Quote:
Originally Posted by sir yun
''With compressible flow things get more complex. The square root law still applies but ONLY when the upstream conditions are held constant. In other words to increase the pressure drop without changing anything else we have to change the downstream conditions i.e suck harder. If we change the upstream conditions then the density of the fluid changes and so does the mass flow rate.''
l'll take a stab
if you push test exhaust (blow outward) the density/''upsteam'' condition changes thereby affecting mass flow. you are ''supercharging'' the exhaust
according to this reasoning it would be better to test by pulling through the exhaust port.
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You are correct. In exhaust flow testing the upstream conditions are not atmospheric and will change at different valve lifts. This is not a problem provided the flowbench properly accounts for everything in its calculations but my concern is whether they all do so.
Last year I was testing a Flowquik equiped bench with my orifice test plates and once we had the inlet side dialled in I ran a test by holding the plate down by hand and testing it with the bench blowing not sucking. The flow figures went all over the place. I forget the exact numbers but a plate that was showing the correct flow when tested in the inlet direction was showing something like 20% higher numbers when tested in the exhaust direction.
Clearly something in the system wasn't working at all and all the exhaust flow numbers the guy had accumulated over the previous five years were meaningless. I knew that anyway by looking at the data for a couple of heads and the exhaust CFM was far too high to be correct.
In the end he rigged up an adaptor and now tests exhaust flow with the bench sucking through the exhaust port in inlet flow mode. The flow numbers gathered like that look to be spot on. Whether the problem was in the Flowquik or the design of the rest of the bench I didn't have time to look into.
However any rigorous test of a flow bench with test plates should test in both flow directions to have any validity.
Dave