Quiksilver540

While scanning my local thriftstore's magazine isle, I picked up the new issue of Sport Compact Car. I ended up buying it because of the interesting article on RX-7s. Written by Dave Coleman, it goes on to say how the new RX-8 Renesis engine is in fact 2.6 liters, and the older versions of the 13b were also 2.6 liters. 12a engines are 2.3 liters and the "monstrous" 20b is actually 3.9 liters. For all of us who was told that the 13b was 1.3 liters, "you were all lied to. Sorry."

The following is all information from his article:

The rotor, you see turns at one-third the speed of the eccentric shaft (crankshaft in piston-speak). And with three chambers, that means each rotor has one intake stroke and one exhaust stroke per revolution, just like a two-stroke engine. Mazda seized on this similarity with the two-stroke, and declared that rotary displacement was the displacement of one chamber per rotor. And since I wasn't there to argue with the engineers, they got away with it.

Dave Coleman goes on to say that the rotary is in fact a four-stroke engine, not two-stroke. Stating it has separate intake, compression, exhaust, and power strokes. Not to mention this is a four stroke world. Also on a four stroke, theres only one intake stroke for every revolution, so displacement is how much combustion chamber volume gets sucked in over two revolutions. For a rotary, with one intake stroke per revolution, that means measuring two combustion chambers per rotor, not one like Mazda says.

Mazda's 13b formula - 654cc x 2-stroke = 1308cc
Dave Coleman's formula - 654cc x 4-stroke = 2616cc

Okay now here is my question! I'm not one to jump in and regard something as right simply because I read it in a magazine. So, I'm asking those who are rotary engine knowledgable, is this guy right? I'd recommend picking up the article and reading it. He also has a lot to say about the new RX-7.

Decay

First off, there is no "new" RX-7, Coleman is just speculating on what he thinks should power it. He makes some good points in his article, but this thread will get about 500 replies and usually gets pretty messy.

Do a search, there was a particular thread that went on forever on the subject...

911GT2

I can see Colemans point, but the developers and pioneers of the rotary engine, including Dr. Felix Wankel, who invnted the thing, measured the capacity in the same manner mazda did, and still does. So I'd trust them before a magazine any day.

rynberg

From a certain point of view, he is correct. But as far as I know, engine displacement for rotaries has always been measured the way Mazda measures it.

Also, I wouldn't read any of the import tuner mags for any accurate statements about RX-7s. In general, they are grossly ignorant and don't even perform proper research for the articles. In fact, it is apparent from the few issues I've looked through that they usually don't know much more about cars than a great deal of their target audience.....if any.

Scrapiron7

As far as I am concerned, it will always be 1.3L until Mazda themselves say otherwise..

Quiksilver540

Thanks for the replies, and I agree with you guys. Until Mazda claims it to be anything else I'll still regard it as 1.3L. Also I didn't mean he had actual information on the new RX-7, just speculation as was stated.

Wargasm

This whole argument comes from the fact that a 4 stroke piston engine has to go down twice and up twice to make power. The rotor in the rx-7 manages to get the job done without wasting any movement...

It's not the rotary's fault that piston engines aren't as efficient in this regard.

Brian

Quiksilver540

I keep getting mixed responses though. Is he 100% correct, and is the 13b actually 2.6L? Or is he wrong?

Everyone seems to think he is correct, but still thinks of the 13b as 1.3L because mazda measures the displacement that way; However, Dave Coleman has strong evidence against this. I retract my previous post, because I actually want to know the truth. Can anyone answer my initial question?

poss

iTrader: (5)

he's trying to compare the displacement of a rotary to a 4 stroke engine!?!?! it's a totally different kind of engine, therefore they shouldn't be equated like that. why don't we try to compare 2 strokes to 4 strokes?.....why not, because they are fundamentally different. Similar to the fundamental differences between a rotary and a piston engine.

Brentis

I'll tell you one thing, if it was measured on gas mileage we'd be closer to a 5.7 liter...

Garrett

I would have to agree with Mazda and say 1.3 liters. All Dave Coleman is trying to do in my view is trying to equate the efficiency of a rotary engine to a piston engine.

A rotary engine gets the job of a piston engine done quicker and more seamlessly. If this prompts some people to disagree with Mazda engineers (who designed the engine), then so be it. I really don't think comparing two almost totally different engines in the same terms will work. Because, in my mind that is what he is trying to do.

It's almost like giving a piston engine a handicap for being such a waste of unnecessary parts to do the same job. However, this is not golf and the rotary engine is not 2.6 liters. All one would have to do is just pop the hood of a comparably powered piston engine to that of an RX-7 to see the disparity of size.

But...I would like to see an engineer of some sorts to debate this with real data and calculations. Those are always fun battles to watch on the forum.

garrett

paw140

Dave Coleman explains it well, and he is absolutely correct, if you are trying to compare the displacement of a rotary engine to a piston engine. They are inherently different designs, so it is nearly impossible to compare the two, but his method is probably the best.

If you measure the displacement of an engine by the amount of air it pumps per two crank revolutions, like every four stroke piston engine in the world, then the 13B is a 2.6L. If you prefer to think of it using Mazda's logic (misleading, in my opinion), then go ahead and call it a 1.3L.

It seems to me that most people like to call it a 1.3L simply for bragging rights. I was talking to a co-worker the other day about my FD, and told him it made about 300 hp. He asked me the engine size, and I told him 1.3L. He was soooo amazed, comparing it to his 70 hp Toyota something, which had around 1.3L. Is that a fair comparison? Hell no!

As far as pulling out engineering calculations... I'm an engineer, so I'm up for an engineering debate, but there is really nothing to it. Hell, you could even call it a 3.9L (6 chambers, 650cc each), and still be technically correct.

In summary, the 13B can be a 1.3L, a 2.6L, or a 3.9L, depending on how you want to think about it.

Houdini

iTrader: (3)

well, if you want to compare it to a 4 stroke piston, it comes out like this. A 4 stroke requires 2 revolutions per chamber to complete a full cycle. A rotary engine has 3 combustion chambers per rotor but because of the planetary gears used in the eccentric shaft, it's revolutions per turn of the eccentric shaft(essentially the same as a crank shaft) gets reduced by a factor of 3. So the rotor has 3 cycles per revolution of the rotor but only 1 cycle per revolution of the shaft.

In comparison then, the 4 stroke requires 2 revolutions of the shaft to complete the cycle when the rotary requires only one. So if you wanted to compare a rotary to a 4 stroke piston engine you get:

1 Rotor = 2 Pistons

13b is a 2 rotor engine which is equivalent to a 4 piston 4 stroke. So, if you use the 4 stroke standard for displacement calculations, you get "4 pistons/combustion chambers" x 0.65litres per chamber = 2.6litres.

I always questioned this myself, wondering how a 1.3l engine was producing 255hp with only 10lbs of boost. I never knew what the reduction in rotation of the rotor was from the planetary gears, but assuming 1:3 is correct, the translation to a 4 stroke is a 2.6litre.

Hey, all those years in college for a mechanical engineering degree finally paid off.

paw140

That's a good way to look at it. Or you can just look at how much air the engine pumps per crank revolution, since engines are basically air pumps. A 2.6L piston motor and a 1.3L rotary both pump 2.6L of air per two crank revs.

Houdini

iTrader: (3)

Yes, my point exactly. That's why I calculated everything based on the shaft revolutions since in the engine, that's what your engine RPMs are based on.

Aviator 902S

There's a couple of other factors to also consider when trying to figure out how such a small engine can produce so much power.
The first is obvious: no valve train means more power available to propel the car.
The second has to do with the duration in degrees of eccentric shaft rotation per power sequence (stroke). With a 4-cycle piston engine, the power stroke sees the crankshaft through 180 degrees of rotation. With the rotary, this power sequence lasts through 270 degrees of e-shaft rotation.
And yes, the 2.6L displacement theory does make sense. If you take a 4-cylinder piston engine (4-cycle), displacement is calculated by figuring the displacement of one cylinder as it travels from BDC to TDC, then multiply this figure by the number of cylinders. In order for all four pistons to experience a power stroke the engine has to complete 2 revolutions. (In one rev, only two of the four cylinders fire.)
The rotary is similar to the 4-cycle 4-cylinder engine in that it also fires twice per rev. (2 x 654 cc = 1308 cc, approx. 1.3L) But remember, it takes 2 revs for the piston engine to fire on it's entire displacement capacity, so it's only fair to compare apples to apples here. The rotary displaces 1.3 liters per rev, so for two revs it displaces 2.6 liters.
In spite of this, I'm still referring to my 12-A (actually 1146 cc, which is actually 2296 cc according to the above theory) as a 1.2L---Because it sounds better!

Crionics

Wrong! For a 4-stroke boinger, after 1 complete revolution of the crank, the engine has in fact displaced it's entire volume. Every piston moves up and back, or down and back. You are referring to power strokes. It takes 2 revolutions of the crank for each piston to make 1 power stroke, which is what everyone who believes the 2.6L myth are actually referring to.

For a rotary, it takes 1 e-shaft revolution to make 1 power "stroke" per rotor. I am at a loss for a better word since the rotary doesn't make strokes per se.

Houdini

iTrader: (3)

Which ever way you look at it, number of revolutions of the "shaft" for one rotor/piston to complete one cycle or number of revolutions between powerstrokes, you come up with the same number. If you read my post a couple above the one you quoted, you can see I said it right. You actually have to study this stuff to get a degree in mechanical engineering. I'm edumacated over here.

paw140

Wrong! In a piston engine, each cylinder DOES NOT displace it's entire volume every crankshaft revolution. There are four strokes: 1)intake - a cylinder draws in the intake/air charge (no displacement) 2)compression - compression of the intake/air charge (no displacement) 3)power - ignition forces the piston down (no displacement) 4)exhaust - the cylinder expells it's entire volume (displacement). This cycle takes place over two crank revs. Thus it takes two revs for a piston engine to displace all it's cylinder's volume.

The 2.6L thing is not a myth, it is fact. I don't know why people can't understand this.

ERAUMAZDA

DISPLACEMENT IS NOT REV BASED!!!!!!!

ERAUMAZDA

For every 1 rotation of the wankel crank, the engine takes a 1.3l breath.

ERAUMAZDA

if u want to be critical the 13b is a 3.9 because all the faces on the rotor are fired after 3 revolutions.

paw140

I said this in one of my other posts. For comparing to a
piston motor 2.6L is the most realistic value, in my opinion.

Houdini

iTrader: (3)

bad comparison...4 stroke piston displaces its entire volume in 2 revolutions. So you have to see what the rotary does in the same amount of revolutions for an apples to apples comparison. Like you said, the rotary displaces it's full 1.3l in a revolution so that's 2.6l for 2 revolutions. The reason we're using RPMs as a basis for these calculations is because that's what the output of the engine is in. That thing on your guage cluster called the Tachometer measures RPMs(revolutions per minute) of the engine "crank shaft". By some coincidence, piston engines have one as well. Since both output horsepower/torque through that shaft to the drivetrain by turning, we'd like to know how much gas is being sucked up by the engine in that turn to produce that horsepower.

paw140

I was just thinking about this, and from an engineering standpoint, putting piston engines aside, it makes the most sense to call the 13B either a 1.3L or a 3.9L. I could make a strong argument either way. Calling it a 2.6L doesn't really make much sense, except for comparing it to a piston engine. But that's really what we're after here, since 99.99% of the cars on the road have pistons.