V8 Pop

[Slow4]

I've only manage to make it happen once in the V8 but have been successful on numerous attempts in the B5 S. This morning enroute to work, I decided to do an onramp launch from 30-90 (there is a law that prohibits this so I don't fully condone this) and managed to pop the engine three times at end of third, fourth and fifth.

I understand its the science of more fuel to air ratio that creates bigger combustion but am not sure how "harmful" it can be or is.

Techs/Engine Gurus?

[Slow4]

Let me rephrase this:

The 4.2 block is NOT popping itself, if it were I wouldn't be asking and I'd start praying.

The backfire, I believe, is an exhaust pop.

[sparkhed]

Ours will pop when shifting between gears when putting the hammer down. My mechanic says its because we are cat less.

[Slow4]

Ours will pop when shifting between gears when putting the hammer down. My mechanic says its because we are cat less.

Harmless or harmful?

[cheetah993]

It is harmless. I happens without Cats because you do not have the same back pressure on the system that you used to. The back pressure on a stock system will keep this at bay, it is just because of the aftermarket exhaust. Nothing to worry about. Every car that I have owned or my friends have owned that we put a new catless exhaust on does this.

Also, X-pipes are known more to do this then H-Pipes due to the way that they handle back pressure and the way they even out the flow on each bank.

[dparm]

You're probably hearing unspent fuel detonating in the downpipes.

[S4orgy]

I have a video of me launching my car and it popped in first, second, third and fourth. Im curious if flames have made it out the exhaust. Such a pop that you can feel it

[AudiA4_20T]

It is harmless. I happens without Cats because you do not have the same back pressure on the system that you used to. The back pressure on a stock system will keep this at bay, it is just because of the aftermarket exhaust. Nothing to worry about. Every car that I have owned or my friends have owned that we put a new catless exhaust on does this.

Also, X-pipes are known more to do this then H-Pipes due to the way that they handle back pressure and the way they even out the flow on each bank.

is it backpressure? Or the fact that the gas doesn't get caught in the cats anymore?

[cheetah993]

is it backpressure? Or the fact that the gas doesn't get caught in the cats anymore?

I am not an engineer, but I believe it is a little of both. I know that as soon as you change the back pressure, you can get the pop, or a backfire. My friend had straight pipes off of the cat on a Mustang, and it still did it, but not as bad.

[dparm]

See my earlier post -- it's unspent fuel detonating in the downpipes. Not the best for cat converters in the long-term as it will eventually melt the substrate. Sounds cool though.

[Slow4]

it's unspent fuel detonating in the downpipes. Not the best for cat converters in the long-term as it will eventually melt the substrate. Sounds cool though.

If I'm full catless, I wouldn't need to worry about this in the long-term, correct? (Thinking outloud)

The back pressure on a stock system will keep this at bay, it is just because of the aftermarket exhaust. Nothing to worry about.

This is where I get confused, if my full catless piggies and FI exhaust allows a more free-flowing system, wouldn't that back pressure be eliminated?

Or is the JHM tune making my air/fuel ratio THAT much richer, thus causing a combustion BEFORE the piggied cats and potentially creating a problem for the engine?

Im curious if flames have made it out the exhaust. Such a pop that you can feel it

I am curious about this as well. My wife, whom has trailed me before, has said that she's seen my exhaust eject a black cloud before with a thick smell of gas that she had to travel through.

Of course, this was back when I just put the tune, piggies, exhaust on. I laughed and never thought much of it.

[dparm]

Slow4, you won't have any cat damage if you don't have cats. There is a fuel cut on deceleration (between shifts), so you're hearing that brief instant of excess fuel. Backpressure is definitely reduced with a freer-flowing exhaust system but not totally eliminated. The stock system obviously is meant to be quiet, so these types of noises are canceled out or kept at a lower volume.

Your car is modified so you need to just learn to live with some of the negatives...noise, higher fuel consumption, reduced reliability, etc.

[Slow4]

There is a fuel cut on deceleration (between shifts), so you're hearing that brief instant of excess fuel. Backpressure is definitely reduced with a freer-flowing exhaust system but not totally eliminated.

And there is my answer for my "thought provoking question". Thank you good sir.

Your car is modified so you need to just learn to live with some of the negatives...noise, higher fuel consumption, reduced reliability, etc.

Indeed I will hence why I wanted to brag about my V8 pops Thank you for the answers though.

[Dudeman258]

Just a technicality but figured you might find this interesting. Most exhaust backfires are actually from a lean condition. When lifting from the pedal to shift the cylinder will see a lean condition. The last few revolutions of the engine after your foot is off the pedal are so lean that combustion struggles to take place. As a result, flame speed is reduced so much that unburned fuel is able to reach past the exhaust valve. This is what you hear %99 of the time a exhaust backfire occurs!

Since catalytic converters generally are well past the ignition temperature of fuel, it greatly/totally eliminates any chance of unburned fuel making it very far down the pipes and thus no exhaust backfire

Finally, any alteration that reduces back-pressure will help the unburned fuel evacuate from the cylinder quickly and also increase the chances of this event happening.

Enjoy!

Walker

[miamifan13]

I have cats, with a 2.5" FI exhaust, and mine does it...but only on cold starts, usually just on the the first start of the day. I only have to blip the throttle to 1800rpm to make it pop a few times. My car is loud already, but when it pops like that in the garage it can be heard 7 floors down.

[paul61]

Most exhaust backfires are actually from a lean condition.

Bingo !

[0000 V8]

Im pretty sure my exhaust will shoot flames out of the sides. I need to do a video to test it.

[ProgMetalHead]

exhaust pops are amazing! I hope my FI system will do it on occasion!

[LJH]

My Ur-q is totally catless and has a 3" turbo back exhaust....on shifts and overrun the thing spits flames and pops, at night it lights up the surrounding area.

I5 FTW....:-).

Cheers,
Jim

[Slow4]

My Ur-q is totally catless and has a 3" turbo back exhaust....on shifts and overrun the thing spits flames and pops, at night it lights up the surrounding area.

I5 FTW....:-).

Cheers,
Jim

Video or its just Forza4

[dparm]

Just a technicality but figured you might find this interesting. Most exhaust backfires are actually from a lean condition. When lifting from the pedal to shift the cylinder will see a lean condition. The last few revolutions of the engine after your foot is off the pedal are so lean that combustion struggles to take place. As a result, flame speed is reduced so much that unburned fuel is able to reach past the exhaust valve. This is what you hear %99 of the time a exhaust backfire occurs!

I agree. I was just confused when the OP was hearing the noise. We're both right.

[AUDIocee]

There's a youtube of someone with a JHM tuned car using launch assist, at the end he does a donut and some flames come out of the exhaust. I'll try to find the link.