Fast XFI and Backfiring

[Fastbird93]

This isn't on a turbo buick, but on a single turbo 93 LT1 car, opti and all. I was recommended to ask here from LS1Tech because of the amount of knowledge and number of XFI familiar people here. I must say that after browsing I'm quite impressed.

Anyway, I've got a fresh motor (383 stroker) with a 76mm Turbo running and driving around off boost for the most part. Needs some fine tuning but it's enough to get it moving. The problem that I'm having is that under low RPM (about 1700 and below mostly) light load and light throttle, it gets a sporadic and consistent very snappy backfire out the exhaust. Air fuel is ranging from 12.8 to 14.5 and I've tried pulling and adding timing, and the only thing that really helped any was severely retarding the timing back.

Was wondering if anyone had any thoughts as to what tuning parameters could be causing this. The ignition system is in good order as well as the fuel system, plugs are Autolite 104's gapped at .30, no burnt plug wires or anything. If I get more aggressive with the throttle it won't backfire, but even at steady state with just minimal throttle to maintain speed it does the backfiring.

I do have logs and .gct files if anyone wants to look at those.

Thanks for any help offered.

 

[Otto J]

This isn't on a turbo buick, but on a single turbo 93 LT1 car, opti and all. I was recommended to ask here from LS1Tech because of the amount of knowledge and number of XFI familiar people here. I must say that after browsing I'm quite impressed.

Anyway, I've got a fresh motor (383 stroker) with a 76mm Turbo running and driving around off boost for the most part. Needs some fine tuning but it's enough to get it moving. The problem that I'm having is that under low RPM (about 1700 and below mostly) light load and light throttle, it gets a sporadic and consistent very snappy backfire out the exhaust. Air fuel is ranging from 12.8 to 14.5 and I've tried pulling and adding timing, and the only thing that really helped any was severely retarding the timing back.

Was wondering if anyone had any thoughts as to what tuning parameters could be causing this. The ignition system is in good order as well as the fuel system, plugs are Autolite 104's gapped at .30, no burnt plug wires or anything. If I get more aggressive with the throttle it won't backfire, but even at steady state with just minimal throttle to maintain speed it does the backfiring.

I do have logs and .gct files if anyone wants to look at those.

Thanks for any help offered.

what year is it? is the optispark still controlling can and crank signals or do you have the 96 version with the crank sensor?

HAve you verified the timing in the XFI matches the timing at the balancer?

Where are you located? near Connecticut?

 

[Fastbird93]

Otto,

It's a 93, running a 95+ Vented opti, using a Brian Macy (Fast Track Performance) piggyback harness.

No timing to match up at the balancer. No timing marks on balancer.

It's ONLY a low RPM light load backfire. Anything else I give it it's fine, be it at idle or being more aggressive with the throttle.

Located just out side of St. Louis.

Thanks!

 

[Otto J]

Otto,

It's a 93, running a 95+ Vented opti, using a Brian Macy (Fast Track Performance) piggyback harness.

No timing to match up at the balancer. No timing marks on balancer.

It's ONLY a low RPM light load backfire. Anything else I give it it's fine, be it at idle or being more aggressive with the throttle.

Located just out side of St. Louis.

Thanks!

I would find a piston stop and make a timing pointer,you need to be POSITIVE your timing is dead on. This may not be your problem but I would not even begin to tune a car without this being my first priority

 

[Fastbird93]

That's something that will have to wait. I know the cam was degreed and found to be correct when the engine builder assembled the motor, the opti is a vented that I personally installed and know is on correctly, and the motor doesn't act like the timing is off (I've worked on LT1's where the physical timing was off and it's not pretty).

I just have a feeling it's something that I haven't hit in the XFI or it's something external that I've not found though I've checked fuel, spark, ect.

 

[turbot990]

I have 96ss with a 383 76mm turbo with a fast iran 990s at 139 mph in my car on 18 psi but otto is 100% on that timing issue that has to be perfect its easy to do i just made my own out of a heavy peice of wire and attached to my water pump bolt jeg hp. sells a tool that will screw into the cylinder head u will need to remove your drivers side valve cover and the ex rocker and the intake rocker arms but before you do remove no,1 spark plug bump the engine over to compression stroke stop then remove the the rockers screw the piston dead stop into the head just until u feel the tool touch the piston bend a small 3/8 circle in the wire and a striaght line off it bolt that to your water pump aim it at your balancer mark the balancer with a white marker then turn the engine by hand counter clockwise just till the piston touches the dead stop lightly now mark your balancer there now you should have 2 marks split the differece between the 2 white marks and there you go now remove the tool from the head make sure to keep your timing marker in the same place DO NOT REMOVE IT OR MOVE IT AT ALL now put it all back together and take your timing light and read your timing with a adjustable timing light see if it matches to your xfi fast system if not adjust as needed.make it match your numbers but there,s more check as well but first do the timing i have my car set up very lite on the tune 20 degrees of timing wide open at 10,9 afr. and with my fuel system 60 ve. we need to no what your fuel system is as well. BeCarefull what you change.good luck bud im getting ready to turn my car up abit for my next pass on the car so far 990s at 139 mph i like these cars but my t type is my favorite still lol.If you want post your log