Setting initial timing - Pontiac GTO Forum
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post #1 of 14 (permalink) Old 05-12-2012, 04:22 PM Thread Starter
 
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Setting initial timing

Need input on setting timing. 400 with stock cam and HEI.

How many rpm's?
Vacuum advance disconnected
how many degrees should I set it to?

Can't find a good clear topic for this. Not building a race car I just want optimal power and smoothness.

If you have a good link please post it. Its already running well just not as strong as it should be.

Last edited by FlambeauHO; 05-12-2012 at 07:43 PM.
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post #2 of 14 (permalink) Old 05-12-2012, 09:58 PM
 
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For detailed instructions on correctly setting the timing, just drop me an e-mail request for my "How to set Timing" tech paper. To set the timing correctly, you need to set total timing - not initial. The initial timing is somewhat irrelevant, and will not optimize your timing for best performance. Pontiacs like 36 to 38 degrees of total timing, and you want the curve to come in as quickly as possible without getting into detonation. This total timing spec can produce an initial timing anywhere from 8 to 18 degrees, depending on the distributor's advance limit slot, so it's very important to check and set the total.

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post #3 of 14 (permalink) Old 05-13-2012, 12:27 AM Thread Starter
 
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Thanks lars, email sent. I will probably have questions to post on here lol.
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post #4 of 14 (permalink) Old 05-13-2012, 05:46 AM
 
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I have the exact '68 H.O. car in red. When setting total timing, do you do it with all vacuum hoses connected?
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post #5 of 14 (permalink) Old 05-13-2012, 09:51 AM
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6 degrees BTDC was the original spec, and yes, set the total with everything hooked up.
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post #6 of 14 (permalink) Old 05-13-2012, 12:29 PM
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To get timing set to the absolute optimum for your car, it's an individual thing. A great deal depends on your heads, chambers, and overall combustion efficiency of YOUR engine. Generally speaking though, for D-port heads, I'd go through a "trial and error" process starting at about 34 degrees TOTAL, then creeping up from there on degree at a time until you find the point where the motor quits liking it. "Not liking it" is defined as either 1) performance quits improving and starts to fall off or 2) you get into detonation.

Finding that point implies you've got a way to check total advance and also a reliable, obhective way to measure performance --- something like an engine dyno, chassis dyno, or accurate set of e.t. clocks. The 'seal of your pants' will lie to you.

You can check total advance with a light like this one:
Actron/Digital advance timing light with LED screen and mode indicator lights (CP7529) | Timing Light | AutoZone.com or something similar.

To check total advance (for performance purposes) disconnect and PLUG any vacuum advance connections, increase RPM while watching the light unitl you're SURE that all the centrifugal advance in the distributor is "in" (the timing marks have quit "moving") -- usually somewhere in the vicinity of 3000-3500 rpm.

If you don't have a way to accurately and objectively measure performance, then D-port Pontiac's "usually" like to be somewhere close to 36 degrees or so total at WOT with the advance "all in". One exception is 67's with closed chamber 670 heads - they often like "more" because their combustion efficiency isn't as good so you have to start the fire earlier to get the best out of them. I've heard that aluminum heads can like more advance too, because they run cooler and that too has an effect. I don't have any first hand experience (yet) with them or "usual" numbers though.

Once you've got the motor "happy" by setting total advance, then you can check it at idle and see what the initial value is (vacuum disconnected) for reference - so the next time you need to set timing you can do it at idle "the easy way".

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post #7 of 14 (permalink) Old 05-13-2012, 05:40 PM
 
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I always thought you needed the vacuum advance connected to check total ???
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post #8 of 14 (permalink) Old 05-13-2012, 08:17 PM
 
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I am confused as well. Some say set the total timing with all vacuum hoses connected, while some say to set the total timing with vacuum hoses disconnected.

Unless I am totally wrong, would you not set the total timing with the vacuum hoses connected? My theory is, they would be connected when you are going down the road at 40 mph, so why wouldn't you connect them to set total timing in park?
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post #9 of 14 (permalink) Old 05-13-2012, 08:21 PM
 
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As far as I know, my engine emissions decal states 9 degrees with vacuum hoses off. This, of course, is the INITIAL timing spec. However, as per my post above, I would prefer to set the TOTAL timing, but I am unsure if the vacuum hoses should be connected or not.
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post #10 of 14 (permalink) Old 05-13-2012, 08:24 PM
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Seems that way doesn't it? But no, you want the vacuum disconnected. What you're setting this way is the optimum total advance for when the engine is operating at WOT (wide open throttle) - full power. At WOT there's going to be zero manifold vacuum so the vacuum advance will be out of the picture anyway. It's very hard to simulate actual WOT conditions with the engine in neutral, sitting in your driveway (at least, for very long ) so you disconnect the vacuum advance to "remove" it to "simulate" the ignition timing the motor's going to see when under load and at full power output. At part throttle cruise, the vacuum advance is going to add a little more advance and that helps with economy and emissions --- but it's way too much for full power operation.

The term "total advance" is misleading because it seems like that would mean the "maximum" amount of possible advance --- but it doesn't mean that. 'Total advance" actually means "total mechanical advance" - initial timing plus full centrifugal - it doesn't include vacuum.

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