wankleler

well, yet again i have another question. and need help proving a honda happy moron wrong. For some odd reason he still insist that the actual SAE displacement of a 13b is 2.6. he claims to have learned that from the SAE. if you ask me hes full of crap. i've search the internet for about 45 minutes now for SAE acceptence on the rx7 motor and failed to find info. if someone could help me out i'd much apprecate it

Bio-Weapon

https://www.rx7club.com/showthread.p...hreadid=167166

Nathan Kwok

Not sure if this is true but it wouldn't surprise me if it is. The rotary is considered a 2.6l in various sanctioning bodies. You'll have to buy the paper yourself if you want to read it:

http://www.sae.org/servlets/productD...PROD_CD=930684

travisorus rex

The 13b is either a 1.3 liter or a 2.6 depending on how you look at it.
I believe the first time displacement was figured for a rotary engine was on a two-stroke engine. Because nobody else ever challenged the displacement for a rotary (at that time) the calculations became standard.
That was years ago and, on a two-stroke engine. When Mazda started making four-stroke engines, they never figured it would make any difference to double the displacement even though it was going from a two to four-stroke.
I am not a scientist but there was a great article in a magazine several months (Sport Compact Car May 2003) back about this subject and it probably makes more sense than my rambling. In fact, the guy that wrote the article is a total car genius. His name is Dave Coleman. Email him and let him set you straight.
Dave.coleman@primedia.com

DaveW

The reason for doubling the actual displacement for equivalency purposes is that each volume gets used for combustion every time it goes past the plugs. The available volume on a rotary is used like that of a 2-cycle piston engine, even though it is a four-cycle. On a four-cycle piston engine, it gets used every other time.

Since the available displacement is used twice as often, it makes sense to double it when comparing to piston engines.

KevinK2

Bet him $100 there is no SAE standard for displacement determination. There are SAE std methods for hp and torque determination, but none for displacement.

Do NOT buy the SAE std linked by Kwok. I have it, and it's only a paper study (not a standard) related to different numerical methods for calculating the swept volume of one chamber of 2, 3 or 4 lobed housings, and related accuracies.

The 13B does ingest the same amount of air at 100% VE, per rev, as a 2.6L honder.

Shad Laws

Hello-

Actually, many sanctioning bodies call our 1308cc a 2354cc engine, following the lead of the FIA.

The FIA has displacement multipliers for various things, right? Most people know the 1.4 multiplier for forced induction. Well, the multiplier for being a rotary engine is 1.8.

This is because, as one obvious advantage, the firing frequency is twice as much as a 4-stroke piston engine, but there are disadvantages. Namely, the thermal and, to a lesser extent, volumetric efficiencies are inferior compared to a 4-stroke piston engine of twice the displacement. So, they "settled" on a 1.8 multiplier.

That means that our 13B-REW is considered a 3296cc engine by the FIA (!).

Of course, the 1.8 thing has a lot less to do with physics and a lot more to do with "making things fair." But, hey, it serves as an interesting number to consider when trying to compare things.

Take care,
Shad

peejay

SCCA also uses a 1.8 multiplier. It really should be 2.0, if you want to convert an apple to an orange so you can compare it to other oranges, HOWEVER... rotaries are much less efficient than piston engines, PLUS the 1.8 multiplier conveniently allows 13Bs to run in the under-2.5l classes.

SCCA Performance Rally goes a step further. There are two Open 2wd classes - Group 2 and Group 5. Group 2 is under 2400cc adjusted displacement, EXCEPT for any forced induction cars OR ROTARIES, which have to compete in Group 5! So even though a 12A or 13B falls under the 2400cc adjusted limit, it's not allowed to play in that class.

Which is OK, the naturally-aspirated engines have NO problem in Gp5, against all the bigger and/or turbocharged engines. The torque curve of a PP 13B is just about right for 2WD rallying - flat curve, smooth response, IDEAL for finding the best traction on a gravel road, while you're trying to keep the car straight enough to not hit the trees!

KNONFS

iTrader: (14)

No he is not, in fact he proved himself to be a TOTAL moron!!

https://www.rx7club.com/showthread.p...y+displacement

KNONFS

iTrader: (14)

Sorry

SPOautos

I didnt see Howard type any info on that thread? Maybe I missed it, after all it is 25 or so pages lol

STEPHEN

DamonB

If we could just convince the rest of the masses of that

As Kevin says this is where the common x2 displacement rule for rotaries comes from in many sanctioning bodies. If you look at the volume of air processed, the 13B in fact gulps in 2.6L; not 1.3L

In order to make the displacement between a piston and a rotary relative you can either halve the piston displacement (after all, only half the cylinders are making power at any time) or you double the rotary. It is not unfair at all for a 13B to have to compete as a 2.6L motor.

silverpain

Why should differing technologies/mechanical pricipals be measured by a single format ?? I don't see too many SCCA rules governing a weight or setup penalty to those with double wishbone suspensions over those with MacPherson/ multilinks, and there is as much of a perfomance difference with those two to make it warrent a ' performance' or ' technology' penalty..

DamonB

The rotary is not being penalized! In order to make a relative displacement comparison of a piston motor and a rotary motor you must double the rotary's Mazda given displacement!

Quit it with the victimizing

KNONFS

iTrader: (14)

Actually, I think you are wrong!

Check the link I posted and look for Rice Racing's explanation

KevinK2

nope, he's a 2.6 er, or an rev qualified 1.3 or 3.9, just like me

from rice:

"13B=
ONE REV 1308cc (one face fired once per rotor) min
TWO REV 2616cc (two faces fired once per rotor) std
THREE REV 3924cc (all faces fired once per rotor) max

I implore you guys to read over this last post carefully and if you use logic you will understand why ALL three are correct and that you need to simply quote one in reference to the number of main shaft revolutions, again like I said way back at the start you guys like using tachometers to compare how hard you rev various engines right? Well your actuall aspirated displacement is ALL relative to this! That is why I use the middle 2616cc capacity because it has the most relevance to a wide audience of people who are unsure about the wankel principle of operation"

Evil Aviator

It depends. If you mean actual displacement, then it is 1.308L. If you mean "capacity", "flow rate", "relative displacement", or other terms of comparison, then it can be 2.616L, or any other number, depending on the defining factors. There is even one particular technically-challenged UK publication that recently placed the Renesis engine in the 2.5-3L "capacity" category due to tax laws, lol. Just be careful of what you read, as the journalists don't always understand technical things very well.

"Displacement" is based on the "displaced" portion of the engine, or "female" portion if you prefer that term. This corresponds to the cylinder of a piston engine, or the housing of a rotary engine. The 13B has two housings, each of which has a 654cc displacement. The "displacer", or "male" portion of the mechanical aparatus (ie piston or rotor) has absolutely no bearing on displacement other than its physical relationship to defining the displacement of the displaced portion. Therefore, "displacement" is a static value, and rpm has no bearing, nor does the number of surfaces of the displacer, the compression ratio, or boost level.

I think what confuses people is that the 4-stroke piston engine is so common that automotive aftermarket parts manufacturers rate their products based on what they call "displacement", when they really mean "flow rate", which is more difficult for the general public to understand. You also see this with the "horsepower" rating of transmissions or clutches (torque is the proper measurement), and the "cfm" rating of carburetors (which assumes a 1.5" Hg vacuum for 4-barrels, or a 3" Hg vacuum for 2-barrels).

peejay

Good point... maybe the main problem is that we're just plain paying too much credence to engine size.

How about a BSFC rating? Measure at low speed cruise, normal cruise, and averaged full power. Or how about factoring HP and fuel economy together, which is similar to BSFC but it gives you a better real-world figuring of the engine's efficiency - you couldn't have a small "ringer" engine making stupid high RPM at the expense of fuel economy. (LT1/LS1 powered cars would *shiiine* with this method... )

But we already *do* have a fairly good measure... displacement vs. HP. Bigger engines feel more powerful (due to higher torque). A small engine that makes big HP will feel like crap to drive, while a big engine that doesn't make much power will still feel like it's got some serious power. Basically we use displacement as shorthand for torque. If you look at enough engine specs, you come to realize that for the VAST majority of engines, torque is roughly 1lb-ft per cubic inch or roughly 60lb-ft per liter, plus or minus depending on engine efficiency. (Naturally aspirated of course!)

The trouble here is that rotaries are not rated for displacement the same way as piston engines. Piston engines use their rated displacement in 2 revs, rotaries in only one. That is why we need to multiply rotary displacement by 2 - to compare oranges to oranges. Simply looking at the torque numbers will confirm this - there is no way in hell that a 1.3l naturally aspirated piston engine will be making 130-140lb-ft of torque. Just not gonna be happening. Again, you're not going to be moving far away from 60lb-ft per liter. However, that amount of torque is just about right for a 2.6l engine. It'd be on the low side, but we already KNOW that the rotary is not as efficient as a piston engine, as evidenced by the higher loss of heat to the cooling systems and to the exhaust. (The more efficient an engine is, the less heat it wastes, and the more torque it makes)

Evil Aviator

Yes, that doesn't really define an engine very well. In fact, the general public has no use at all for the displacement statistic.

LOL, I think the general public would have their brains explode if you started talking about BSFC. I think that cruise gas mileage, horsepower/weight ratio (of the vehicle, not the engine), and peak torque at the wheels would probably suffice for the masses.

No, they are rated the same for displacement.

The problem is that you cannot use the displacement as a comparison for flow rate between engine types. That is why we multiply the displacement of rotary and 2-stroke piston engines by two when we want to compare them to 4-stroke piston engines based on their indicated flow rates, or we add other factors to compare their volumetric flow rates, density flow rates, or other similar comparative values. The important thing to note that is that once you multiply the displacement by a modifier, it is no longer the displacement.

I would also like to point out that technically, the rotary and 2-stroke engine displacement should stand, while the 4-stroke piston engine displacement is halved for flow rate calculations based on a more sensible 1-rev of the output shaft, but that would make the "leading brand" look bad, so we know that will never happen.

peejay

They might use the same *method* of measuring, but they're not *rated* the same. Rating is separate from measuring.

Just like the 2-strokes, yes... you can measure the displacement the same way (bore area times stroke times number of cylinders) however the rating of the 2-stroke is volume displaced per rotation while the rating of the 4-stroke is of volume displaced per 2 rotations.

Uggghhh... Could you just imagine it if they altered how 4-strokes were rated. All the Honda-Boyz would be creaming their shorts in juvenile bliss as their beloved S2000s magically crossed the 240hp/l threshold....

But we wouldn't want to ruin their joy with reminders of what a good racing 2-stroke could do, eh?

- Pete (100hp 80cc engines.... !!!!)

KevinK2

Evil understands actual aspiration, but when he speaks of rotary 'static' displacement ratings, keep in mind he considers rotors with big holes in 2 of the 3 faces, so each housing of that sorry 13B is in deed rated at .65L.

I choose not to invent a rating system, and the common boinger 'total swept volume' method brings in the ugly 3.9L rating, as my rice quote noted.

IMHO, swept volume method was based on piston pump ratings, that predated the IC engine. A pump rating method would work for the 13B, as noted in this edited paste from another thread:

there is no displacement standard.

piston pumps came well before otto cycle ic engines. they look like an engine, but just have inlet and outlet check valves in the head. pump displacement rating was volume pumped per rev. for piston pumps with cranks, one rev gets fluid in and out, with 2 strokes. displacement was just number of holes x pumped volume per hole, which for piston engines is the same as swept volume method. all positive displacement pumps are rated at volume/rev, at least all the ones I have analized.

i suspect this practice was applied to ic engines when they were invented, thus the method resulted in same size ratings for 2 vs 4 strokes, and the obvious 2 stroke power advantage.

looking at the wankel as a pump, where just 1 exp and contraction is needed to pump a chamber's worth, the pump method 'one rev' rating can be applied. chamber goes thru 2 exp-contr'n events in 3 revs. in one rev, you have 2/3 of the 'up down' event per chamber, so 13B displacement would be 2/3 x 6 x .65 = 2.6L.

Note this 13B pump would need check valves instead of ports (or valves on a boinger). inlet check valves along the NW and SE housing arcs, and outlet check valves at 2 contraction zones at NE and SW zones.

this old method would provide consistent ratings for all 4-stroke engines, regardless of cycles needed to complete the total engine cycle. With all 4 strokes, it just doubles the actual air injested per rev. Mr Wankel (rip) was indirectly suggesting this method, but made it confusing with his geometric approach.

Evil Aviator

OK, I see what you are saying now. You are pointing out the difference between a "displacement" rating and a "displacement rating".

No, Evil does not even consider the faces of the rotor at all. The amazing thing about Evil's warped method of determining displacement is that the results are consistent with the displacement figures of every 2-stroke piston engine, 4-stroke piston engine, single and double-ended dyna-cam engine, and Wankel rotary engine. Of course, Evil's method is pretty boring because it's the usual thing taught in universities, and it doesn't allow for the much more exciting conspiracy theories about how displacement really isn't displacement.

Then why don't you measure the displacement by dividing the length of the eccentric shaft by the diameter of the flywheel, and then multiplying by the number of spark plugs squared?

MikeC

It's actually a 3.9litre engine:

http://www.mikesdriveway.com/misc/rotor.doc

rxtasy3

iTrader: (2)

why don't we just send this arguement to the Mazda engineers and see what they come up with? this might be interesting. have to include all these links u all posted.

Evil Aviator

Um, they came up with 1.308L a long time ago. I would rather not send them these links because I don't want them to think that their fans are a bunch of morons.