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MG MGA - How high does the spark advance go?

Hi everbody,
I noticed the other day that when I rev the car up to about 6000 the spark advance goes up to about 40 deg btdc. I don't think I have ever put the light on it at speed before and was wondering if that is normal ??
I don't know if this is good, bad or indifferent. It is the 1500 engine.
Does anyone have a curve for the dizzy?
Thanks/ gil
gil

Gil,

If you open up your distributor the arm that pivots has the degree's of advance written on it. See below

http://www.jcna.com/library/tech/tech0015.html

As a rule of thumb 36 Degree's without the vacuum advance connected max is about right, but this includes the static degrees also.
I use to rev my race engines to 6000 and with an extra 36 degree marking on the pulley and join the dots... great fun holding 6000...NOT !!!

There's all sorts of calculations on that site above which should help you.

Cheers <MARK>
Mark Hester

Hi Mark,
That is great information!!
Thank you--
/ gil
gil

Gil, it is easily possible to get more advance than you have measured. You should get about 32 degrees of mechanical advance and more than another 10 in vacuum. You will get vacuum advance when the engine is revving and not under load as the throttle will require very partial opening to get up to revs.
I would think 36 degrees with vac disconnected is way too much, obviously Marks race engine was not on the limit of compression otherwise I would have though it would suffer pre ignition. Most racers in this country would use a figure of 32 degrees max and I have known times when I have had to settle for 28 degrees with really aggressive CR.
I always use this method to set the ignition timing on every engine I tune, that is rev at 4.5K and set the timing at 32 degrees with the vac disconnected. This will set the thing perfectly. It is not necessary to rev it to 6K as all the mechanical advance is well in by 4.5K.
Bob (robert)

Bob- How do you figure out how far is 32 degrees on the pulley? Gary
gary starr

Gary

You do it with the timing light. You dial 32 degrees of advance into the instrument. Run the engine up to 4.5k and the notch in the pulley should show agianst the TDC mark. If it doesn't, rotate the dizzy until it does.

Steve
Steve Gyles

Gary

Sorry, I think I misunderstood your question in the previous post. Having read the topic from the beginning you obviously know how to use the timing light.

What Bob is saying is that you set 32 degrees with mechanical advance as I described. You then reconnect the vacuum pipe and get the extra 10.
Steve Gyles

Bob,

Sounds like you have a better timing light than mine. I think I've seen the type your using which allows you to dial in the advance on the gun.
In my brothers race engine we run 36 degrees, with 11:1 on avgas without any problems.
but our distributor was remapped so it advances all the way up to about 5500-6000.
Are you still running the stock advance that stops at 4500 RPM ?
We don't use Vacuum advance either as theirs no point when racing.

Cheers <MARK>
Mark Hester

Mark

I just assumed most people used these 'dial-in advance'timing lights nowadays. They cost about £50ish. One of the best investments I have made in the workshop (engine hoist was the other). Makes timing the engine so easy.

Steve
Steve Gyles

Most of the good ones also have a tach built in also.
Other wise you would need to accurately make a new notch on the pully.
R J Brown

That's what I use to do, with the aid of the cam timing plastic wheel that comes with the Vernier Timing chain kit.
Mark Hester

Yes it is a good question Gary! I used to use a protractor to mark front pulley many years ago and that surficed me for a long time. But as has been said above I too now have the electronic timing light that gives me the ability to dial in 32 or any other number and then the flash is happening at top dead centre mark on the pulley.
Mark I can see the point you are trying to make but on tests that I have done an advance figure of more than 32 degrees even at 7K revs does not increase power. Perhaps in theory it ought to but it never has with me. I have run engines on 98 octane at 11.5 to1 CR and 32 degrees without problems although they were on cams of 285 degrees and over which have poor trapping at low pre ignition revs of maybe 3K.
Bob (robert)

Thanks guys. I don't have a timing light like that, but the protractor idea will help. Gary
gary starr

This thread was discussed between 08/09/2005 and 10/09/2005

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