ibanezeros

i tried to search this and i'm not coming up with much. So hopefully somebody can explain this to me. I'm comparing the compressor maps for various MHI (mitsubishi) turbos, btw I have a td05-14b, and I'm going to upgrade the compressor wheel. For my application an H23a, (prelude si) my calculations leave me for 5psi 360cfm at redline, and 200cfm at the boost threshold. The redline is way off of my 14b compressor map. OK, so my question is if I follow the 14b map up from 360cfm the first point is at 10psi. If I run 10psi on my 14b to produce 360cfm of air, will it be the same as If i'm running 5psi of boost on a 20g wheel with 360cfm? I wanted to run about 5 psi at first, but for 5psi I need 360cfm, but for 350cfm I need 10psi. you can see how this is confusing me.

claytonsmith

<-- confused as well

but i can tell you that cfm ( cubic feet per min) is a measure of volume... and psi is a measure of pressure... but you prolly knew that...

B16Drag

get a new turbo to accommodate for ur nice and large displacement motor

hackish

I'm not an expert in this area but here is what I think... The airflow versus PSI is going to be dependant on your cam, your head, your RPM, even your mixture. For example, really rich, it won't combust quite as much and the exhaust will be cooler thus less exhaust flow through the turbine.

Ignoring the exhaust restrictions imposed by the turbine there is a way you could determine the compressor flow into the engine...

You could calculate your VE as a result of your fuel requirements to achieve a certain AFR under certain conditions. IE to make 12.5:1 at 5 PSI of boost and you inject X grams of fuel (based on injector size, fuel pressure and pulsewidth) then you can reasonably see the actual number of grams of air that went into the cylinder.

-Michael

ibanezeros

I'm going to upgrade the compressor wheel to a 20g, and get it all rebalanced etc.. a 20g will flow good for my engine at 5psi or 20, but i was just wondering about this.

ibanezeros

yea, i know the flow, i know the psi, and i know the VE, I'm just trying to figure out what the difference is between 5psi flowing 360cfm, and 10psi flowing 360cfm. What is safe for my engine.
I want to keep the boost low, ~5psi on the 20g, but I was noticing that the air flow volume of the 20g is the same as the 14b at 10psi. Not that I'm doing this, but is it safe for me to run 10psi on the 14b?, I know my top end wont be what it would be with the 20g, but when reading corky bell, he derives his horsepower figures based only on the pressure, and i dont see how 5psi flowing 250cfm can create the same power as 5psi flowing 360cfm. Am i missing something important here?

this is my first attempt at posting pics, so here goes

Here is the map for the 14b. I plotted differnt values of pressure, vs. cfm



Here is the map for 20g

scarecrowX

cfm at a given psi is based ENTIRELY on the displacement, VE and rpm of your engine, not on the compressor. think about this: your engine has a maximum displacement of 2.xL. if it consumes 360cfm at redline at 10 psi, the ONLY way to cram more air volume into the engine is to raise the boost. when you see the same cfm at a lower pressure ratio on a bigger compressor wheel, the ONLY thing it's changing is where your engine airflow plot lies on the efficiency map. it does not make your engine consume more air volume at the same rpm and a lower boost. the outlet temp is lower, but not enough for a huge difference in density.

with the 20g, you may only need 9.75psi to get the same performance as the 14b at 10psi. the real difference is once the 14b begins to choke off, the 20g will allow you to keep pulling. there's negligible performance difference before that, only lag.

which brings me to my next point, the flow of the 20g will also require an external wastegate if you are still using the 7cm exhaust housing as it begins to choke off just as the compressor is spooling up. you will get boost creep if you don't have one.