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I've read the timing 101 article, but am still confused about the mechanical timing.
I have a 496 with a hyd roller cam, 238/243 dur 634/643 lift 112lsa, Accel HEI.
My best vacuum idle is 14.5/15".
I have the initial timing at 20*, vacuum advance hooked to manifold vacuum.
The motor seems to like it best this way for the power brakes.
When I unplugged the vac advance and ran the motor to 3000 rpms the mech advance was 35*, +20* would be 55* total. That seems very high to me.
I changed the dizzy springs a few times and got a best of 32* mechanical. So that + 20* initial =52*. Still too high?
The car runs good, don't think I hear any pinging. Im just afraid of ruining my new motor. I'm pretty unfamiliar with playing with mech advance. Any other way to lower it? Or is 52* total fine for my combo? Vacuum advance hooked to manifold is definitely necessary on it. It made a world of difference when I changed it to that from ported.
 

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If you are reading 35 degrees advance at 3000 rpm, that is your total advance.
You do not add your initial setting to what you read on your harmonic damper.
With the vacuum line unplugged, what you read at any rpm is your total.
 

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1970 El Camino SS396, VortecPro 467, ATI TH400, Moser 12 bolt /TruTrac 3:42, Hedman headers
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When you did your initial timing of 20 degrees your info indicates the vacuum advance was still hooked up, initial timing should be with the vacuum DISCONNECTED for valid results. Vacuum advance should / would be non-functional at full throttle and low vacuum conditions (acceleration, etc.) Do your timing measurements with the vacuum advance hose disconnected and the manifold vacuum source plugged. I would suspect a starting point for total advance (initial + full mechanical} might be in the 34-36 degree range and go from there depending upon C.R., combustion chamber shape, iron or aluminum cyl heads, fuel, timing curve, etc. Total timing (not total advance) can be in the 50 degree range with the vacuum advance hooked up when under cruise and LIGHT throttle conditions because the leaner cruise fuel mixture requires more timing to properly fire vs a WOT full on rich acceleration. Hope this helps.
 

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1. Disconnect and plug vacuum advance
2. Set total to 35 degrees all in at 3000
3. Return to idle and check timing (this is initial, record it)
4. Hook vacuum advance back up
5. Check timing at idle (Record it)
6. Slowly rev engine up to max timing and record. (Should be somewhere around 50*. This will be your light load cruise timing. This will disappear when you go WOT and return to 35*)

I think you had it right the first time. You just got confused and added your initial to your total.

Do this and come back with your numbers. If you have too much timing at number 5 and 6, you’ll have to limit vacuum advance.
 

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With vacuum disconnected, you need to go higher than 3000 rpm. Run it up until it stops advancing and note the total and the rpm if stopped at there. Then you subtract what your initial is set to and that will be your total mechanical advance. Then set up your vacuum advance so that your total with vacuum is around 50.
 

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496... 4.25 stroke. My experience has been significantly less timing than your running. I can run that much but it just over heats a lot.

right now 14 @ 1000rpm with a v-advance of 6
32 @ 3500rpm with v-advance of 10
 

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52 is OK if it includes the vacuum advance. if you lean on it the manifold vac goes away and so does the advance. If it still makes you nervous take 5 out of it but it'll cost you gas mileage.
 

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1970 El Camino SS396, VortecPro 467, ATI TH400, Moser 12 bolt /TruTrac 3:42, Hedman headers
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Vacuum advance....drive your car into a long gradual hill section with a steady throttle to see if the motor “rattles” or pings under lean throttle conditions, this is an easy way to see if there is too much vacuum advance in your vacuum canister. Remedy’s..disconnect and plug the vacuum line and try again to be sure the pinging stopped, if no more pinging you can 1) change the can for one with less advance, 2) you can limit your current cans’ advance with various limiters (Crane/MSD, etc.) or if you’ve got a sharp guy with a dist. machine he can silver solder a small washer to limit the plunger travel of the can. Also remember, GM put a small piece of rubber tubing on the plunger to “cushion” and limit over advance of the can...over the years these have dried up, shrunk or just plain fallen off and may contribute to your vac, advance issues when using a stock style distributor so you may want to check this first.
 

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If you are reading 35 degrees advance at 3000 rpm, that is your total advance.
You do not add your initial setting to what you read on your harmonic damper.
With the vacuum line unplugged, what you read at any rpm is your total.
Correct. But, the 'reading' would be the same w/vacuum advance connected because the 'advance' from the virtual zero vacuum (at 3000 rpm) would be nil.

Just a clarification.

Pete
 

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Correct. But, the 'reading' would be the same w/vacuum advance connected because the 'advance' from the virtual zero vacuum (at 3000 rpm) would be nil.

Just a clarification.

Pete
Not necessarily - if you are sitting in park/neutral, revving the engine to 3000 rpm and holding it there to check timing, there is not much load at all and there will be plenty of vacuum and the vacuum advance will still be in play. The only way to check definitively is to disconnect the vacuum advance.
 
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If you don't know how much mechanical advance your distributor is capable of, the first thing you want to find out is how much it is capable of. Do this with vacuum advance disconnected:

1) Put in the lightest possible mechanical advance springs you can. Then do an RPM test where you measure the timing, slowly bringing up the RPMs until the timing stops advancing. You will see it stop advancing early in the RPM range, as the light springs let the weights fly open much, much sooner.
2) Now put the heaviest springs in possible, and measure the timing at idle. The heaviest springs will keep the mechanical advance from engaging at idle.
3) The difference between the maximum you measure between (1) and (2) is how much mechanical advance your distributor will add.

Now, say you want to get to 36* total timing when at WOT. Take 36* and subtract the reading from (3) above (the max mechanical advance amount) and that is what you set your initial timing to.

Example:

(1) we measure this, and wind up at 29* is the most we ever see (this is the most total timing we see)
(2) we measure this, and wind up at 8* (this is where current initial timing is set at)
(3) we calculate this, and wind up seeing that the mechanical advance in this distributor will generate 21* maximum.

So, say we want 36* total timing when at WOT. Set the initial timing to 36-21 = 15*.

Now, for a good baseline, adjust the springs in the distributor so that you are getting to your wanted 36* at around 3000 RPM.

You now will have your engine running with 15* initial timing, 21* of mechanical advance and the curve has been tuned to be all in by 3000 RPM for a total of 36* of timing.

Note that I haven't mentioned anything about vacuum advance here (yet), because it has been disconnected the whole time.

Next, you want to figure out how much vacuum advance your engine can safely take. That is where knowing how much vacuum you have at idle and having a limit on the amount of vacuum advance comes into play, along with looking for detonation. Dave Ray has a great writeup on the best ways to limit vacuum advance.

You need to figure out how much vacuum you have at idle. You then need to get either a vacuum canister that has adjustable vacuum settings, or one that is fully deployed about 1-2" of vacuum below your idle vacuum. You then plug the vacuum canister into full manifold vacuum and then use a vacuum advance limiter to limit the total number of degrees. I usually start with a safe 10* limit on the engines I use vacuum advance on.

Now, what you will see is that you now have your 15* initial timing plus the 10* of vacuum timing when at idle, which means you will see 25* if you measure timing at idle with vacuum advance connected. You will also see that during light cruising, when you have lots of vacuum, if you are humming along at 3000 RPM and could measure timing, you would see 46* of timing (initial + mechanical + vacuum.) If you smash the gas pedal, vacuum will drop below the amount of vacuum to engage the vacuum advance and the vacuum advance will drop out and your max timing will now be limited to the 36*.

You can get away with the additional advance added by the vacuum advance unit at light throttle because there is not much load on the engine.

How much timing (WOT and cruising) is engine dependent, the numbers above are just a rough baseline starting point that I have found works well on many engines, but each engine can/should be tuned to what it wants and how it runs best in each individual situation.

Hopefully someone finds this explanation useful.
 

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Yup, My timing it set to 36 TOTAL. Hell I forget what initial is. as it dont really matter.

PS depending on your compression, 10 degrees may be a bit much vacuum advance. This isnt a 8.5:1 80's smog motor. I havent even turned mine on yet, bit tis limited to 3-4 degrees is all. ( that was the builders call, so as not to detonate and ruin a superior build trying to scratch out anotherh 1-2 mpg.)
 

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Whats your altitude and fuel? Plays a role too.
The more towards sealevel, the denser the charge and the less advance in general the engine needs.
You can keep a aggressive timing, but then use premium or better fuel for detonation protection.

I have a 496 with a hyd roller cam, 238/243 dur 634/643 lift 112lsa, Accel HEI.My best vacuum idle is 14.5/15".

For comparison:
Living at sealevel using 96=(102+90)/2 fuel: 496, 238/248 Dur. - .578/.608 lift - 110 LSA - MSD 6AL - Vacuum at idle 1000 rpm ~10-12"
(i expect my gauge to have 1-2" offset - powerbrakes work perfect)

I run 18° initial + 18° mech. (all in at measured 3800 - 2 heavy silver MSD springs) -
+ 6° vacuum (So 24° idle timing or 42° cruise timing).

I expect there is room for more timing.. but i just want it to perform and still be way on the safe side. Gas mileage is no object. Safety is..
 

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Whats your altitude and fuel? Plays a role too.
The more towards sealevel, the denser the charge and the less advance in general the engine needs.
You can keep a aggressive timing, but then use premium or better fuel for detonation protection.

I have a 496 with a hyd roller cam, 238/243 dur 634/643 lift 112lsa, Accel HEI.My best vacuum idle is 14.5/15".

For comparison:
Living at sealevel using 96(!) fuel: 496, 238/248 Dur. - .578/.608 lift - 110 LSA - MSD 6AL - Vacuum at idle 1000 rpm ~10-12"
(i expect my gauge to have 1-2" offset - powerbrakes work perfect)

I run 18° initial + 18° mech. (all in at measured 3800 - 2 heavy silver MSD springs) -
+ 6° vacuum (So 24° idle timing or 42° cruise timing).

I expect there is room for more timing.. but i just want it to perform and still be way on the safe side. Gas mileage is no object. Safety is..
Your 96 octane if pumpgas is meassured in RON same as Us 92 MON just a a clarifaction so its not confuses since theres different ways to meassure octane. Dont you have 98 RON in germany, like we have in Sweden?
 
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