why would a GT35R making 20psi make more power than the 20g making 20psi? isn't 20psi 20psi, regardless of where it's coming from?

 

I had the same question once. Use the search function. Volumetric efficiency

 

IT has nothing to do With VE, IT's actually CFM that matters. all Boost is is back pressure in the manifold. More boost equals more air pressure being built up.

Thats why a honda boosting 20 psi makes 300hp and a LS1 boosting 20 psi makes 1k+

The more air your motor can flow the more the lower the PSI would be in the the manifold at the same setting.

 

you go and make a technical correction to someone and then you say something like all boost is is back pressure in the manifold? boost is air pressure inside your intake tract, technically it is "backpressure" but then on that token it's also "forepressure."

then you say more boost equals more air pressure built up. which is absolutely true.

so in long, a gt35 at 20psi is moving more air mass than a 20g at the same psi.

now a dumb illustration: a big bucket that is 75% full will move more water than a small bucket that is 75% full.

now this illustration really sucks becuase it doesn't in any way explain how pressure works so i'll do that. pressure is a force per area so since it's a ratio you can achieve any number of similar pressures by modifying the two contributing factors force and area. so another illustration that is better is that you can have a small box holding 30psi of air and that is less than a big box holding 30psi of air.

now that illustration also sucks because it doesn't explain how turbos work. so the gt35 pushes more air mass because 1) it's bigger physically, so it modifies the area part of the equation a bit 2) it's more efficient so it pumps colder air which means in a roundabout way it lowers the force part of the equation. there are more reasons but i think you're starting to get the picture.

 

I think everyone on here could figure out what i ment about the back pressure, i should have said backed up pressure instead.....

a better example would be if you had two fans, a small desk fan and a industrial fan (the kinds used in dyno rooms) bothe have a low, medium, and hi. No matter how hi you turn up your desk fan(boost) you will never flow as much air as the industrial fan on low(lower boost but more CFM).

does that help

 

woooooooooo nice metaphors!!

 

bigger is better.



that always seems to sum it up

 

+1

 

Well, you're effectively pressurizing the intake manifold, yes? So the same intake manifold, different turbos....same area to fill, same pressure....this means they have the same amount of air put into them in the same amount of time...aka, cfm doesn't change...I'm still confused!

 

You missed the key point in trinydex's response: "it's more efficient so it pumps colder air."

Every motorhead needs to know PV=nRT. The "n" is how much air (you want it big) so n = PV/RT. At the same pressure and volume you have more air if the temperature is lower. Manifold air pressure by itself is meaningless, manifold air density (proportional to P/T) is the important thing.

Dave

 

Physics is something I understand! Thanks djh. Didn't know that intake temps changed depending on the turbo!

 

So now you are comparing two different motors is size. a honda is 2.0L, a LS1 is 5.7L, when both boosting 20 psi, obviously, the bigger engine will make more hp.
But how about if the size of the motors is the same ?

thus, i think the key difference between both turbo boosting 20 psi is the efficiency

 

ok guys here we go.......

Now the one on the bottom is a GT44R and the one on the top is a Evo 3 16g....now just by looking at the picture what turbo do you think would produce more at 20psi??? any takers?

this isn't rocket science

 

sooo what we come up with is that the shear mas of the gt44 will produce more CFM at 20 psi then the 16g

 

Forget about my comment about the Differnt motors

All you really need to know is that a Larger turbo will flow CFM (cubic feet per minute) at the same boost pressure.


The more CFM you Flow the more power you will make.... Thats why Smaller turbos have to spin faster to make up for there innability to flow cfm drawback is that the faster you spin the turbine the hotter it gets the less efficient it becomes.


So

Big Turbo=Cooler air, more cfm, less boost required, longer to spool
Small turbo, Quick Spool, Less CFM, Fall out of efficiancy at high RPM and Boost.