This equation will tell you the air flow rate of an engine at any rpm, thus giving you the a more calculated way of choosing your turbo. The example I have is for a
302 cid v8 engine. I am still trying to figure the equation for a 1300 cc rotary engine maybe someone can help.

air flow rate = cid * rpm * 0.5 *Ev/1728= xCFM

1728=converts cubic inches to cubic feet
cid=cubic inches of displacement
0.5 is due to the fact that a four stroke engine fills its cylinders only on one half the revolutions.
Ev=volumetric efficiancy=85% or .85

airflow rate of a 302 engine at 5000 rpms=

302*5500*.5*.85/1728=408cfm at 5500 rpms
Thus a N/A 302 v8 at 5500 rpms produces 408cfm

If we wanted to run 12 psi of boost we would

Pressure ratio = 14.7+boost/14.7
=14.7+12/14.7 = 1.82

So our airflow rate at 12psi = 1.82psi*408cfm=743cfm

Knowing that my maxflow at 12psi is 743cfm I can go look at turbo charts that show max efficiancy at 743cfm.

Thats easy. Lets see if we can combine our brains and figure out this eqation for a rotary engine.

First, we have to convert 1300cc, if I am wrong please correct me, to cubic inches.

Second, if 0.5 is for a four stroke engine that fills its cylinders only on one half the revolutions, what would be our CONSTANTt for the rotary engine? I am guessing .3 because a rotary engine fiils its "cylinder" every 3rd of a cycle.

I am gonna crunch some numbers, any help would be appreciated. If we want our turbo's sweet spot to be at 5000 rpms and running a max boost of say 15 psi we can find the perfect turbo no matter what sweetspot you desire and how much boost you wish to run.

Its early but lets get our BRAINS goin. If anyone can find out what our CONSTANT is through a friend or any other way that would be awesome!

 

If a 4 stroke engine fills have its cylinders every revolution than a rotary engine fills both, making our constant = 1. 1300cc = 79.3 ci.

If a 302 cid v8 at 5500 rpm=408 cfm, then 79.3 ci rotary using constant of 1 at 5500rpm = 214cfm
It sounds low but if you calculate cfm to ci ratio you get
1.35 cfm/ci for the 302 and,
2.69 cfm/ci for the 13b

So, if I want to flow at least 15 psi of boost yielding psi ratio of about 2 than (2*214=428cfm at 5500 rpm and 2 273=546cfm at 7000 rpms) I need a turbo with thats most efficiant between 428cfm and 546cfm.

These numbers sound right to me and are correct as long as our constant is correct. Again, if I am wrong PLEASE PROVE IT. Of course this does not count for porting but I dont think porting an engine will very the displacement more than 20 to 50 cc's.

In conclusion, if you know what RPM range you want your turbo to do its magic on then punch in your numbers and compare to flow charts.

I want a my boost threshhold to be at 4k rpms and be at max efficiancy for as long as possible.

1300cc=79.3, (79.3*4000*1*.85/1728=156cfm(N/A))
(79.3*7000*1*.85/1728=273cfm(N/A))
street boost of 15psi=Pressure ratio=2
track boost of 20psi=PR=2.4

at 15 psi at 4k cfms=156*2=302cfm
and 15 psi at 7k cfms=273*2=546cfm

at 20 psi at 4k cfms=156*2.4=374cfm
and 20 psi at 7k cfms=273*2.4=655cfm

I want my car to be more efficiant at 20 psi than at 15 psi so I will focus on the higher numbers.

Looks like turbonetics 60-1 would be ideal for me!

 

Most graphs are in pounds/min so just do this.

Xcfms * .081 = x pnds/min

655cfm * .081 = 53.055

 

Good question. I've tried this also and couldn't come up with any good numbers.

Someone has to know, probably Rice

 

What a waste!