5 replies to this topic

[CPC]

Hi guys

 

Just looking for some general information on this really.  I run a megajolt so have full control over my ignition timing and have noticed lots of the base maps have very high advance (40-50 degrees on some maps)  it higher rpm with no load ( ie overrun when you lift off the throttle from say 4000/5000rpm).

 

I have tried these maps and it runs great,  but say if you lowered the advance in those conditions would it induce some popping and a exhaust burble on the overrun?

 

 

 

[tiger99]

Ideally you would have fuel injection and that would give zero fuel on the overrun, as in most diesels, so the ignition timing would be immaterial as there would be nothing to ignite. But that usually is not done with a petrol engine because somewhere between fuel on and fuel off is a zone of very weak mixture which could cause damage. Unless, of course, the throttles were closed tight too.

I would be inclined to keep the timing quite moderate on the overrun. You are not trying to make torque, just keep the engine in good order. Not overheating or detonating. If it is making lots of noise it is generating unnecessary stress on the engine.

[mini13]

its all about flame travel speed,

 

on over run there is not a lot of air and usually its leaner so the flame speed is lower so the advance can be craned up a lot more,

[Ethel]

I expect it also helps to stop you dumping unburnt fuel in to the exhaust.

[mini13]

Yep, actually that would be the main reason, emissions.

[tiger99]

The emissions would be helped if you had the so-called plasma ignition system that used to be available. For some reason (maybe because it would be lethal if touched?) it disappeared from the market a long time ago, followed by capacitor discharge ignition, leaving only the inferior inductive discharge systems that we still have.

 

For those who don't remember, the plasma system had the ignition coil secondary earthy end come out on an insulated terminal, which was fed with about 1500V DC. That would not break down the gaps at the rotor arm and plug, until the 30KV or more ignition pulse was applied, whereupon a sustained arc would persist until the rotor arm moved away from the segment. The advantage was that there was continuous ignition of any residual mixture all through the piston stroke. It was especially beneficial in the case of weak mixture.

 

A better system uses a high frequency high voltage source, coupled on to the standard coil output via a filter circuit. It has less tendency to erode the plug and rotor arm electrodes and is safer.

 

These old ideas should probably be revisited in conjunction with modern mapped ignition and fuelling. There are almost certainly gains still to be had. And there are even more clever things that can be done with plasma through a 14mm hole.

 

But to get back on topic, the less that the engine does on overrun, the better. I have given it some more thought, and with a carb you could possibly arrange for the butterfly to close completely, therefore no fuel to combust, with a solenoid controlled stop that operated only at or near idle to prevent it cutting out. Some of the advantages of fuel injection without the complexity. A drive by wire throttle conversion, which do exist as retrofits, with added speed sensor, e.g. tacho signal, could do all that.