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Old 07-27-2008, 03:33 PM
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Distributor curving

Engine is a 392 FRPP stroker, with a Mighty Demon annular booster 750, Weiand Stealth dual plane, GT40 ali heads etc. I have changed the cam the crate engine came with for a more mild duration, 221/229 at .050 with LSA of 112.
The motor runs well, but I'm getting some pre-ignition under heavy load. I have initial timing set at 12º, with all-in at 34º at around 3000rpm. Distributor is an MSD billet, and I suspect I need to change springs to a more suitable curve. How do I go about this - is it just trial and error, maybe with a dyno session?

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Old 07-27-2008, 03:44 PM
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Quote:
Originally Posted by rsk289
Engine is a 392 FRPP stroker, with a Mighty Demon annular booster 750, Weiand Stealth dual plane, GT40 ali heads etc. I have changed the cam the crate engine came with for a more mild duration, 221/229 at .050 with LSA of 112.
The motor runs well, but I'm getting some pre-ignition under heavy load. I have initial timing set at 12º, with all-in at 34º at around 3000rpm. Distributor is an MSD billet, and I suspect I need to change springs to a more suitable curve. How do I go about this - is it just trial and error, maybe with a dyno session?
12 initial will be ok if you run manifold vacuum to the vacuum can.
With those heads your total mechanical probably needs to be about 30-31*.
I think you need to reduce the amount of centrifugal advance from 22 to about 18 and then try it with the same springs.
With part throttle high wacuum and low load your total should come in around 45-48*.
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Old 07-27-2008, 03:51 PM
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Should have left the old cam in. It was matched to the static CR. With a milder cam with wider lobe sep you probably have a bunch more cranking pressure. Doesnt make sense to me to go with a milder cam and then kill the power further by raking back timing.
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Old 07-27-2008, 03:51 PM
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Timing question

Distributor Tuning and Theory - Part 1

http://www.gofastnews.com/board/tech...implified.html

I'm not talking about cutting the timing back, but recurving.

Last edited by ScoTFrenzel; 07-27-2008 at 04:07 PM.
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Old 07-27-2008, 04:18 PM
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WOW ScoTFrenzel, thanks for the links. If I couldn't figure it out after reading all that, I should take up knitting.
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Old 07-27-2008, 04:23 PM
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Quote:
Originally Posted by techinspector1
WOW ScoTFrenzel, thanks for the links. If I couldn't figure it out after reading all that, I should take up knitting.
http://www.learntoknit.com/instructions_kn.php3

Just in case
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Old 07-27-2008, 04:29 PM
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Quote:
Originally Posted by rsk289
Engine is a 392 FRPP stroker, with a Mighty Demon annular booster 750, Weiand Stealth dual plane, GT40 ali heads etc. I have changed the cam the crate engine came with for a more mild duration, 221/229 at .050 with LSA of 112.
The motor runs well, but I'm getting some pre-ignition under heavy load. I have initial timing set at 12º, with all-in at 34º at around 3000rpm. Distributor is an MSD billet, and I suspect I need to change springs to a more suitable curve. How do I go about this - is it just trial and error, maybe with a dyno session?
Go into the MSD site and in the support section they will have a download for the distributor install and tuning. You wll see the selection of springs that should have been included with the distributor and the color combinations needed to slow the rate of advance down. What you need to do is likely remove one of the springs and replace with a heavier, slower advancing, one. Very simple operation.
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Old 07-28-2008, 07:33 AM
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Put better gas in it.
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Old 07-28-2008, 08:30 AM
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Quote:
Originally Posted by F-BIRD'88
Put better gas in it.
I'm already using what's sold here as 98 octane super-unleaded - nothing better available!

This cam has made the car much more driveable than it was before. Pick-up is smoother, idle is smoother and there seems to be pretty good power there. I need to get into this distributor to chase the over-advance as the pinging won't be doing any good, even with initial at 12-14.
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Old 07-28-2008, 09:36 AM
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Does it ping when you leave the vacuum advance disconnected and drive it?
(testing purposes) Is the vacuum advance adjustable?
In what different driving mode(s) does it ping? what different driving modes (transistions)
does it ping and not ping? Hook up a vacuum gause to manifold vacuum so that you can watch the gauge while driving. You wnat o to know and remeber at what load points ( throttle position,rpm and vacuum guage reading causing pinging. eg rolling into the throttle from 2000 to 3000 at x vacuum reading

eg going full throttle from 2500 rpm cruise (vacuum reading)

eg mashing the throttle from and idle

eg high speed comming off full throttle and suddenly right back on full throttle

When cruising along at speed limit on the hiway in high gear, what is the tach rpm and vacuum gauge reading? eg i have 18" vacuum at 100KPH at 2200rpm Does the engine surge or buck or ??? at any speed/rpm/vacuum gauge or particular driving mode?

With just centrifical advance not vacuum advance using a timing light at what rpm does the advance start climbing as you rev the motor in neutral. How much advance at 2000rpm, at what rpm does the advance reach its max point 2500,3000,3500,4000 or higher?

The centrifical and vacuum advance rate in/out and vacuum adv limit stop must be balanced.
usually the light springs/fast centrifical all in by 2500rpm is too fast.
A smoother curve with more initial but less advance slope peaking at around 3300-3500 rpm is better. Combine with a smooth vacuum advance controled by ported vacuum and limited to around 10-12-15deg max at high vacuum cruise.
The new cam with less valve timing and overlap lacks the exhaust reversion effect at low engine speed (egr effect) combined with higher in cylinder combustion pressure/temps that before allowd/ required a very agressive low rpm timing curve rate. So you got to tame down the timing curve.
cooler hat range spark plugs will help. richer main jets will help (1-2 jet sizes) if you get pinging at heavy throttle/high load/low vacuum guage readings.
a power valve opening point value change my be needed.
Need to know which driving modes/load/rpms/vacuums cause pinging and which do not.

Last edited by F-BIRD'88; 07-28-2008 at 09:59 AM.
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Old 07-28-2008, 10:02 AM
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Curving Instructions For MSD Billet Distributor.

Only if you have the advance kit.
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Old 07-28-2008, 01:55 PM
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Quote:
Originally Posted by F-BIRD'88
Does it ping when you leave the vacuum advance disconnected and drive it?
(testing purposes) Is the vacuum advance adjustable?
In what different driving mode(s) does it ping? what different driving modes (transistions)
does it ping and not ping? Hook up a vacuum gause to manifold vacuum so that you can watch the gauge while driving. You wnat o to know and remeber at what load points ( throttle position,rpm and vacuum guage reading causing pinging. eg rolling into the throttle from 2000 to 3000 at x vacuum reading

eg going full throttle from 2500 rpm cruise (vacuum reading)

eg mashing the throttle from and idle

eg high speed comming off full throttle and suddenly right back on full throttle

When cruising along at speed limit on the hiway in high gear, what is the tach rpm and vacuum gauge reading? eg i have 18" vacuum at 100KPH at 2200rpm Does the engine surge or buck or ??? at any speed/rpm/vacuum gauge or particular driving mode?

With just centrifical advance not vacuum advance using a timing light at what rpm does the advance start climbing as you rev the motor in neutral. How much advance at 2000rpm, at what rpm does the advance reach its max point 2500,3000,3500,4000 or higher?

The centrifical and vacuum advance rate in/out and vacuum adv limit stop must be balanced.
usually the light springs/fast centrifical all in by 2500rpm is too fast.
A smoother curve with more initial but less advance slope peaking at around 3300-3500 rpm is better. Combine with a smooth vacuum advance controled by ported vacuum and limited to around 10-12-15deg max at high vacuum cruise.
The new cam with less valve timing and overlap lacks the exhaust reversion effect at low engine speed (egr effect) combined with higher in cylinder combustion pressure/temps that before allowd/ required a very agressive low rpm timing curve rate. So you got to tame down the timing curve.
cooler hat range spark plugs will help. richer main jets will help (1-2 jet sizes) if you get pinging at heavy throttle/high load/low vacuum guage readings.
a power valve opening point value change my be needed.
Need to know which driving modes/load/rpms/vacuums cause pinging and which do not.
I will start getting into these investigations just as soon as I've sorted out the steering, which has gone all heavy on me (not sure why - no p/s, so out with the alignment gauges).

I have the MSD instructions and the spring kit for this distributor so will investigate further.
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