How much CFM needed to produce PSI

tjh566 · Jan 3, 2007

Ok, semi-stockish mustangs have in the 500's for the CFM number, How much higher than that do you need for boost in CFM, like 100 more CFM = so many psi?

Ive searched around and cannot find it, thanks for the help guys

BlueOvalStangGT · Jan 3, 2007

No a leaf blower will not produce boost

Carry on normal tech conversation....

Umbra_Lupis · Jan 4, 2007

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

Synned · Jan 4, 2007

Umbra_Lupis said:
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

Makes sense

Black95GTS · Jan 4, 2007

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

mustangt562 · Jan 4, 2007

N/A is spoken in terms of vacuume and Forced Induction is spoken in terms of PSI right?

Apples and oranges?

Black95GTS · Jan 4, 2007

mustangt562 said:
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

tjh566 · Jan 4, 2007

Well Im just trying to figure out how fast i need to spin my blower to get 6-7 psi, on my stock-ish car, Im trying to order a pulley for the blower and Im trying to calulate what size I need

tjh566 · Jan 4, 2007

Black95GTS said:
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

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?

Umbra_Lupis · Jan 4, 2007

Black95GTS said:
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

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

Umbra_Lupis · Jan 4, 2007

oh and btw tjh566, im not so sure this eaton m90 is going to be too happy powering the 5.0 engine. its gonna run out of potential waaaay early

tjh566 · Jan 4, 2007

Ive done some research, it'll be fine, I only want 6psi out of it, that'll prolly be its max anyways, I shift at 5300rpm, it'll work.

Black95GTS · Jan 4, 2007

Umbra_Lupis said:
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

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

tjh566 · Jan 4, 2007

We'll see, it'll be an interesting project, it has been done before, on a fox, he made it work, theres some vids floating around of it.

But still, how am I goning to calculate boost here?

tjh566 · Jan 4, 2007

Aha!, Found the formula

25.58 X (Blower displacment) X drive ratio / CID -14.7

Yobi1Kanobi · Jan 4, 2007

Wow my head hurts

Black95GTS · Jan 5, 2007

tjh566 said:
Aha!, Found the formula

25.58 X (Blower displacment) X drive ratio / CID -14.7

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

final5-0 · Jan 5, 2007

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

urban96 · Jan 5, 2007

id like to see what my P combo would do with a few PSI

tjh566 · Jan 5, 2007

Black95GTS said:
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

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.

How much CFM needed to produce PSI

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