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MG MGB Technical - Ignition timing

Hi Guys,

I'd like some thoughts on ignition timing. My engine is a 'standard' class race engine, with a 25D distributor which has no vacuum advance, but has about 20 deg of mechanical advance. I have had conflicting advice from various experts on where to set my timing at tick-over.

I have set it to the specified 14 deg. BTDC, however, it has been suggested that 7 deg BTDC would be better. Somebody else has suggested I should just keep advancing it until I get pinking and then wind it back a bit.

Also, if anyone can explain the effects of too much retard and advance on the engine and performace, I'd be very grateful.

Thanks

Antony

Anthony. If a race engine, you want a rolling road to determine the most desirable advance curve, then have a dizzy built to that specification. Generally speaking, insufficient advance does not allow the car to develop the maximum power because the spark is not firing at the optimal point, but after the piston has reached the optimal point (about 20 deg after top dead center for full burn as I remember it.) To hit this optimal full burn point, the actual burn has to be started somewhat in advance, hence our timing of before top dead center to reach a full burn after top dead center. So, late timing equals loss of power and extra heat--the mixture may still be burning as the exhaust valve opens. Too early an ignition timing starts the burn too early and can cause the engine to be inefficient because the full burn is taking place at the wrong point. In a worst case, the piston is still moving upwards or just at the top of its stroke at full burn which may result in engine damage. Pinking/pinging is a result of over advance. Neither condition is optimal, but slightly retarded (not advanced as much) timing is safer than over advanced timing and most cars are set to operate this way. For a race car, one which will be operating at high revs for extended periods, maximum performance will require a custom dizzy with a rolling road determined advance curve. Les
Les Bengtson

Antony,
All of what Les says is true. Back to your question 34 degrees total (14 initial + 20 advance) is a good starting point. A rolling road is the best place to check maxmium advance as the operator can see the HP drop off before pinging sets in.
I do question the need for a trick distributor in a 'race' engine; on track my engine doesn't get below 5,000 rpm, well beyond the trick distributors curves.
Leland Bradley

Les/Leland,

Thanks for your comments. Whilst my engine is in my 'race' car, it is a standard set up. It is overbored by +40 thou, it has the head with the bigger exhaust valves (1.625") which is converted for unleaded, all the rotating bits are balanced, the flywheeel is slightly lightened and it has a tubular exhaust and K&N filters.

I have just had it on a rolling road, which was rather a frustrating experience. The operator was convinced that 7 deg was correct on the dizzy, giving me approx 7 + 20 deg of advance. However, since researching further I am convinced that this is too retarded and am trying to find a way forward without paying for another rolling road session. Hence why I am trying 14 + 20 degrees as specified in the Peter Burgess book.

My other issue may be selecting the right carb needles. At the moment I have ABD needles - but that's a whole new thread I fear.

Antony
Antony

Antony,
You do need to get your mixture correct as it affects the advance. A lean mixture
doesn't need/support as much advance. With less fuel to burn you don't have to
start the burn as early so your rolling road operator was probably seeing the HP curve
go flat at 27 degrees total. The archives have extentive discussions on needles.
Leland Bradley

Anthony. I should think that a proper rolling road session would first set the mixture and ensure the carb needles were correct over the entire range of engine speeds. Only then would optimal ignition timing be addressed. Might I suggest Peter Burgess? It is my belief that he has a rolling road service available. If not, he should be able to tell you where to find someone competent to tune your car. While this may involve some inconvenience on your part, perhaps event taking a couple of days of your annual holiday/vacation, getting the initial set up done correctly may well be worth it. Once you determine the correct needle, whether the piston spring is ideal and get the mixture set and balanced, you can come up with the best advance curve. As a general idea, I have found the AAA needles have worked very well with both the factory and K&N air cleaners and the early spec distributor (40897) using vacuum off the carb is a good place to start tuning. Unless your car is only used on the track, I would recommend using a vacuum advance. I do so with my "race car" which street driven more than track driven. Les
Les Bengtson

This thread was discussed between 23/05/2002 and 24/05/2002

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