large CFM difference between intake and exhaust

[SuperRunner]

I have a plate that flows 308cfm on the intake, but it seems it likes to flow 340cfm on the exhaust.

My orifice mounting surface is a 7"x7" block of 3/4 particle board mounted to the chamber divider which is also 3/4" particle board. There is a 4" hole through both pieces. There there is a 1.25" drop before the orifice on the exhaust side. My orifice that I am using is 2.8".

So, going from 4" to 2.8" with a 1.25" ledge all the way around, would that create that large of a difference of 32 cfm?

[SuperRunner]

Was also thinking I might need to do a larger hole like 6" and use a thin metal plate for my actual mounting surface instead of particle board. That way I don't cause any venturi effect.

[cspeier]

Is your cd the same for intake and exhaust? What kind of calibration plate are you useing? If it has a taper, you need to turn it over. The cd for the exhaust will be different. The orfice hole is effected by sucking or pushing. My cd's are way different. My intake is .640 and my exhaust is .740. The number for the exhaust is high, but it's what it wants to flow what the plate calls for.

Chad

[SuperRunner]

I don't know if my CD is the same, I am still in the calibration phase.

My calibration plate is a sharp edge, so I would assume my orifice would be the same CD on both directions, but I guess I don't understand that part yet...d'oh. Time to put on my deep thought thinking hat again. I got a headache trying to figure out some numbers for a head I flowed last night. I got the intake numbers done, but I just can't seem to figure out the exhaust yet.

[cspeier]

What do you have for a calibration system? If you have excel, I'll send you the calbration form I received for my bench when I used manometers. It's an easy spreadsheet.

I'll promise the cd will be different. That's all your trouble. If you want that sheet, email me. I can walk you thru it.

Chad
cspeier@media-net.net

[larrycavan]

The flowpath is different on the exhaust side of the plate and is probably affecting the Cd for that direction just like Chad pointed out. You can prove your plate by flowing it on intake at a given pressure. When your readings are stable, shut the motors off, leave your flow control valve where it is and turn the plate over. Fasten it down and fire up the bench. If the Cd is the same for the plate in both directions, you're bench should climb back really close to the same reading it had when you shut it off.

If it does, then it's likely the 1 1/4" lip affecting the flow path and the Cd.


Larry C

[Mouse]

[SuperRunner]

Thanks.

I will have to re-design that part and use steel instead of wood.