'65 GTO 389 Timing Specs w/modified engine - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 03-26-2016, 07:43 AM Thread Starter
 
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'65 GTO 389 Timing Specs w/modified engine

I'm wanting to get the best tune possible for my 65 GTO. I know the factory spec for initial timing is 6*BTDC but since my engine has been modified, I wanted to see what expert advice or resources were recommended.

The mods are as follows: '65-389 bored 30 over = 395 cu in, Wolverine Blue Racer cam #WG 1169K for excellent mid-range power 2000-4600 rpm, '69 #48 heads, Edelbrock Performer intake with soon to be installed '68 428HO Q-Jet, GM HEI - vacuum advance distributor, Hooker Headers, and TH400 Tranny.

Any advice on timing specs would be greatly appreciated.

Thanks, Roqetman!
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post #2 of 6 (permalink) Old 03-27-2016, 11:18 AM
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I've posted on here several times about how to find the correct timing settings for your Pontiac engine. You should be able to find the information about the procedure using the search tool. Short version: Initial timing setting is mostly unimportant. What matters most is the total mechanical timing (vacuum advance disconnected, since when the motor is pulling hard at wide open throttle, it's not making vacuum to activate the vacuum advance). Set your total timing to the value that produces the best engine performance - then use a light to READ (not change) that value at idle so that in the future you'll know how to return to the same setting. (Reaching in to adjust the distributor on the engine with it running at 3000 rpm or so can be a little daunting.) If your distributor is "weird" (i.e. damaged or has been trifled with by someone in the past such that the advance curve is way out of whack) and causes your engine to be very hard to start (i.e. advanced so far that it trys to run backwards during startup) or overheats (because it's extremely retarded) - then you'll want to repair/replace the whole distributor. Otherwise, the 'total' setting is the way to go. A good starting point for most Pontiacs with open chamber iron heads is usually around 34-36 degrees total. Closed chamber (1967 670 and others) and aluminum heads tend to "like" a little more, depending on combustion chamber design.

Example: My 69 with ported aluminum heads seems to like (i.e. make the most power) with 36 degrees total. My distributor (Davis Unified ignition HEI) has 20 degrees of mechanical advance baked into it. That leaves me with an initial timing setting of 16 degrees, which the engine is fine and happy with.


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post #3 of 6 (permalink) Old 03-27-2016, 07:13 PM Thread Starter
 
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Bear, thanks for the Information! Very helpful!
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post #4 of 6 (permalink) Old 03-29-2016, 05:23 PM
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In '69, Pontiac made two different #48 heads: a pressed-in -rocker-stud 64cc version for the 350, and a screw- in rocker stud 72cc version for the 400. The 64cc version will limit the amount of timing you can run on pump gas. If you have the 72cc version, you will be better off. I am running the 64cc #77 heads on my .030" 389 and a rather large Sig Erson single profile cam with a 4 speed, headers, and 3.36 gears, and run my initial at 10 degrees for a total of 38 degrees all-in. Due to my compression, I need to run about 98 octane fuel. What timing you can run will vary with your compression ratio, camshaft, transmission, and rear end ratio. You can get by with more compression and more timing if you have a stickshift and a stiffer rear ratio, like a 3.55 or 3.90. My 9.3:1 TH400 068 cammed 'stocker' '67 GTO ragtop never pinged on hot days with the 3.36 rear end. Now that I'm running a 2.56 gear out back, it will ping a bit on 100 degree days pulling a long grade at cruise. There are a lot of variables that will determine the 'best' timing curve to run. As a down-and-dirty rule of thumb, though, if you have a bigger cam and exhaust, you can run a little more initial timing due to loss of cylinder pressure at low speeds.
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post #5 of 6 (permalink) Old 03-30-2016, 03:55 PM
 
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The guys are spot on, and like Bear said find out the mechanical and then dial in the base for no more than 36 total....

Put in a 16 degree (crankshaft) B28 Vacumn can in the distributor....from autozone....they are all from the same makers....

And you will have 52 degrees for light throttle cruise, which is perfect....

Or you can use an adjustable Vacumn can...like crane etc....base & Mechanical is total advance....Vacumn advance is not included because like Bear said when throttle wide open, not much Vacumn....

But you don't want to much vac advance ...cause that can ping at cruise if you have too much.....
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post #6 of 6 (permalink) Old 04-01-2016, 07:58 AM
 
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what everybody said plus as you go up in altitude the engine will want more advance
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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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