i finished putting the car together and got it tuned. here's the set up

stock jdm gsr w/itr cams
ramhorn mani on a 57trim
3in dp
9psi

it was street tuned by two local guys. the car runs good but around 3,000 (give or take some rpm's) deceleration or a steady throttle there's a backfire at times.

i won't take it back to the guy's that tuned it. long story.

any input on why this could be ? recommendations on someone in the area that could help ?
thank you

 

Is it a backfire or more of a hesitation?

It could be a lean misfire at a certain load if it happens at a steady throttle state while cruising.

 

it doesn't hesitate, and my wideband reads 12.5-13.5 in mid throttle.

 

it doesn't backfire in acceleration at all. only decel and at a steady rpm

 

......

 

my car always pops out the exhaust on certain deceleration scenarios. nothing wrong with it .its when it pops out the intake you should worry.

 

it sounds like it's coming from my down pipe. it's parked for the winter but i just wanted to find out some info. thanks you guys. i do want it retuned just before summer though, but i'm upgrading some parts first.

 

If it's 12.5:1 afr on stead-state or decel, that's much richer than it needs to be. It's probably just that, a backfire. The higher-vacuum condition (less dense) burns air much slower, and rich conditions burn even slower. So you're ejecting unburnt fuel into the manifold, which continues to burn or is re-ignited by another exhaust event.

If you enable fuel cut AND ignition cut on a motor at the same time, you can get massive backfires at redline from this same idea. And I mean gunshot loud. It basicly cuts ignition after fuel has been injected, and then that fuel is ignited when ignition re-starts.

FYI cruising or light accell should be 14-15:1, light decel should be 15-16:1, and 0% throttle decel should read full-lean at most rpm's. You can get away with a much leaner mixture with lighter loads. My car will cruise as lean as 16:1 before I start feeling lean misfires. Saves gasoline, but bad for NOx emissions.

 

thank you. makes perfect sense.