Help, problem with consitant orifice numbers

[SuperRunner]

I am not sure what to do about this, but here is the problem.

I have an orifice plate and several calibration plate, all tests are done at 28"

The first of the calibration plates flows 275cfm and on my bench that is 86%. This means my orifice flows 320cfm.

Ok, so that is great an all, but I have another calibration plate. This one is supposed to flow 269.3cfm, and on my bench, that is 83.5%. Well guess what, now the orifice flows 322.5cfm.

Not so bad, just 2.5 cfm, but this is where it gets intersting.

I have yet another calibration plate that flows 255.4cfm, which shows up as 78.5 cfm. Well this no means that my orifice would have to flow 325.5 cfm. Now I have a 5cfm difference from my largest calibration plate, and so far it is a upward trend as the calibration plate gets smaller. Seems like for every 10 cfm of flow that is increased through the calibraition plate, the orifice flows less.

And now the most intersting part. I have another calibration plate that lows 120cfm, and on my bench, it reads 39% which calculates my orifice to be 310cfm. GRRRRR, this is really ticking me off. Now the amount that the orifice flows has dropped instead of increased. Of course this is really below the threshold of accuracy, but I have tested it 10 times, and is rock steady at 39%

What might be going on here?

[SuperRunner]

Anybody want to take a crack at this?

Why does my orifice flow not linear with different sized calibration plates?

[Thomas Vaught]

There is no guarantee that you have exactly the same cd on each
plate. Did you do multiple points on a given range with each plate.

Even the plates from Bruce read differently vs his calculated cd/
orifice hole diameter and the Ford "Standard Bench"

Just some questions to help you get started on the analysis process.

Tom V.

[SuperRunner]

Is it my orifice, or is it the plate that is giving me strange readings.

No, I did not have the guy that tested my calibration plates flow at different ranges. Oh, by the way, they were tested on a brand new SF600 that had just been calibrated.

Oh, and just as another test, I tested a head and compared againts the flow numbers of a similar head. Our numbers were increadibly close on everything but .2" which is where my biggest problem is on this paticular orifice. The rest of the flow numbers of this head followed almost exact as to my calibartion plates.

Can the CD of my orifice change that much from 160 CFM to 250 CFM?

[SuperRunner]

Here are some pictures that might help on seeing a flaw in my design. It is about 2 inches from my opening to my baffle. I don't know if that is a problem.

[SuperRunner]

After doing some reading on CD, I am thinking it might be my bench design. I have been needing to re-design my orifice mount so it is near flush to the chamber divider as possible. This should help with the increased CD I have on exhaust. I believe itis probably greater than .61. I also think I need to add a baffle between the orifice hole and the control valves.

What do you guys think. Any problem with the CD changing if you don't use a baffle where I just stated?

[SuperRunner]

After getting some help from John at Flow Performance I have some new information that should be helpful. Thank you John for that help.

My orifice is about 3.38" I say about because I left my calipers over at a friends house.

And now to re-quote myself from the first post with inches of water instead of %

I have 4 calibration plates that were calibrated on a SF600

#1 - 275
#2 - 269
#3 - 255
#4 - 120

According to plate #1 my orifice flows 320 cfm at theroetical 8.57"(100% on the inclied) drop accross the orifice.

#1 gives a 6.34" drop for 275 cfm
#2 gives a 5.98" drop for 267 cfm (-2cfm)
#3 gives a 5.28" drop for 251 cfm (-4 cfm)
#4 gives a 1.30" drop for 125 cfm (+ 5cfm)

So, what do you think?

[Thomas Vaught]

Please copy and post the calibration card info from the 600 cfm SF bench and the Bench number.

Tom V.

[SuperRunner]

Hmm, I don't know when I will talk to the guy again, but I will see what I can do.