Piston Engine Equivalence
 

Here is a copy of a letter I wrote to the SCCA's Club Racing Board (CRB) and Solo Events Board (SEB) about how the rules treat Rotary engines vs. piston engines.
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The SCCA has traditionally hit rotary engines with a 2x displacement equivalency factor compared to piston engines (making a 1.15L 12A equivalent to a 2.3L piston engine, and a 1.3L 13B equivalent to a 2.6L piston engine). This equivalency was probably correct in the 70's & 80's against pushrod and 2 valve engines. Today, against modern DOHC 4 valve engines it is too much of a handicap, especially when considering the preparation limits typically placed on rotary engines. For reference, remember that turbocharging, proven to be the most effective way of making lots of power, has only a 1.4x equivalency factor.

The club racing C Sports Racer class has allowed bridge ported 12A's and street ported 13B's to run against 1.6L 4 valve engines for a while now. And experience shows that the 12A produces virtually the same power as the Toyota Atlantic style 4AG engine (~250hp, 1146cc vs. 1615cc, for an equivalency factor of 1.4). The street ported 13B seems to be less competitive, with power output estimated at ~220hp, but such an engine is significantly cheaper and longer lasting than a 12A bridge port (1306cc vs. 1615cc gives a 1.23x equivalency factor). While all of these engines have different choke or throttle body size limitations in CSR trim that complicates the comparison, this data strongly suggests that the traditional 2x factor is not correct for modern racing engines.

Solo’s D Modified legalized bridge port 12A’s to run against up to 2.0L piston engines this year (1.15L vs. 2.0L, for a 1.74x equivalency factor) with no apparent impact to the class leaders. The current proposal for 2006 would break the class into two subclasses of up to 1.8L, and 1.8 – 2.0L at a higher weight. The current version puts the bridge port 12A in the lower sub-class against the 1.8L engines (1.15 vs. 1.8L, for a 1.57x equivalency factor). This provides another precedent for using a lower equivalence factor for rotaries.

Some may ask what about the threat of peripheral ported rotary engines? A comparison here is harder to make given than peripheral ported rotary engines are typically illegal for most classes. There is no doubt that a peripheral port can produce more horsepower than a bridge port engine, but given the increased cost, reduced time between rebuilds, narrow powerband, and limited legality; a peripheral port engine is not a factor for most classes. For those classes that do allow it, a separate higher (but still less than 2x) equivalency factor should be used.

Others will surely point to the new Renesis 13B rotary as a new threat. While their power potential in race trim is still unknown at this point, I suggest that is it highly unlikely to significantly exceed that of peripheral port engines. In stock trim, a Renesis from a manual transmission RX-8 has almost exactly the same power as a 2.0L engine from an early Honda S2000 (238 vs. 240hp, 1.3 vs. 2.0L for an equivalency of 1.54). The less peaky, revised 2.2L S2000 engine makes significantly more power than the less peaky automatic transmission version of the Renesis engine (still 240hp vs. 197 for the automatic Renesis, requiring a 1.39 equivalency factor to yield the same specific output). Overall, this comparison results in a slightly higher equivalency factor than exists in CSR but it is still well below the 2x factor traditionally used.

Suggested equivalence factors:

Street port - 1.3
Compared to the 1.23x currently used for CSR 13B street port engines, conservative in consideration of the varying induction restrictions on most CSR engines.

Bridge port / unported Renesis - 1.5
Compromise between the 1.4 used for CSR 12A’s and the 1.57 currently proposed for DM. The Renesis value is close to the 1.54 vs. the S2000 for stock engines, slightly lower on the basis of a desire for a common value and because a piston engine allowed cam and port work should more than make up any gap on a unported Renesis.

Unrestricted porting (Peripheral, ported Renesis) – 1.7
Based on an equating a 238hp 1.3L Renesis with a 2.2L 240hp S2000 engine, and continuing the +0.2 gap between bridge post and street port engines

In summary, the current standard 2.0 equivalence factor used for rotary engines does not properly account for their power potential verses modern piston engines. The use of both piston and rotary engines competitively in CSR has demonstrated that the real equivalency factor against their modern 4 valve competition is much lower. I request that the equivalency be readjusted based on that experience, especially for those racing classes were the minimum vehicle weight is tied to displacement or where a category is subdivided based on displacement.
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If you agree (and are a SCCA member), you might want to write something similar (feel free to copy anything here you like).

Bruce F.

Bravo, Bruce. Finally somebody that shares my same thoughts, but has the brains to do the math!

I'll be stealing a few paragraphs from you. ;)

/Heads back out to look at the Crossle and daydream about a 12A in there instead of the friggin' Pinto motor....

Lucky they don't go by the real displacement which is 3.6 litres for the 12A.

You just kidding around right? Right.....?

Bruce F,

You didn't say in your letter which class you wanted the weight or displacement adjustment applied to. I think you need to be very specific when trying to get rules changes in SCCA.

Since you reference CSR class a lot, how did the rotaries finish in the runoffs this year in CSR? The finish at Mid-Ohio seems to affect more road racing rules than anything else. The 12A bridge in CSR has to run tiny 34mm chokes, smaller than I run on my street car 12A street port which uses 37mm chokes.

I am big on consistant rules, so I was suggesting it as a general change for those classes that have a weight/displacement formula or those that divide the classes by displacement.

I was partially succesful in that I got street port 13B's allowed in D Modified (Solo2) vs 2.0L piston engines for my Lotus 7 clone. I would be nice if I could run Super Production - Under 2.5L (club racing) instead of Super Production - heavy cars with big V8's (I don't want to play chicken with 600hp, 3400lb cars with my 190hp, 1300lb car) , but I am not sure I am ever going to wheel to wheel race it. And if so, NASA has a couple of categories where it would be a better fit.

You bringing out the RX-7 race car to the TAC Car Show?

Mike C - are you high? Try reading the manual - 12A is 1146cc displacement.

Says who? ;)

Mazda

ooo.. what underlying meaning does that statement really have?

Why not just do it by power/weight? Jeeze.... if they're factoring in how a larger displacmeent motor has a bigger powerband, why not just have a rev decide, or figure a forumula for how big the powerband is relative to the whole revrange or do a torque/hp ratio?

Volumetrically its the same as twice what mazda says it is. Mechanically its 3 times waht mazda says it is but has to spin 3 times, not two, to use all of it... making it effectively a 2.6 that way too.

But, well, a coefficient of "2" is not fair as theyre inefficient thermodynamically and dont have the same specific output.

So, why not screw it all and do it case by case?

Also, I see GTORX7 is as stubborn as ever :wallbash:

The way Mazda computes the displacement of the rotary engine has no equivalence whatsoever with the way piston engine displacement is calculated.

Rotary engine displacement vs piston engine displacement

Grintch's argument is well and good however because he's not really arguing about displacement, he's arguing about equivalency rules that are applied to different powerplants running in the same class.

Because the rule book would become thicker than the Hong Kong telephone directory and be nearly impossible to comprehend.

Remember that no one claims a 500cc 2 stroke piston engine displaces 1000cc just because it fires every revolution rather than every other revolution like a 4 stroke. But yes I was trying more to find a performance equivalence rather than some theoretical equivalence of two drasticly different methods of operation.

Also remember, most of the classes that would be effected by such a change allow virtually unlimited changes to a piston engine (pistons, crank, bored out blocks, ported heads, cam(s), etc.) while a rotary is typically stuck with the stock rotors, shafts, and housings (and often the porting is restricted).

For reference, I think FIA used a 1.8x equivalence factor with any extent of porting.

Looks good for the most part. The only problem with it as is, I think, is that the bridgeports from CSR that you're using as comparison are forced to use quite small chokes, though you did touch on that. Unrestricted, the numbers might crunch out to more like 1.6-1.7x than the 1.4-1.5x or so.

Note the semi-standard for the class Toyota Atlantic style 4AG also has a choke/throttle body size restriction.

In 2006 SCCA C Sports Racing,
the 4AG Toyata DOHC 4 valve is allowed four 42mm chokes = 5541 square mm flow area
the 12A bridge port carb is allowed two 34 mm chokes = 2036 square mm flow area

If you want SCCA to change a rule, you better be really specific about what class and exactly what you want them to change, unless you are an insider or a manufacturer with money.

Remember the Golden Rule of Racing: Whoever has the most Gold, makes the Rules