389 engine - original timing curve - Pontiac GTO Forum
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post #1 of 2 (permalink) Old 10-22-2012, 12:04 PM Thread Starter
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389 engine - original timing curve

Hi all,

New here :-) Just a quick hello from the Netherlands!
I decided to join this forum as i'm having some technical questions regarding my '63 389 engine.
Unfortunately i'm not driving a GTO or Lemans (great cars!), but a 63 Pontiac Bonneville with a pretty serious engine for a fullsize car in my opinion.
Decided to join the forum as it looks like there are more Pontiac performance freaks over here

Not sure, but if i'm right (correct me if i'm wrong) my 389 engine is also being used in later models includes the GTO/Lemans. Some specs:
1963 389 engine
4 barrel (AFB Carter)
original 797 heads (62cc)
10.25 or 10.5:1 compression ratio.
Shop manual says something about 330HP and 430ft.lbs torque, not bad for an engine of almost 50 years old.

Engine is still fully original, but half a year ago I started to have issues with the stock point distributor. As it was quite rusty I decided to swap it with a new HEI ignition.
At this moment i'm busy with recurving my HEI, as the cars runs on propane (since a couple weeks an Impco 425 LPG carb directly on the intake instead of the petrol carb.
Cars runs great, but i'm missing some performance which I know can be found in the ignition. The strange this is that when the engine has a total mechanical timing (initial + mech) of 32 degrees it starts to ping @ WOT which is quite strange as propane has about 110 octane - a lot more than the average pump gas. Propane likes (better performance) less total advance, but so far I know it shouldn't ping earlier than petrol.

Does anyone know the default timing curve / total advance for a 389 engine with it's original points distributor? My shop manual says only 6 degree BDTC, but nothing regarding the total timing. The slots in my point distributor seem to be shorter compared to the HEI - it looks like it has less mechanical advance.
I thought Pontiac engines @ pump gas did like about 32-36 max timing, but maybe it's different with my engine.
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post #2 of 2 (permalink) Old 10-22-2012, 12:43 PM
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It does depend on the specific cylinder head quite a bit, but you're correct that most Pontiacs seem to like about 32-36 degrees total ignition lead at full throttle, above about 2500 "or so" rpm. That's with iron heads, on gasolene. Aluminum heads tend to want more advance, so do the closed chamber iron keads like the 1967 670's.

When I converted my 69 GTO to HEI (aluminum heads, 9.9:1 compression, 93 octane) I wound up with 38 degrees total. The HEI I'm running (Davis Unified Ignition) has 20 degrees in it, so my initial is at 18 degrees. I've not yet done any concentrated testing to find the setting for best power, but I feel like it's reasonably close.

I've zero experience with propane though with regard to ignition advance or propensity for ping/detonation.

But... welcome to the forum. How about some photos of your car?


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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

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