piston vs rotary engines!!??

Did anyone read this dec issue of turbo mag? They started a series on rotary engines and they explain how they work and some issues with them in detail. I'm seriuosly considering an RX-7 now. I just wanted to know does anyone think that maybe rotary is the best way to go for drag and just high speed driving in general. They do have much less moving parts it seems so less shit that can brake! It just spured my interest in rotary and want to know if any of you guys have any experience with them, or can anyone compare from experience cylinder vs. rotary angines reliability and race potential!? What are some of the advantages or better yet biggest disadvantages of rotary engines??? And how hard are they to work on for the do-it-yourselfers as compared to piston engines???

It just spured my interest in rotary and want to know if any of you guys have any experience with them,


Well, since I'm not a guy, I guess I cant answer any of your questions...

Oh well.

Well, if you want to go rotary then forget the RX-7. You're looking at all sorts of reliability issues. Go with the RX-8 if you decide to get a rotary. The one thing I don't like about the rotary engine design is that you don't have a lot of rotational force like you do in a piston combustion engine produced by the crank and rods. This causes the rotary design to have a loss in torque so i wouldn't suggest one for a drag racer. However; they have a very high redline making them great track racers.

thankx for your input guys and GALs! Sorry dayna240sx for excluding you! I have heard of the reliability issues with the rx-7, but i wonder if a race built motor would eliminate most of them or is it still just too much trouble?! The concept of rotary just seems really cool cuz it's relatively small displacement but capable of pushing some mad power! really fascinates me! Anyone alse wanna take a shot at this? Any links for some info would be appreciated!

Well, if you want to go rotary then forget the RX-7. You're looking at all sorts of reliability issues.


And thats differnt from people calling Cryslers unreliable because? :rolleyes: :rolleyes:



The very early Rotory engines certianly didnt have the longevity of thier piston engine counter parts, but with new seals, and proper maintiance a recently rebuilt one will last for an easy 100,000ks plus.
The new engines are of course even better, and when properly maintianed will last just as long as a piston engine.
The RX8 funnily enough has yet to have been in the market long enough to have its reliability tested.


The torque issue is also a myth, you have to remember they are relativly quite small capacity engines, and do infact have a very linear power curve, which when it comes to actualy going somewhere is far more important than any peak numbers.
Any of the RX series make great drag cars, there are plenty of sub 10sec cars, and Iv seen plenty of pics in Ausie mags of rotory powered cars flying the front wheels when launching.



Maintiance for the average owner is not an issue, they all still have an ignition system, and some form of either carberation or fuel injection, and they require regular oil changes just like any other car.
There is some specialist knowledge required when getting into rebuilding the engine, but its knowledge that is readily avliable, and anyone that can rebuild a piston engine can rebuild a rotory, usualy in less time as there are less parts.


Basicly they are not any better, nor any worse than a piston engine, they are simply differnt, and anyone who tries to tell you otherwise is mis-guided and ignorant.

Moppie, polygon never said that mazda's engines are unreliable....he specified a model...and you have to agree there are a lot of rumors saing that the rx7 engines had some issues...even the mazda engineers admited them....it seemed that the biggest problem was valve overlap...problem which they aparently solved trough the repositioning of the valves in the renesis


and you know too that the 1.3L its better measured as a 2.6L.....so its displacement its not that small


also I agree with polygon....i cannot see the rotary engines being capable of as much torque as the piston ones since they lack the leverage of the rods pushing on the crankshaft's lobes

Moppie, polygon never said that mazda's engines are unreliable....he specified a model...


True, but it was still an un-qualified statement.
The cars themselves are very reliable, its only the early engines that ever had problems, all of which will have either been fixed, or are easily now fixed.

How the displacement is measured is another issue, Iv heard a variety of theorys, but yes the most common one is to double the pyhsical capacity of the combustion chambers, so yes a 13b would be a 2.6l engine. But then most people also say that the functioning of the rotory is so differnt to a piston engine that you can't really compare them on capcity.



But torque certianly does not seem to be an issue with the engines, thier design means they have to be revved higher than most piston engines to make similar amounts of hp, but that in itself is not a bad thing, its just as I said above, differnt.
But remember that a rotory engine applies a more direct force to the crank than a piston engine does, and the diameter of the rotor I suppose gives the same effect as the length of rod and crank.

I am yet to understand the obsession with torque. Torque is not what accelerates a car, horsepower is. Yes torque is a factor it determining the HP, but so is RPM. YOu dont see people going crazy because their car has a higher redline, poeple would laugh at them. If torque were such a big deal then why arent more deisels race cars??? BECAUSE THEY DONT HAVE VERY HIGH HP.

Now on the topic of rotaries, the port overlap of a RX7 to my knowledge has no significant affect on the performance of the engine. It is a problem due to unburned hydrocarbons that caused it to fail emmisions.

One of the other problems with a rotary engine is the seals. They are definately the weakest part of the engine. There are many companies out there working to find the "perfect seal". The problem comes with durability vs performance. They make seals that can handle very high rpm and boost, but they would have to be replaced more than you would probably like, but taking apart an RX7 engine isnt all that tough.

but i would get an rx7 because they look sweet and car be fixed up for drag racing or you can do some sweet drifting in it too

....it seemed that the biggest problem was valve overlap...problem which they aparently solved trough the repositioning of the valves in the renesis

i cannot see the rotary engines being capable of as much torque as the piston ones since they lack the leverage of the rods pushing on the crankshaft's lobes

What valves?

You cannot have valve overlap, if there are no valves...

adding width to the rotor face is like lengthening the stroke in a piston engine... the wider the rotor face, the greater the torque.

Hey ~Ms. Rotary Bandit~
how about you put a twist on this discussion! Do you agree what everyone's saying about the rotary or .....? To me it seems like there realy isn't any reason to go with rotary other then liking the actuall cars that have rotaries. I just thought that maybe rotary had a lot more race potential and maybe more reliabitity - up to about 60k miles that is, after which they begin to die it seems! But if the biggest problem with the engine are the seals then with some r&d someone will find a way to overcome that problem. Turbo mag suggested that there are some aftermarket seals that do significantly better than stocks. Well, this is good info so far. Any rotary experts out there?

What do you want to know?

There a three good sets of aftermarket "seals" out there... And I assume we are talking about the Apex Seals.

Mazdaspeed motorsports has come kick ass Carbon apex seals. they really aren't good on a daily driven car because they really do not seal well until reletively high RPM..

another option is the Hurley seals
http://www.hurleyrotary.com

I have used their long life seals in basic rebuilds and plan to try their self lubricating seals on my RX-3 engine.. (its goal is to see 300+K miles w/o a rebuild/oerhaul)

their race seals are also very impressive.

The third choice are the seals made by atkins rotary,

there is another brand out there made by "rotary aviation" which i believe is the same as atkins..

they claim 700% greater bending strength and 33% harder seals than all other seals... I have never used them, but they sound tempting...

I have a nice chart in my RE handbook where you can calculate the forces on the apex seals, the extra "bending" would make a longer lasting apex seal, and cause less wear on the rotor housing...

what else would you like to know?

As far as i am concerned the Apex seal issue has been fixed for years.... really ever since they killed the 6mm apex seal in the early 12a's...

well the diameter of the rotor gives the torque in a rotary engine but you cannot make a direct analogy with the torque aplied by a rod in a piston engine


the rods in the piston engines aply 100% of the force to the point with most leverage (crankshaft lobes)

on the other hand inside a rotary the force its spread all over the face of the rotor and not on its apex where it would have the most leverage


dayna about those intake and exaust ports...i called them valves just because they do the same thing as in a piston engine and i'm very used to the therm valves....but nevertheles all the info i found on the rx7 atributed its poor gas mileage and problems to the overlap between the intake and exaust ports

and disco 192....torque is every bit as important as horsepower for perfomance...you don't belive me try autocrossing.....you hit a hairpin turn you drop out of the powerband and you'll see how long it takes to bring the rpm back up

Well unless you are going slower than 15 mph i dont see why you would be in the bottom part of your powerband anyway.

And what you are trying to say is low end power, not torque. Torque does not accelerate a car, horsepower does.

you hit a hairpin turn you drop out of the powerband and you'll see how long it takes to bring the rpm back up


A little tip, try changing gear ;)

A little tip, try changing gear ;)


well very often those autocross courses have very very tight turns and changing gear would mean putting it in first at speed...and i'm not gonna do that in my daily driver


and disco 192 if you would ever had raced before you would know that torque is as vital as horsepower....why do you think honda engineers raised the torque in the new s2000 because they are stupid?and don't say marketing ploy because most people when they buy cars don't even know what torque is

I realize that it is important to have a flat torque curve and power in the bottom of your powerband.

They gave the s2000 more torque so they could have more power in the lower end of the power band. They also lowered the redline, but they didnt do it so much for performance, they did it for driveability. They both run very similar 0-60 and quarter times, but the new one is a hair faster.

And acceleration equals the area under the torque curve, which is essentially the average HP. Top end HP isnt the only factor, but due to gear ratios it is a large one.

And yes, i know that torque is important but it just seems like everytime someone says the T word people cream their pants. Most people dont even truely understand what it really is.

well very often those autocross courses have very very tight turns and changing gear would mean putting it in first at speed...and i'm not gonna do that in my daily driver



Then learn to heal toe :p

I certianly never had a problem getting my SiR into 1st at 40kmh, you just have to give the throttle a little blip, and it, being a Honda, had one of the worst gear boxs Iv ever used.



But Rotorys do not have a problem with a lack of torque, thier extremly succesful history in rallying is proof enough, its always been regarded as a sport where who has the most torque wins.
And it shouldnt be that hard to find a picture of an old RX3 or something lifting its front wheels in the air at a strip, which should require at least a small amount of torque.

I was not generalizing that Mazda makes bad cars when I said that the RX-7 had reliability issues. I was focusing on the fact that the 13B isn't the best engine and is very high maintenance. What I am about to post are facts that have been admitted by the engineers that designed the 13B.

The engine had a pesky port overlap problem. This meant that exhaust gases were allowed into the combustion chamber during the intake process. This caused a few problems like excessive heat, which contributed to bursting apex seals. The turbo also contributed to that problem. This meant that N/A RX-7s were less prone to that problem. This also caused the RX-7 to have poor gas mileage and emissions. It also caused the car to stall at idle which increasing the engine speed at idle solved. These problems can't be completely negated by meticulous maintenance or better cooling systems. The problems will still exist and will still cause these reliability issues. Don't argue with me that the RX-7 wasn't very reliable and call me ignorant, because that is proof enough that it wasn't reliable compared to the other sport compact cars of the time.

As for the torque issue. Torque is VERY important in any kind of racing. Horsepower sells cars but torque wins races. In the simplest terms I can think of to explain it horsepower is how fast you can go in a gear and torque is how fast you get there. That isn't exact, but it is as simple as I can get. As for rotary engines and torque I stand by statement. They do not produce the rotational force that a piston engine does so the ratio is way off and the maximum torque is produced much higher in the power band. The specs on the 13B Turbo show that it has 255HP and 214 ft/lbs of torque. It has a descent amount of torque but ideal would be an even amount or more torque than horsepower. Also the maximum torque is produced at 5,000 RPM. In racing and especially drag racing it is very important to have that torque produced in the low end of the power band and hold strong throughout. However, this same pattern can be seen in some piston engines like engines using variable timing and lift systems. Remember that it is easy to increase horsepower but torque is quite another story. In racing that is just more important. I am also not saying that you can't get a good amount of torque out of a rotary I am just saying I don't think that you can't get as much.

As far as the Renesis goes, reliability will remain to be seen but I feel that they have fixed all the problems that the 13B had. They have removed the turbo and gone N/A. There is also no more pesky port overlap.

good stuff ya'll! this definately taught me alot! I'm no expert either, but i drive a 1991 sentra SE-R and those of you who know, it has one of the torqiest 4 cylinder engines ever made. i have driven many 4 cylinders to compare, and off the line my car pulls the best I think. This I definately attribute to the amount of torque it has!

A little tip, try changing gear ;)

In many autocrosses, there ARE turns that are tight and slow enough that they're 15-20 MPH. This drops the car out of the powerband, and most cars have a lockout that makes it very difficult to get the car into 1st gear with the car moving. Heel-toeing does not help. Even on a road course, coming out of slow corners the SCCA ITS Mazda RX7's get pulled by cars with similar power but less torque (Porsche 944 8v comes to mind).

Physics lesson: HP = Torque x RPM x (constant). If the RPM is low, the torque number is more indicative of the acceleration of a car. What this multiplication results in is the elimination of one degree of time as a factor in HP calculation, where it is a factor in the torque calculation. Higher HP = higher top speed, but for a lower torque and a constant period of time, it takes a longer period of time to get there. Fast lap times are about fastest times, not how fast you're going at the end of the straight. In my example, even though a slightly higher HP (or HP:weight ratio in SCCA ITS trim) Mazda RX7 may be going the same speed at the end of a straight as a Porsche 944, the Porsche will take less time to get down the straight because though the acceleration has begun to trail off, it has already made up the time at the beginning of the straight with its torquier engine and steeper acceleration curve. This is all assuming the same exit speed for the two cars, same shift points (at a higher RPM for the RX7 than the 944), and same length of time in gears. (In reality, the shift points of the 944 and RX7 are very similar, but due to the lighter minimum legal weight of the RX7, their cornering speeds are usually higher and entry speed onto the straight is higher as well. Because of this, the RX7's are usually a bit faster on most tracks than the 944's) I know someone who runs a 91 Sentra SER in ITS as well, and the top end speed is usually about the same as the 944's and RX7's, even though they have less power and are slower in the corners. That's probably better than the example I gave, though the dynamics of the SE-R is totally different than the 944 or the RX7.

there shouldn't be any debate over whether torque or horsepower is more important. fact is, they're both equally important in a fast car. thats the reason why diesel powered semis with loads of torque couldn't burn cars on a regular basis, even if they didn't have a huge trailer behind them. think of it like this...if torque is the force behind your engine, then horsepower is the ability of the engine to gain speed. that's the closest i can get to the proper explanation of it now... im kinda out of it.

rotary powered cars are cool, get over it

hy i have a mazda rx7 1980 13b 1/4 9.047 the diference is if you put the two motor together for race the mazda rotary you spend $5000 in the motor to run in the lows 10 second 1/4 but to put the piston motor to run lows 10 second 1/4 cost you 15,000 to 20,000 because you has to built all motor. for the rotary only you need a good compresor to port the plates and 2 or 3 trucos. in rpm my car easee touch 15,000 rpm wen i race i take my car at 10,500 rpm 0 to 60 in 2 second the 1/4 at 156.65 miles



Estos son los integrantes del Club de los 7 Segundos© para Orgullo de Puerto Rico.
Siguel Racing
Motor Rotativo 6.98 @ 199 mph


Siguel Racing
Motor Rotativo 6.98 @ 199 mph


Nibo Race (RX 7)
Motor Rotativo 7.76 @ 176 mph


Rafaelito
Motor Rotativo 7.49 @ 184 mph



Sammy Promotion
Motor Rotativo 7.61 a 177 mph

Nibo Race (RX 7)
Motor Rotativo 7.76 @ 176 mph



New Vivian
Motor Rotativo 7.77 @ 173 mph



Nibo Race (RX 7)
Motor Rotativo 7.76 @ 176 mph

Imprimir Listado
Ultima Actualización 28 de Agosto de 2003
Copyright 2002 PuertoRicoDragRacing.com © Do not copy, duplicated any parts of this website.

Did anyone read this dec issue of turbo mag? They started a series on rotary engines and they explain how they work and some issues with them in detail. I'm seriuosly considering an RX-7 now. I just wanted to know does anyone think that maybe rotary is the best way to go for drag and just high speed driving in general. They do have much less moving parts it seems so less shit that can brake! It just spured my interest in rotary and want to know if any of you guys have any experience with them, or can anyone compare from experience cylinder vs. rotary angines reliability and race potential!? What are some of the advantages or better yet biggest disadvantages of rotary engines??? And how hard are they to work on for the do-it-yourselfers as compared to piston engines???
http://www.rotaryengineillustrated.com/animations.html

Ok...short and simple. Torque gets you off the line and HP gets you down the road. Nuff said. The thing that impresses me about the rotary is its efficiency. Think rotational mass vs. recipricating mass. In a piston engin the piston/rod mass has to go a cirtain direction and suddenly come to a complete dead stop and change direction-recipricating mass. In a rotary engine the mass is rotating in one direction and never has to stop-rotational mass. Put a ball on a 3 ft. string and try to get it to go up and down the distance of 30 ft. then use that same set-up and swing the ball in a circle the diameter of 30 ft. what's more efficient? Duh. Also...the design alone of the rotary enging is responsible for the tremendous gains in power by adding small performance parts that would normally give the average Honda 3Hp. Rotaries Rule!!!

Ok...short and simple. Torque gets you off the line and HP gets you down the road. Nuff said. The thing that impresses me about the rotary is its efficiency. Think rotational mass vs. recipricating mass. In a piston engin the piston/rod mass has to go a cirtain direction and suddenly come to a complete dead stop and change direction-recipricating mass. In a rotary engine the mass is rotating in one direction and never has to stop-rotational mass. Put a ball on a 3 ft. string and try to get it to go up and down the distance of 30 ft. then use that same set-up and swing the ball in a circle the diameter of 30 ft. what's more efficient? Duh. Also...the design alone of the rotary enging is responsible for the tremendous gains in power by adding small performance parts that would normally give the average Honda 3Hp. Rotaries Rule!!!

1. You want to talk about efficiency? I don't think that allowing exhaust gasses in on the compression stroke is efficient. I also don't think running rich at idle is efficient. Suddenly, rotaries aren't sounding very efficient.

2. In a piston based engine, while the engine is running none of the pistons EVER come to a stop. If they do you have a VERY serious problem. Not even under severe detonation do they come to a stop.

3. You can't compare a rotary to a piston-based engine, size for size, when adding mods. Displacement isn't the same between the two.

1. You want to talk about efficiency? I don't think that allowing exhaust gasses in on the compression stroke is efficient. I also don't think running rich at idle is efficient. Suddenly, rotaries aren't sounding very efficient.

2. In a piston based engine, while the engine is running none of the pistons EVER come to a stop. If they do you have a VERY serious problem. Not even under severe detonation do they come to a stop.

3. You can't compare a rotary to a piston-based engine, size for size, when adding mods. Displacement isn't the same between the two.

actually each revolution the pistons come to a stop - twice

iirc, the problem with rotaries (besides having too little rotating mass to apply the clutch smoothly) is with emissions. after each 'fire' the volume of the chamber gets rather large rather quickly with respect to crank angle. Because of Mr Boyle, the resulting mas rapido loss of pressure cools the currently burning mix, slowing/stopping the autoignition of the advance reactions. This results in a rather large HC output. Correct me if I am wrong, but it was the inability to meet tier 2 emissions is why mazda/ford pulled the rotary for a number of years - I have not bothered to read up on how they solved this problem.

actually each revolution the pistons come to a stop - twice

iirc, the problem with rotaries (besides having too little rotating mass to apply the clutch smoothly) is with emissions. after each 'fire' the volume of the chamber gets rather large rather quickly with respect to crank angle. Because of Mr Boyle, the resulting mas rapido loss of pressure cools the currently burning mix, slowing/stopping the autoignition of the advance reactions. This results in a rather large HC output. Correct me if I am wrong, but it was the inability to meet tier 2 emissions is why mazda/ford pulled the rotary for a number of years - I have not bothered to read up on how they solved this problem.

1. No, like I said, the pistins NEVER come to a full stop.

2. The 13B had poor emissions because of port overlap and because the engine ran rich at idle. They solved this by moving the ports.

1. No, like I said, the pistins NEVER come to a full stop.

2. The 13B had poor emissions because of port overlap and because the engine ran rich at idle. They solved this by moving the ports.

I beg to differ, in a recip motor, at TDC and BDC the piston is indeed not moving or being acted on (force applied no movement) in any direction - for an instant[1]. there is no '3 second rule' :wink:

of course they 'bottom dwell' and the tdc reversal is more violent
(leading to nasties like rod stretch etc) but they do in fact stop

notes:

[1] the sideways force to the thrust side gets to its lowest point, but I think due to angular momentum of the small end of the rod, there is at least a dick hair of force still applied, but I might be wrong....its been a few years

The piston in a cylinder goes up and stops COMPLETELY then goes back down. What the fuck else would it do? The rod may not completely stop due to it's rotation at TDC but as far as a 180 degree change in direction it does completely stop. The design flaw, if that's what you want to call it, of the rotary enging may not be as efficient but the mechanical movement of engine parts in a rotary are. Increase the same psi in a rotary and piston engine and which one gains more power? Hmmmm? Oh...and don't give me that bull shit about increasing psi in a rotary vs. a fucking McLaren F1 V12. But then why compare the two...it's comparing apples and oranges right? So what's the point of this whole thread? Just to argue I guess. Bring it!


P.S. Don't get pissed, it's all in fun and games!

Well unless you are going slower than 15 mph i dont see why you would be in the bottom part of your powerband anyway.

And what you are trying to say is low end power, not torque. Torque does not accelerate a car, horsepower does.

Here's a funny thing though, throughout your gears, your horsepower stays exactly the same at any point in the powerband. Yet in lower gears, the higher torque multiplication accelerates you faster, as does adding torque from the engine, but if you ONLY add horsepower by revving higher (assuming your torque doesnt drop off too fast) then you dont accelerate faster at all, you just stay in gear longer.

First, I'm not pissed. Second, the piston does have to fight the inertial forces on it and if you really want to delve into physics I suppose it does stop but for a nearly immeasurable amount of time.

Still, it is far more efficient than a rotary.

First, I'm not pissed. Second, the piston does have to fight the inertial forces on it and if you really want to delve into physics I suppose it does stop but for a nearly immeasurable amount of time.

Still, it is far more efficient than a rotary.

So we can get this out of the way, I'll delve into the semantics of the physics of the internal combustion engine. All below are with respect to the engine block and negating the actual movement of the vehicle, and are true regardless of RPM (assuming nonzero RPM), change in RPM, or throttle input.

Position is constantly changing, and the rate of change of position is constantly changing.
Piston Velocity is zero at TDC and BDC, but the rate of change of piston velocity is constantly changing. The time the piston is stopped is 0, but there are two points where it is stopped.
Piston acceleration is zero at the midpoint of the stroke (this is only true for constant RPM, with changing RPM the location of zero acceleration can move slightly), but the rate of change of the acceleration of the piston is constantly changing. The time the acceleration is stopped is 0, but there is a point where there is no acceleration.

I hope that clarifies a few things.

Efficiency of an internal combustion engine is higher than for a rotary engine. Exactly why this is the case is beyond my knowledge base, a good indicator of this is the HP:Fuel Mileage ratio. A normally aspirated gen II RX7 makes about 160 HP and gets about 20 MPG, where a Porsche 944 is about 150 HP and gets about 25 MPG. A gen III RX7 Turbo makes about 270 HP and gets about 18 MPG, and a Nissan 300ZX Twin Turbo makes about 290 HP and gets about 20 MPG. There are a lot of other factors involved, but generally the efficiency for an internal combustion engine is around 20-22% and is about 18-19% for a rotary. This is all based on memory, so the exact numbers could be off, but the general idea is the same.

The problem with increasing boost in a rotary is the reliability of the apex seals. You do get a higher percentage gain in power and tourque for a rotary, but you can run more boost in a internal combustion engine more reliably than in a rotary.

Acceleration on a piston in a piston driven engine is NEVER zero. While one cylinder might be at tdc or bdc, another one is in it's combustion stroke. Since all cylinders are attached to the same crankshaft, the combustion in one cylinder would add acceleration in another. And all the pistons would have gravity acting on them.

Also:
Yes, velocity at the top of the piston reaches zero, but the bottom does not. Velocity is a measure of both speed and direction in a three dimensional space. At tdc and bdc, the crank may change direction vertically, but it is always moving around the crank, thus always has velocity.