RXTII400

What is the generally accepted figure for volumetric efficiency for our cars? I have been reading maximum boost, a book by corky bell, and one of the chapters discusses turbo sizing. A key part of the equation for selecting a turbo that has a good fit is the volumetric efficiency of the engine you are going to be using the turbo on but it does not discuss how to make an estimate or get the figure. The book just uses 85% for a ford 302 cubic inch V8. Any thoughts or help?



Thanks.

j0rd4n

I've read that the N/A RX7 VE is around 94%. I'm not sure exactly how accurate that is, as it was treated as a 2.4l 4-cylinder in the VE formula since, to my knowledge, there isn't a rotary VE formula.



I actually just started reading about VE and such, so does it mention that VE can be greater than 100%??? It's ridiculous.

j9fd3s

i'm not really sure either, and it changes with rpms. just throwing figures at a calculator, about 85-95% is a good figure.



another method of turbo sizing, would be to look at the stock turbo specs, and just upsize, stock compressor is 63mm turbine is 62mm, notice how close they are sized, hint hint



another method would be to see what people are running with similar power goals, take with a grain of salt, and choose a turbo that way.







the KEY when choosing a turbo, is to be honest about what you want the car to do, are you looking for the highest hp #? instant response? only 260hp? the better you define what you want the engine to do, the easier it is to do it.

j0rd4n

oh yea, i forgot to mention that. your greatest VE comes at your highest torque rpm, which is....4500? i really forget :/

j9fd3s

yeah, its prolly really close to, if not just over 100% around 4500...

j0rd4n

What I read (and, again, this guy's formula is used for piston engines, so I'm not sure of the error rate) is that he calculated it to be 95% at peak torque.



This makes sense, since engineers had many restrictions in terms of developing the intake. I believe that 95% is pretty good for a stock intake. A porting, etc, helps increase VE, I'd like to see someone do a calculation of VE on a modded intake and exhaust (yea I said exhaust. A better-flowing exhaust will not cause as much resistance for the intake air being pushed into the intake area in the rotor).



For those who don't know, VE and HP have a direct (linear?) correlation. Raise VE 10%, and you've raised HP 10%.