I have a '95 Z-28 that I put the Borla stainless cat-back system on. Every time I rev the engine, as the rpm's come back down, I get small backfires through the exhaust. I know it is unburned fuel gettimg burned by the hot exhaust. My question is, Why is there unburned fuel getting into the exhaust? Am I not getting enough backpressure? (I have the least restrictive plate that came with the system in the exhaust pipe.) or is this a timing issue? The car runs great under load and never backfires when I get on it while driving, just in park and neutral. Any help is greatly appreciated.

 

mine has the SLP exhaust with SLP superflow muffler....does the same thing...a friend of mine has a 95 Formula with BBK headers , y-pipe , and flowmaster muffler...does the same thing.....guy i work with has a 96 Z28 with flowmaster exhaust....does the same thing.....so i'm guessing it's normal

 

It is normal, enjoy it. You don't hear it with stock exhaust because of better muffling. the throttle valve is closing faster than the computer can change the fuel delivery rate, so you have a momentary rich mixture.

 

pete , are the LT1 cars batch fired like the TPI cars were? or are they a true MPFI ?

 

I get it too, but the best sound I get is when my car is cold and i accelerate and i get really loud cracks around 2200-2500 rpm...

 

If you have a high stall tq converter, 3800 like mine works better, or it might work with the stock converter, and a good exhast system.when your cruizing around 50 mph or around there let off the gas, downshift into drive and tap the gas quickly it will rev up and then make a real mean slight crackle/loud roar, sounds awesome on our cars.

 

Ya, if I time it right, I can get the same thing right when I pass that riced out honda too, but I don't shift down to drive, I just punch it and let it rev then let off.

 

The LT1s at least 94 and up, not sure about the 93, went to SFI, sequential fuel injection AKA timed injection. Fuel is injected only when the intake valve is open.

 

so they aren't the old batched fired systems like the TPI.....i ran a TPI set-up using a TBI computer a few years ago...i was looking at a wiring diagram of the TPI and found the PCM only had 2 injector drives. i got the idea from a guy that lived beside us at the time. he had a fullsize chevy truck with factory TBI , then he put TPI on it and sent his wiring harness to some company that " modified" it to use with TPI and sent him a "custom" prom to use in the TBI computer......

 

The TPI and TBI computers are basically the same, the difference being int he MEMCAL prom, which contains the actual instructions for the computer. The other main difference is the fuel system where the TBI requires much less fuel pressure than TPI. These computers had two drive leads and and alternated the firing ofthe injectors. One injector at a time on TBI and one bank of injectors at a time on TPI.
On SFI computers there are 8 drive leads and and the injectors are fired on at a time in sync with the opening of the intake valve. AlsoSFI computers do not have removeable MEMCALEPROMs and are programmed by flashing the memory via the diagnostic port.