calibration- with or without bore adapter?

[Greg]

I have been playing with my bench this weekend and made some sharp edged orifice plates.

When I fit the plates directly to the bench, I get a different reading than what I do with them on top of my bore adapter.

The adapter is 86mm, which is what I usually work on for the engines I do. I gain around 15cfm with the orifice on top of the adapter, I'm assuming this is because it has the bore adapter tube under the orifice. The gain is fairly consistent at 100, 200, 300cfm.

Any ideas?

[Flash]

[Greg]

I've checked for that, with the plate completely blocked on the adapter I get zero flow.

[slracer]

Greg, How are you clamping the plate to the bench vs to the bore adapter? Remember that the flow is really coming into the plate horizontally so different clamps and/or arrangment, spacing, etc could disturb the flow pattern. Just a thought.

Doug

[GordonE]

Hi Greg!
You should test them without bore. If you are using a cylinder you must compensate for this when the orifice diameter is getting nearer to the bore diameter. I was very confused of that before too...

I dont have a settling chamber on my bench, just a 110mm plastic tube. So I have to use a corrected value with my 307CFM orifice.

Regards Gordon