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Old 02-24-2005, 02:12 AM   #31
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Quote:
Originally Posted by CarEXPERT
Are you sure. I thought compression ratio was like,
cylinder head volumn + gasket volum + swept volumn DIVIDED by something elese I cant remember.
I remember it was more that that to get CR.

Walley can you please correct one thing that was wrong that I said so I know that im wrong.

yes I am sure. Clearance volume is basically everthing on the compression side of the top ring and piston top = combustion chamber, piston dish, gasket volume, top ringlands, etc. Swept volume is the actual full stroke displacement, not cylinder volume.

no I won't attempt to correct this thread, there are too many issues.
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Old 02-24-2005, 02:40 AM   #32
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Quote:
Originally Posted by Wally
yes I am sure. Clearance volume is basically everthing on the compression side of the top ring and piston top = combustion chamber, piston dish, gasket volume, top ringlands, etc. Swept volume is the actual full stroke displacement, not cylinder volume.

no I won't attempt to correct this thread, there are too many issues.

Is what ChrisV said about compression ratio true? How would you test a compression ratio of a cylinder? Do you have to buy something to put it on the spark plug?
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Old 02-24-2005, 03:03 AM   #33
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Quote:
Originally Posted by CarEXPERT
Is what ChrisV said about compression ratio true? How would you test a compression ratio of a cylinder? Do you have to buy something to put it on the spark plug?

I'm sure you know the answer to both questions, but for the readers:

no you can't accurately dynamically test your CR with a gauge. You can however get a very poor ballpark figure by measuring absolute manifold pressure and absolute compression pressure on a dead pot while the engine is idling, but it is very dependent on valve closing angles, rod length and stroke.
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Old 02-24-2005, 03:09 AM   #34
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Quote:
Originally Posted by Wally
Remember this for posterity:

CR = (swept vol. + clearance vol.)/ clearance vol.
i.e. = (swept vol./clearance vol.) +1

The rest of the thread isn't worth correcting.

Bahh, what would you use a term like swept volume and clearance volume? You have to keep in mind that he does'nt understand the basic concepts of an engine. Wich is probably why he is more confused than helped by your "information".

I'll try to put this as simple as I can.

Compression Ratio is the relationship between the cylinder volume (clearance volume) when the piston is on TDC(Top Dead Center) and the cylinder volume when the piston is on BDC(Bottom Dead Center)

TDC- Top dead center is the maximum distance a piston can travel up in the cylinder.
BDC- Bottom dead center is the maximum distance a piston can travel down in the cylinder.

Now lets say you have an engine at BDC inside of a cylinder. From the Tip of the piston head to to top of the cylinder wall measures 6". So thats a total of six inches between the top of the cylinder and the top of the piston at BDC

Now when the piston moves up to is compression stroke at TDC lets say you have 1" of distance from the top of the piston to the top of the cylinder wall. It its obvious that 6" of cylinder volume has been compressed into 1". So therefore you have a compression ratio of 6 to 1(6:1)
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Old 02-24-2005, 01:00 PM   #35
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[quote=CarEXPERT][quote=ChrisV]All superchargers and turbochargers compress air AND THEN push it into the intake manifold. That's the point. it's conmpressed IN the supercharger or turbocharger, not in the intake manifold (kind of like how a shop air compressor compresses air IN the motor part on top, then STORES it in the tank and is USED in the hose to the air tool)
Quote:

ChrisV I 99 percent sure your wrong on this. the ROOTS s/c doesnt compress air IN the s/c. It pushes the intake air to the manifold which compreses it. That is why the roots gets the hottest air compared to the other superchargers. The twin screw is the one that compresses air IN the S/C because the screws spin inward to compress it while a roots spin ******d to push.

Thanks, that cleared up the compression ratio issue

*sigh* You can mount a roots type to the side of an engine and run a long tube to the intake manifold if you want. The compression, like in ALL compressors (latham type, roots type, centrifugal, etc), is done IN the compressor. The increased pressure is FELT in the intake manifold on all types, as well. You've got the act of compressing the air mixed up with where the compressed air ends up.



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Old 02-24-2005, 01:01 PM   #36
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Quote:
Originally Posted by Wally
The rest of the thread isn't worth correcting.


Oh? We're trying to simpify things for the beginners.
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Old 02-24-2005, 01:14 PM   #37
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Quote:
Originally Posted by ChrisV
Oh? We're trying to simpify things for the beginners.

I think you do a very good job and thus avoid taking issue with you, but I have two reservations:

1) that the forum seems by default a nursery when it comes to anything technical;
2) I'm not convinced that trolls are extinct on the forum or else God help us all.
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Old 02-24-2005, 01:30 PM   #38
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Quote:
Originally Posted by Wally
I think you do a very good job and thus avoid taking issue with you, but I have two reservations:

1) that the forum seems by default a nursery when it comes to anything technical;
2) I'm not convinced that trolls are extinct on the forum or else God help us all.

1) unfortunately true. But, to much tech at once confuses people regardless of being on this forum, or any type of knowledge. Getting them thinking abiout the baiscs correctly can lead to understanding of the systems, so they dont'have a problem incorporating teh exceptions to the rules into thier knowledge base. Giving them to much formulaic knowledge up front usually limits their ability to incorporate teh exceptions, and makes their thinking rigid and say things liek, " but I learned it this way, so how could that be?" and then argue with you because the exception to the rule doesn't fit their formula. That makes them bad diagnoticians later, as they are unprepared for the variety of things that are out there OR be able to incorporte new thinking.

Once they understand the basics and the processes, then you can give them the formula, as they will fit the formula to what they see, rather than try to fit what they see to the formula.

2) I really have ceased to care about trolls. It takes too much energy to try and separate trolls from regular idiocy. If they are misinformed, educate them. If they argue, shoot them. Pretty simple, really.
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Old 02-24-2005, 09:59 PM   #39
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Quote:
Originally Posted by DSMer
Bahh, what would you use a term like swept volume and clearance volume? You have to keep in mind that he does'nt understand the basic concepts of an engine. Wich is probably why he is more confused than helped by your "information".



Now when the piston moves up to is compression stroke at TDC lets say you have 1" of distance from the top of the piston to the top of the cylinder wall. It its obvious that 6" of cylinder volume has been compressed into 1". So therefore you have a compression ratio of 6 to 1(6:1)

ChrisV already said what compression ratio is, but it just cant be that easy because I read a magazine that said the formula for c/r and it was like, piston volumn + gasket voulum+ cylinder head voulum DIVIDED cylinder voulum +.....+.... = compression ratio. C/R like ChisV said is the pressure of the air when the piston is at BDC of the intake stroke to the pressure of TDC of the compression stroke. It depends about the pistons length and rods and the bore and stroke like you said of the cylinders.
DSMer I dont think it can be just that easy to find the compression ratio like that and I know what swept volumn is and know the basics of an engine. You all think I dont know basics about engines but i do.

ChrisV, I know the other S/Cs compress air in the S/c but the ROOTS s/c doesnt have an internal compression ratio. Like I said in twin screw s/c the 2 screws spin inward with each other which compresses them. In roots the lobes are spinning ******d with each other which pushes the air to the outlet of the s/c.I read all this in a magazine so I think im right
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Old 02-24-2005, 10:29 PM   #40
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Important Questions

When the air/fuel ratio is 14.7 is that the best to get the most power out of the engine? What about forced inductions, should the ratio be the same? I noticed that on air/fuel dynometers or charts where they test the ratio when the speed goes it, the air/fuel ratio is never the same. Like when the wheel speed goes faster, the ratio of air/fuel is less to like 11:1 which is rich. Is it supost to get richer to get the most power when the wheel speed go up or would you get more power with a 14.7 ratio?

When your running too lean on fuel you get less power but if you run too rich will it be better or worse? I think the richer is better because it insures all the oxygens get burned from the fuel but I dont know.

With forced inductions, I know that you have to add more fuel cuz of the more air pressure but why do you need aftermarket fuel pums, injectors, rails just to get mroe fuel into the cylinders. Can't the OEM fuel things just add more fuel pressure when the MAP senses more air pressure in the manifold? I read that fuel pumps typically be able to flow about 1/2 gallons per minute! Thats impossible to use all that in a minute. What im asking is when you have more air pressure coming in, the computer should sense it and add more fuel so why are there aftermarket fuel things such as injectors that flows 50 lbs per hour! I dont think the engine uses that much.

When the engine is at idle, what happens to the throttle plates? Does the throttle plate open a little to get the air in or does the air bypass the throttle through an air bypass control.
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Old 02-24-2005, 10:36 PM   #41
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Quote:
Originally Posted by CarEXPERT
ChrisV already said what compression ratio is, but it just cant be that easy because I read a magazine that said the formula for c/r and it was like, piston volumn + gasket voulum+ cylinder head voulum DIVIDED cylinder voulum +.....+.... = compression ratio. C/R like ChisV said is the pressure of the air when the piston is at BDC of the intake stroke to the pressure of TDC of the compression stroke. It depends about the pistons length and rods and the bore and stroke like you said of the cylinders.
DSMer I dont think it can be just that easy to find the compression ratio like that and I know what swept volumn is and know the basics of an engine. You all think I dont know basics about engines but i do.


I can assure you ChrisV said nothing of the sort. He mention nothing about pressure of air. He said VOLUME. He even put the words VOLUME in italics. If you're going to sit there and argue what you thought you knew against what the truth is then stop asking for help. You repeatedly tell us that you "know the basics" when you repeatedly ask so many BASIC questions.

Why is that one whom can't says he can?

If you knew the basics, this discussion would'nt be eminent. If you're confused about something basic, then you don't know it. Just admit you don't know(its ok you have to start somwhere), stop arguing, read, and re-read until you understand.

The information I gave you on what a BASIC concept of compression ratio was from a highschool basic engine functions books. Thats how its figured out. Measurements and other formulas come LATER once you understand the basics. Its the difference between the VOLUME of the cylinder at BDC and the Volume of the cylinder at TDC. Thats what ChrisV said. Its just that simple, and thats all you need to know for now until more complex variables, definitions, and forumlas come into the equation.
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Old 02-24-2005, 10:47 PM   #42
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Quote:
Originally Posted by CarEXPERT

I read all this in a magazine so I think im right

Of course you do, so why bother asking anyone?
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Old 02-24-2005, 10:50 PM   #43
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Measuring Air/Fuel Ratios
Before a programmable management system can be effectively tuned, the air/fuel ratio needs to be measured. As described below, the air/fuel ratio will need to vary in different conditions, and so the meter needs to be accurate across a wide range of ratios. While the oxygen sensor found in the factory management systems of all cars can determine rich/lean scenarios, it is not accurate enough to be used in the tuning of programmable management.

Air/fuel ratios are typically measured using a so-called "wideband' air/fuel ratio sensor. This is usually just a normal oxygen sensor that is a little more linear in its behaviour away from the 14.7:1 'switchover' point (where the sensor output voltage suddenly changes from high to low) than a typical oxy sensor. More sophisticated sensors use UEGO or oxygen pump designs, but in tuning workshops these are still almost unheard of.

In addition to this high speed measurement, some workshops use a slower speed gas analyser, logging its results during dyno power runs so that they can compare those readings with the oxygen sensor system. The disadvantage of gas analysers is that they are too slow to get the instant results which are needed when tuning real-time. But for setting the steady-state light-load cruise mixtures, for example, a gas analyser is fine.

Most workshops have high-speed air/fuel ratio metres than read too rich at the rich end. All meters will be able to read around 14.7:1 mixtures in light-load, closed loop cruise - but that same meter may read a full ratio too rich at 10:1 air/fuel ratios. Meters typically read too rich because the exhaust gas temperature compensation is poor. Mixtures around 9-10:1 (ie ultra rich) will cause the car to blow black smoke, but even when workshop meters are displaying that figure, smoke is rarely seen. However, a meter reading richer than reality is in many ways a safe meter - the tuner won't set up the car dangerously lean. But a key question to ask of tuners is: how long since you replaced your air/fuel ratio sensor?

Ratio Requirements
A well-tuned engine used in normal road conditions has an air/fuel ratio that is constantly varying. At light loads, lean air/fuel ratios are used, while when the engine is required to develop substantial power, richer (ie lower number) air/fuel ratios are used.

Bosch state that most spark ignition engines develop their maximum power at air/fuel ratios of 12.5:1 - 14:1, maximum fuel economy at 16.2:1 - 17.6:1, and good load transitions from about 11:1 - 12.5:1. However, in practical applications, engine air/fuel ratios at maximum power are often richer than the quoted 12.5:1, especially in forced induction engines where the excess fuel is used to cool combustion and so prevent detonation.

There is no one air/fuel ratio where all emissions are minimised. At an air/fuel ratio of 14.7:1 oxides of nitrogen peak, while hydrocarbons and carbon monoxide (CO) increase substantially as the air/fuel ratio richens.

1. Cranking and Idle
The amount of fuel that needs to be added during cranking can best be determined by experimentation. This enrichment may be configured by just a one-dimensional variable based on engine coolant temperature, or it may be able to be controlled in a more sophisticated manner. Examples of the latter include post-start enrichment and enrichment decay time. Cold start is one of the dirtiest times in regard to emissions, and so if emissions requirements are to be met, a sophisticated ECU with multiple starting enrichment and decay maps should be used. Reducing the cold start enrichment but increasing cold acceleration enrichment will reduce the total amount of emissions. Some factory systems open the idle air bypass during cold deceleration, presumably to act as a form of exhaust air injection.

The air/fuel ratio required for a smooth idle will depend on the engine's combustion efficiency and the camshaft(s) used. Some engines with hot cams will require an air/fuel ratio as rich as 12-12.5:1 for a smooth idle, while others will run happily at 13-13.5:1. Engines with hot cams that are fitted with sequential injection management systems can run leaner idle mixtures than systems using bank or group fire. Those engines that can be configured to run in closed loop at idle will use an air/fuel ratio of about 14.7:1 when fully warmed, although they will still usually idle better at a slightly richer air/fuel ratio. However, keeping the engine air/fuel ratio as close to stoichiometric as possible will benefit emissions because the cat converter works most efficiently at this ratio.

2. Cruise
Light-load cruise conditions permit the use of lean air/fuel ratios. Ratios of 15-16:1 can be used in engines with standard cams, while engines with hot cams will require a richer 14:1 air/fuel. If a specific lean cruise function is available, air/fuel ratios of 17 or 17.5:1 can be used, normally at the standard light-load ignition advance. However, running too lean a cruise mixture will cause the cat converter to overheat. If a dyno and exhaust gas temperature probe is available, the cruise air/fuel ratio can be leaned out until exhaust gas temperature becomes excessive for these load conditions (eg 600 degrees C+), or torque starts to significantly decrease. Remember, an engine in a road car will spend more time at light-load cruise than in any other operating condition. The air/fuel ratio used in these conditions will therefore determine to a significant degree the average fuel economy gained, especially on the open road.

3. High Load
A naturally aspirated engine should run an air/fuel ratio of around 12 - 13:1 at peak torque. The exact air/fuel ratio can be determined by dyno testing, with the ratio selected on the basis of the one that gives best torque. Rich air/fuel ratios can be used to control detonation, and this is a strategy normally employed in forced induction engines. Thus, on a forced induction engine, the mixture should be substantially richer: 11.6 - 12.3:1 on a boosted turbo car and as rich as 11:1 on an engine converted to forced aspiration without being decompressed. As is also the case for ignition timing, the air/fuel ratio should vary with torque, rather than with power.

Most factory forced induction cars run very rich full load mixtures, with 10:1 being common. This is done for engine and cat converter safety reasons - in case an injector becomes slightly blocked, or the intake air temperature rises to very high levels. These cars will normally develop more power if leaned out. Note that emissions testing does not normally take place at full throttle, so full load emissions can be high without legal problems.

In the engine operating range from peak torque to peak power, a naturally aspirated engine should be slightly leaner at about 13:1, with the forced induction factory engine about 12:1 and an aftermarket supercharged engine staying at about 11:1.

4. Acceleration
During acceleration the engine requires a richer mixture than during steady-state running, with the extra fuel provided by acceleration enrichment. Under strong acceleration, the air/fuel ratio will typically drop 1 - 1.5 ratios from its static level. The amount of acceleration enrichment that is required is normally found by trial and error, and this is best done on the road rather than the dyno. The acceleration enrichment should be leaned out until a flat spot occurs, then just enough fuel to get rid of the flat spot should be added. This approach usually gives the sharpest response. Note that both over-rich or over-lean acceleration enrichment will result in flat spots, and that a greater amount of acceleration enrichment is needed at lower engine speeds than higher speeds.

5. Over-run
In road-going vehicles, deceleration enleanment is used to reduce emissions and improve fuel economy. This normally takes the form of injector shut-off, with the shut-off often occurring at mid-rpm (such as 3000-4000 rpm) and the injector operation re-starting at 1200-1800 rpm. High rpm injector shut-off can, in some cases, have the potential to cause a momentary lean condition.

Conclusion
How a car drives on the road is a pretty damned important part of owning a modified car - and in both the power that is developed and the driveability, air/fuel ratios are a vitally important ingredient

**NOTE**
On the subject of aftermarket fuel rails, injectors, and fuel pumps. Stock fuel components are good enough to supply enough fuel for the maximum and extreme conditions of the original engine characteristics. Once you bring forced induction to a N/A engine, or larger turbos and higher flowing fuel maps the stock parts won't be able to deliver the ammount of fuel that will be needed to supply an efficient enough a/f ratio. Thats why people purchase higher flowing fuel pumps, larger CC injectors, and larger fuel rails.

Also the flowing at .5 gallons a minute is just a measurement. A Corvetter may produce 400HP and 400TQ ar its maximum peak but it never continuosuly produces that ammount of power. Same as with car audio, the units are usually measure by their peak output. The pump can probably flow .5 galons of fuel per minute at one point in its peak of operation, but it should'nt need to flow that much fuel. Its just an operation measurement to give consumers an idea of the equipment they are buying.
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Old 02-24-2005, 10:51 PM   #44
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Of course you do, so why bother asking anyone?

I just want to make sure cuz you know how sometimes you read something and you still dont understand it so I just want to make sure...
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Old 02-24-2005, 11:09 PM   #45
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When the air/fuel ratio is 14.7 is that the best to get the most power out of the engine?

Not necessarily. best power is achieved when EGT is in a small temperature range (about +55C) on the rich side of peak EGT. Some guys may over fuel to provide some cooling of the air and limit det during boost (look up latent heat of vapourisation).

Quote:
Can't the OEM fuel things just add more fuel pressure when the MAP senses more air pressure in the manifold?

Only if they have the available duty. If the pump can't deliver sufficient fuel at a pressure req'd by the injectors to overcome boost pressure, the injectors will under fuel and give a poor spray pattern. Jap turbo cars tend to have sufficiently sized pumps for additional boost. Of course fueling is a little more complex than just recieving instructions from a MAP sensor.

The injectors must also have the capacity to deliver enough fuel for increased boost/power.

Quote:
When the engine is at idle, what happens to the throttle plates?

They close. You answered your own question.
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