Timing question
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Timing question
Today I measured my harmonic balancer and divided the measurement by 10 giving me 2.55" I marked it on the balancer and unhooked the vacuum line at dist. I also changed to a soft set of springs which allowed me to bring all the timing in by around 26-2700 rpm. I locked down the dist. hooked up the vacuum and runs great. My initial timing is 8* at 750 rpm. The timing is locked at 36*. My question is, should I reinstall the original springs in the dist? Should I leave this soft set in? Is my total timing really at 36*? This is an HEI distributor.
Thanks!
Thanks!
Melting Slicks
Joined: Apr 2004
Posts: 2,083
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From: Montreal Qc.
36* advanced (mechanical only, vac unplugged), all in at ~2500 is where you want to be. Leave the new springs in, that's what's giving you the "all in at ~2500". Go back to stiff springs and you'll get the same total but at different (higher) rpm. When you plug in the vac advance that will add to your "total timing", which varies depending on load, rpm, and other things you can't measure in your driveway. Don't worry about the timing at idle, who drives around at idle? 
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
Vette64 -
If your total max (36 degrees) is coming in at 2600-2700 you have an ideal timing curve. This should provide very good throttle response and power. Leave it like that. As Star stated, re-installing the stock springs will still give you 36 total, but the total will come in at a highly elevated rpm. Do check your vacuum advance, though, and make sure it's not pulling in any more than 16-18 degrees (producing something less than 54 degrees maximum total combined timing). If you're pulling more than 18 degrees vacuum advance, change the vac advance unit out for one with a shorter curve, such as a NAPA VC1838.
Lars
If your total max (36 degrees) is coming in at 2600-2700 you have an ideal timing curve. This should provide very good throttle response and power. Leave it like that. As Star stated, re-installing the stock springs will still give you 36 total, but the total will come in at a highly elevated rpm. Do check your vacuum advance, though, and make sure it's not pulling in any more than 16-18 degrees (producing something less than 54 degrees maximum total combined timing). If you're pulling more than 18 degrees vacuum advance, change the vac advance unit out for one with a shorter curve, such as a NAPA VC1838.
Lars
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Sorry to seem so dumb, but, how do I go about checking the vac advance? Curb idle with the vac disconected is 8*, when I hook it back up it (the timing mark is well above the timing tab which has 16* as the last measurement.
Team Owner
Joined: Jun 2001
Posts: 23,367
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From: Virginia USA
Sounds like you are using manifold vacuum to run your vacuum advance. You will be pulling a vacuum on the can even at idle using this source. You can switch to ported vacuum and not have any vacuum advance at idle and WOT. This will allow the vacuum advance to only be used at cruise.
Here is Lar's paper that will clear it all up for you:
Lar's Vacuum Advance paper
Another good Lar's Timing paper to read
Here is Lar's paper that will clear it all up for you:
Lar's Vacuum Advance paper
Another good Lar's Timing paper to read
Last edited by theandies; Sep 30, 2005 at 06:00 AM.
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by vette64772
Sorry to seem so dumb, but, how do I go about checking the vac advance? Curb idle with the vac disconected is 8*, when I hook it back up it (the timing mark is well above the timing tab which has 16* as the last measurement.
Using a non-adjustable light, and with the balancer marked at 36 degrees, you can check your vacuum advance as follows:
First, set total mechanical timing to 36 degrees as you've done. This will occur when your newly marked line on the balancer aligns with the "0" on your timing tab at elevated rpm. Once verified, hook up your vacuum advance. With the vacuum advance hooked up, rev the engine just as you did before. The new timing line should not go any farther advanced than the 16 degree mark on your timing tab (about 18 degrees is still OK). If it does, change it out for a unit with a shorter advance. Since the total timing is 36 degrees when the new line is aligned at "0", the total combined timing with the new line aligned at "16" (with vac hooked up) is 36+16=52. The max you want to see is 54. Above 54, you stand the chance of the car "jerking", "kicking" or "bucking" at cruise speed.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
The HEI is actually pretty darned good all by itself once correctly curved. I doubt you'll see any noticable gains by adding the MSD. I have found, however, that the MSD does compliment the stock points-style ignition systems nicely, and will produce a smoother idle and a measureable improvement in throttle response. It's the only ignition system upgrade where I have actually seen changes in the dyno numbers before/after. I have not done a before/after dyno run with stock HEI versus HEI + MSD, so I cannot say with certainty what you could expect to see.
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Just got back in from checking the vac. timing.............dead on 16*. So, I guess I am sitting at 52* overall and man, guess it is as good as it is gonna get? Thanks for the guidance guys........by the way, it runs so much stronger now that it has been recurved. Throttle response is much improved! Think I will just skip the MSD expense.........