Not at all. For the purpose of determining capacity you can forget about rotors, pistons, valves, ports, seals etc. All that is important is the chambers and the output shaft. Both engines work by allowing chambers to expand and contract. Through some magic in both engines these expansions and contractions are turned into output shaft revolutions. These engines are actually very very similar and a direct comparison can be made.

The RPM of the rotor itself is completely irrelevant. All that matter is the chambers. However, if the drive was taken from the rpm of the rotor wouldn't that make the engine a 7.8 litre?

That depends on the engine you look at. Its easy to tune an engine to run at higher or lower rpm but I would say that in general a rotary does about 1.5 times as many rpm as most 3.9L sixes. Rotaries way back in the rx3 days were revving to 7000rpm standard while the sixes generally only did 5000 standard.

The gearing does not give you any advantage or disadvantage, the diff ratio is just changed to compensate. Most rotaries run approx 1.5 times bigger ratio in the diff. If gearing is applied to an engine of 1.5 times higher then the torque goes down (consistant with a rotary) and the revs go up (also consistant). But power remains the same so there is no advantage.

Um, this is *your* logic. I am saying that no matter what the gearing the capacity remains the same. You are saying the capacity is measured over 2 output shaft revs so must change with gearing.

Interesting, I didn't know that. It could produce some interesting racing, give everyone 50 litres for 50 laps let them do what thay want with it :-)

 

Scott--- Last time you shot me an email i could not respond, shoot me another and i will try again,

I finished tacking the steering rack mounts together and it is in the car, I have to make the sway bar mounts now and then it is ready for the real welder to come over and have a field day, I am ordering some of the parts that i need today, all and all it is going well with nothing really that unexpected jumping up and slowing progress, Thanx for the good conversation and I'll keep you all posted....

Shaun
Here are some pics

 

another pic, remember these are just the tack welds, because i am a serviceable welder at best but no guru, so i am having a buddy take care of the finish welding. Also someone asked if i owned my own shop and yes I do it is Upgrade Performance in Tallahassee, FL

Shaun

 

Good point, I never thought of it that way. I think there is no good way to compare a rotary and a piston motor, as far as displacement. Fuel consumption sounds pretty fair.

 

I'm not defending anyone here, but here's my take on this..

I KNEW about the issues the 3rd gen had before I bought one. I was willing to put up with them because of how well the car performs by itself. Personally, I would only install an engine from the same family as the car's. I just see it morally wrong to install an engine from another manufacturer into a different manufacturer's car.

I didn't see the 787b win Lemans with a Porsche engine. I don't see Audi winning lemans with a Viper's engine.

Personally, I don't just look at a manufacturer's car. I also look at the heart of their cars. Ok, so the rotary needs a little extra more tunning than other cars. If it was such a fluke of an engine, it never would have won Le Mans, and to this day, it remains as the ONLY Japanese engine to win the 24 hours of Le Mans. Most people do not wish to deal with the extra tuning of the rotary and most dont even want to deal with the extra coolint required by a rotary, either.

Personally, when I see an engine swap like this, I see a person that "didn't want to deal with the extra TLC the rotary needs"

This forum allready has a few guys on it running their rotaries on 600+hp and streetable. Get it tuned right and get it cooled enough, and your chances of failure will be minimal. ANY engine of high HP capacity will have tight tolerances.

 

ok personally im torn. i see the whole argument that rotary is the heart of the beast and to go with anything but would be to kill the car. but i also see the other side. The rx is a beautiful car, it also has an amazing chassis which is very light and handles extremely well. I dont discourage swapping out our weakest (but also most identifible) link, the 13b. the rx7 is a good platform for an engine swap while it may be difficult.

i look foward to watching the step by step of these swaps.

keep it up. oh and post as many pics as pissible

-mike

 

I might be going out on a limb here, but I think Mazda might have put a wee bit more R & D into building the LeMans motor than the 13B. Not to mention that motor only HAD to run for a day, and even at that I'm sure they had a spare or two in the pits. Wait, nevermind...a little of that LeMans strategy was used while developing the produciton car. Mazda just forgot to tell everyone to have a couple spare motors in the garage

 

Because you've got a bunch of 16 year olds (whether chronological age or simply the extent of their mentality) that thinks that owning (or wanting, it seems, in a few cases) buys them into some exclusive club.

I'll be sure to get everyone's 'approval' here before I do my next mod. Sheesh, it's a car, not a cult.

 

What, you didn't get your "Exclusive, Rare and Cooler than Everyone Else" membership card?

Have jimlab mail you one, he has extras.

 

(Just FYI, this response isn't directed solely or even especially at MazdaMike... just a general rant...)

Identifiable by whom?? Rotary enthusiasts?!?

Many of you absolutely loathe anyone who doesn't automatically know what a rotary engine is (as if that information were issued at birth and they'd had the audacity to forget...) or how it works. What you're forgetting, is that at some point, you didn't know either...

Those same people absolutely love to gloat over someone asking them "how many cylinders" their engine has. As if it weren't a completely valid question, because 98+% of the cars on the road are powered by piston engines... At least they liked your car enough and cared enough to ask. What a bunch of pompous ******** you people can be at times.

And that, more than anything else, is why I'm absolutely embarassed to be associated with many of you, even if it's only because we own RX-7s.

Get off your high horse. You didn't join some elite club just by buying a rotary powered car, and just because you have one doesn't mean that you're any better (or worse) than anyone who has a piston engined car. In other words, grow the **** up, already.

And many of you have the gall to call Supra owners ********??? Good Christ...

 

Damn. I am not on the forum for a couple of days and look what I miss.

Very interesting choice for an engine swap. I await the end result.

You have my respect for your willingness to be different from the pack.

 

I love it.

Maybe this means you are all out of those membership cards?

 

i'm personally content with my FD.... 416RWHP @ 13psi.... it will do that ALL day long..... its his car he can do whatever he wants with it i love mine

i have two friends with sr20det's in their 240sx's they run pretty good. i like the engine but wow they are expensive !!!!! $3500 for everything *almost* to install it into a S13 chassis car. anyway just my thoughts

 

Yes I agree, its a very intresting swap Idea, and I knew I would see this type of swap eventually... Now I'm waiting to see when the next choice will be a Honda motor........

 

That's funny because that is what i wanted originally but there are several problems, plus i could only use the newest generation of K or F motors like the RSX or S2000, very pricey...


Shaun

 

y take out the rotary and put in a SR20???

 

I can see the logic in the swap if he has the time and skill to fab. as necessary and do the swap himself. The SR20 will live longer, be easier to tune, make huge power, get better mileage off boost, and generally be an easier engine to live with. Yes, the rotary is the heart of any RX-7 in terms of its ability to make rev and make easily attainable power but if he can do this successfully he deserves some credit. I've seen supras w/ 3SGTE motors, 300z's w/ SR20 motors etc...sure they were swaps of like makers. I wouldn't condone someone shellin out $$ to pay for this kind of work but if one were inclined and skilled enough to do it themselves THATS damn cool!!!! G-LUCK!!

 

I think they can be compared exactly. Everyone talks about an 'equivelent' capacity but a direct, exact comparison can be made.

 

just curious but the 20 in sr20 stands for 2.0L just as sr26 is 2.7L or am i wrong?

 

An FD runs 4.1:1 the equivalent for your 3.9L 6 is 2.7:1 seems a little low?

Let's look at your 1.5:1 3.9l 6 -

In 2 flywheel revs it has displaced 2616cc. It has turned over 1.333 times. The equivalent 13b has turned over 1.333 times as well for the same displacement. So both consume 1962.5cc per internal rev.

mechanically the 3.9l six will kill it for sheer power output. Why? Because the 3.9l 6 can (before the 1.5 gearing) crank out between 5500rpm and 7000rpm depending on the design. The humble rotary can only manage 2500 (eg rx-3 - s1/2/3 rx-7) to 3000rpm (rx-8?) before e-shaft whip starts to wreak havoc. The eccentricity of the motion shifts it's operating range away from that of a piston. Even the lowliest 6 can manage 5500rpm these days but nobody can get that out of a rotary.

Since both your geared motor and the rotors are spinning at the same rate the winner will be the one that can turn the fastest - thus making more power by flowing more air. The reality is that rotor speed will always be about 1/3 slower(more like 1/2 these days) than any piston engine equivalent, thus it's flow rate will be 66.7% of that of the 3.9l 6. This works out nicely at 2616cc.

If the rotary engine design was such that the primary displacing mass had a hope of achieving the same angular velocity as that of a piston motor, I'd think your comparason was fair, if anything piston engine speeds are increasing faster than rotary engine speeds as time goes by so it's only going to get worse.

-pete

 

jimlab...when's your car going to be done?


j

 

Hey Mike,

That's absolutely correct, but the "direct, exact comparison" that you speak of is the one that compares a "1.3L" rotary to a "2.6L" piston engine.

To correctly compare engine displacements we must first rigidly define the word "displacement" and the units with which it is measured. This is not clearly done by the manufacturers, and that is the real source of confusion.

As an example, let's take a piston engine with a manufacturer specified "displacement" of "2.6L". So what exactly does that mean? The fact that the engine displaces 2.6L is meaningless unless you specify how long it takes to displace that much air. Indeed, if the engine is not running, it displaces no air and hence makes no power: At that point in time it is a 0.0L displacement engine. When the engine IS running, the amount of air displaced (and therefore the amount of power it makes) varies dependent on the engine speed. By looking at the mechanics of the engine, we can actually see that what the manufacturer really means is "2.6L per two output shaft revolutions". Without specifying "how long" we really know nothing about the engine's output potential.

Now if we look at a rotary engine with a manufacturer specified "displacement" of "1.3L", we must again ask the question: What do they mean? They didn't tell us how long it takes to ingest that much air. Looking at the mechanics of the engine, we can see that they really mean "1.3L per one output shaft revolution."

So, obviously to compare the two displacements, we can't simply just put the two numbers side by side since one is specified in different units than the other. But we can directly measure one engine in the units of the other: We can measure the rotary diplacement per two output shaft revolutions as "2.6L" or we can measure the piston displacement per one output shaft revolution as "1.3L" When the two "displacements" are measured in the same units, you can see that these two engines are indeed equal. The problem here is really no more difficult than when people try to measure the height of one object in meters and another object in inches. At first the results seem confusing, but if you just measure them in the same units, everything becomes clear.

Going a step further, you pointed out that Mazda could have alternatively specified the rotary as a "3.9L" because that is the sum of the displacements of all the combustion chambers. But again, to be meaningful, we have to ask "3.9L in how long?" What are the units? Clearly when the engine is not running, it's not displacing anything and therefore not making any power. What Mazda would have really meant had they specified the engine this way would have been "3.9L per three output shaft revolutions" which is the same as "2.6L per two output shaft revolutions" which is the same as "1.3L per one output shaft revolution". It's all equal in the end. One inch or 25.4 millimeters, your choice....in the end it's all the same length.

The important thing to note is that power production is determined by how much air goes into the engine. If it weren't, we wouldn't be so concerned with free flowing intakes and exhausts and denser air charges from turbo/superchargers. Measuring the displacement of the engines in the same units, you will see that at any given RPM a "1.3L" rotary will be ingesting just as much air as a "2.6L" piston engine at the same RPM.

This is the comparison that matters.

 

Not really. Remember they can put different diff ratios in the exact same car so the 1.5 comparision is never going to be exact, but in general the rotaries run *approx* 1.5 times bigger diff ratios.

The gearing is applied to the 6 only because the rotary already has this gearing due to its internal design. If you compare both the engines then they have both displaced 3.9L over 3 revs.



Thats all irrelevant. Both engines still have a capacity even at 1 rev per minute.

I understand what you mean very very well. I used to believe it myself but then I worked out why it is not correct. The reason it is not correct is that it is easy to change. Just by attaching the output shaft to the camshaft of a piston engine I have effectively doubled its capacity. If engine capacity is the amount of air sucked per 2 revs then this has got to be true, right? You are most likely going to say 'thats ridiculous' but it is not, this is exactly what has happened in the rotary due to its design. Isn't the ratio between the rotor gear and the stat gear 1.5?

Why does it have to be output shaft rpm, why can't it be per X expansions and contractions of a chamber? If you calc the capacity of a 3.9L six over 2 expansions/contractions of the first chamber you get 3.9L of air sucked. If you do they same thing for a rotary you get 3.9L also.

 

No, it is not ridiculous. You are absolutely right, by adding gear reduction/multiplication to the output shaft, and calling that "part of the engine" you are indeed increasing/decreasing the displacement per rev of the engine and simultaneously you are changing the number of revs. As an example:

Take two identical 2.6L per two revs piston engines. Their displacements (and hence outputs) at 1000 RPM(or any other RPM) are equal. Now let's add a 1.5:1 gear reduction to one of those motors so that the crank shaft (no longer the output) rotates at 1.5 times the speed of the new output shaft. You have just multiplied the displacement of the original engine: You now get 3.9L displacement per 2 output shaft revolutions for this newly created "cyborg" engine. Here's the rub: Simultaneously, your 1000 RPM at the output became 667 RPM at the output because of the gear reduction. Now let's compare the two engines:

To accurately compare these two engines we have to run them at the same RPM. What ends up happening is that the "real" engine inside the "cyborg" engine is actually running at 1500 RPM when the output shaft is at 1000 RPM. The amount of air flowing through this new engine at "1000 RPM" is 1.5 times the amount of air flowing through the original engine at 1000 RPM: The displacement has indeed increased and will be 1.5X at every given RPM.

So why can't you calculate it based on X expansions/contractions? I think the real answer is: You can. The trouble is, that's not the way "displacement" is defined. As I said before, you need to know "how long" for displacement to be meaningful. The way displacement is defined, "how long" = "1 revolution". You are trying to redefine "displacement" using "how long" = "as long as it takes for every combustion chamber to complete a full 4 stroke cycle". One way is RPM dependent and combustion cycle independent, the other way is the opposite. Neither is right or wrong (based on it's technical correctness), it just depends on how you want to measure it. The way you choose to measure should be the way that gives the most useful results in real world comparisons.

So which way is more useful and gives the best comparisons? I'll argue that the RPM dependent method is the most useful. Why? Let's first ask ourselves what we want to know when we compare the displacement between two engines. Basically, we want to know roughly how the power producing potential of the engines compare. So let's look at a real world example. Take a maxxed out p-ported N/A 13B. This will probably make in the neighborhood of 400 HP. To compare this 13B to an N/A 3.9L piston engine doesn't make much sense. Production cars (ie. not maxxed out) are surpassing the 100HP/L mark. The power output of a maxxed out 3.9L piston engine would be significantly higher than that of the 13B in question, but if we compare the 13B to a maxxed out 2.6L piston, we are much closer to a match. As a secondary example, you can generally count on smaller displacement engines having much more revability (if that is a word) than larger ones. Though a 13B should be able to out rev a 2.6L piston, the revving potential of the 2.6L is much closer to what the 13B is capable of than some big 3.9L beast.

Basically, this all comes down to a matter of convention and, with good reason I think, the convention is the RPM dependent version of displacement.

 

what is the poitn of gearign teh flywheel, and are u forgettign the transmission gears, there is nto only rear end. and eventually teh power you end up with at a given rpm is very comparable to an in line 6cyl 2.6 liter engine. this should tell u sopmethign if u are gettign that ****. compare a rotary with a 3.9 liter piston for power output at same rpm. you will see what happens. power is only air and fuel and how fast it all happens.