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I know this subject has been beaten to death, but please bear with me. I've read Duke's paper MANY times. I learn something each time. Here is what I have:
My 62 has what is essentially a 1971 LT-1 engine. That is to say, I bought a boneyard 350, bored it .030 over, align bored, etc. etc., and totally rebuilt it using LT-1 parts. It has run great for over 30 years, but it has always overheated. I have replaced every part of the cooling system, still not happy. Then I read the part about retarded timing causing overheating. My engine was set @ 8* BTDC as per specs for 1971 LT-1. But when I checked, it maxed out at only 32* (24* centrifugal advance). Now 24-26 centrifugal is supposed to be right for SHP SB, but only 32 total? Hmmm. I cranked it up to 16* BTDC (40* max). No overheating, but I was getting ping when pulling from low rpms. (Up hill in 4th @ 25mph). I tried 38* total - better, but still pings a bit. At 36* total, it seems happy. So, I have 12* static + 24* centrifugal = 36* total.
Just for chucks and grins, I then checked my vacuum advance. Engine pulls 12" @ idle. Using a vacuum pump I got the following: 0-6" - 0*, 6"-2*, 8"-6*, 10"-11*, 12"-15*, 14"-18* which was max. If I understand it correctly, the 2" rule says that I want vacuum advance all in by 10", not 14". It would appear that I have a B22 vac, but would be better served by a B28. My question is what would I gain by changing to a B28? A bit more advance at idle. My idle is a bit lopey, but most LT-1s are. It starts easily and idles well - if lopey - but no stalling or stumbles off idle. What else? Would it make a significant difference over the rest of the rpm range? What about overheating? Or fuel economy? FWIW, I run a 625 cfm AFB on an aluminum high rise 180* manifold (Edlebrock) and vacuum takeoff is not ported.
A B22 is rated at 16 crank degrees of advance at 14-16 Hg vacuum so if you do have a B22 it’s over performing a little. With a B28 you would probably pull in about the same vacuum advance at idle and light throttle cruise, assuming you have the vacuum advance hooked up to manifold/full time vacuum. But the B28 can will be a more consistent performer and probably meet the 2” rule as you said. With either vacuum can on manifold vacuum be sure to watch for an over advanced condition at highway cruise that feels like a continuous light miss. If you get that then you will need to limit the amount of vacuum advance. I don’t think either can will affect engine cooling as long as you stick with 12 deg initial timing and vacuum advance hooked to manifold vacuum.
To answer a few questions:
Yes, full vacuum (left port on AFB) I confirmed that port was pulling 12" at idle.
Up hill @ 25 mph in 4th - looking for knock. I figured that would do it if anything. It did.
What about an adjustable can like this? https://www.summitracing.com/parts/acc-31034?rrec=true
I could set it to 10" (2" rule) and adjust total advance as well. Too good to be true? Does anyone have experience with these?
To answer a few questions:
Yes, full vacuum (left port on AFB) I confirmed that port was pulling 12" at idle.
Up hill @ 25 mph in 4th - looking for knock. I figured that would do it if anything. It did. What about an adjustable can like this?https://www.summitracing.com/parts/acc-31034?rrec=true
I could set it to 10" (2" rule) and adjust total advance as well. Too good to be true? Does anyone have experience with these?
A lot of Mopars use those and I didn’t know they were available for Chevy. But you need to have one that pulls in the max amount of vacuum advance you want. If I remember right, the only thing you can adjust on those is the vacuum range it operates in. The ones I’m somewhat familiar with have a fixed amount of advance that doesn’t change with the adjustment. There’s a screw and spring inside that applies tension on the diaphragm to allow adjusting the vacuum point at where it begins to move. By adjusting for a lower vacuum threshold you also change the amount of advance pulled in at different vacuum levels and probably engine rpm. But the max amount of advance does not change unless you rig up a limiter of some kind on the arm.
I have a 1970 LT-1 short block in my 61 and had similar issues.
Like you, when advancing it to the DUKE recommended point, I had predetonaton (pinging) on hot days.
The best gas we can get here in CAL is 91.. With the domed pistons and 11:1, I think our LT-1s would like a few more octane points to be at their optimal tuning!
What Dan says is right.. if you have too much advance from VAC, you will get a "bucking" at light cruise. Not fun.
So it's a compromise... I have experimented with a few vac cans as well as one with an adjustable limit.
The adjustable vac can was the best solution for me.. but I still run hotter than I'd like idling for long periods on hot days.
The static timing ended up at 14 btdc.
To answer a few questions:
Yes, full vacuum (left port on AFB) I confirmed that port was pulling 12" at idle.k. I
Up hill @ 25 mph in 4th - looking for knoc figured that would do it if anything. It did.
What about an adjustable can like this? https://www.summitracing.com/parts/acc-31034?rrec=true
I could set it to 10" (2" rule) and adjust total advance as well. Too good to be true? Does anyone have experience with these?
Up hill @ 25 mph in 4th, at what rpm?
Sometimes the L76 centrifugal advance curve is too aggressive, adding advance at too low in rpm to result in detonation at WOT combustion chamber pressures. This can be addressed by fitting a stiffer advance spring combination to delay the 24* advance all-in rpm (move from 2200 rpm to 2800 or 3000 rpm, a setting indicated by the rpm when WOT ping/knock stops in load testing), or you can just avoid WOT at lower rpm.
I say forget idle timing. Power time you car. Use your timing light to see when Max Timing is in. Slightly loosed dizzy, then set Max Timing (ie:34 degrees?) at that rpm. Your max timing should be All In by 3000rpm. If not, using the chart above you can swap out the distributor curve springs buying a Moroso Spring set for about $25. Either way.....set you timing at Max RPM, tighten your distributor then re-check . Then adjust your idle mixture for closest to your liking . Power Timing is the way to go.
I say forget idle timing. Power time you car. Use your timing light to see when Max Timing is in. Slightly loosed dizzy, then set Max Timing (ie:34 degrees?) at that rpm. Your max timing should be All In by 3000rpm. If not, using the chart above you can swap out the distributor curve springs buying a Moroso Spring set for about $25. Either way.....set you timing at Max RPM, tighten your distributor then re-check . Then adjust your idle mixture for closest to your liking . Power Timing is the way to go.
Only 34 for max?? What I've read suggests 38-40 which is a BIG difference.
Power tuning is exactly what I did - set to 40 max. That had too much ping at partial throttle, so I have progressively scaled back - 2* at a time - and now I am at 10* initial and 34* max so maybe you are right about that 34*.
Others have comments?
Only 34 for max?? What I've read suggests 38-40 which is a BIG difference.
Power tuning is exactly what I did - set to 40 max. That had too much ping at partial throttle, so I have progressively scaled back - 2* at a time - and now I am at 10* initial and 34* max so maybe you are right about that 34*.
Others have comments?
Is the overheating resolved?
Too much ping at partial throttle, at what RPM?
A fatter travel stop button can get you 12* static + 22* centrifugal = 34* total (what I run), or with stiffer advance springs 14* static + 22* centrifugal = 36* .
A fatter travel stop button can get you 12* static + 22* centrifugal = 34* total (what I run), or with stiffer advance springs 14* static + 22* centrifugal = 36* .
Overheating was resolved @ 14* initial, but too much ping. Yesterday I had some overheating when I turned it back to 12*, but the temps were in the 80's. I'll see tomorrow how it is @ 10*.
The ping (or rattle) comes just off idle (my tach isn't working and I haven't installed the new electric replacement yet) - pretty much any time I step on the throttle after coasting with throttle closed. Also with each shift.
Coming at it from a different angle, I tried using a stiffer spring on the step rods (AFB) which should make the richer 'Power' mode kick in at a lower vacuum, but it didn't seem to help.
FWIW, I confirmed that my vacuum advance unit is a B26.
Also note, the car runs fine. Plenty of power and if I were just a bit deafer I wouldn't think there was a problem at all.
Overheating was resolved @ 14* initial, but too much ping. Yesterday I had some overheating when I turned it back to 12*, but the temps were in the 80's. I'll see tomorrow how it is @ 10*. The ping (or rattle) comes just off idle (my tach isn't working and I haven't installed the new electric replacement yet) - pretty much any time I step on the throttle after coasting with throttle closed. Also with each shift.
Coming at it from a different angle, I tried using a stiffer spring on the step rods (AFB) which should make the richer 'Power' mode kick in at a lower vacuum, but it didn't seem to help.
FWIW, I confirmed that my vacuum advance unit is a B26.
Also note, the car runs fine. Plenty of power and if I were just a bit deafer I wouldn't think there was a problem at all.
The 12* static + 24* centrifugal = 36* is probably not a problem.
Off idle ping with the vacuum line off and plugged?
The final tune may need a B28 can, or stronger springs on the advance weights.
Off idle ping with the vacuum line off and plugged?.
No, the ping is with vacuum advance working. I have not tried running it without direct full vac adv.
I was actually wondering if going to ported vacuum might help despite no one recommending that.
No, the ping is with vacuum advance working. I have not tried running it without direct full vac adv.
I was actually wondering if going to ported vacuum might help despite no one recommending that.
See if you get the same off idle ping with the vacuum line off and plugged (eliminate the vacuum advance).
Still pings = >>> centrifugal advance springs are too weak (too much low-mid rpm advance, @ 1200-2400 rpm)
No ping = >>> a B28 vacuum can may help along with less static advance.
OK, I looked and the cent. adv had way tiny springs. I went to stiffer springs (and I also got my tach working!) so here is what I have now:
Cent adv. starts just over 1000
6* @ 1250
11* @ 1400
23* @ 2250
27* @ 2300
Vac adv (this is stamped B26, but these are the actual measurements I got):
0" = 0
5" = 1*
7" = 4*
10" = 11*
12" = 16*
15" = 19*
With the tach now working, I can say that the ping I'm hearing occurs between 1000-2000 rpm if under load. Transient on upshifts, but if I'm pulling up a steep hill at that rpm, I will hear it until I reach the top or if I let up on gas. Since the cent. is not all in at that rpm, I'm thinking it is a problem with the vac adv. Of course, I have no way of knowing what vacuum I have in those circumstances. I bought an adjustable vac can, but I cannot figure out how to remove/install it on my MSD distributer, but I'm going to start a separate thread for that and try to keep this focused on timing.
Interestingly, I read a thread recently on reading your spark plugs. The linked video (
) described how you could read timing from a spark plug, so I pulled one.
To my eye, the transition from burned off to unburned occurs just after the bend, but definitely before the base which he says indicates retarded timing, not over advanced.
I'm confused.
As quick setup on stock engines I push the edge on initial as though I'm not using a vac can. 15-23 degrees. depending on the year. Then typically check and choose the lightest and a medium spring choice combined for the mechanical advance. In the vacuum advance area I make a simple horseshoe (screws in on the inside of the dist can mount) that provides an adjustable mechanical stop so I can choose any degree choice, I usually choose from 5-10 degrees on manifold vacuum. I find with todays fuels this gives a nice seat of the pants feel with out all the time involved with locating the "right" vac can. I'm on southern east coast and the humidity is high. I cant say how well these settings would work in a dry cli or at altitude though.
08-01-2023, 04:56 PM
Timing questions (NOT AGAIN!)
