All of a sudden a story came to mind from my dyno days at Chrysler when we were doing the rating runs ( numbers for advertising). I was running "full spread" of runs from full gross to full net (which is the way the engine is equipped with in the car) all the way to a complete car exhaust system for a 383 Road Runner engine. So I got to the last run where I had to put on the chrome exhaust tips and run a final test. We were really surprised when we lost around 5 HP by putting the tips on. So we ran the test 2 more times and the same thing happened so we just used the numbers for the test without the chrome tips. Could never figure out why it happened. When I ran the same type of tests on the 440 there was no loss so it was one of those quirks you can't figure out.