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MP test repeatability problems

blaplante

Well Known Member
I'm working on doing some changes to the air filter inlet size and length and so need to see what manifold pressure (MP) improvements are made with the changes. So I need a baseline.

I eventually realized that I need to be testing at a constant PRESSURE altitude, so that any changes in MP are against a consistent ambient pressure. [ergo, don't test at a constant density altitude.]

I've got two baseline runs, and the results make no sense. I'm testing at 7500' with my Kolsman set to 29.92", hence 7500 pressure altitude. Wide open throttle and lean to best power.

Test 1: Actual barometric pressure 29.98. OAT 16C. At this altitude the ambient pressure should be 22.7", but I see 23.4" on the JPI. [Yes the JPI baro sensor is calibrated, as per JPI's instructions.] Very odd... stock system has 0.7" of boost? With a stock air filter.

Test 2: Actual Barometric pressure was 30.08, OAT was 11C. Once again flown at 7500' PA. JPI says the MP is 22.6".

RPM and Speed of the tests were within 1% of each other. But yet one test has 0.8" less MP than the other. Now if the MP is actually down, then why is the speed nearly identical?

I sure wish I had direct measurement / recording of static pressure. Anyone have ideas on why my tests are inconsistent? I can't test ram air modifications when I can't get a consistent baseline.

Thanks much in advance!
 
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