Is there a chart that shows the different diameter tubes and how much cfm flows through em? I know, cfm can change with the velocity, but, like a general idea I guess? Thanks!

SKeTchy

 

It depends on the surface finish of the tube and how many/what kind of bends it has in it. The density of the air will also have an effect on the flow. There is a lot of variables.

 

CFM can also change with the pressure differential. Fer' instance, heads are usually flowed at a vacuum level of 28" of water. 4bbl carbs are flowed at vacuum level of 1.5" of Hg (Mercury) which is about 3" of water. Don't quote me on that conversion rate. I don't recall the conversion rate from H2O vacuum to Hg vacuum but I think it's about 2:1 or thereabouts. Of course, I'm talking VACUUM here- pressure below atmosperic, not in situations where you're seeing boost (pressure above atmospheric).

There are formulas that you can figure the CFM regardless of what the pressures (or lack thereof) are on either side of the tube. I'm sure somebody would be happy to post them.

Actually, I don't mean to hijack this thread, but I'd like to know what the conversion rate H2O->Hg actually is.

 

Air density does not have much impact on volume flow (cfm). Pressure differential, shape, surface finish and the length of the passage all do. If you’re comparing things like throttle bodies, 4bbl carbs (2bbl and smaller are rated at a different pressure differential), exhaust… you can usually get a usable approximation using something in the range of 122cfm/in^2

 

Flow thru a tube depends mostly on the pressure differential, as WS6 TA said.

My $.02

 

So in order to do that formula....take the tube diameter, ie. 3"square that and multiply that by 122 and that's a basic figure on how much flow you'd get through a 3" dia pipe? Cause I came up with 1098cfm for a 3" pipe. Is that right? Maybe I just suck at math.....

SKeTchy

 

d= diameter
a= area
PI= 3.14

a= ((d/2)^2)*PI

cfm = a*122

or

cfm = (((d/2)^2)*PI)*122

 

just some random thoughts,...

in terms of constant flow, i guess inside area would be a good way to approximate the flow potential of a tube. however, in an ic engine, the flow is never constant (and quite often is reciprocating) and this means you need to get into resonant theory to calculate at what point flow in the tube becomes optimized and then you can calculate the approximate flow potential of the tube.

flow numbers (cylinder heads, carbs, etc.,...) are measured using constant flow, and therefore should be used as basis of comparison not as an absolute measure of maximum flow potential.

 

Well that's easy enough. Ok, I need to flow about 400cfm. With that formula, a 2.25in id pipe flows about 485cfm. Now with the whole 'reciprocating air flow' thing stated by ZWILD1, will that decrease that number, the 485cfm, or increase that number? Thanks guys.

SKeTchy

 

It won't change, he's just talking about the effectiveness of that volume of air filling something if it has to stop and start. Most engine parts have to deal with that, so this will flow the same as any other part put in the same place (I have no idea of what you're doing...)

 

Giving too much thought to a CAI

SKeTchy

 

SStroker- thanks for clearing that up. Like I often say at work: "man, I was WAY off on that." People love me at work, and you can probably tell why. Glad I'm in sales, not engineering or accounting.

Appreciated. That'll go in the old memory banks. 14:1 conversion. Not 2:1. Heh heh. I was only off by what? 7 times? That's pretty close, right?

 

4" is enough for 1100hp

 

1100 ponies from a 4.3l V6.....it CAN be done!!!!! Hehe, it's for my S10 man....

SKeTchy

 

If you are contemplating a CAI, don't make it smaller than the MAF or it will become a restriction.

What do you have the 4.3 in? What are you plans?