Scruff

As the subject says, what is over run. I hear people talking about it when they say I get pops and bangs from the exhaust on over run. Can someone explain in muppet terms.

Thanks

Alan

blubell

oh I know this one!

When you turn the ignition off the engine keeps 'running' - only for a little bit and I cant remember what causes it - I m sure someone else out there can enlighten us further

BarryK

It's when the engine keeps going when you switch the ignition off.

It indicates a number of things are either not working right/need tuning/are bust/want replacing.

This is not a good thing. Unless you have a turbo timer.

fast bloke

when you let off the accelerator, all the fuel is not completely burned in the cylinder. Some of gets into the exhaust system and is ignited by the heat. This causes a pop or if you have removed all the cats etc you might get a large flame melting your bumper. This is what people are referring to when they say over-run.

BarryK

No that's mis-fire. Or popping. Probably.

fast bloke

I think Bluebell and barryk are talking about something slightly different. When you kill the ignition the engine continues to run for a few seconds. I don't think this happens on fuel injected cars. It happens on carburetted cars when the timing is substantially off. The engine is basically using residual heat as the ignition source, and is sucking the petrol through the carb. On injected cars, the injectors should stop as soon as the electronics are turned off, so you shouldn't get the same problem. You won't get pops or bangs with this

fast bloke

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>Originally posted by BarryK:
<B>No that's mis-fire. Or popping. Probably. [/quote]

Misfire is when none of the fuel is burned until it reaches the exhaust. I.E. no spark in the cylinder

Scruff

I think fast bloke got it right, explains why I get the pops with my straight through exhaust.

Thanks for the help and speedy replies. You all must be bored at work.

Alan

bren simpson

Bluebell and Barry are talking about 'running on', not over run. Fast blokes explanation is correct though, the car basically runs as a diesel when the timing is sufficiently out.

However the popping and banging is not over run.

The over run is when you completely throttle off, i.e. the period of time when the engine is deccelerating.

The popping and banging is caused by excess fuel being passed through the system and hitting the hot exhaust pipe where it explodes.

Bren

bren simpson

Btw Scruff, you wouldnt have a vent to atmosphere dump valve fitted would you?

Bren

Scruff

No, I have a standard dump valve on the car and i am running it at just under 1.3bar boost pressure. Still I get those nice pops out the back do you reckon this is a problem?

Alan

bren simpson

Probably not, my Cossy pops and bangs and throws BIIIIIIGG flames out of the back when I throttle off after a hard charging run through the gears! And, no, Im not running a dump valve

Bren

Dragon

Afternoon Bren, fancy bumping into you over here

Yep, agree with the above - overfuelling is where the unspent fuel is ignited in the outlet manifold/exhaust; this gives you large bangs and lots of flames - most noticeable at night as it lights the entire road up!!