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Discussion Starter · #1 ·
My 1973 302 just got a fresh timing chain (comp cams double roller, advanced the cam 2*), and some nice new timing marks on the dampner. It has a pertonix iii module in the new stock dist. I set it at 6 BTDC to start, vac line disconnected.

When I brought the RPMs up the mech timing took over (vac advance disconnected) and as the rpms increased the mech timing of the new dist just kept going up. (nearly to 40 with just mech advance alone before I decided to let off!)

what would cause the mech advance to be so high? total timing should be all in (mech and vac) by 28-3200 rpm with about 36*. Correct? timing will not continue to advance after it is 'all in' correct?
 

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My 1973 302 just got a fresh timing chain (comp cams double roller, advanced the cam 2*), and some nice new timing marks on the dampner. It has a pertonix iii module in the new stock dist. I set it at 6 BTDC to start, vac line disconnected.

When I brought the RPMs up the mech timing took over (vac advance disconnected) and as the rpms increased the mech timing of the new dist just kept going up. (nearly to 40 with just mech advance alone before I decided to let off!)

what would cause the mech advance to be so high? total timing should be all in (mech and vac) by 28-3200 rpm with about 36*. Correct? timing will not continue to advance after it is 'all in' correct?
what would cause it ???

the cause is the mech. adv. what was it before the Pt/module install?
should be the same advance/rate
 

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Discussion Starter · #3 ·
what would cause it ???

the cause is the mech. adv. what was it before the Pt/module install?
should be the same advance/rate
I never bothered to check the rate before. i was wondering if i am getting too much mech advance, or how mech and vac affect each other, they do both add to the total? or does mech advance overtake the vac advance when you get up to rpm???
 

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I never bothered to check the rate before. i was wondering if i am getting too much mech advance, or how mech and vac affect each other, they do both add to the total? or does mech advance overtake the vac advance when you get up to rpm???
where you getting to much befopre ?
how it works has been coverd a number of times here .
 

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If anyone knows the answer, please post it. Thanks!
 

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When I brought the RPMs up the mech timing took over (vac advance disconnected) and as the rpms increased the mech timing of the new dist just kept going up. (nearly to 40 with just mech advance alone before I decided to let off!)

what would cause the mech advance to be so high? total timing should be all in (mech and vac) by 28-3200 rpm with about 36*. Correct? timing will not continue to advance after it is 'all in' correct?
Let's look at it this way..
1) A timing light will show you 2 things.
a) What your initial timing is at idle(warm)
b) What your total timing is ( initial + mech.)

2) Notice I said nothing about Vacuum??

3) You never said at what rpm you stopped..
a) Were YOU looking at the light and and working the throttle?
b) Back the timing down till it just starts and see how high and where
the advance stops.( now you know total timing and mech. advance)

3) Now you will know if it's an issue with the dist. or maybe the timing
marks are off....

4) Sometimes people forget that with the vacuum hooked-up you'll
never get a good reading, because a timing light can't figure it out.
(not you, but I'm adding that for the one's that don't know)

Good Luck
 

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Discussion Starter · #8 ·
Let's look at it this way..
1) A timing light will show you 2 things.
a) What your initial timing is at idle(warm)
b) What your total timing is ( initial + mech.)

2) Notice I said nothing about Vacuum??

3) You never said at what rpm you stopped..
a) Were YOU looking at the light and and working the throttle?
b) Back the timing down till it just starts and see how high and where
the advance stops.( now you know total timing and mech. advance)

3) Now you will know if it's an issue with the dist. or maybe the timing
marks are off....

4) Sometimes people forget that with the vacuum hooked-up you'll
never get a good reading, because a timing light can't figure it out.
(not you, but I'm adding that for the one's that don't know)

Good Luck
I don't have a tac right now, so I have no idea how much rpm I am turning.

I just reconfirmed the timing marks very carefully, so I know those are spot on.

I had the light on it and was working the throttle underhood. The more I revved it, the higher the advance went, upwards of 40 before I let off. I am going to reconfirm how much advance I have when my next batch of parts come in in the next few days, but I was curious to see why I would have so much mech advance. The plate for mech advance has a 13 on it, so I would be getting 26* + my 6* of initial for 32*, but when I brought the rpms up, it went well past the 32 mark.
 

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Excess mechanical can be caused by a few factors. Advance limiters originally had neoprene sleeves, and if missing will give more advance than the plate indicates. Also, timing chain and gear tolerances cause advance by inertia at higher revs. Modern ECMs actually have systems to compensate for this slop. You and Gator are on it - with vac advance disconnected (just purely mechanical), your initial and mechanical advance should equal your max mechanical timing. Example: 6° initial plus 26° mechanical advance = 36° max mechanical. Your vac advance would be in addition to this at idle and midrange (manifold vac) or just midrange (ported vac). Let's leave the vac out of it for the moment.

Each engine combo has it's own best curve (rate of mechanical advance increase with rpms), but the typical V8 street engine has "all in" by 2500-3000. High-perf has all in sooner and mild engines tend to be later. Initial advance is largely based on the dynamic compression (DC) near idle set primarily by the cam timing - low DC cams (hi-perf type) need more initial advance and low-perf need less.

Why? First, understand there is a best place (crank angle or degrees after TDC) for peak cylinder pressure (PCP) to occur in the engine's power stroke where it is most efficient in producing power from the burning mixture. This best crank angle is based on the geometry of the engine, and is most efficient for any purpose - torque, horsepower or mileage included. The job of ignition timing to to light the fire at the point where PCP will occur at that best crank angle every time and under any conditions.

As conditions change, the rate of the charge burning and the time it has to burn change, and the timing must change to compensate to hit that PCP sweet spot. It's mostly based on cylinder filling and fuel ratios, where less filling (with lower DC) and lean or rich air:fuel ratios both burn slower, requiring more relative advanced timing. This is why we need vac advance at cruise, as the cylinder filling is very poor (high vacuum and low DC) along with lean mixtures for efficient mileage (slower burning). This is why 3000 rpm at 25% throttle in cruise may use 45° total combined advance, but 3000 rpm and wide open throttle only needs perhaps 34°.

Back to your situation. To test your mechanical advance, slowly rev the engine until it shows no increase in advance. Note the rpm if you can. Shoot your timing light to see what your actual total mechanical advance is, which is normally from 20 to 30 degrees on-top of your initial. Adjust your distributor (at that high rpm) until you see the proper total advance for your engine - probably 34-36 for a '73 302. Lock it down and idle it. Shoot your timing at idle to see what your new initial timing is and note it for future reference. Done.

Hook your vac back up and drive. The timing should be just as the factory intended - if the distributor mechanisms are working correctly and it is the original distributor. Replacement and rebuilt distributors usually have the wrong curve. Unless you are willing to make changes to the timing curve and amount, this is the best you can do at this point.

David
 

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Excess mechanical can be caused by a few factors. Advance limiters originally had neoprene sleeves, and if missing will give more advance than the plate indicates. Also, timing chain and gear tolerances cause advance by inertia at higher revs. Modern ECMs actually have systems to compensate for this slop. You and Gator are on it - with vac advance disconnected (just purely mechanical), your initial and mechanical advance should equal your max mechanical timing. Example: 6° initial plus 26° mechanical advance = 36° max mechanical. Your vac advance would be in addition to this at idle and midrange (manifold vac) or just midrange (ported vac). Let's leave the vac out of it for the moment.

Each engine combo has it's own best curve (rate of mechanical advance increase with rpms), but the typical V8 street engine has "all in" by 2500-3000. High-perf has all in sooner and mild engines tend to be later. Initial advance is largely based on the dynamic compression (DC) near idle set primarily by the cam timing - low DC cams (hi-perf type) need more initial advance and low-perf need less.

Why? First, understand there is a best place (crank angle or degrees after TDC) for peak cylinder pressure (PCP) to occur in the engine's power stroke where it is most efficient in producing power from the burning mixture. This best crank angle is based on the geometry of the engine, and is most efficient for any purpose - torque, horsepower or mileage included. The job of ignition timing to to light the fire at the point where PCP will occur at that best crank angle every time and under any conditions.

As conditions change, the rate of the charge burning and the time it has to burn change, and the timing must change to compensate to hit that PCP sweet spot. It's mostly based on cylinder filling and fuel ratios, where less filling (with lower DC) and lean or rich air:fuel ratios both burn slower, requiring more relative advanced timing. This is why we need vac advance at cruise, as the cylinder filling is very poor (high vacuum and low DC) along with lean mixtures for efficient mileage (slower burning). This is why 3000 rpm at 25% throttle in cruise may use 45° total combined advance, but 3000 rpm and wide open throttle only needs perhaps 34°.

Back to your situation. To test your mechanical advance, slowly rev the engine until it shows no increase in advance. Note the rpm if you can. Shoot your timing light to see what your actual total mechanical advance is, which is normally from 20 to 30 degrees on-top of your initial. Adjust your distributor (at that high rpm) until you see the proper total advance for your engine - probably 34-36 for a '73 302. Lock it down and idle it. Shoot your timing at idle to see what your new initial timing is and note it for future reference. Done.

Hook your vac back up and drive. The timing should be just as the factory intended - if the distributor mechanisms are working correctly and it is the original distributor. Replacement and rebuilt distributors usually have the wrong curve. Unless you are willing to make changes to the timing curve and amount, this is the best you can do at this point.

David
timing chain cause more advance ??? at high rpm???

I'd like to see that happen
 

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timing chain cause more advance ??? at high rpm???

I'd like to see that happen
I thought you would have known about that, Dan. Timing belts, gear drives and chain drives of various types have tendency to take slack out of their system in various ways. At idle they usually just bounce around with timing jitter. But at various rpm bands above idle, the can advance and retard at various points, though usually only by a couple degrees if the belt/chain/gears aren't trashed. I had this with a BBC and silent drive, where it would jitter 2-4° at idle, show smoothing through the timing curve, then advance timing a couple degrees through about 5000, then retard a couple degrees to 6500. A 452 Mopar I had did almost the opposite, showing stable at low rev's, retard through about 4500, then advance with jitter to the 6300 rpm redline. This is all one reason ignitions have gone almost exclusively to crank triggering, avoiding gear, chain and distributor slop and harmonics. Electronic systems designed for distributor use often have an adjustment to compensate for advance or retard, depending on what your system does with the harmonics and rpms it uses. Go find a high-mileage engine and shoot it with a light through the rpms to see how that particular engine acts. It's pretty weird.

David

PS: I watched a video of a AA blown alky engine on a dyno where the drive side of the drive belt is ballooned out, effectively 'advancing' the blower drive. Pretty amazing considering the power being pulled through it. While it's not a timing belt or chain, the same effect can occur with those, especially with heavy chains. Like adding an invisible tensioner on the inside of the drive segment. I'll see if I can find it to show the effect.
 

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I thought you would have known about that, Dan. Timing belts, gear drives and chain drives of various types have tendency to take slack out of their system in various ways. At idle they usually just bounce around with timing jitter. But at various rpm bands above idle, the can advance and retard at various points, though usually only by a couple degrees if the belt/chain/gears aren't trashed. I had this with a BBC and silent drive, where it would jitter 2-4° at idle, show smoothing through the timing curve, then advance timing a couple degrees through about 5000, then retard a couple degrees to 6500. A 452 Mopar I had did almost the opposite, showing stable at low rev's, retard through about 4500, then advance with jitter to the 6300 rpm redline. This is all one reason ignitions have gone almost exclusively to crank triggering, avoiding gear, chain and distributor slop and harmonics. Electronic systems designed for distributor use often have an adjustment to compensate for advance or retard, depending on what your system does with the harmonics and rpms it uses. Go find a high-mileage engine and shoot it with a light through the rpms to see how that particular engine acts. It's pretty weird.

David

PS: I watched a video of a AA blown alky engine on a dyno where the drive side of the drive belt is ballooned out, effectively 'advancing' the blower drive. Pretty amazing considering the power being pulled through it. While it's not a timing belt or chain, the same effect can occur with those, especially with heavy chains. Like adding an invisible tensioner on the inside of the drive segment. I'll see if I can find it to show the effect.
blower belt with a long unsupports span , ok . short timing chain , even if worn NO. long chains in overhead cams have shoes for slack.
timing advance NO , retard timing , even thats questionable. forgot retarded timing being in question is for Ford or any front drive dist. sbc/bbc with rear dist. thats a given that they will retard but never advance
 

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Discussion Starter · #13 ·
Ok, so after about 45 minutes of poking around (and thanks to PSIG's suggestions) I found a seemingly excellent combo.

I found I had just shy of 30 mech advance with the new dist, plus approx 6 with the advance. I pulled apart my old dist, and found it had a 10L plate, and a rubber spacer. I couldn't figure out how to get the advance plate out, so I just borrowed the rubber spacer on my 13L plate. I set my timing to 8 with the advance hooked up. I had 41-42 total advance. Little too much. So what I did is took the vac advance off. Bumped the initial to where I had 36 total and it settled in at 12 btdc, with 36 total. Makes sense if the spacer I took out of the old dist made the 13L plate into a 12L plate. So I would have my 12 initial and 12L (24*) advance plate for 36 total.
Found my engine likes the additional initial advance in previous tuning, so I think it is pretty damn close to the best setup for my bronco. I also don't have any lag between my primaries and secondaries anymore.
 

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Ok, so after about 45 minutes of poking around (and thanks to PSIG's suggestions) I found a seemingly excellent combo.

I found I had just shy of 30 mech advance with the new dist, plus approx 6 with the advance. I pulled apart my old dist, and found it had a 10L plate, and a rubber spacer. I couldn't figure out how to get the advance plate out, so I just borrowed the rubber spacer on my 13L plate. I set my timing to 8 with the advance hooked up. I had 41-42 total advance. Little too much. So what I did is took the vac advance off. Bumped the initial to where I had 36 total and it settled in at 12 btdc, with 36 total. Makes sense if the spacer I took out of the old dist made the 13L plate into a 12L plate. So I would have my 12 initial and 12L (24*) advance plate for 36 total.
Found my engine likes the additional initial advance in previous tuning, so I think it is pretty damn close to the best setup for my bronco. I also don't have any lag between my primaries and secondaries anymore.
did you check it at max rpm the engine will ever see ?

max rpm the chain mighrt want to advance it more . (not) lmao on that one
 

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Nice redtail. It's interesting how it affects more than what you'd think, eh?
Dan, he won't have that problem with the fresh timing set he installed. No reason to test.

David
 

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Discussion Starter · #16 ·
Nice redtail. It's interesting how it affects more than what you'd think, eh?
Dan, he won't have that problem with the fresh timing set he installed. No reason to test.

David
Yes, It is quite interesting to see how many things it affects.

When I get my proper coil in the next few days here (Slow fed-ex :mad: ) I am going to see if it likes 8 degrees of inital with the vac advance on ported for a total of 38*

I noticed a quicker throttle response with advance hooked up, but I will have to reconfirm this when I retime.
 

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Nice redtail. It's interesting how it affects more than what you'd think, eh?
Dan, he won't have that problem with the fresh timing set he installed. No reason to test.

David
new or old a chain or belt will never be the cause of timing advance.

take the blower belt example you gave .
you said the belt could be seen bowing out . where did the extra lenght come from ? the non load side ? that would mean it jump a tooth , so thats not it. . belt stretchingv ? stretch would be it . would'nt that be a longer distance ? then with the bow , the best it could be would be a wash .

Say it again , would like to see what you claim happen. plus the gear drive claim . those rubber gears on that drive ?
 

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Discussion Starter · #18 ·
new or old a chain or belt will never be the cause of timing advance.

take the blower belt example you gave .
you said the belt could be seen bowing out . where did the extra lenght come from ? the non load side ? that would mean it jump a tooth , so thats not it. . belt stretchingv ? stretch would be it . would'nt that be a longer distance ? then with the bow , the best it could be would be a wash .

Say it again , would like to see what you claim happen. plus the gear drive claim . those rubber gears on that drive ?
I can see that if on an old engine that has never been timed had the chain stretch causing the cam to be in a slightly retarded, thus changing the position of the timing in relation to the crank from where it was when everything was new.
 

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Discussion Starter · #19 ·
Got my new coil, and rigged it up to get a full 12v. Didn't feel any better, so I decided to wait to put it on, I have a few other wiring projects that I am goiing to do all at once.

it loves that 11-12 initial and 24 all mech advance. running the best it ever has.
 

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Interesting post, I have certainly observed the timing moving around beyond the maximum mechanical advance, typically when engine speed is > 5-6k.

In my case it always seemed to add a couple of degrees. This was with a Motorcraft/Autolight distributor that I had recurved.

I can't recall if my 408 (MSD distributor) does the same thing, but I don't think so, I need to get out the timing light and refresh my memory;-)
 
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