PH please explain backfires and popping on overrun to me

[xr287]

I was watching this video http://www.youtube.com/watch?v=4Glp0Scj6GM&fea... (Mclaren mp4-12C GT3 at Monza with ridiculous amounts of flame on the overrun) which got me thinking.

Can I assume the Mclaren shoots so much flame on the overrun because it has turbos and allowing a small amount of fuel to be injected which then detonates in the exhaust will keep the turbos spooled up a bit and give for better throttle response?

I also explained a normal backfire to myself using the logic that at the point you lift off the throttle some fuel has been injected but the spark plug has not yet fired, since you have removed the input for throttle the spark plug does not spark and the fuel then passes out of the exhaust valve and detonates in the hot exhaust. This would explain normal lift off backfiring and backfiring when blipping on downshifts?

My real confusion is why do some race and road engines pop and crackle on the overrun with no throttle input? Why would there be fuel being injected when the throttle is at 0%? What would be the point other than to make it sound "racey"?

[HustleRussell]

Mine pops, bangs and spits two foot flames out of the side simply because the Webers hose so much fuel into the engine that much of it ends up igniting in the exhaust!

[Mastodon2]

xr287 said:

My real confusion is why do some race and road engines pop and crackle on the overrun with no throttle input? Why would there be fuel being injected when the throttle is at 0%? What would be the point other than to make it sound "racey"?

I'm no expert engineer, but my understanding is that if the car is not travelling fast enough with a closed throttle to overcome the compression that we feel as engine braking by pumping air alone through the cylinders, some fuel must be injected and sparked or the engine would not be able to turn over, resulting in the car shunting and rapidly losing speed. If you've ever experienced a fuel cut in a (sickly) turbo powered car at high revs you will know what I mean. No fuel goes in, to protect the engine when it thinks it is about to experience a detonation event, and the engine speed drops instantly, at which point you will nearly headbutt the windscreen. Even though only a little fuel is injected on a closed throttle, it's enough to keep pushing the pistons. This is the same stuff that is flashed off with pops and bangs as it touches the hot exhaust.

You have probably also seen rally cars which crack and bang like mad on a closed throttle, is the anti lag system working to keep the turbo spinning, so large amounts of boost are available with less lag than you would see on a car that did not have an anti lag.

Those McLarens look ridiculous too, they look more like those flamer kits that McDonalds car park kids used to have than anything else.

[Steffan]

It depends on the engine. If there are explosions occurring in the silencer or part of the exhaust there must be sufficient fuel for these to occur. Therefore the throttle closure cannot have removed all the fuel passing through the engine.

It is a very interesting subject, if you are so inclined. There are a whole gamut of factors that can affect this phenomena.

Primarily the working temperature in the cylinders may become too high and cause pre detonation and firing at any point in the combustion cycle. Cylinder heads and plugs can literally be glowing red with heat particularly where unburnt carbon has attached to the engine internal surfaces.

Textbook cases I have seen of this include several seriously tuned cars I owned years ago when the cars would run on happily and rev up when hot with the ignition turned off. No spark was needed and the only way to stop this was to stall the engine.

In my youth this was thought to be caused by over rich mixtures but with fuel injection I find this difficult to credit as an explanation. If this is a fuel injected engine then the hot spots on the head or combustion chamber must be the explanation.

The sound in overrun is another interesting effect. If you are into that sort of thing.

To this day, a tuned A series still produces an instantly recognisable rasping sound for me on overrun in changing up. The waffle of a big V8 is another such sound, caused by harmonic waves in the exhaust.

Harmonic waves are of course a subject in itself. Enjoy the bangs.