So I have heard from a couple people that a larger compressor can make more power at the same psi level, because "it's moving more air".

I don't understand how this is possible, as the amount of air going in the engine is more or less a function of displacement, volumetric efficiency, rpm, and psi. If all these are the same, how can increasing the compressor size change the amount of air? What I'm saying is that if it moves more air, it must cause the pressure to go up.

For example, a 2 liter engine with 80% volumetric efficiency, at 5000rpm (2500 intake strokes per cylinder per minute), and 14.7psi (2 atm absolute) should have
2 * 0.8 * 2500 * 2 = 8000 liter/minute = 282 cfm of air going in.

So how can a larger compressor move more than 282 cfm of air at the SAME psi?

I do understand that there are other factors which come into play:
- Heat/efficiency - a small compressor driven outside its range will heat up the air a lot more, resulting in a loss of power. But this depends on the compressor map, which shows whether or not the cfm (as computed above) is within the higher efficiency ranges
- Exhaust restriction - a small turbine will choke the exhaust resulting in a loss of power. But this is more a function of turbine size rather than compressor size.

Am I missing something or is the heat/efficiency factor really the only reason a larger compressor can make more power at the same psi?

 

You're basically on the right track. Its kind of sad how this is beat up across the internet. A lot of people say its based on the CFM, but as your formula just shows, the amount of possible CFM is irrelevant if you're maxing out the cfm your motor can take at an rpm with both. Picking a turbo is directly related to:


1) Exhaust restriction as you said. Generally, turbos with smaller compressors are smaller overall.
2) Heat. There can be a BIG difference in the heat a turbo produces from one to the other. I don't have exact numbers on the different heat ranges from turbo turbo, but I've noticed the difference as I've switched between them. Even a 20-30 degree air temp change (seems reasonable) can result in 15ish whp difference at the same psi, assuming like a 250 or so base starting point.
3) Efficiency. You've already solved for this as well.


PSI is a measure of pressure in pipes. If you change the turbo, and leave the piping it runs through the same, you're gonna have the same pressure output. CFM is a flow-rate. If you're already at the maximum flow rate your motor can take in at a certain RPM, flowing more air won't help as well. Sometimes this isn't the case.

 

Thank you!!


Still, if you're not at the maximum flow rate that the motor can take in, then the psi will go down... For example with a compressor that's too small you might have 14 psi at 4000rpm but only 10 at 6000rpm <- is this actually possible?

 

I believe I'm wrong and you are correct on that one. Smaller turbos do have a tendency to fall on their face, especially at higher rpms, and have the PSI drop.

 

For starters, a bigger compressor doesn't always make more power. The goal is to get the best size compressor. Look at a stock EVO on Gen Coupe dyno chart. If the torque curve peaks early and then drops off, the engine is consuming the air faster than the turbo can supply it. A bigger compressor will make more power here. If the torque curve is flat, then the turbo is properly sized and a bigger compressor won't do much without other engine mods.

 

Yes, that's why you want to consult the compressor map to see if it fits your application. You could be to the left of the efficiency areas (compressor too big) just as easily as you could be to the right (compressor too small).

 

you need to take in fatc that a larger compressor wheel and housing will turn slower for the same boost/CFM
so this means less heat

less heat means, colder air
colder air means, denser air
denser air means,more air
more air means, more power

 

^ the added heat is quantified by the 'efficiency' range, the ones that show up in the compressor map, am I correct? Or is there more than the efficiency factor that tells you how much heat is added?

 

if you stay in the compressor sweet spot, yes it's much better
but most of the time when you go wit ha larger compressor you will also get a larger turbine
more turbine flow = less restriction = less boost for the same HP level

but i did see some cars on wich we gained some power with only a larger compressor side when the already in use compressor was in is sweet spot

again, some cars like oversized compressor while other won't gain anything exept more lag

 

So, are you trying to compare a 13b turbo for example to a gt40 at say, 10psi and asking why one has more power than the other?

Or are you comparing a evoIII 16g for example paired with a 18g compressor wheel?