Total timing question
Thread Starter
Intermediate
Joined: Dec 2015
Posts: 28
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From: Fullerton Ca
2015 C3 of Year Finalist
Total timing question
I have a 72 350 LT-1. Pretty much stock less smog, comp cam 12-701-3. The other day I had a mechanic adjust the timing to stop it from pinging at high RPM. After a few test drives he said it seemed to like 36 degrees total timing. When I got home I put my timing light on and read 36 degrees at 4 degrees after top dead center at about 2800 rpm. Is that right I thought it was read at 0 degrees. Bottom line is its not pinging anymore but would like to know where I read timing mark in crank mark.
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
I have a 72 350 LT-1. Pretty much stock less smog, comp cam 12-701-3. The other day I had a mechanic adjust the timing to stop it from pinging at high RPM. After a few test drives he said it seemed to like 36 degrees total timing. When I got home I put my timing light on and read 36 degrees at 4 degrees after top dead center at about 2800 rpm. Is that right I thought it was read at 0 degrees. Bottom line is its not pinging anymore but would like to know where I read timing mark in crank mark.
Your mechanic set it to 32 degrees which is a lot better than 36 degrees for a fairly stock engine. The 36 degrees is a valid number for high octane fuel but for 87 octane the 32 degrees is more forgiving and that's what I run in my beefed 454" engine.
Burning Brakes
Joined: Jul 2015
Posts: 823
Likes: 57
From: Austin TX
I have a 72 350 LT-1. Pretty much stock less smog, comp cam 12-701-3. The other day I had a mechanic adjust the timing to stop it from pinging at high RPM. After a few test drives he said it seemed to like 36 degrees total timing. When I got home I put my timing light on and read 36 degrees at 4 degrees after top dead center at about 2800 rpm. Is that right I thought it was read at 0 degrees. Bottom line is its not pinging anymore but would like to know where I read timing mark in crank mark.
Was this car pinging with 36 degrees total even on premium fuel? If so, it would be worth checking to be sure the index mark on the balancer really is TDC.
Dr. Detroit
Joined: Mar 2012
Posts: 10,095
Likes: 4,022
From: New Braunfels Texas
Does it read 32 degrees when the light is set for 0? If so, you might want to call your mechanic and ask him about it. Either he made a mistake, or something is changing.
Was this car pinging with 36 degrees total even on premium fuel? If so, it would be worth checking to be sure the index mark on the balancer really is TDC.
Was this car pinging with 36 degrees total even on premium fuel? If so, it would be worth checking to be sure the index mark on the balancer really is TDC.
A 1972 LT-1 should run excellent all day long on 93 octane assuming the motor is fairly stock.36 is a tried and true number as well.....what is the timing at idle?
Jebby
Thread Starter
Intermediate
Joined: Dec 2015
Posts: 28
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From: Fullerton Ca
2015 C3 of Year Finalist
In Ca. 91 octane is as good as it gets unless using "racing" fuel or an addditive. I'll have to check when I get home, I don't remember now but it seemed like it was real low, less than 4 degrees BTC. with vacuum advanced attached. It stumbled a bit on take off, richened it up seems to be better ( Holley 4150 780 CFM, factory correct carb)
Dr. Detroit
Joined: Mar 2012
Posts: 10,095
Likes: 4,022
From: New Braunfels Texas
In Ca. 91 octane is as good as it gets unless using "racing" fuel or an addditive. I'll have to check when I get home, I don't remember now but it seemed like it was real low, less than 4 degrees BTC. with vacuum advanced attached. It stumbled a bit on take off, richened it up seems to be better ( Holley 4150 780 CFM, factory correct carb)
I like to use the MSD timing tapes.....but barring that you will need a dial back timing light.
Also check for proper dwell first. This can alter timing. Set at 30 degrees.
Then dial in your 36. Wherever the initial falls is where it falls unless the distributor is curved with different or modified weights.
It should still run well at 91 octane as the 72' LT-1 was a 9:1 compression ratio engine.
Jebby
Last edited by Jebbysan; Mar 8, 2016 at 02:03 PM.
Dr. Detroit
Joined: Mar 2012
Posts: 10,095
Likes: 4,022
From: New Braunfels Texas
Yes...I failed to mention that but I believe it does spell that out on the spec sticker under the hood.Here is what the original sticker looks like for 72'.....I stuck one on mine...just FYI.
Jebby
Last edited by Jebbysan; Mar 8, 2016 at 02:20 PM.
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Burning Brakes
Joined: Jul 2015
Posts: 823
Likes: 57
From: Austin TX
Like someone else said, the vacuum advance port on your carb/intake needs to be disconnected and plugged for these measurements.
www.corvette-restoration.com/resources/technical.../Vac_Adv_Spec.pdf
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
No way. Unless you've got a high-overlap cam, vacuum advance is worth about 16 degrees at idle. You'd be set at 12 degrees after TDC. I'm not sure a motor will even run like that.
Like someone else said, the vacuum advance port on your carb/intake needs to be disconnected and plugged for these measurements.
www.corvette-restoration.com/resources/technical.../Vac_Adv_Spec.pdf
Like someone else said, the vacuum advance port on your carb/intake needs to be disconnected and plugged for these measurements.
www.corvette-restoration.com/resources/technical.../Vac_Adv_Spec.pdf
Last edited by SH-60B; Mar 8, 2016 at 05:43 PM.
Thread Starter
Intermediate
Joined: Dec 2015
Posts: 28
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From: Fullerton Ca
2015 C3 of Year Finalist
I checked a couple of checks tonight before,I was interrupted.
Vacuum advanced disconnected & plugged.
Idle 900 rpm 8.2 degrees at 0 TDC
At 2400 rpm 25.6 degrees at 0 TDC
Much more advance and it wants to ping at higher rpm when getting on it.
By the way
Dwell is set at 29.8 at idle
Vacuum advanced disconnected & plugged.
Idle 900 rpm 8.2 degrees at 0 TDC
At 2400 rpm 25.6 degrees at 0 TDC
Much more advance and it wants to ping at higher rpm when getting on it.
By the way
Dwell is set at 29.8 at idle
Last edited by 1972LT1; Mar 9, 2016 at 12:32 AM. Reason: oops I forget
Melting Slicks
Joined: Mar 2008
Posts: 2,989
Likes: 198
From: Meriden CT
I think whats confusing me is the way you're stating the advance. Timing is either BTDC or ATDC. So the way I'd state it is you simply have 8.2 degrees of initial spark advance at idle. That sounds completely normal. When you say you have 25.6 at 2400 RPM, has all the mechanical advance come in at that RPM? If it has you can use stronger springs in the mechanical advance to get it "all in" at a higher RPM, like 3600 RPM. That's about 17.5 degrees mechanical that you're seeing in your distributor, IDK if that's all there is available in there, but it seems a little low to me. IIRC it should be about 22 degrees.
Sorry to sound like a lecture, but IDK what your experience is with timing.
Sorry to sound like a lecture, but IDK what your experience is with timing.
Last edited by SH-60B; Mar 9, 2016 at 04:51 AM.
Burning Brakes
Joined: May 2014
Posts: 884
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From: Rhine Georgia
My HEI that came on my crate gives only 18 mech, (verified many times). Only thing I've changed in it are the springs. I've read you can expect around 20ish but could be as much as 25. Those are internet numbers so I don't know if they are accurate.
Race Director
Joined: May 2006
Posts: 18,664
Likes: 2,564
timing
I checked a couple of checks tonight before,I was interrupted.
Vacuum advanced disconnected & plugged.
Idle 900 rpm 8.2 degrees at 0 TDC
At 2400 rpm 25.6 degrees at 0 TDC
Much more advance and it wants to ping at higher rpm when getting on it.
By the way
Dwell is set at 29.8 at idle
Vacuum advanced disconnected & plugged.
Idle 900 rpm 8.2 degrees at 0 TDC
At 2400 rpm 25.6 degrees at 0 TDC
Much more advance and it wants to ping at higher rpm when getting on it.
By the way
Dwell is set at 29.8 at idle
Unless you are willing to correctly modify the distributor, different springs, bronze stop bushing, vacuum advance limited, etc. just set your timing to stock and you will have no problems.
When you state in your post "0 TDC" I believe you are referring to setting the dial on your dial back timing light to zero degrees so the TDC is confusing.
Last edited by MelWff; Mar 9, 2016 at 11:50 AM.
Burning Brakes
Joined: Jul 2015
Posts: 823
Likes: 57
From: Austin TX
First, it's possible the balancer ring has slipped, and you have much more advance dialed in than you think. This is a common problem for this age of vehicle, and it needs to be checked before you do anything else. No other tuning work will be valid unless you can be absolutely certain that ignition timing is correct. Someone needs to verify that the TDC mark on the balancer really is TDC.
Second, if the timing is accurate, it shouldn't be pinging with 26 degrees total mechanical advance. Assuming 26 degrees really is the total amount, it's possible this is a fuel mixture issue, not timing.
Finally, if the motor has a lot of miles, carbon buildup in the heads can increase compression and reduce the quench area. The result is susceptibility to early detonation.
Good luck with this, it's probably a simple fix but you have some detective work to do.
Thread Starter
Intermediate
Joined: Dec 2015
Posts: 28
Likes: 9
From: Fullerton Ca
2015 C3 of Year Finalist
I think whats confusing me is the way you're stating the advance. Timing is either BTDC or ATDC. So the way I'd state it is you simply have 8.2 degrees of initial spark advance at idle. That sounds completely normal. When you say you have 25.6 at 2400 RPM, has all the mechanical advance come in at that RPM? If it has you can use stronger springs in the mechanical advance to get it "all in" at a higher RPM, like 3600 RPM. That's about 17.5 degrees mechanical that you're seeing in your distributor, IDK if that's all there is available in there, but it seems a little low to me. IIRC it should be about 22 degrees.
Sorry to sound like a lecture, but IDK what your experience is with timing.
Sorry to sound like a lecture, but IDK what your experience is with timing.
Melting Slicks
Joined: Mar 2008
Posts: 2,989
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From: Meriden CT
Had a 1976 L-82, 4-sp
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