Backfire

[alan gt4]

hi new member here, may take me alittle time to work out how to use these forums,I'm learning how to use this computer, always tried to steer away from them,

anyway i need a few ideas to chuck around, i have a gt4 st185 (91) uk spec wide arch, that i have owned for the past five years,

i decided to do a few mods to her and at moment have a backfire on the over run (sounds like a gun going off) not all the time only sometimes

but i think its getting worse as the other morning it happened about three times in the space of a mile, so i put her back on the drive and haven't looked at her yet..

i haven't really done any miles since i done the mods,(like about twenty) it maybe something silly but obviously un burnt fuel is being ignighted in the exhaust causing the explosion.

mods are ,

steel head gasket,

skimmed head,

arp studs,

stage 2 turbo,

dump valve,

de-cat-pipe,

s/steel pucker exhaust 3"

magnecor leads,

new dis/cap&plugs,

fox rs7 wheels&tyers

samco hoses,

black diamond disc alround &pads,

goodridge hoses,

polished the intercooler&throttle housing,

but what as you can see i have not fitted as yet, but have sitting on my tool chest a fcd, fuel reg,electronic boost gauge,

[Karma Supra]

I would say with those mods its fairly normal for a car to pop bang on over run, the increased boost from the turbo will mean there is more fuel being pumped in, and the BOv will be venting metered air causing a bit more overfueling. 3S-GTEs tend to run rich anyway. Sounds fine to me.

My car pops and bangs and shoots flashes of flame out of the tailpipe.

I consider this a good thing.

[alan gt4]

yer i can see what you mean by the bov dumping metered air making it richer, but she don't pop she bangs!! like i have someone riding shot gun,, interesting point though cheers, flaming exhaust sounds cool man

I would say with those mods its fairly normal for a car to pop bang on over run, the increased boost from the turbo will mean there is more fuel being pumped in, and the BOv will be venting metered air causing a bit more over fueling. 3S-GTEs tend to run rich anyway. Sounds fine to me.

My car pops and bangs and shoots flashes of flame out of the tailpipe.

I consider this a good thing.

[Jones~32]

Sounds normal to me

Soon as you de-cat the 3s-gte it backfires like a good un

[Sketchy]

As above, won't worry about it, 3S-gte on stock ecu really do overfuel quite badly when running above stock boost levels, i.e. lower than 10:1 most modified fours with stock ecu's seem to go off the wideband scale on rolling roads.

This results in an amount of fuel entering the exhaust that would give the tree hugging hippys heart attacks... Which in turn when the exhaust is hot causes the fuel to ignite, get a mate to follow you and I have a feeling when you stop they'll come running over and ask you if you've just fitted a flamer kit

On the 185 the ecu uses a flow sensor to adjust the fueling rather than a speed/density system like the 205, this mean that (As mentioned by Karma) your dump valve is dumping already metered air and the ecu doesn't know this is being removed so is still fuelling for it thus increase the overfuelling problem even more.

Anyway it's not technically backfire it's just the excess fuel igniting in the hot exhaust, proper backfire is when the timing is to advanced and the combustion mixture ignites to early causing to much pressure to be built up in the cylinder before the piston has been able to get high enough in it compression stroke. This really is quite bad as it put excess stress on the engine in the wrong direction, this also causes banging noises but is also accompanied by a hesitation/power loss and tends to occur while accelerating rather than on over run.