Backfiring on gear change ?

[Ozzy]

Has anyone experienced backfiring on gear change when u take your foot off the loud pedal ?

I've only had around 0.8 bar dialled in (bleed valve), but I backed it down to around 0.6-0.7 but no change....I don't think it has anything to do with boost.

Any ideas ?

[doxximus]

every skyline backfires so i've heard

mine even did when it was n/a

it still will when its turbo

[T0nyGTSt]

Mine pops and farts badly on down change esp. if it's a high rpm downchange. I use 0.8 - 0.9, backing down though as it's too uncontrollable at this boost.

I pulled the silencer and it even flames

T.

[rob77]

An atmo bov will increase the backfires as its venting air to the atmo instead of back into the air intake making the afm compensate with more fuel, which means car runs richer and backfires. It used to happen on 90% of my gear changes so I got rid of the atmo bov and went back to the std one, though when I downchange at 3000rpm or more it backfires too. Need to get an SAFC to lean it out a bit.

[red17]

Ive always had it, figured the aftermarket ECU caused the majority of them. Then i got an atmo BOV and its increased slightly, probably hear 6 or 7 in an hours worth of driving. Doesnt really do any harm. ALOT of turbo nissans do it with aftermarket exhausts.

But if its doing it on 90 percent of your gear changes then thats just ridiculous.

And its not strictly a backfire in my opinion.... backfires cause people to hit the pavement (see Uncle Buck).... Skylines seem to just make nice snap crackle pop noises.

Red17

[SHUTO-BOY]

Funny,my car used to make all the loud popping crackle whatever because the first exhaust was catback only, now after i put the full exhaust,its smooth and doesnt do that anymore.Any explanations?

13.6

-Outlaw Racing-

[Ronin 09]

yep, i believe that this 'backfiring' as described is actually 'over-run'. I think it has something to do with the fluid (exhaust) flow through a pipe and the effect of massive changes in flow velocity as you change gears (i.e very fast gas flow to much slower)... sort of a reverberation within the pipe. This happens more with aftermarket exhausts due to the increased diameter resulting in a greater flow speed differential. Of course if your mixtures are rich then this will also cause exhaust noise as the unburnt fuel is ignited outside the combustion chamber.

This is only a guess, but i think it is reasonable given the aerodynamics i studied through my (unused) mechanical engineering degree... feel free to correct me if i'm wrong.

[Ozzy]

OK...youve given me some ideas...thanks.

Some great theories, and it's intriguing that others suffer the same problem.

It is actually a popping noise, rather than a backfire, still, it is a bit distressing. Is it damaging though ??

I do have an aftermarket exhaust...a Kakimoto NS309....but the problem has only recently started....possibly after I was stuffing around with the BOV tension.

I might try tightening up the BOV (an atmo) and see if that has any effect.

cheers Ozzy

[ed]

I thought the popping was just unburnt fuel being burnt in the exhaust system :confused:

[NISMO]

same here ED!

The Skylines are known to run very rich. Up the boost and the ECU over compensates even more. Mine is starting to sound like a rally car on the up changes ( running 0.9 bar ).

Could be wrong but I definately though it was unburnt fuel from running too rich.

[slayer_1971]

Yeah....same thing happens to me....I beleave my Turbosmart BOV is the reason why...

[TTT]

yes it is due to running rich.

Skylines run rich as standard and when you add a external venting bov and more boost, you run even richer.

and of course you're going to get back fires and poping and so on.

Every skyline does it.

[Ozzy]

......but will it cause any damage ?

[Doctor]

Just foul your plugs a little thats about all. And maybe stuff your cat over a period of time.

Doc

[GILTY]

BUT it sounds COOL...RIGHT???

Maybe not 90% of your driving time tho, that'd be a little annoying! I can get mine to back fire any time usually, just nail it and the back off real quick, if no back fire, lightly tap the Loud Pedal and you should hear the fruits of your labour.

I think my plugs need replacing now, but that's OK...

[rob77]

Yeah its just a pop, not a huge backfire... yeah its cool, there is one corner on the way home if I get the green light I can wind it down from 80km/hr to around 40km/hr and drop it into 2nd and I got a nice pop out the back... I suppose if I had a dump+front pipe and no cat I'd get flamage

[HoeTrain]

I bet if you put the standard BOV back on you wouldn't hear a sound...

Yes the ECU expects an extra burst of air from the plumb back which is expelled from the intake system, so it compensates for this by adding fuel. This also solves the mystery of why some after market BOV make your car stall (over fuel).

A mate of mine solved this by installing an AVO atmospheric/plumb back BOV (all in one).

But if you don't mind the sound, forget about it, it's doing any harm.

Have Fun.

[Duncan]

Standard BOV and exhaust, no running rich, no backfires.

[Bleach1]

Ozzy, sorry this is off the topic. is that just the standard front GTR bar in the little pic of you're car? (im assuming its yours). It looks wicked> better than alot of the other gtr style bars i have seen around latley.

[Ozzy]

Yesiree, it is mine.....thanks....I think it looks good too. It came from Japan like that....I believe it is a standard factory R33 GTR bar.....Its good in several ways....you can remove the bottom lip when its been scratched to hell and repaint it off the car....(the beast has Tein's all round and is set on the stops)....it also allows you an absolute shirtload of room for a FMIC....which will come when the MRS lets me spend some more (of my) money.

cheers Ozzy