pops and bangs on the overrun

[shalmaneser]

How would you go about making a car do this if it doesn't already?

I doubt I'll ever get round to it but I'd be interested to know. Is it basically just overfuelling with the excess fuel being burnt in the hot manifolds?

I guess the first move would be decats then?

[thetrash]

Why?

[GreenV8S]

Usually, an air leak in the exhaust will do the trick.

[Steve_D]

Any petrolhead would recognise the sound as an engine in a poor state of health/tune.
Why would you want yours to sound that way?

Is this some subtle way to convince your local scrots that it's not worth nicking?

Steve

[shalmaneser]

http://www.pistonheads.tv/clip940

Come on, you can't tell me the overrun on that doesn't sound ace!

[fatjon]

Steve_D said:

Any petrolhead would recognise the sound as an engine in a poor state of health/tune.
Why would you want yours to sound that way?

Is this some subtle way to convince your local scrots that it's not worth nicking?

Steve

Total rubbish, TVR Ceberas and many other highly tuned engines do this as standard. Ever watched a scooby going full tilt in a rally stage. Are we to believe it because their engines are knackered?

The answer is a remap and a little less fuel and some retard on the over run, although even then it's only going to happen if the cam is quite wild. Although I have to agree that it's a bit antisocial on the street. I remapped my Cerbera to get rid of it although I do have a map switch so I can turn it on when I fancy scaring the crap out of pedestrians.

Jon

[Marf]

fatjon said:

Ever watched a scooby going full tilt in a rally stage. Are we to believe it because their engines are knackered?

No Jon, its called anti-lag. Bit different to a TVR popping and banging as standard.

Edited by Marf on Friday 22 February 14:11

[fatjon]

Yes, granted it's a different principle on turbo motors where its caused by lobbing loads of fuel in and retarding it to make it combust in the exhaust manifold but the simplistic statement that pops and bangs mean the engines fubar still holds no water. That said I can't see why you would want to encourage an overrun that sounds like a night in the Lebanon when it's not the natural characteristic of the engine. If you want to get some more HP out of do the tuning and you may well end up with the desired sound AND the performance rather than just a racket and no extra power. To each his own and all that.

Jon

[Transmitter Man]

Jon said: when it's not the natural characteristic of the engine.

I don't like cars where it's not natural

http://www.youtube.com/watch?v=Gy984GqjidE

Phil
79 De Tomaso Longchamp GTS

[Herman Toothrot]

Pops and bangs are unburnt fuel,...

unburt fuel is present in the exhaust for a few reasons :-

1. Anti lag, my ECU can do antilag at the click of a button, I won't though as big red letter flash up that state "seriously reduces turbo and manifold life"

2. Poor tuning with serious over fueling AFRs richer than 10:1, pointless when max power is acheived at 12:1, turbos can benefit down to 11.3:1 to keep temps down.

3. Delay in over run fuel cut to smooth gear changes, then really it should be set for little enough time to not cause pops and bangs.

Basically its not desirable if you want the car to last, excess fuel = bore wash, burnt valves, cracked manifolds, cooked turbos & knackered CATS.

[Transmitter Man]

I forgot this one in my favourites:

http://videos.streetfire.net/video/bca78155-5b7e-4...

Phil
79 De Tomaso Longchamp GTS

[JonRB]

Herman Toothrot said:

Pops and bangs are unburnt fuel

[snip]

knackered CATS.

Yup. That was my understanding too. Which is why it never bothered me on our pre-cat Lotus Elan SE Turbo, and why I'm a litle bit concerned that my new (to me) Sagaris does it.

[Rude Girl]

For the TVR engines at least, pops and bangs are indeed caused by unburnt fuel. However, on the overrun it's a function of the engine running lean, not overfuelling. When you come off the throttle, there is still fuel in the injectors, but not enough for the Lambdas to pick up, so it doesn't spark. The unburnt fuel gets pushed through with the exhaust gases in the next 'suck, squeeze, bang blow' cycle, where it ignites in the hot manifold. That's why you can get your car to pop and bang by coming off the throttle, then feathering it - what you are effectively doing is starving the engine, then pushing through the unburnt fuel.

Ten points for the first person with a "listen love..." reply.

[Marf]

Rude Girl said:

For the TVR engines at least, pops and bangs are indeed caused by unburnt fuel. However, on the overrun it's a function of the engine running lean, not overfuelling. When you come off the throttle, there is still fuel in the injectors, but not enough for the Lambdas to pick up, so it doesn't spark.

Lambdas dont detect fuel, they detect oxygen resulting from the combustion process, n'est pas?

And besides, even if they did detect fuel, they sit in the exhaust, so how could the plugs ignite the fuel anyway, as its passed out of the combustion chamber into the exhaust?

Can I have my 10 points now?

Edited by Marf on Saturday 23 February 21:41

[NightDriver]

When you run lean (i.e. close throttle) the flame speed in the combustion chamber is significantly slowed down. As a result the combustion is still happening when the exhaust valve opens. This causes the combustion to finish in the exhaust, giving a pop.

If you are having unburnt fuel burning in the exhaust then you have a misfire or you are running anti lag.

On a race car running a full throttle shift system you can sometimes get pops and flames when you change gear because the spark is being cut but the fuel is still being injected - then the fuel simply burns off in the exhaust.

Edited by NightDriver on Saturday 23 February 22:44

[JonRB]

I know unburnt fuel is really bad news for a catalytic convertor.

I'm releived to hear that pops & bangs aren't necessarily unburnt fuel!

[NightDriver]

JonRB said:

I know unburnt fuel is really bad news for a catalytic convertor.

I'm releived to hear that pops & bangs aren't necessarily unburnt fuel!

Unburnt fuel is VERY bad news. On a modern Cat - 3 misfires and they can be fit for the bin Its why spark plug technology has had to become so much more reliable in recent years, the life of a spark plug is huge compared to ~15yrs ago and thats all due manufacturers not wanting to fork out for new cats during the warranty period just because of a duff plug giving a misfire.

[v8ian]

My Cortina can be prompted to give a wee burp if you nail it, lift, then go back to part throttle, It can be tempted to throw big yellow flames out of the tailpipe, only ever seen it once when in the Holsmdale? tunnel on the M25 when I caught the reflection off the walls, Cause, possibly down to the way I have the engine set up, I have a very slow decay on the ign advance, so on a quick lift off, the timing is slightly off on over-run
Also, under full throttle, and loaded engine, I have been told there is a small blue cone of flame at the exhaust tip,

[shalmaneser]

v8ian said:

Also, under full throttle, and loaded engine, I have been told there is a small blue cone of flame at the exhaust tip,

That's so cool!

I guess you're on carbs given its a 'tina?

[v8ian]

Nope,






Ian