rx7tt95

I've noticed I'm registering higher knock readings on my PFC when I lift off the gas than on. I'll make a drag pass, no real readings to speak of, but let off the gas and I'll sometimes, but not always register a number in the 30-65 range, even greater on occasion. I did recently change my trailing maps, not my leading, to eliminate what many think is too much split (upwards of 17 degrees), but the changes weren't drastic. Again, my leading timing was unchanged and I never registered numbers as high before. The only other change I have made is the addition of premix (2-stroke synthetic) in the gas, fairly small ratio, and the adjustment of trailing timing.

I have two theories; one, the premix lowers octane a bit (but then I'd get knock readings on acceleration, no?), and the second is that the trailing ignition times cause more "severe" backfires as I lift off the gas, setting off the knock sensor. You know...the trail of burbles as you decelerate of the gas and the occasional flame shooting spectacle? I'm not talking massive explosions in the tailpipe, the backfires are nothing large or unusual for a rotary.

Any theories or facts on why the readings are higher or what causes them on deceleration?
Michel

MikeL

Maybe now that you've changed the trailing split, there is too much timing in the high vacuum area of your map.

rx7tt95

could be...didn't think of that..but on deceleration, there's no fuel. Usually it's right as the throttle is closed, and not continuous as the engine drops revs.
Michel

MikeL

Then I would try to change the timing and/or fuel map at the exact point where the problem occurs. Sorry, I don't know your computer's lingo, I have a Haltech but, I have had to deal with similar things.

MikeL

P.S. Fuel under deceleration makes big pretty flames

HWO

I , like MikeL dont am speaking in terms of haltech not PFC however, somewhere you should be able to turn the fuel cut on deceleration to OFF. HITman - one of the most knowledgeable tuners of aftermarket efi systems on rotary turbo's strongly advises against running fuel cut on decel. on a rotary. Could be for the EXACT reason you are experiencing