Dip in exhaust flow curve

[stef-1]

Hi All,

I have been noticing a trend during exhaust flow testing that I don't know if anyone else has had experience with. With the heads I test (normally four cylinder engines 1.3L-2L in size) when I do a range change about midlift I get a dip in the flow curve. This is a recent example below;

Lift (mm)/ Orifice dia (in)/ Percentage %/ Raw CFM
4mm/ 1.26"/ 73.5%/ 43.3
5mm/ 1.26"/ 88.75%/ 52.3
6mm/ 1.494"/ 53%/ 43.9
7mm/ 1.494"/ 61%/ 50.5
8mm/ 1.494"/ 68.5%/ 56.7

The attached picture shows the range of the plates used in the test. I'm not sure what is going on. Any ideas?