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Old 07-31-2004, 02:01 AM   #1
Zalight
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Rotary VS. Piston; The never ending battle!

First off, if you don't know how rotaries work go to http://travel.howstuffworks.com/rotary-engine.htm

Pay special attention on the producing power section.

Now, I've heard a lot on this subject and done a lot of research on my own. But I want to hear some more opinions on the subject.

Rotary engines are my prefrence, and while they may seem complicated, if you learn more about them they are actually incredibly simple engines, especially when compared to piston engines; they only have 3 moving parts compared to piston engines which have hundreds. They also rev incredibly high, but unfortunatley this produces an extreme amount of heat, which is its main flaw. Another big flaw are the apex seals (the seals that seperate the combuston chambers) which are very weak at around 100k miles. But if you get a good cooling system and upgrade the apex seals to a racing setup, then you will have an incredible engine that can easily handle 350 hp without changing anything else. And once you upgrade the output shaft(crank shaft to you piston lovers) you will be able to push around 600 horse without any worries to your engine. Now turbos, tranny, chassis, and rear end are a different story at 600 horse, as with any car.

But any way, let's here some opinions on the subject shall we?
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Old 07-31-2004, 02:51 AM   #2
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this thread will end up getting closed really soon, im sure of it.

but i guess ill just lay off my very simple opinions without going into details.

first, they are called eccentric shafts.

that out of the way. well, i can bet over and over again, that if one tuner builds a piston engine and a rotary engine with the exact same quality, the piston engine will last longer cuz of obvious reasons. it is said that rotary engines can rev at high speeds, but only if built correctly. the RX-8 is able to do this cuz of their new SiC seals and some other nice mods. its pretty much a modified 13b.

the advantage rotaries have are, they produce very nice power levels of their small displacement. they are also alot more compact than the average conventional piston engines (obviously). they are cheaper to tune due to alot fewer parts, so cheaper to build too. also, no problems with center of gravity like in piston engines, such as inline 4's needing 2 balancer shafts, just like other engines.

the rotary disadvantages would be that, well, very few mechanics actually work with them. many of those few dont even know wat they are really doing. even though they contain very few parts, they are hella expensive. they need alot more attention than piston engines too. when they break, its bye bye engine, unless you have hurley seals or something similar. another thing is their low compression levels, which is a problem in NA vehicles, atleast more of a problem anyhow. these also need a more sophisticated fuel system.

piston engines.... everyone should already know how they work if they are browsing around here, else they need to pay a visit to howstuffworks.com and read a few manuals. disadvantages of these engines are that they contain alot of moving parts. this means that there is alot of vibration that needs cancelling (balancer shafts come into play when needed). some engines dont need this balancing, such as inline-6's, V12's, and boxer engines, but they still cause more vibration than rotaries. these also weigh alot and need a larger displacement than rotaries to produce equal power. also, since there are so many parts in these engines, it takes alot longer to fix things and find problems.

advantages of the piston engines. for starters, they obviously last longer and are alot more reliable in any sort of climate. they are far more efficient than rotaries fuel-wise. compression in these engines can be easily controlled. you can throw more crap at these engines than in rotary engines with a higher room for mistakes. these engines have also proved themselves to be able to rev at high limits, it just depends on the application and internal components, which are usually defined by the purpose of the vehicle (raw massive torque power or high revving drifting vehicle). a bit more expensive at first than rotaries but parts for piston engines are actually alot more cheap compared to rotary engines, specially if u know where to look (connections help so much). every mechanic should know how to work with these engines, so help is easily found. its even really easy for people to learn how to deal with these engines.


ok, so i got into a bit of detail, but screw it. i could have gone even further but im not even gonna bother since it will get closed after everyone starts fighting.
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Old 07-31-2004, 06:29 AM   #3
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This subject crossed my mind before and I came to a little thought.

THe piston engine has had more time and money to be developed. But the rotary engine, even though the idea has been around for a long time, hasn't had the same amount of attention and money put into it. If the rotary engine and the piston engine were to "compete on a level playing field," I think the rotary might be a lot more competitive if not surpassing the piston.

I can't say I know all the specifics on both engines, but that's just something that I thought of.

Also, I think I read somewhere that rotary engines are unable to produce as much torque as piston engines. But since that wasn't mentioned, is that true?

Personally, I like the rotary engine. I like how it's sort of the underdog. Kinda inspirational in a metaphorical kind of way.
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Old 07-31-2004, 12:12 PM   #4
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rotary engines havent been given the same amount of research cuz all manufacturers, except mazda, say they are too inefficient for practical uses and such. with today's really strict standards, rotary engines need alot of development to keep up with piston engines in every single aspect. even though rotary engines are simple, they are complicated to work with.


it would be impossible to put an equal amount of research into rotary engines, since piston engines have had so many more times amounts of research put into them. its already past the "point of no return". its just like the Miller Cycle piston engines.


as for the whole torque issue, yea, it's true. many people will argue that they have less torque capacity due to their incredibly smaller displacement, but lets face it, rotary engines dont like producing that much torque. the displacement is just something huge rotary fans use as an excuse. others will just say, "fine, ill just get a 20B or a quad rotor engine (the 2.3liter one)". fine, you do that, but 3 rotors are even more expensive by A LOT. while you would be spending around $330 per ceramic apex seal, and there are 3 per rotor, and its 3 rotors. you do the math . dont forget the even more complicated fuel system and such. theres also the extra machine work, which hardly any mechanic knows about.
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Old 08-01-2004, 06:51 AM   #5
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Rotaries do produce a lot less torque then piston engines, but they certainly aren't lacking tourque. And the idea that if somone put in as much money, research, and time into rotaries they would equal to or greater than piston engines is a very valid point of view. Just look at the differences between the 13b(RX-7 engine) and the renesis(RX-8 engine).

The renesis makes 235 horsepower naturally aspirated, the 13b makes 255 pushing 10 psi...think about the power out put of a NA 13b. Thats about 150 horsepower, see my point. Mazda made a jump of 85 horse power by changing two major things; the exhaust port was moved from the side housing to the housing face, the same location as the intake. This put a stop to wasted fuel by cutting down on the amount of unburned fuel to be expelled with the exhuast. It also added huge amounts of power by stopping something called rotor overlap. This is when leftover exhaust in the combustion chamber before the intake stroke because the motor starts drawing in fuel and air before the exhaust stroke is complete... Get that? Basically the engine starts drawing in fuel and air while the exhast port is still open. This does two things, it allows unused fuel to escape, and used gases to be reused. Not much of either, but enough to cut out about 50-60 horsepower. the renesis put a stop to this. Unfortunatley, for reasons of which I am unaware, this also means a huge loss in back pressure. Which means the renesis can't spool a turbo. Still, it doesn't make sense why mazda didn't release a superchaged version of the RX-8, more power than the RX-7 with all the wonderful new goodies of the RX-8. But that's not important, the second way the renisis makes more power is that mazda added another fuel injector that sprays an incredibly fine delivery of fuel that is easier to break down and therefore easier to combust.

Anyway all this rambling was to make a point. With just two major engine modification the renesis produces about 80 more horsepower then the 13b, think of the number of these modifcations that have already been done to piston engines. just wait till mazda finds a new way to squeeze more power out of there wonderful rotars.
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Old 08-01-2004, 10:25 AM   #6
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actually, the RENESIS is just a modified 13B with natural aspiration.
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Old 08-01-2004, 09:27 PM   #7
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Quote:
Originally Posted by Inygknok
actually, the RENESIS is just a modified 13B with natural aspiration.

That's exactly my point. They only changed very little and got tremendus horsepower out of it.

I don't think anyone is really posting on this because, like you said, not many people know how they work.
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Old 08-02-2004, 01:24 AM   #8
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Quote:
Originally Posted by Zalight
That's exactly my point. They only changed very little and got tremendus horsepower out of it.

I don't think anyone is really posting on this because, like you said, not many people know how they work.


yea..... i dont blame them though. rotary engines arent common at all, like boxer engines. though, boxers are piston engines so they really aint so complicated to understand. a lot of people say they know how rotaries work, but when it comes down to details, they really have no idea wat they are talking about.... so maybe others dont wanna make mistakes in order to get a flaming fest on them.....
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Old 08-02-2004, 08:31 AM   #9
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its always been quite common knowledge among rotory owners that just a little bit of port work results in huge gains. relative to other engines. rotory engines are probably the easyest engine to mod for performance, due to the inefficiancy of the factory set up. My old colegue is a rotory fanatic that drives me bonkers!

I'm not a rotory nut or anything, but I do like them! we got a qaud rotor over here n/a that's pushing out 447kw (i forget the hp conversion, i think about 600hp). thats with no turbo, no blue bottles, nothing!

I admit i know nowt about how they really work, but looking at an engine caseing, it seems to be a type of 2 stroke motor? of sorts anyway?
and i do know, that from the series 6 i just delivered to a customer, they are great fun
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Old 08-02-2004, 08:50 AM   #10
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I have had the chance to drive a RX-7 turbo (engine manifold and turbo manifold cracked even) and I have to say that I have never seen that performance out of any other car. I do love my little gas efficient Honda, but man! **Drools** Too bad those cars are so finicky! That poor thing is rotting in a driveway now because someone engaged the turbo repeatedly on a rebuilt engine before it had reached 100 miles.
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Old 08-02-2004, 04:01 PM   #11
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A rotary engine will inherently have less torque than a piston engine: torque is force multiplied by the distance it is applied over. Think of a weight on a see-saw: the longer the see-saw, the more torque is applied.

With a rotary engine, the force is applied to the side of the rotor, which is a shorter distance from the centre of the crankshaft than on a typical piston engine.

People often confuse torque and power. Power is basically torque, multiplied by rotational speed (rpm). Here's where a rotary engine comes into its own: there's no piston constantly accelerating, decelerating anc changing direction, so it can easily spin much faster. That means that it is more powerful than an equivalent piston engine, because although it has less torque, the crank is spinning faster.

Unfortunately, rotary engines also use a lot of fuel. For most applications, rotary engines offer more problems than they fix, when compared with a piston engine. They're only really suitable for light sports cars or motorbikes, where people are happy to use high revs.
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Old 08-02-2004, 11:15 PM   #12
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Yeah, which is why one of my favortie cars of all time was the Datsun 510. A rotary powered, all wheel drive, ultra light wieght, rally beater that revvs till the cows come home!
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Old 08-02-2004, 11:31 PM   #13
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ok, a rotary could concievably reva farther than a pisotn engine. In theory. In reality, the flame front across the large face of the rotor limits it, as well as total breathing. You can increase the breathing, but what passes for low end driveability goes away in a hurry.

A stock 13B revs to about 7500 rpm. A modded one that retains useable drivebility (i.e a street port, rather than a bridgeport) doesn't rev that much higher. it takes a fairly radical porting jopb to get a 13B to rev past 9000 rpm, and it will not be very streetable at that piont, nor will it last very long. the 12A could rev higher (my long street port 12A powered RX3 made peak power at 9250 rpm) but it makes noticeably less overall power than the 13B. the 20b makes more power yet, but the added rotating mass linmits overall rpms.

The old IMSA RS engines that I helped with for the IMSA RS RX3 team I crewed with could turn 12k, but not for long periods of time (the front runners could go to 14k, but would only last one race at a time).

Contrast that to modern 1 liter sport bikes, with piston enignes that turn over 10k regularly. or the old Chevy DZ302 from the 1967-1969 Z/28 that turned 8000 rpm in stock form (yes a pushrod V8) and with minor mods, could turn 9000 rpm regularly in streetable form.

My Ford 302 in my RX7 turned the same peak rpm as the original 13B in it, but made 3 times as much hp and 4 times as much torque, for less money than a swap to a stock TII engine, and vastly less than the equally heavy 20BTT. In fact, my swap gave me better weight distribution and handling, as well as vasly more power down low without sacrificing any high rpm ability that the 13B had.

I love rotaries in little light cars, and I think a 20BTT in a '65 Mustang fastback would be cool (weighs the same as a 2nd gen RX7...). But the rotary is not a "better" way to make power, merely a "different" way to make power.
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Old 08-02-2004, 11:39 PM   #14
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I forgot to mention, besides the IMSA RS RX3 I mentioned, The team also had a rotary powered Formula Libre. I've also owned an R100, RX2, 2 RX3s, a pair of first gen RX7s (one with a highly ported 13B from an RX4) and the second Gen RX7 that was a rotary powered autocrosser for teh first year I had it before the V8 swap. I've also put rotaries into other cars, including an MG Midget I had.
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Old 08-02-2004, 11:48 PM   #15
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Quote:
Originally Posted by Zalight
First off, if you don't know how rotaries work go to http://travel.howstuffworks.com/rotary-engine.htm

Pay special attention on the producing power section.

Now, I've heard a lot on this subject and done a lot of research on my own. But I want to hear some more opinions on the subject.

Rotary engines are my prefrence, and while they may seem complicated, if you learn more about them they are actually incredibly simple engines, especially when compared to piston engines; they only have 3 moving parts compared to piston engines which have hundreds. They also rev incredibly high, but unfortunatley this produces an extreme amount of heat, which is its main flaw. Another big flaw are the apex seals (the seals that seperate the combuston chambers) which are very weak at around 100k miles. But if you get a good cooling system and upgrade the apex seals to a racing setup, then you will have an incredible engine that can easily handle 350 hp without changing anything else. And once you upgrade the output shaft(crank shaft to you piston lovers) you will be able to push around 600 horse without any worries to your engine. Now turbos, tranny, chassis, and rear end are a different story at 600 horse, as with any car.

But any way, let's here some opinions on the subject shall we?

I don't know if anyone has mentioned this yet, but the Rotary is a very inefficent motor. It burns gas MADLY. I saw somewhere where one review site got the SAME mpg from it's Rx-8 as it did it's Nissan Titan. It may be small (1.3L, but really 2.6L in comparison to a piston motor due to the cycle of the engine compared to a piston motor) but it's a gas guzzler.

My girlfriend's 13b-rew just crapped out a few weeks back in her 94 Touring RX-7...it lasted to right before 100k....stock motor (yes, it was a apex seal)..cool thing is, she surprises me before we go on our trip with a shane's racing 20b we ordered and she installed it right before our trip. It's nice...big power (ported out...etc.)...best of all it's N/A and doesnt have that extra heat from forced induction.

One thing I want to know is this...has anyone ever supercharged (if it's possible) a rotary?
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