Pops & bangs

[paul gotts]

Hi all, I'm interested in understanding what causes the satisfying crack on the over-run / lift-off for gear changes with the RV8 (although I suppose it applies to most 4 stroke engines). I'd always thought that it was a rich mixture resulting in unburnt fuel in the exhaust gas being ignited in the hot exhaust pipe. I could do this easily by revving the engine to c. 5K and lifting off, causing a flame visible from the tail pipe. However, since replacing my exhaust box with a new one, which does not have the small holes and poor manifold sealing of my original, the tendency to produce these effects is lessened. I've also had advice that leaning off the mixture at high revs and no load would cause more banging. Anyone able to explain these effects?

Thanks,
Paul.

[leorest]

paul gotts said:

Hi all, I'm interested in understanding what causes the satisfying crack on the over-run / lift-off for gear changes with the RV8 (although I suppose it applies to most 4 stroke engines). I'd always thought that it was a rich mixture resulting in unburnt fuel in the exhaust gas being ignited in the hot exhaust pipe. I could do this easily by revving the engine to c. 5K and lifting off, causing a flame visible from the tail pipe. However, since replacing my exhaust box with a new one, which does not have the small holes and poor manifold sealing of my original, the tendency to produce these effects is lessened. I've also had advice that leaning off the mixture at high revs and no load would cause more banging. Anyone able to explain these effects? Thanks, Paul.

I recommend reading this book…

"Engine Management: Haynes High-Performance Tuning Series Optimising carburettors, fuel injection and ignition systems"

Which can be found here…

www.haynes.co.uk/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&storeId=10001&productId=11025&langId=-1

I think the satisfying pops and bangs are due to valve overlap allowing some of the charge to pass straight through the engine, unburned, this is done to ensure that the combustion chamber receives a full charge.
I am sure someone who actually knows what they are talking about will give a better explanation
More sophisticated fuel injection systems cease injecting fuel when they detect they are on over-run (i.e. engine revs above a set rpm with a throttle pot number below a set value).
The Emerald M3D ECU referenced in the book is laptop programmable so you could have different maps for different occasions.
Hangover map - Cut off all fuel in overrun conditions.
Hooning about/everyday map - No overrun cut-off.
MOT/valet map - Optimised for minimal emissions & silly low rev limit to stop the inspector thrashing your pride and joy!

I believe that some engine management systems can store two maps, which can be swapped while the engine is running, at the flick of a switch! Could be useful if you get stopped for a random emissions check.

Leo

[tallbloke]

The leaks in your old system would suck in air as you lift off and the closing of the throttle causes a partial vacuum. This mixes with some unburnt fuel and is ignited by the high temperature of the exhaust manifold.