For this reason if you drive an economy car or a car that makes shit power down low, you have no problem getting good economy. If you drive a V8 that isn't geared super low or with out devices (such as cylinder deactivation or alot of EGR) you are going to consume. Thats all. When I drive the explorer shifting before 1000 rpm I get phenomenal gas mileage, ya the power isn't good, but it is a trade off. If I were to actually have the ECU set up properly to not throw it rich down low then it would be even better.

If his ECU leans out the mixture based off of the O2 readings or something similar, than ya, he will be getting better mpg. Until he pays the extra $XX at the pump for the premium.

posted by  rudypoochris

Wow! No matter how many times you repeat something you read in "Sixth Grade Basic Engine Theory" magazine, it still doesn't make it right. Let me supply you with some facts, in no particular order, that I'm sure you won't let get in the way of your opinion.

1. The ECU in your Exploder DOES NOT "throw it rich down low", no matter what you may think. Once in closed loop, the ECU, based mostly on input provided by the oxygen sensor(s), will keep the engine running as close to stoichiometry (14.7:1 for a gas engine) as possible. The only time it richens the mixture is at large throttle opening, usually WOT. This is in no way relevant to your described scenario.

2. Lack of "power down low" in no way increases fuel economy. In fact, a vehicle propelled by an engine with a weak low RPM power curve will normally get worse fuel economy than an engine with a strong lower power curve. An engine is most efficient when running at or near it’s torque peak. Torque rules.

3. If you take a car with any engine and gear it "super low", you will always increase fuel consumption in the real world, not decrease it. I don't know where you get your information, but I suggest you throw the magazine in the trash.

4. If you take a car with an engine that is optimized for 87 octane, such as an '05 Elantra, and run 93 octane gasoline, you will see a concurrent reduction in power, performance and fuel economy. Because it is optimized for 87 octane, it needs the additional BTU’s carried within the lower octane fuel to run as designed. Increasing the octane will cause it to require more fuel to create the same amount of power per combustion cycle. That mean’s you’ll make less power at any given RPM or need more RPM to attain a given level of power.

These are all proven and documented facts. I know they do nothing to further your ill informed and closed minded position, but they are the facts. Until you are able to repeal the laws of physics, they will remain the facts.

posted by  vwhobo

Also, when cold, I have a bit of valve buddy who is in school for mechanics said its running lean when its cold, and said replacing the spark plugs might solve the problem...this happens when the engine is still cold and tapers away after 3krpm usually...

Also, does seafoam really help clean the valves and such? We put it through his mustang, and my friends accord, but not my car cause I was a bit skeptical

posted by  newyorker

:roll: Calm down dude... Im not trying to argue with you at all. I hardly think I have a closed mind about things. I appreciate your opinion and perspective on things. Seriously though, cool your jets. Don't try and start an argument for the sake of arguing...

You don't need to speak down to me, seriously. Thats rude. I didn't talk down to you, don't talk down to me. You don't know anything about what I know, read, or do. You may THINK you are all knowledgable and that the facts (regardless of if they are or aren't) you are supplying are invaluable. I simply put out what I believe to be true... A little respect on your end would be MUCH appreciated. Thanks.

You make the big assumption, just like you assumed last post that I was talking about different engines, that I am not at WOT down low. In fact I am going WOT. I use it for good economy. I go wot below 1000rpm where the ECU fuel tables are vague at best. I BELIEVE it enriches the fuel beyond normal because A, it burns my eyes behind it at that RPM and B, I can blow black smoke (think it is carbon...) out the tail pipe after driving low rpms for a while and C, because my brother who happens to have an AFR meter and a Ford EEC-IV equipped vehicle DID tune his EFI below 1000rpm and found that it was throwing out a rich mixture (possibly to keep from dying, I dunno???). That is what I based my comments upon. Take it for what you will, but that si what I believe from my experiences. Weather or not it does enrich down low like that is not that important or relevant to the topic anyway...

Maybe an engine is most efficent near its torque peak, FOR A GIVEN POWER, maybe it isn't. I don't know or care. I don't need anywhere near as much power as I get at torque peak when going around town. I just know that going to my torque peak when driving around usually has me at 1/4 throttle or something light. There is no reason for me to have the TB restricting air into the engine and causing it to work harder. SO I shift and gear it down to make the motor work a little harder at a lower RPM. There is a reason the Camaros got 28mpg when they added skip shift (amongst other things). They didn't force you to shift 2nd to 4th because it hurt your economy.

By super low I mean putting in a lower numerical ratio. Or simply shifting to the next gear up, say taking 5th instead of 4th. Sorry if that was confuzing or if I didn't use your certified terminology. Im not reading any magazine. I really am kind of baffled that you are straight up attacking my experiences for no apparent reason. Its extremely rude. I don't think you would talk to me like that in public so I don't see why you should over the internet.

I know... I said that it would lose power AND economy. So I don't know why your trying to correct me. I did say it MAY be possible if the O2 sensors leaned out the mixture that you could realize better economy. I went on to say that that "I DOUBT THIS WOULD BE HAPPENING THOUGH".

Thanks... I guess you know it all. :ohcrap:

posted by  rudypoochris

No. I do not have access to SAE papers. I am studying for my Mechanical Engineering degree right now and am/was (its summer) on the FSAE team. Im not saying VWHobo is wrong... I never did... I don't understand why you guys keep putting words in my mouth. I don't get the extremely argumentative nature. Im not retarded I have been working on cars a while and I build Megasquirt EFI computers. I know a thing or two... Im sure I could find papers that state what I am and what Hobo is saying and they would both be right. I just don't have the time or the will honestly.

Okay... symantics. I meant the injectors lean out the mixture from a signal from the ECU which in part is based off of the signal from the O2 sensors. So the O2 sensors are leaning out the mixture if they feel it is needed. Its saying the same thing. At WOT the computer tries for less than 14.7:1 since 14.7:1 is not ideal for every condition. Less than 14.7 is richer, more is leaner. These calculations are done impart by the O2 sensor. So I said the O2 sensor can lean and richen up the mixture, which is true. Sometimes the computer completely ignore the HEGO's it really depends. It bases these calculations off load and other things as well, depending on your EFI maybe it is different for EEC-IV it is that way. I still don't know what this has to do with what I said before...

:umh: Sorry if you don't believe me. Thats fine it really isnt a big deal. Doesn't change anything either way.

posted by  rudypoochris

Thanks... What is it exactly that you want me to find?

You want me to prove that the ECU will lean or richen the mixture based off of O2 sensors?

You want me to supply the fuel tables?

You want me to prove that having a lower numerical gear ratio gets better fuel economy?

You want me to prove that if you take the same engine and make more power on it that it will consume more fuel?

These are all pretty basic things. I don't understand exactly what you want? Hence why I don't understand VWHobo calling me out on things I didn't necessarily say. Im new to this forum, but I am hardly an idiot. I just don't see the reason to argue with me over things I did not say then request sources and citations over mostly common knowledge to disprove someone else who supplied... zilch.

I do what works for me and supplied what I feel to be true. As I said before I have and do build EFI computers, I might know something. If you have certain points you want me to supply some info for I can try. As I said though, limited access. Honestly Im not here to argue thermodynamics and never said I was. I simply stated what the ECU MIGHT do if premium fuel was introduced and it was tuned for regular.

posted by  rudypoochris

My point is that the O2 sensor MIGHT pickup on the premium fuel due to differences in the emissions. I didn't claim to KNOW that it would or wouldn't. Also it most likely depends highly on what the ECU's capibilities are. There definately are mights, maybes, and I don't knows in engineering. Maybe you don't like to admit it?

I never came on here claiming anything out landish or unreasonable. You all jumped down my throat for no apparent reason. If you say I know nothing, then I must not know anything. Your the boss :/

As for all of your acredited technological knowhow and SAE certified master skills, you might want to work on your attitudes and social skills slightly. Members of YOUR forum are PM'ing me about how little you all know and how petty you argue using "" as your technical resource. Just a heads up. :doh:

posted by  rudypoochris

C'mon man ... no need for the holier than thou.

Seems a bit silly to point someone to a dictionary website when you mad a grammar mistake yourself in a 5 line post... and also shortened information to "infor", which doesn't seem to be a standard abbreviation as far as I'm aware. Silly mistakes to point out, right?? Well, so's trying to pick on his spelling.

[EDIT] : read my bit about google again, and removed it.... I retract all statements made :mrgreen:
I don't think I quite hit the mark I was going for.

posted by  windsonian

Maybe you should not be in the help section. Did you ever come to that conclusion? I know I did.

An O2 sensors primary functions is to relay a voltage to the ECM. It derives this voltage by measuring the amount of oxygen in the exhaust manifold.

You're thinking that because higher octane fuels burn slower(not burning all of the fuel) that the O2 sensor would pick up the rich mixture and send the voltage to the ECU which would, in return, supply less fuel. Hence better fuel economy.

Well you're wrong. VW already tells you why.

"4. If you take a car with an engine that is optimized for 87 octane, such as an '05 Elantra, and run 93 octane gasoline, you will see a concurrent reduction in power, performance and fuel economy. Because it is optimized for 87 octane, it needs the additional BTU’s carried within the lower octane fuel to run as designed. Increasing the octane will cause it to require more fuel to create the same amount of power per combustion cycle. That mean’s you’ll make less power at any given RPM or need more RPM to attain a given level of power."

I even referred to my literature for the given subject because I didn't understand it at first. However, it is in there. I don't know what engineering program you are in but its not a very good one if they haven't thought you rudimentary physics. So your "point" was proven to be incorrect. Now what are you still trying to prove?

posted by  DSMer

hehe.. and now I notice that I misspelled "mad" as well :laughing:


can you let us know what literature this is?

posted by  windsonian

No, that was not what I was thinking.

I was thinking that possibly the O2 sensor would recognize the increased emissions resulting from a higher octane fuel and lean the mixture out in return. It was just a rudimentry hypothesis which depends entirely on how an O2 sensor actually meters. Hence MIGHT.

I am in the UC Davis Mechanical Engineering program. I don't think I shall be switching. I am sure you are in MIT though. I am not trying to prove anything, it was just an idea as to how something might happen. I still don't believe my "point" (if you cna even call it that... it was just a rudimentary hypothesis) was disproven. That quote simply said premium in an 87 octane car will make less power and have worse economy. I know the performance will be reduced. I said the economy MIGHT not be. Yes lower octane fuel has more energy in it... The whole quote assumes that the car KNOWS how much power it is making and that the engine will want to compensate and make that same power. That is not necessarily true. The car does not necessarily know how much power it is making, nor does it care. Thus I don't see any reaosn for it to want to try and compensate by spilling more fuel.

posted by  rudypoochris

"Internal Combustion Engine Fundamentals", "Combustion Physics" and "Theoretical and Numerical Combustion". A few of the MANY required readings in my program. If you want any more information than that you're going to have to buy these overly expensive books yourself. My 30k a year tuition doesn't cover YOUR learning. Sorry.

No, not MIT, Cal-Berkely, G-Tech or Stanford. I attend UIUC. So I guess 5th best in the nation will have to do. I'll break it down to you nice and simple, just like they used to do Freshman year.

If you require MORE fuel to produce a proper combustion on a higher octane gas, how will leaning out the mixture better your fuel economy if you're inevitably going to need MORE fuel than is normally necessary?

posted by  DSMer


You're not the only one with book learning champ.
i was merely asking about your sources. Maybe other people's engineering degrees used different texts. Does that make yours the be all and end all? $30k a year is wasted if you don't learn to learn ... if you follow me.

posted by  windsonian

No I don't follow you. I don't care to follow you and more importantly I don't want to follow you. I don't think I need to take lessons from you on "learning to learn" or whatever the f*ck that means.

Save your bullshit philosophy's for someone who actually feeds into that kind of crap.

posted by  DSMer

my point is that you're paying $30k per year to believe everything your teachers tell you. you're not learning to learn things for yourself.

you do not, and will never know it all ... and your school probably doesn't make you better than the rest of us with formal training.

posted by  windsonian

No I'm paying $30k a year to receive a piece of paper that will make me more qualified. Its a science. Its must be learned, understood, and applied. It has nothing to with what my "professors" tell me to believe. More to do with what IS fact and what is NOT.

No one knows everything. I don't care.

I don't need a school to make me better than you. My self proclamation that I am better than YOU makes me better. Who is this "us"? I don't recall myself talking to anyone other than YOU.

posted by  DSMer

30k a year?? much do courses cost on average over in the US?

posted by  GreekWarrior

You ASSume that the car knows what a proper combustion is on the higher octane gas. This assumption implies that the car knows how much power it is making, more importantly a proper combustion on premium, and that the car understands premium when it was set for regular. That is the whole issue. The car DOESN'T know the difference. The only way it can know is through its sensors. I was merely pointing out that since burning premium fuel results in higher emissions that MAYBE the O2 sensor would pickup on that, read it as "rich", then compensate by leaning it out. YES you will make less power this way, NO you won't consume more gasoline. Got it? Thanks for schooling me like I'm a freshman with your overly priced education at the fine University of Illinois. I will take my University of California Davis education and be quite content with keeping my mind open to new possibilities and ideas. Maybe that way one of us can actually LEARN something???

I could go on or the registrar site and simply find all the engineering books offered in every subject at Davis and post them all here. That doesn't change a damn thing. Until you put in english a quote that EXPLICITLY says what you are trying to say from a reputable source, preferably a couple, then it doesn't make it any more true. You can have your hypothesis, I have mine. The only difference is that I can admit that there is a decent chance that I am WRONG.

posted by  rudypoochris

As long as you believe that. It won't take any skin off my sniffer.

If you read my quote, I was talking about those of "us" who have had formal training. I'm not the only one (as rudypoochris points out above), so why would I claim to be?

And Chris, you seem pretty dead on to me. The control system is not smart, no matter how much we'd like it to be. It takes its inputs (ie sensor values), and will interpret the same set of parameters exactly the same way each time, regardless of what set of circumstances led to that state.

posted by  windsonian

Hello there. I am confused. Why does running PULP result in higher fuel consumption, reduced power and lower performance?

I would be very interested to see the published independent data that two members have referred to as bible in their comments. There should be no excuse not to provide this data as the one gentleman who stated 4 points of supposed fact and the other engineering student both seem fairly emphatic it is correct and accessible. I don't understand the reluctance of the student in providing the proof unless he is being deliberately awkward.

Am I correct in assuming the '05 Elantra is fitted with a knock sensor for closed loop control of ignition timing? What are the published calorific values of standard gas and PULP gas in the USA? Do Hyundai admit to selling a car that loses power, performance and economy with PULP?

Enquiring minds need to know.

posted by  BestPractice

You've still failed to answer my simple question. Until you CAN answer that I'll hold your opinion in the "everyone has an asshole" box with the rest. I would like for you to explain the simple solution of.

1.) Combustion ratio does not change. It will always be the same optimal ratio regardless to what octane you have in the car.

2.)Doesn't matter if the car knows how much power it is making. You will give the car more throttle if it is not making the proper power to propel you to a given speed.

Again to my simple question.

If your are making less than normal power per combustion stroke. How will this save you fuel if you are inevitably going to need more fuel to make a normally powerful stroke?

posted by  DSMer

Who is this us? Can you account for anyone else's formal training other than your own? Do you have everyone else's credentials? Thats exactly what I thought. YOU are alone in YOUR own opinions.

Real MEN stand behind their own thoughts and morals. They don't need the backing of other people to support their own theories and beliefs. You're too old and "formally trained" to be a p*ssy. Stand for yourself and stop relying on other people to back you.

posted by  DSMer

How is it then, that I get better gas milage than it's EPA rating? I read in some places on the internet that premium does have other ingrediants than gas that improve fuel economy. My car improved 4-6 miles per gallon when I switched to premium. My wifes PT cruiser only gained 2 miles per gallon. One time I only paid 13 more cents for Shell premium. Is it true that some companies ad ethanol or toluene to thier gas? Octane ratings in the US are a measure of anti-knocking.

I read that compression ratio increases when carbon builds up. How do I clean carbon out? With Lucas or Gumout? If my transmission has been replaced with a new or rebuilt transmission, would I still have the carbon build up?

posted by  whicky1978

Higher octane fuel does have a lower energy content than lower octane gasoline. Higher octane fuels allow for a higher combustion ratio inside of a given engine. Packing more fuel and air into less space will produce a stronger and hotter combustion, which is more efficient. When these fuels are introduce to an engine with a lower compression ratio there would indeed be a lower overall power output because they contain a lower energy content. The lower compression of these cars tuned for regular are not able to optimize the benefits higher octane fuels and overcome the lower power due to the lower energy output of higher octane fuels.

Ethanol has a lower energy rating than regular gasoline. And ethanol is often used to increase the octane rating of premium gasoline. I don't know the exact blend of all gasoline(because it varies so widely) but there is a definitely an overall energy difference between regular and premium gas. But....

Will it be enough to dramatically effect your fuel economy?

posted by  DSMer


posted by  windsonian

By combustion ratio do you mean the air to fuel ratio? If so I realize that the AFR is largely set by the O2 sensor. If there is more C8H18 in the gasoline it stands to reason there will be a greater amount of emissions in the exhaust gas. IF the O2 sensor picked up on increased emissions from premium the O2 sensor MIGHT send a rich code to the ECU which in turn MIGHT lean it out. That is what I was getting at pages ago. I don't think it is necessarily possible, but it could be. I am not saying that is what the computer does, I was simply saying that is one situation in where it COULD be possible that the ECU would lean the mixture out which would inturn reduce consumption.

Actually it does matter... which is one of the reasons why 400hp cars typically get much worse fuel economy than 70hp cars.

You simply don't NEED all the power the engine can provide. Yes you are making less, but you don't necessarily need more. At cruising speed a vehicle only uses a fraction of its total potential power. This means that the throttle body will be very much closed. A closed throttle body is a large restriction which results in increased fuel consumption. The very fact that we cruise at say 2000 instead of 6000 is testiment to the fact that making less power results in better fuel economy.

As far as higher octane fuel having less power potential than lower octane fuel, that largely depends on the rest of the mixture. Yes if all things are the same a higher octane fuel has less potential, NO not all things are the same. Refineries DO often mix their premium blends to carry more energy than their regular blends.

Look guys. I am not flat out saying that he will get better economy by running premium in a regular car. That would be dumb. I never made ANY indications that I believed this to be true. I simply pointed out a hypothetical scenario in which it COULD work. Just thinking out of the box. Take it for what you will. Was just an idea, after all it could be right. I don't see any way that I could prove my side or anyone else could prove their short of actually running multiple regular gasoline motors on engine dynos at various speeds running premium with a stock ECU setup. I don't think I will find anywhere in any college text that would support or disprove my concept. I am talking about changes int he ECU strategy and how that results in economy gains or losses, not necessarily how the combustion itself occurs. Thus I find it kind of hard to find support in texts that limit themselves to thermodynamics. I DO admit though, now and long ago in this thread, that he would still probably end up paying more since premium is another 20 to 30 cents a gallon.

posted by  rudypoochris

I'm confused at your apparent about face on this issue Tbaxleyjr. Early the subject you whole heartedly endorsed the comments made by VWHobo and haven't corrected DSMer in his incorrect ascertions. You asked members to produce discounting proofs, but did not equally ask for proofs for the originating statements which is traditionally required by engineers to support theorems.

Supporting documentation is difficult to acquire, because the data is generally company sensitive. In the absence of fact, the auto industry thrives on tenuous comments which gain credibility by adoption, eventually moving from folklore to blind acceptance.

Comments like these are just not true:

They are predicated on a myth and unlikely to be found in the course books offered up as supporting. Unless you change the molecular structure of the gasolene with additives it will burn at the same rate as the gasolene from the same drum without additives. If high octane fuels made the burn slower they would not be used on high performance engines which typically must release energy faster than a moderate redline street car running regular unleaded. The exposed surface area of the molecules to oxygen gives rise to the volume of burn, but the rate of molecular burn remains the same.

The octane additive is included in the fuel to inhibit detonation. It helps to break the fuel into a homogenious burn pattern rather than one or two almighty explosions followed by a trivial afterburn. It allows for a controlled ignition and if tuned correctly a more complete burn. Even well tuned engines don't achieve complete burn, so there is scope for improvement.

The blanket argument that a fuel injected car tuned for 87 will be adversely affected by higher octane fuels is highly questionable. Anecdotal evidence by everyday consumers alone suggests there are noticeable benefits, not withstanding the evidence amased by technicians intimately involved in engine tuning, this is no slight on service mechanics, but the correct testing apparatus is not something typically found in a repair business workshop. Driver attitude is an influencing factor, but can equally be helpful and detrimenetal to fuel economy. Car makers are quite open in suggesting poorer fuel economy and possible rough idling, but they rarely suggest loss of power, when using very high ethanol content pump fuel.

If the engine has been designed and constructed to run most efficiently on 87 there may not be any improvement in performance and if oxygenated additives are used,a loss of economy. For instance the Elantra engine is designed for 95 RON fuel. If the quoted 87 AKI meets the RON specification then there there may not be any tangible improvements to be had, but in this instance it doesn't appear to. Heating values for gasolene vary, but in many parts of the world the potential energy of premium pump fuel is at least equal if not higher than standard pump fuel.

At the outset the type of fuel to be used is a factor in the engine geometry and construction. If the engine was to rely soley on the spread of flame speed of gasolene in air it would never complete anything approaching complete burn or desirable rpm, so the mythical argument about gasolene burn rate differences becomes minor in the overall scheme of things anyway. The metalic octane fuels like lead enhanced didn't rely on the metal combusting, it was primarily there to inhibit detonation associated with high compression ratios, secondary benefits to valve seats were a bonus.

Knowing the regular pump grade fuel knock index and the availability of premium fuels allows the designer and calibration engineer the opportunity to construct an efficient combustion chamber which propogates and accelerates multiple flame fronts rather than one that results in a big bang at the instance of ignition. Automatic detuning of the engine for regular grade pump fuel after development means there is always scope for improved performance using higher octane fuel. The program and tables in the engine controller should account for this by adapting via the various sensor inputs, perhaps some controllers are not so bright as others.

The reliance of AFR figures is also no longer as credible as it once was. With direct injection and fuel stratification now being employed the ratios are substantially less, yet power output is commendable.

posted by  BestPractice

Ok, there seems to be a definate need for some clarification in this thread.

1) Octane Rating.
Alot of shit seems to revolve around this, probably due to it being called 'premium'
Truth: Petrol is made up of hydrocarbons (usually with 4 to 12 carbon atoms) derived from petroleum. Originally that’s all it was. A fraction skimmed off petroleum during refining that was usually just burnt off. People were more after the heavier oils and heavier fuels like kerosene for lanterns.
It wasn’t really till the invention of the petrol engine that anyone used it for much.
And it really wasn’t till the world wars when engineers were trying to develop high performance plane engines that people started to give a hoot about octane ratings.
Truth: Octane rating is a measure of how the ANTI KNOCK properties of a particular fuel blend compare to a mixture of pure octane and Heptane, (C8H18 and C7H16) ONLY. Burn rate, energy content, detergent whatever content are not governed by octane rating at all.
An example is fuel with an octane rating of 98 displays the same resistance to detonation in a test engine to a mixture of 98% octane to 2% heptane by volume.
Yes you can have fuels with octane ratings above 100. This is because Octane is not the highest detonation resistant compound there is around For example ethanol, often used a racing fuel (that’s right, its actually good!) has an octane rating of about 130.
I must apologise, I do not know how octane ratings above 100% are calculated, I just know it can.
What I do know is the different octane ratings can be achieved with many different additives and blends, some of which increase energy content a bit some don’t. Anyway same end result, High octane fuel isn’t really much different to standard fuel.

2) Fuel energy content.
The energy content in different grades of petrol does not differ too much and if it does its just due to things like where the oil came from, who distilled and how exactly they did, whether it had more heavy hydrocarbons in it whatever. It probably only differs by about +-0.5MJ/kg or something. Petrol is usually about 44 to 45 MJ/kg.
High octane fuels DO NOT give you extra mileage in the same engine.
To prove this, diesel engines. 5 to 6 litres per 100km say. A similar sized petrol car would probably get 8 to 9 litres 100 km. that’s a 50% to 60% increase in mileage. Diesel has between 45 and 46 MJ/kg. only 1 MJ/kg increase. 2% more energy. Think about it.
This leads onto the next point, well second next point (does that work?), Compression Ratio.

4) Thermal efficiency.
Simply a ratio between the rate chemical energy is supplied to the engine (in the fuel) compared to the rate of mechanical energy (work, crankshaft spinning) is recovered from the engine.
High thermal efficiency, more power more miles less petrol. Happy driver.

3) Compression ratio.
Compression ratio is a simple ratio between the volume in the cylinder and combustion chamber when the piston is high at TDC over the volume when the piston is low at BDC. So it’s the difference between the big volume when the piston is down low and the little bit that’s left when the piston rises to the top.
This value is often critical to the thermal efficiency of an engine, so high compression ratio, generally, high thermal efficiency, more power more miles less petrol. Happy driver.
As to why, the explanation lies within thermodynamics which is not really the topic of this post, so we will just accept that it does for now (explaining it could take a while).
So why don’t all engines have massive compression ratios?
Well we all know the piston compresses the petrol air mix on the compression stroke, and we know the higher the compression ratio the greater the change in volume. Lots of gas, tiny space. As the pressure of a gas rises so does the temperature. Things like fuel catch fire at high temperatures, called DETONATION or KNOCKING and this is what limits the compression ratio in petrol engines.
Detonation is when the air fuel mix ignites before the spark fires. This is bad because the spark is precisely timed to ignite the fuel slightly before the piston ‘rocks’ over top dead center so that when this happens the fuel has just finished combusting. The pressure (and also the temperature) in the cylinder is at its maximum here and begins to pres the piston down. As this happens the pressure and temperatures relieve while the piston is pushed down. Energy is passed to the piston in the form of mechanical motion.
If the fuel burns to early two things happen. First, the peak pressure is reached while the piston is still rising. This puts massive strain on your bearings and totally destroys any efficiency the engine had. Second the temperature peak is reached at the same point, and this does not relieve very much until the piston starts moving down again. This means heat has more time and conducts into your piston and head more. Again wrecking performance and possibly your piston. Aluminium alloys melt.

So we want a performance engine. Among other things like high RPMs, good head flow and nice bearings (turbos and superchargers are another kettle of fish when talking compression ratios) were going to try and push the compression ratio up, to get more thermal efficiency and by that more power.
This is why we have high octane fuel, for these engines. The additives to make high octane fuel, mentioned above, cost money so average cars designed for soccer mums and grandmas have an engine that can use cheaper, lower octane fuel. It makes pure economic sense. They can easily make the choice to part with a few kilowatts and a small efficiency loss if they can save 15c per litre at the pump. Truth is most of us can.

Now, before we go any further, TO CLARIFY: You may notice I just spoke of an increase in mileage and in the same sentence octane fuel ratings. Critical difference, the difference is the ENGINE not the FUEL.
Engines with high compression ratios are more efficient. They require high octane fuel however. Engineers would rather use the lowest octane.
Lower performance engines with lower compression ratios can run on lower octane fuel. They can also run on high octane fuel. This really (because of the reasons mentioned previously) doesn’t effect them at all. It won’t stuff your cat, it wont lower your performance, it will make absolutely no difference. It will just hit your hip pocket. That’s why it’s a false economy to put it in a 2001 Hyundai Elantra.

4) Rev ranges, Torque vs power and shit.
Ok first things first. Power. Usually measured in watts it is the rate that energy flows through something or is ‘used’. Yep the word rate is in there, that means it has a relationship with time.
Torque. A turning or twisting force. Measured in newton meters. This is what actually moves your car along.
RPM. Revolutions per minute. A common measurement for rotational speed. Speed is a rate. Has a relationship with time.
In these forums, mostly performance ones and usually when comparing jap 4s with (often slower) V8s you hear ‘well your 4 banger has more power but the V8 will smoke you with its torque’. Two words, often used together as one word, bullshit.

Lets go back to power, specifically, shaft power, which is what comes out of an engine. It is the product (two things times each other) of torque and shaft speed (in radians per second, another form of shaft speed). The formula literally looks like this:
P = T x ω ( P power, T torque, ω shaft speed)
This means both torque and high redline are a way towards high power.

Now I did say before torque is what moves your car along, so im going to mention something else.

Gearbox. Wonderfull invention, gears are fun to change.
We all know its what is in the middle of your drive train, the group of components that transfer the energy from your crankshaft your wheels. We also all know its what matches the speed of the engine to that of the wheels. So it changes speeds.
It also changes torque!!
Your energy going through your wheels follows the same relationship as your engine, as far as torque and power are concerned. So our good old formula P = T x ω still applies.
So two cars. Both making 5kw, both going 60km/hr, one the engine is going at 900rpm, the other at 2500rpm.
Road speed is the same, wheels are the same size so they are spinning at the same speed, powers the same, TORQUE AT THE WHEELS is also the same!!
This is why torque does not mean shit when you are talking fast cars because an appropriate gearbox changes it to match the load on the wheels. Aren’t they great?
In the example above the slow reving engine will be producing a higher torque reading but the high reving engine is making up for that with its shaft speed.

The two cars would be different to drive. The slow engine will be nice cruizy and lazy, the fast engine will feel highly strung and it will feel like you are driving it harder. Its subjective though.
Did I mention F1 cars? 2.5l or something, 20,000rpm redline, next to no torque, would smash the fark out of a 5l V8 with maybe 1 and a half times the torque rating?

Its cool, you can like your V8s, just don’t say it will smash an evo or something with1 and a half times the power and 2 thirds the weight. Like wise, a worked V8 with massive torque and more power but lower redline might smash a jap car whatever.
Don’t start an argument about this please, its been done before.

Anyway, one price you can pay with high revving small motors is a loss of efficiency. Because everything is moving faster, pistons going up and down faster etc there is more friction in relation to the power produced. So there it goes, your efficiency can just fly away.

Anyway, fark im getting vague


PS heaps of books on this stuff read em if interested

PSS this is not written well enough to be used as an assignment, go copy Wikipedia

posted by  donkeytits1

Oh and a side note, there is heaps of shit you can do in engine design (if your an engine designer for toyota) to inhibit detonation and allow high compression ratios.

Things like swirl velocities, squish (basically a way of creating swirl) and sheer engine speed can help an engine achieve a high comp ratio with low octane fuel.
Have a look at motorbikes.
GPX250. 11.5:1, standard 91 fuel. Engine designed in the 80s when allot of cars were around 9, 9.5

posted by  donkeytits1

Oh yeah, rudypoochris, thermal efficiencys and mechanical efficiencys are the main factors that determine your mileage in a car and alot of factors influence these figures.
Just saying more power means less milage is too simplistic and just plain wrong in this case.
Perhaps they havent taught you this stuff in your engineering course yet

posted by  donkeytits1

You seem to be having a good time.:wink2:

posted by  Wally

If only someone was listening......

oh... and hi y'all by the way... haven't been round for a while.

posted by  windsonian

Wow, where do you idiots get yer info from? Especially YOU Hobo. I have to assume from yer "title" you're into air cooled VW's. Have you been so brainwashed by the oil companies and the powers that be, that you actually believe the bull.... you're spewin. HIGHER octane full makes more horsepower and gives better mileage, don't believe anything but. Dig out your old Hot VW's magazines and look at some mileage tests a reader did with his Ghia. 54 m.p.g. with mild modifications. He quit his tests when ????????????? Fuel octane and quality dropped so badly. THATS why cars now have all this engine monitoring equipment, TO COMPENSATE FOR THE SH.. GASOLINE. It costs the oil companies more $ to refine gas to a 93 Octane rating vs. 87. Is any of this sinkin' in ? You're embarrassing me and the entire air cooled community. Pull the tapeworm outa' yer a.. yeah, yeah, I know yer one of these so called gurus that will tell ya' they build 250 H.P. V.W. motors with 11 to 1 c.r. runnin' on pump gas. Yeah, O.K. Remember kiddies, runnin' higher octane fuel than recommended by the manufacturer WON'T HURT ANYTHING. Especially on newer vehicles, yer vehicles brain will adjust accordingly, like more timing means more power . Anyway, remember, don't believe everything the "experts" throw at ya'. Like 911 was caused by terrorists. Happy motorin ya'all, no matter what yer' wheelin:wink2: :wink2:

posted by  lawrencejr

Correction, higher octane pump fuel does not make more power. The specific heat value (or calorific value) is the same. The fuel isn't refined any differently. Additives are cream profit for the oil companies. Octane additive coupled with high compression and opportunistic ignition timing just allows more of that power to reach the crankshaft.

posted by  Wally

WHAT !!!!! So what you're sayin' in yer last sentence is higher octane makes more power. I didn't say it, YOU just did. If you can refer to a site written by experts concerning refing techniques and octane ratings, you'll make me a believer. Otherwise, I stand my ground. Thanx, and keep the shiny side up !!!

posted by  lawrencejr

If your car doesn't require higher octane fuel, there won't be any difference in performance if you put higher octane in. Higher octane fuel won't prematurely combust at higher compression ratios.
That said, virtually all cars on the road today have advanced enough timing systems so they can take 87 fuel and run fine (I'm not suggesting it, I'm just saying).
Also, in your previous post, it seemed like you were suggesting for people to put premium in their car even if it doesn't need it. Please tell me I misunderstood that post?

posted by  chris_knows

No I'm saying you don't know what you profess to know. I am an expert. I don't need to defer to internet sites.... read my avatar rank.

posted by  Wally

Wow! Just f*cking wow! I haven't posted on this forum or even looked at it in months, and I still have self appointed experts who are actually not very well disguised morons calling me out.

I'll keep it short and sweet just for you douchebag. You don't have a clue of what it is you speak. You are an idiot. Go crawl back under your rock.

posted by  vwhobo

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