Exhaust cackling/popping

[djones]

Hi,

What causes exhausts to pop/cackle (whatever you want to call it) on over run?

Here is a perfect example:

http://www.silcs.co.uk/ring.wmv

It start at about 10 seconds in.

What causes it?

In a strange kind of way it's similar in noise to when an F1 car is heavy on the TC.

[Scuderia_Russ]

Interesting lines through there.

[modbaraban]

Ithink that's called backfire
http://en.wikipedia.org/wiki/Back-fire
also read this related one about misfire http://www.f1technical.net/glossary/m

I'm looking for proper explanation form someone who knows better too.

[djones]

"Interesting lines through there."

Yea that was me

It was my fist ever lap of the Nurburgring though so I didnt have a clue what was coming next. It's was also chucking it down, I have never been on a road as slippy apart from ice!!! I think it must have been water sitting on the rubber on the track or something.

From that wiki page:

"Afterfire occurs in engines that have an emission system malfunction (air injection system diverter valve), exhaust leak or unburnt fuel in an exhaust system in which the catalytic converter has been removed. When a driver shifts up and lets off the accelerator, the engine has a moment of running rich or with insufficient oxygen. This causes an incomplete burn which causes the fumes to explode in the exhaust system. The leak itself is the most dangerous aspect. Without it, the mixture would cool enough not to explode. A fuel injected engine may backfire if an intake leak is present (causing the engine to run lean), or a fuel injection component such as an air-flow sensor is defective."

Er... that doesn't sound good

EDIT

ps. It only does this when the exhaust is VERY hot, so maybe it's not that bad.

[modbaraban]

Not sure what the reason is. But that's common case with different racing cars. And it happens when the TC engages too (as you mentioned it).

F1 backfire

[Belatti]

djones, your car engine is standard, or has some valve overlap modification?

[Tom]

The line through that corner, I can't see the exit but its clearly damp, that might be a very good line because you'll apex later so the car is more stable on exit when you need to put the power down.

[Scuderia_Russ]

"Interesting lines through there."

Yea that was me

It was my fist ever lap of the Nurburgring

Enough said mate. (Very jealous that you've had the privelidge)

[djones]

Tom, I was pretty much taking the lines that felt safest and most stable given the conditions, setting a lap time was of no importance. I didnt want to upset the car by throwing it in at a sharp angles trying to hit the apex's as you would do in the dry. Basically like you say I was trying to smoothly flow through a late apex so the exit was as stable as possible.

Near the end you can tell how slippy it was as im on/of/on/off/on/off trying to find grip on the exit of the next right hand corner. That was in 2nd gear in a car that has very little torque (180 Nm).

Belatti, apart from an aftermarket exhaust and air intake system the car is standard.

[MrT]

The sounds come from unburnt hydrocarbons (fuel), resulting from poor combustion or possibly rich mixture, overfueling (which is why you hear it alot on race cars). The unburnt fuel is that which sits behind the piston rings, or which the flame front doesn;t reach (the flame may be quenched by the cool cylinder walls before combustion is complete). The fuel then enters the exhaust (on overrun) and combusts in the hot exhuast creating the backfire sound.

Mr T

[djones]

I had the car rolling roaded and they told me they could re-map it to run a bit leaner and get maybe 10 BHP or so.

I later found out Honda set the car up to run slightly rich to help cooling or something due to high revs (9000 RPM limit).

Does that sound about right?

I know every Integra I have seen has a really sooty black exhaust and if its not been washed in a while soot up the back of the car.

[MrT]

Well, the soot definetly suggests rich mixture, unburnt hydrocarbons...

There is a concept of using the extra fuel as a cooling mass, although I've not heard it used on road cars specifically. You could be right but it does suprise me a little.

Maximum power is 0.85 Lambda (slightly rich). The reason slightly rich mixtures are slightly more powerful is because of fuel dropplet size. In a normal stoichimetric mixture not all the fuel in a droplet sees air (the bit at the middle for instance) and therefore the mixture which actually combusts is not stoichiometric. lambda 0.85 actually results in a mixture near stoichiometric combusting and therefore maximum power.

[djones]

Very interesting. Thank you.

If it was dark do you think these 'pops' will be visible as flashes?

[modbaraban]

Very interesting. Thank you.

If it was dark do you think these 'pops' will be visible as flashes?

EVO
McLaren F1
Formula1 Minardi-Asiatech

[MrT]

It depends on how overfueled the mixture is....most of the time the fuel combusts in the manifold so no flash etc is normally visable.... boy racers etc sometime overfuel so much it makes it to the exhuast and thats when you see the flames.... you see it occasionally in videos of f1 engines on dynos too.... I have even heard of people intentionally spray fuel directly into the exhuast to get flames.... can;t see the point myself!