How bad is backfiring?

ZCAT3 · 07-21-2004, 11:20 AM

I just had my first track day with my "new" 1971 911T. What a blast that car was on the track. So much easier to drive than the other 3 911s I have driven at the track (Carrera, 930, and 993TT). All you early car guys should really stop patting yourselves on the back for being quick around the track in those "antique" cars ; ) Why, oh why, did Porsche make the steering and handling so much heavier heading into the 80s??

In any case, the car has an SC case engine with 3.2 Ps&Cs and is running an electromtoive EFI system. I have driven the car maybe 1500 miles on the street and it never back fires under those conditions. On the track, however, I was getting about 3 backfires per lap. Basically anywhere I let off the throttle after high RPM WOT (such as downshifting into a tight turn) I would get a very loud backfire. The exhuast is custom buillt SCRS headers and a dual out muffler (that I presume is gutted).

It was a very hot day at Thunderhill - over 100 degrees - but the car ran within safe temps - never really over say 205 degrees while on the track. The car ran great. I am just wondering if backfiring like this is bad for the car? If not, I guess I will not worry about it.

89911 · 07-21-2004, 12:15 PM

If its backfiring right and the end of an upshift, that is common with cars that are have some sort of fuel enrichment, (carbs, etc). The worst that it can do is knock your muffler apart after a while. Might want to check your richness.

ZCAT3 · 07-21-2004, 12:37 PM

It backfires right when you get on it after a downshift - say going into a turn you are at WOT in 4th, clutch in, brake, downshift to 3rd, get on gas as you enter turn and blam -

I spoke to my mechanic about it and he says the Haltech EFI computer systems dumps fuel at WOT, which is then being burnt off when I get back on it. As you noted above, he says this will not harm the car - but may scare the drivers behind me.

jpnovak · 07-21-2004, 12:49 PM

It does sound like unburnt fuel from the WOT is causing your backfires. Can you try a slower transition to off-throttle?

What kind of throttle bodies are those? Looks good with the EFI. Nice and clean

ZCAT3 · 07-21-2004, 12:57 PM

Quote:

Originally posted by jpnovak
It does sound like unburnt fuel from the WOT is causing your backfires. Can you try a slower transition to off-throttle?

What kind of throttle bodies are those? Looks good with the EFI. Nice and clean

Actually my wife does not get as much backfiring as I do as she tends to allow the transmission to slow her down more and relies less on the brakes. I find myself braking harder into the turns - her method is probably better - but then she is also probably a bit faster around the track.

I think they are PMOs?