My question is, can this be applied to rotaries?

http://www.rceng.com/technical.aspx

Obviously, the rotary uses a lot more fuel per cc than a piston engine so the formula doesn't work as is. But I found that by adjusting the BSFC to .75 instead of .50 (piston engines), the results seem pretty realistic.

And to adjust the formula to rotaries and uneven injector size, you do the formula backwards since its the total ccs that matters at WOT.

Example:
(720cc x 2)+(550cc x 2) = 2540cc

2450cc= 612cc
4

612cc= 58.25lbs/hr
10.5

(58.25lbs/hr x 0.80%duty)= 62.1bhp per injector
0.75 BSFC

62.1bhp x 4 = 248.5bhp @ 80% duty.

(based on stock TII fuel pressure)

Does this make any sense? Anybody can tell me if this can be used on rotaries?

Spec.

 

Absolutely

I feel your pain. I have had a lot of trouble trying to reverse-engineer the numbers due to the extreme variances in rotary engines, such as porting differences, tuning quality, oil pre-mixing, etc. A BSFC of .75 would probably be about right for a high-boost engine that is not tuned by a professional, while a typical well-tuned mid-boost engine seems to be around .62-.64, and NA engines seem to be in the .5's range. I have seen dyno sheets of professionally-tuned NA and supercharged aviation and marine rotary engines in the mid .4 range. It's really difficult to estimate fuel requirements when you don't have good BSFC numbers, so there is a fair amount of guessing here.

Yes, if using the RC Engineering calculator, just enter "1" for the number of injectors and then figure out which combination of injectors will yield the total CCs.

Just keep in mind that not all injectors are rated the same. For example, some are rated at 43.5psi while others are rated at 40psi, and some are rated with gasoline while others are rated with higher viscosity mineral spirits. Also, not all injectors have the same latency.

In summary, just use these types of calculations for basic estimates, and when in doubt use the next larger injector.

 

I've found the best way to size injectors is to explain your setup and your goals to really experienced people/tuners and follow their recommendations. Rules of thumb work well enough.

 

So I would assume that to calculat it on a boosted NA with a supercharger I would use a BSFC of .70.
Meaning I would come up borderline with 4x550cc injectors at 6-8PSI with a walbro255lhp. Using 550cc x 2 and 720cc x 2 would give me more freedom with a fuel pressure regulator and a SAFC.

Which injectors set-up would you choose on a supercharged NA @ 6-8PSI?

Spec.

 

In all modesty, I have a whole lot of experience. And the reason I am posting this is because a member of our club is doing an SC set-up as we speak and he bought 4x550cc injectors. I am pretty sure that he would be borderline duty wise so I recommended him some 720CC secondaries (because like E.A. said, when in doubt use the bigger injectors). So the guy in question showed me that link and I played with the formula, used my logic and got to this conclusion. Hence my post.
Also, SC on RX7s are rare as pope ****, so finding "experienced" people on the matter is not that easy.


Spec.

 

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According to the guidance from Atkins/Camden, the stock 13B NA fuel injectors (4x460cc) are good for 6-8psi boost and 176bhp on a stock-ported 13B 6-port engine with their Roots supercharger kit.
http://www.camdensuperchargers.com/index.php?pag=10

Since you did not specify the supercharger type or engine porting, you can just use the above base information and adjust as necessary.