How much CFM needed to produce PSI
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BlueOvalStangGT
Active Member
isnt in this case the psi a measure of the amount of resistance of the air after the s/c going into the engine? and out of curiousity where did u get taht 500 cfm number? well ill just assume for half a sec that 500 is the number of the day for ur n/a engine consuming at 1 atmosphere. and 14.7psi (?) is +1 atmosphere right? so for the engine thats flowing 500 cfms at that point would req another 500cfms from the s/c to read 14.7psi. soooo... 500cfm/14.7=34.01 cfm for 1 psi.. at that point.
btw, im talkin out of my ass a bit here, but ill try to figure this out anywho
btw, im talkin out of my ass a bit here, but ill try to figure this out anywho
Black95GTS
Active Member
You can't convert a pressure into a flow rate when talking about an engine. Your intake, heads, etc will only flow a certain amount of air at a certain point. If you read a lot of boards, lots of peeps refer to this as "bottlenecking."
Example 1: You have a 408 with Vic Jr. Heads, Custom Cam, all the supporting goods, but with a stock 351 truck intake. No matter how much air your components could flow, at a certain point the air that enters the engine will be limited by that truck intake.
Example 2: If you put a 10 PSI turbo on a stock engine, and then put a 10 PSI turbo on an HCI engine, the HCI engine will flow WAY more CFM, all else being equal.
So to quote Mark O'Neal.... it depends.
Adam
Example 1: You have a 408 with Vic Jr. Heads, Custom Cam, all the supporting goods, but with a stock 351 truck intake. No matter how much air your components could flow, at a certain point the air that enters the engine will be limited by that truck intake.
Example 2: If you put a 10 PSI turbo on a stock engine, and then put a 10 PSI turbo on an HCI engine, the HCI engine will flow WAY more CFM, all else being equal.
So to quote Mark O'Neal.... it depends.
Adam
mustangt562
Member
N/A is spoken in terms of vacuume and Forced Induction is spoken in terms of PSI right? 
Apples and oranges?
Apples and oranges?
Black95GTS
Active Member
N/A is spoken in terms of vacuume and Forced Induction is spoken in terms of PSI right?
Apples and oranges?
The compression ratio in an engine is the measure of how much the compression stroke forces the air / fuel mixture together in comparison to atmoshperic pressure (14.7 PSI).
For example, an engine with a compression ration of 10 - 1 compresses an air/fuel mixture of 14.7 to 1 PSI to 147 to 1 PSI in the compression stroke.
When you use forced induction, you stuff more air (and fuel) into the compression stroke, thus raising your compression ratio, and increasing power. This is why a typical forced induction engine uses a smaller compression ratio then a NA engine. With a smaller initial compression ratio, more boost can be stuffed into the initial compression stroke without causing the engine to detonate.
Which brings us to your original question. People measure boost in terms of PSI because that is what the forced induction device ADDS to the pressure in the cylinder's compression stroke.
If I got that wrong someone correct me, but I think it's correct!
Adam
But Even if that were the case, say example one flows 700cfm and example 2 flows 550cfm n/a. Im sayin how much over the max number would produce boost, like An addtional 100cfm = so many PSI?
ok. so in ex2, since the HCI engine flows more cfms, wouldnt that same 10psi read lower than the 10psi measured on a stock engine? even if the turbo is moving the same amount of air
cause when u remove restrictions after the blower/turbo, boost gauges always drop the measured psi, even tho theyre turning teh same speed as before
Black95GTS
Active Member
Yes. You just said the same thing I did
My point being that depending on the engine, 10 PSI of boost can be a whole world of different CFM.Think about it this way: you got a 1 inch diameter tube (stock engine) and a 3 inch diameter tube (HCI engine). 10 PSI more pressure in the 3 inch tube will be exponentially more flow then 10 PSI pressure in the 1 inch tube. It's not a linear correlation even in the most basic scenario.
If you guys do some googling you'll come up with some good info. Just do some searching for things like 4 stroke engine, compression stroke, forced induction, etc. I also recommend turbomustangs.com as those guys are on the forced induction up and up.
I'm pretty sure the Eaton M90 is more of a passenger car supercharger.... are you sure its up for street/strip duty? I don't know anything more about it, so I don't want to risk giving you bad advice.
Adam
Yobi1Kanobi
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- Apr 9, 2003
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Black95GTS
Active Member
Where did that come from? 25.58?? I'm just going to reiterate to you I would be nervous using CID as a constant. Like I've been saying, some 302s are not the same as other 302s.
I'd be interested in learning something, do you have the link for that? :SNSign:
Adam
Hey Now
Something different goin on in this thread
Good stuff by all
I find it interesting how some peeps relate boost measured on a gauge
as the be all and end all of how much power they are gonna make.
Bottom line here is those values are nothing but
a measurement of resistance to the airflow.
Simply put ... Higher boost values don't always mean more power
Generally Speaking ... For a Street Car .......
I'd rather have lower boost on a more efficient combo
than
Higher boost on a stock or close to stock combo
Just a thought or two
Grady
Something different goin on in this thread
Good stuff by all
I find it interesting how some peeps relate boost measured on a gauge
as the be all and end all of how much power they are gonna make.
Bottom line here is those values are nothing but
a measurement of resistance to the airflow.
Simply put ... Higher boost values don't always mean more power
Generally Speaking ... For a Street Car .......
I'd rather have lower boost on a more efficient combo
than
Higher boost on a stock or close to stock combo
Just a thought or two
Grady
I found the formula on the supercoupe forums, (sccoa.com), Someone was doing a custom project with the m90 and a user replied with that formula to try and calcualte boost, No idea what the 25.58 is, But From what ive seen and calculated the result of that formula is close to what I thought it would be.
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