This would mean I could take a 5 litre V8, gear it up 5 times and run it in the 1 litre class. The capacity of the engine is still really 5 litres and the per expansion/contraction of a chamber method still gives 5 litres, where the per 2 revs method gives 1 litre. Remember that gearing the engine up or down gives no advantage or disadvantage either way.

I've heard from a mechanical engineer that its defined as swept volume x no of chambers. Either way the definition could be wrong, as you said a comparision for the real world is needed.

A 2.6L 4 and a 3.9L 6 have exactly the same revving potential because the pistons are the same size. The rotary will always outrev the piston motor at the crankshaft. Fuel can only burn at a certain rate, say, as an example it burns in 5 milli-seconds for a swept volumne of 654cc's. So when a piston motor takes 5ms for the piston to get from min-vol to max-vol the output shaft is spinning at 6000 rpm. But for the same fuel burn time the output shaft of the rotary is doing 9000 rpm. I call it fake revs, when the chambers are expanding contracting 6000 times per minute the output shaft is doing 9000rpm. But for the piston motor 6000 expansion per minute means 6000 rpm. Thats the 1.5 times gearing I keep talking about.

The problem with this sort of comparision is that it is possible to get 2 engines of the same capacity and get different power from them. I mean this sort of comparisoon will always be up for debate. However, I think a 13B develops significantly more power than a 2.6 and is closer to a 3.9L. Maybe it doesn't quite reach a 3.9L engine because of its poor combustion chamber shape or that the shaft bends too much at high boost but that has no effect on capacity.

 

The local tunning shop HAS over here in Okinawa has that tranny in their 600 hp 180sx and they have broken 3 output shafts in the last year.. food for thought...

 

OK you lost me here. Take 1 spin of the rotor to be 360 degrees internal rotation. Take 1 spin of the crank to be 360 degrees of piston motor rotation (or the time taken for the same piston to return to the same point).

Piston engine:
If the power stroke takes 5ms to do 180 degrees of power stroke, you're reving at 100rev/sec = 6000rpm.

Rotary:
if the power stroke of the rotary also lasts 5ms (not that fuel burn rate = piston / rotor velocity) you just completed 90 degrees of rotor rotation in that time. This gives us rotor RPM (true displacing mass angular velocity) you're only revving at 50rev/sec = 3000rpm

Forget flywheel RPM here, it's all gearing as you say. Can you see why I say that the rotary can't hope to match the piston motor rev for rev now because it has to work twice as hard? Your 3.9 litre vs 13b comparason will never match up because:

a) A piston motor will always have more RPM to trade for torque compared to a rotary (before gearing)

b) The mechanical limitations of the rotor itself will mean that although it's displacement looks the same on a timing diagram (eg mine) they will never line up because the piston motor doesn't have to suffer the same 3:1 gearing to have equivalent operating ranges.

I know you're going to come back and say that RPM doesnt come into it, but your wrong. The idea with comparing motors is to equalise the rate of WORK done. But work is simply POWER which is a function of the maximum pumping airflow at the mechanical limit. The rotor will always spin at a lower speed to a piston motor because it completes the operations twice as fast. So all torque developed will be at a lower RPM which has to be geared up to match the same as a piston motor.

-pete

-pete

 

Good post.

 

Joe D,
Are you feeling okay today?

 

I agree w/ JoeD, good post jim

 

Hi Pete,

Forget about the speed of the rotor, its not relevant (to my point). Just think about the chambers and the crank. Forget even about the shape of the chambers, all that matters is that there are 6 independant chambers that get bigger/smaller and this change in volume somehow gets translated to output shaft rpm.

So if the chamber in the rotary takes 5ms to get from min volume to max volume then the output shaft is doing 9000rpm. But 5ms in the piston motor only equates to 6000 rpm. Thats why the rotary revs higher and that is why I say the rotary has an internal gearing in its design of 1.5 (9000/6000 = 1.5). Its a gearing between the chambers and the output shaft. Revs per expansion for the piston engine is 1:1, but it's 1.5 for the rotary.

You could compare the rotary and piston engine at X rpm but I think it should be the rpm of the chambers not the output shaft. If you have a standard rotary going at full song (about 7500rpm) it will suck the same amount of air as a 3.9L 6 at full song (approx 5000rpm). The rotary will suck plenty more air than the 2.6L going flat out. Remember this is not an exact comparision because different 2.6L piston engines could have different max rpm.

 

Lets take a look at a two stroke engine, lets say its a two cylinder 440cc. As you say, the displacement is simply the swept volume per chamber multiplied times the number of chambers. In this case, 220cc per cylinder, with two cylinders.

Can you directly compare this to a four-stroke 440cc two cylinder engine based only on displacement? Not really, because per rpm, the two stroke will pump twice as much air as the four stroke engine. It all comes down to how much air can be pumped through the engine. This is one of the reasons why two stroke engines can make so much more power than their four stroke counterparts. You can gear things any way you want, but it all comes down to volumetric air flow.

 

as i was reading through these posts, i was just thinking of using that analogy.

each cylinder fires per revolution of the crankshaft, just like a 2-stroke. obviously, a 4-stroke only gets half of the cylinders firing per revolution.

I say 2.6L equivalent
but then again i do not claim to be any kind of expert in this field.

 

2 strokes are a different ball game altogether. If someone said that 2 strokes have an effective capacity of 1.x times a 4 stroke then I would tend to agree. But rotaries and piston motors are very similar engines in this regard. Most people concentrate so much on the differences between the 2 engines that they fail to see how similar they are.

The rotary is equivenlant to a 3.9L piston motor that has a gearing up of 1.5 applied to the output shaft. Do the maths, every single figure matches between these 2 engines. If capacity is different then it is the only figure that is. If the rotary is a 2.6L then the geared up 3.9L six is 2.6Litres also.

The rotary sucks plenty more air going at full speed than a 2.6L simply because it revs higher. So going by this logic it has to be a 3.9L.

 

I think the 3.9L 6-cylinder with the gear-up is interesting because many of the items do match up (firing frequency, power stroke duration, etc.). But, I don't find it very satisfying as a diplacement rating because you'd have to give the gear up explanation every time. Most people are interested in displacement when they ask for displacement, not displacement with a gear-up caveat. And besides, there a plenty of piston engines that rev just as high as rotaries, so I think the gear-up thing fails as you rev it out. The displacement thing is not that complicated -- a 13B has the same displacement as a 2.6L 4-stroke piston engine.

-Max

 

Since the piston engine's displacement is based on two revolutions of the crankshaft, the most direct displacement comparison is when the eccentric shaft makes two revolutions. Thus 2.6L.

But you're kind of comparing apples to oranges (or at least macintosh to granny smith).

 

This was the essence of Group C racing (and still is for Indy Racing)
In Group C not only was the total amount of fuel per race limited but the RATE a car could be REFUELED was limited to 100L/Min so to fill a 50L fuel cell took a full 30sec (as compaired to about 6 sec for a F-1 fueling rig) an added twist was that NO other work could be done on the car while it was being refuelled. The interesting thing about it was that Large displacement/Low reving engines were VERY competitive (7L Jag V12, 5L MBZ Turbo V8)

 

You were doing well until the last bit

@7500rpm the 2.6 litre whatever (4/6/8/10) is sucking down the same amount of air as a 13b, that is why the comparson is so fair.

IMHO it is better to have the caveat (thanks max) of the 2.6litre saying "but the 13b has longer duration cycles" than that of having to add "with a 1.5 gearing reduction" for the 3.9l comparason.

At the end of the day, it doesn't really matter that the cycles are of a different length or the number of firing points doesnt match up, the important thing is, that in competitive motoring, the pumping rates are fair. Now you can't honestly say that 5000rpm is good for a performance 6, is WAY off and you know it, a 7500rpm 2.6l 6 is a far more realistic comparason.

-pete

 

7500 rpm for a performance six is not way off, but you are comparing a performance 6 to a standard rotary. 7500 for a 3.9L six is pretty big revs whereas big revs for a rotary is 10000+ rpm. So a rotary will always suck more air than a 2.6L motor.

 

Keep the sr20's in our NISSANS! You obviously must've bought an Rx7 for the rotary, you should keep one in there.

 

A standard rotary is a performance motor. They're too inefficient to be used for anything else.

10,000rpm for most rotaries = no more motor 7500 is maybe a little high for a 2.6 (about 7000 is probably fair)but 5000 is way too low so I think the 2.6 is the closer comparason.

-pete

 

I disagree, but the problem with this sort of arguement is that it is very debatable. I mean we could say a 2 litre fiat motor revs to 10000 rpm where a 2 litre toyota motor only revs to 5000 rpm, so the mazda engine must have twice the capacity, so a 2 litre motor is bigger than a 2 litre motor. All I can say is that if you do the equivelant modifications to a rotary it will always rev higher than a 2.6L 4 with similar modifications and always suck more air.

 

I ballsed that up as usual. I should have said

we could say a 2 litre fiat motor revs to 10000 rpm where a 2 litre toyota motor only revs to 5000 rpm, so the fiat engine must have twice the capacity.

 

Note that the highest revving engines in production and racing cars are piston engines. Though surely the average rotary revs higher than the average piston engine. Is that particularly meaningful? No, but I think you need to divorce redline from the displacement rating. The best answer for someone asking for a displacement rating for the normal purposes of estimating performance is that the 13B is a 2.6L. Rotaries make lousy power if you call them 3.9L, and they aren't even that great at 2.6L, but 2.6L matches up better based on the fact that they flow like a 2.6L and in reality have a similar redline.

Holy thread hijack, Batman! We are way off topic in this one.

-Max

 

Come on max, the 13B kills a 2.6 for sure. It may not be quite up to a 3.9 due to its large chamber surface area but it is well above a 2.6L

But they have much smaller swept volume per chamber which makes a huge difference to rpm.

They are the best threads

 

How's the SR20 swap coming?

 

It is at a bit of a standstill waiting on parts that i ordered last week that should have gotten here already, either way i will get alot more done this weekend.....

Shaun

 

Hurry up! I want to see some more pics!!!

 

You think? Have a look at the RB26DETT used in the GTR. They hit the same ceilings we do. Generally speaking they're a very good match.

Now look at the 4.0L 6 from the Falcon XR6 Turbo - see how little trouble that motor has cranking out 240kW? It's pushing just 5psi and even then the ECU is pulling back timing and boost to keep the power down. You can't tell me that is a fair comparason.

-pete