flow thru an orifice equation

[fcrcmach]

Hi everyone

I know there is a post on here that has thee equation for determining fow thru an orifice.

My question is I have the PT black box and their pro software for my bench. In their book the equation is
CFM Flow=13.29 X 1.5 squared * sqare root of test pressure

the equation I see here uses 13.55

I am sure there is a simple explanation for this but I am not bench smart! I know how to build cylinder heads not flow benches. what is the reason for this? Thanks Shawn

[Mouse]

Shawn,

The difference is for the CD (Coefficient of Discharge) of the orifice. The one posted here is for a standard Cd of .62 - A good safe number if you do not know what the actual Cd is. The other equation is for a Cd of about .59 or .60 or so.

[Tony]

The 13.55 figure seems to be one that most people find corresponds best after a fair bit of testing, and comparing flow between different flow benches.

If put to a popular vote, 13.55 wins.

If like most of us, you are starting from absolutely nothing, then 13.55 is a pretty good basis to work from.

After all, the difference between 13.55 and 13.29 is only 2% and probably not something to worry about too much.

[fcrcmach]

Thanks guys that sounds good I will go with the 13.55 in my calcs. I just didnt understand why they would suggest that number for custom bench calcs and everyone here was using the 13.55 as they were both for as you say starting from nothing. sounds good I will go with the majority vote