ludeowner

Hey Everybody,

I had a question for all of you, I wanted to know how to figure out CFM for an RX7 motor BEFORE the turbo. In other words how much CFM would a 3G RX7 flow with 0 boost?

I know how to do it for a I-4 Piston engine, but how do you do it for a rotary?!?

for a reciprocating engine I use these calculations:
(CID * RPM * VE * 0.5) / 1728 = CFM
using my car as an example (2.2L, 7400 RPM, 95%)
(134.2 * 7400 * 0.95 * 0.5) / 1728 = 273 CFM

and If I wanted to know how much CFM that would be under boost I just multiply it by the Pressure Ratio.

Now how would you calculate airflow (in CFM) for a Rotary?

BTW my sources for the calculations is Maximum Boost by Corky Bell.

TIA

EricM

Others can jump in and correct me if I'm wrong, but I think you can eliminate the 0.5 in the calculation. So, just treat a 13B as 2.6 liter piston engine. hehe.... nice huh ? BTW, the 4 rotor 787B was regarded as a 4.7 liter by FIA back in early 90s.

ludeowner

So then my next question is what would the Volumetric Efficiency be? I used 95% for a quick calculation, but I wanted to be as accurate as possible.

Also what is stock boost level? 10 or 11 PSI?

And what is a FD's stock redline?

Right now here is what I have:

(79.3 * 7500 * 0.95) / 1728 = 327 CFM (Basic Engine CFM)

CFM under 10 PSI of boost @ 7500 RPM = Approximately 550 CFM

And thats without taking into consideration the Air Temp., actual turbo flow characteristics, Because I'm not interested in boosted CFM.

EricM

well, again I'm only guessing...

but I think street port is good for 90-95% VE. My buddy told me that a shop near his place in Japan did a PP engine which is suppose to be 120-125% VE. Don't know for sure though.
What turbo are you planning to buy ?

KevinK2

The effective VE is low due to turbo backpressure, say .85 , then drop cold flow rating due to temp rise say .9 factor.

drops 550 to 440 cfm. At 6500 rpm, that's 380 cfm ... about right for 255 hp stock.

ludeowner

QUOTE]Originally posted by EricM
What turbo are you planning to buy ? [/QUOTE]

Oh there is just something I'm trying to figure out, someone on "one of those Honda" sites made a (silly) comment about putting a TT on an I-4, Specifically a Honda Prelude 2.2L. Altough the size and airflow of the "Lude" makes a TT a bad idea, this got me thinking that *maybe* an S2000 can use a sequential TT setup.

Maximum Boost by Corky Bell states that "sometimes" an engine with a CFM of 300+ CFM can have a TT setup. The closest thing to the 2.0L S2000 was the 1.3L Rotary.

According to my rough estimates, an S2000 engine has roughly:
320 CFM @ 8000 RPM's
158 CFM @ 5000 RPM's

VE% = 116% peak

I wanted to compare the CFM of an FD to the numbers I already had. I just dont know if there is a different calculation that is done for rotaries, and I also dont know what the VE % is for a Rotary. I was just wondering that if "theoretically" a Sequential TT setup would work on the S2000.

ludeowner

Is that CFM "before" or "after" the turbo?

If the turbo was not not on there how much CFM would just the engine be flowing? this without taking temperature into consideration.

Dyre

RICERACING knows all these variables, as do alot of the guys that hang in the single turbo forum- Might want to PM RICE RACING and Crispeed- They should be able to help you.

They will probably never see this thread in this forumn though-

ludeowner

cool I'll look them up, thanks!

EricM

I think kevin meant VE for the whole system, I believe that with properly setup single turbo, FMIC, street port and stuff it should be near 100% for the whole system.
Again, that 255 hp stock is stock port.

yea, ask RICERACING, he should know.

RICE RACING

This is complex, for me to calculate out the exact VE of you're set up I need to know things such as;

IC eff, Porting, turbo (series,trim,housings), exhaust (manifold, system), Intake (type, modifications) all of my calculations are corrected for std temperature and pressure days (this can have an affect as well).

For a 3rd gen with std hardware the ve% @ 6500rpm is around 70% which is around 212CFM (no boost)

Where as a 3rd gen with street porting and all properly tunned accessories has a ve% @ 8000rpm of around 93% which gives 374CFM (no boost).

If you add the BSFC component to these two examples along with the a/f ratio you can calculate the BHP, The ported engine will produce around 227BHP and the std one around 140BHP (In n/a form i.e. NO BOOST).

ludeowner

Thats what I was looking for, just the VE with "standard equipment" and NO boost.

(79.3 * 6500 * 0.70) / 1728 = 209 CFM

thanks RICE RACING