...and the process is completely useless to any of us with aluminum suspension members ;)


Rotary engine vs everything else threads always find the rotary guys wearing rose colored glasses. The rotary is a neat and fundamentally different engine compared to a piston motor, but it does not have any real performance advantages over a piston motor. If it did poeple would be falling all over themselves to run rotary engines and all the piston motors would be in junkyards. You have to compare both sides equally and objectively. What ceramic coatings do for a rotary they do equally well for a piston motor. Ceramic coatings are not some silver bullet that makes the rotary superior.

I can certainly find some detail advantages to a rotary, but overall as a package it's at best equal to but most often inferior to piston engine installations IMO.

That's my car, but I'm not so anal-retentive that I Magnaflux everything.

There are methods for crack-checking aluminum, too. They use a dye instead of strong magnets.

not trying to bust your balls. I know you were making a point that alot of top level race teams will test for cracks after each race(whether it is magnafluxing or dye crack testing). I think this has even gone out of vogue with most top level teams just replacing parts instead of testing or checking the old ones.

That is complete BS! Nothing on a 50k mile rotary is worn enough to warrant replacement or a rebuild unless something is broken. If you had ever taken apart and speced a high mileage rotary, you know that all the internal seals would be perfectly with-in spec. The internal seals (minus the 3 piece apex seals) will easily last over 150k with proper maintenance. What your experiencing is the carbon sticking of the seals from owners who only drive their cars less than 6k a year to keep their miles low. Carbon sticking is what lowers the compression due to stuck seals and granny driving. My stock Fd has over 104k on the original engine and turbos and will still trap 100 mph in the 1/4 mile as if it were brand new. If my engine was due for a rebuild over 50k miles ago care to explain why it's still running so well?

Sorry if I got a little animated, I just hate generalized non factual statements like that.

Well, cermet coating have a very certain advantage to us. For one, when building a race engine, typically one would buy two new rotor housings (or three!), which are about $450 each. JHB takes your worn out, or scared housing, machines it down, and coats cermet on the surface, then re-machines it. Total cost is $380 or so, so its CHEAPER to have a cermet housing than a new one. The rotary also has a much higher ratio of surface area to be subjected to heat, and more surface area for friction. So a Rotary with full cermet would have much higher % gains, than a cermet piston engine. The final problem is shock loads A.K.A., piston slap, makes cermet/ ceramic no so good for high performance piston engines, as its very hard and could crack. Just what I have read so far about it.

Like I said, you have to think things through full circle. You know WHY the rotary has much higher % gains with cermet coating? Because it was so much worse to start with!!!! You cermet coat the rotary and make a big gain in heat rejection of the combustion chamber compared to before and yet you're still worse off than a piston motor.

I was referring to more aggressively modified turbo rotaries, not a stock FD or T-II.

I guess I am a optimistic guy. And when a engine, like the rotary is behind, I wont jump ship and badmouth it in the process. Rather I would try to see why its behind, and what I can do within my ability to improve it. It is fun and educational for me. And besides, the rotary does very well, its by far not the worst engine out there, and can lay down plenty of a bitch slap to alot of what is out there.

The fault you make is letting this become some sort of emotional argument for you :rolleyes: This is about engines and engines don't have feelings.

I enjoy my rotary powered FD and I'm certain I race it more than nearly everyone here. I have a blast with my car. Just because I enjoy my car does not mean I automatically insist it be the most wonderful thing ever invented and ignore its faults. I insist the rotary is far from perfect compared to a piston motor but that doesn't mean I can't still have fun with my car. I must have a better therapist or something as it doesn't bother me to think this way :scratch:

This is not entirely true. People race the motors that are available to them from the manufactures and also what the race classes allow. After Mazda won Lemans with a rotary the sanctioning body put very harsh restrictions on rotary powered race cars to ensure that it wouldn't happen again. Turbines are also a superior racing powerplant that have dominated in racing and then quickly been banned.

I'm not trying to claim that the rotary engine is the ultimate engine. I'm just clafiying that the fact the everyone does not use them has little to due with if they are "better" or not. The racing industry has a big interest is preserving the status quo and using the piston engines that are used in 99.9% of production vehicles.

The evil conspiracy theory again. All that was done was make certain the rotary powered cars did in fact have their displacements rated in such a manner that it was equivalent to a piston motor's displacement. The 13B for instance is NOT a 1.3 L engine. Besides, LeMans is only one race that happens once a year and has its own rules. You cannot use LeMans as the reason why you don't see more rotary cars; it's an oddball just like the Indy 500 is.

Bah. Using that logic explain the flex fuel and diesel racers we're beginning to see.

Let's make a supposition:

The rotary is as good or better than a piston motor.

Okay. Got that? Now let's continue on:

Mazda builds rotary powered vehicles and has done so for decades. Mazda has developed several generations of rotary engines and knows more about them than anyone. If the rotary is so much better than a piston motor why does Mazda insist on shooting itself in the foot as you would claim by putting piston motors in 8 of the 9 platforms it currently offers? Afterall, the claim around here is always that other manufacturers haven't developed rotary motors and rather than take on that burden they use what they already have and understand; piston motors. Mazda on the other hand has developed rotary motors and yet they refuse to use them except in a single model.

Once again why not try thinking this through full circle and applying a little logic. Mazda has already taken on the burden of developing the rotary and yet they themselves refuse to install it in everything they sell save one model. Instead they suffer the expense to develop and manufacture piston engines for all of their other models. I suppose Mazda is just completely stupid. Instead of installing these rotary engines that are just as good (or better!) they're willing to take on all of the expense and overhead in developing and building different piston motors to stick in their cars. What a bunch of stupid idiots.

The rotary is not a better solution to a given problem. It's a marketing gimmick. If it were inherently better than a piston motor Mazda themselves would install them in every (or at least more than one!) car and truck they sell. Mazda itself would never tell you the rotary is a better motor than the piston; the cars they build prove that point.

Before everyone gets creative with their refute and starts inventing reasons and explanations they themselves can't even comprehend, ask yourself if you're smarter and better educated than the company who essentially designed and built the rotary in the first place. My money is on them.

Well now you're talking production street cars. I'd agree that you're probably better off with a piston engine in vehicles used on the streets. But this thread is about racing.

I think its mutually accepted theyre not as high strung as a piston motor in NA racing, but Im still trying to find out about Turbo usage.

I have found through this thread that dealing with the heat produced is much more of a problem with turbo rotaries than turbo piston engines... but are they rebuilt any more or less frequently or at any more or less cost than a turbo piston engine?

Turbo rotaries tend to cost more to rebuild than NA ones because when the boosted rotary blows there are typically broken apex seals that ruin the rotor, rotor housing, side housings and possible the turbo itself as opposed to a motor that's just plain wore out and needs seals replaced.

As for costs vs a piston engine it depends on the engine. A pushrod 2 valve 4 cylinder is of course going to be much cheaper to rebuild than a 32 valve DOHC v8.

13BT vs a cookiecutter DOHC turbo-4?

I was wondering, why are you mostly interested in turbo application in racing? From what I can see, between harsher rules for turbo (both rotary and non) more complex systems (rotary and non), and a much less user friendly race car to drive, most race cars are N/a. When at mid-ohio, I'd would go as far to say 98% of the race cars are n/a. So I was just curious, if your interested in entering racing on a spec class, or just want to know how a rotary stacks up in general?

My engine has feelings, its always a happy go lucky engine, and loves to exercise on a daily biases, I just have to feed it low octane, and keep its air filter clean. Seriously though, I admit the rotary is far from perfect, I stated that Very Very early on. But I do believe in racing, it has many advantages, and sadly its VERY true that rule makers dont like the rotary. I have spoken with countless race teams, and worked on one for over a year, so I watched it happen.

In General.

ah, well, this wont help much, but I never have seen a turbo rotary do well at road course track. Pettit was there at Mid-ohio once with the turbo FD track car, and it got second to last. But it did finish with no problems.

I was referring to maintainance schedule and expenses (how often do you rebuild, and are they any diff than an NA rebuild...), and secondary and tertiary concerns (radiator, oil cooling, state-of-tune, pricing for different turbo sizes on a rotary vs a piston engine of equivilant displacement) and the like.

We all know a 13b will spool like a 2.6 liter piston motor with VERY healthy exhaust side flow and properly paried runners to the turbo basically out-of-the-box, so you can use a bigger turbo for that advantage, plus it has pretty good flow so you dont need to crank up the Pressure Ratio as high as you would on many other motors (stock headed anyway).

However, it takes more lbs-min of flow on a rotary to make the same ponies a piston will, granted its not an inefficient piston motor or the rotary isn't cermat coated (which more and more looks like the possible deus ex machina for rotaries) and for the concerns of wanting to keep the air charge cool you wanna go big with the turbo and relatively low with the pressure. I was just curious as to how this ends up changing costs and any impact on the rebuild frequency or expense per time.

Assuming your supposition was true, we also must conclude that history itself is altered by that basic fact, and thus Mazda would likely be selling more rotary cars than piston cars. You basically said "Let's assume this is true. So how come it's not true?"

A rotary doesn't spool like a 2.6L piston motor, at least not any that I've ever seen. Hotsides on most rotary turbos are quite a bit larger than "comparable" (displacement-wise) piston cousins. The rotary just doesn't efficiently use all the air entering it, so the hot side needs to allow for more flow (also for the fact that there are constant exhaust pulses). Like you said, the rotary needs more air to make the same power. I can't say what effect this would have on the intake-to-exhaust manifold pressure ratio, but it would be interesting to find out.

I think the hot nature of the rotary means that turbo selection will have to be more critical and better suited to the pressure, airflow, and powerband of the engine. Or maybe the rotary guys need to start looking into pre-compressor water/meth injection... Honestly I don't know how many people in the RX-7 community have tried it.

Ok, a 2.6 liter with a good paired header, a wild exhaust side and cam, and really retarded spark timing. (Good approximation for the inefficiency a rotary has leading to its hotter exhaust gas... you should see just what you can get on a piston motor by retarding spark sometime when playing with turbos)

Though, you have a point, you do need a bigger turbo both in outright size and A/R - but you dont need necessarily to support a high pressure ratio. Then again, is there a reason nobody has ever tried to use stock ports and higher pressure ratios? Is detonation that much of a concern?

I know what happens when retarding spark on a turbocharged car. That's how the antilag system on my car works. And I sure wouldn't want to drive with it retarded like that all the time.

I honestly think that porting w/low pressure versus stock porting with higher pressure is just a means to an end. Meaning, if you choose to run high boost levels, you can find a turbo that has good efficiency at high boost with less flow. Or if you've got a well-ported motor that's just gobbling up air, there are plenty of turbos that flow a lot at low pressures very efficiently. That's why, to me, it comes back to turbo selection. You can make almost any situation work.

Now, the thing is, a ported motor should have less pumping losses and therefore better efficiency than a stock motor. The fact that porting can also change port timing has double the effect (for instance, my car has a stock head with cams, and the cams helped overcome probably 85% of the inadequacies of the unported head). So with the right porting and turbo selection, pretty much everything in the system should be operating at a less demanding level.

It's kind of like back in the day with turbo cars, everyone just figured that there was no need to bother spending time or money on the head, because you could just turn up the boost. But then people started realizing that you didn't have to run as much boost if you make the engine breathe better. You start throwing in porting and cams and intake manifolds and now you're running down and to the right on the compressor map... the wrong direction. So then a new compressor wheel or even a whole new turbo must be considered to better suit the setup. Sure, maybe you could stick with the old turbo and max it out (34 lbs/min out of a small 16g at 20 psi? Damn near maxed out and then some!), but your compressor efficiency is likely crap and your IAT's are high.

I guess basically, it just makes more sense to run lower boost levels, and yes it's probably easier to control detonation (though I'm assuming the higher boost setup would offer better air/fuel mixture).

rotaries have a high surface area to combustion chamber volume ratio, making it less likely to knock than a piston engine.

"rotaries have a high surface area to combustion chamber volume ratio, making it less likely to knock than a piston engine"

that sounds bass-ackwards.

the surface area in relation to the combustion chamber volume is high. This makes it less likely to detonate. I never said rotaries don't have more severe consequences when they knock, and i never said rotaries aren't more likely to fail when knocking, i just said that they are less likely to knock.

read the book: rotary engine by Yamamoto, and read some of the tech articles at this website of a rotary manufacturer. www.freedom-motors.com

Rotary Advantages:

Somewhat self sealing at high rpm. If it will build enough pressure to start, it will make very close to original power at high rpm. No need to start rebuilding because your leak down test is down 5%.......

Over-rev capability! The occaisonal grab for 4th gear may result in engaging 2nd gear. Doesn't matter how many rev limiters you have, you just hit 13,000 rpm if the drivetrain didn't grenade. Seems to be pretty fatal to piston engines. Doesn't seem to hurt much on a rotary. Plus, you get to show off the tell-tale tach wrapped 1/2 way around past 9,000 in the pits!

Over rev capability part two: You fall a bit behind in a corner and need a good run down the straight to stay in front. Move your shift point up a bit. (We don't all have perfect trans ratios...) If you need that extra little oomph out of the last corner up to the start line for an extra 0.1 secs off your qualifying time, just don't shift until after the timing stand. Not many piston guys driving big buck pro built motors are going to run the motor out 1,000 rpm past the motor builders recommendations.

Lack of valves make for very strong wave tuning potential.

And finally: No piston motor ever built can have it's hp doubled with a die grinder.

No rotary engine can either, show me ONE completely bone stock rx7 NA or TII with stock everything (boost levels, cats, headers, tune etc. etc.) with only a port job that doubled it's hp.

Yeah, but in general, no valvetrain is a big plus for the rotary. The valvetrain is usually the casualty on a piston engine over-rev, and not having to buy cams, valves, springs, and worrying about guides and valve seals is nice, too.

EDIT: oh yeah, not to mention rod stretch, parasitic drag from reciprocating motion, piston-to-valve interference, etc. All those things aren't an issue with a rotary.

O.K. See attachment.

84 12A with big street port, Nikki with large Primary Venturi (done with lathe, but die grinder would work in a pinch. Same for the jets, a small drill bit or really small die grinder will do.....) The exhaust manifold is stock, and while the 2.25" single pipe isn't stock, taking the guts out of a converter and the baffling out of the stock muffler would get you in the neighborhood. The flywheel is stock, and all the internals are stock, unbalanced, and unlightened. Although you could use a die grinder to lighten And balance it if you desired.

While I find it hard to believe you would want to claim that the manufacturers claimed 104 flywheel hp would equal more than 90 rear wheel hp, realize that the drop in power is where it ran out of fuel. That motor later tightened a bearing up at 11,000 rpm when the fuel situation was corrected. It kept making power until it blew up. And no, it wasn't a stock fuel pump, but the fuel lines were.......

In short, this was a dyno mule made out of junk stock parts that no one in their right mind would have used to build a motor that they wanted to last. Apply die grinder, test theory, then try again.

If this is a problem with a piston engine, it's not one that's doing very well in any race.

great info here! Honestly, I had just about given up this site as a bunch of flamers until I found this thread. It's nice to be able to find a piston vs rotor thread with no flaming. :beerchug: