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my vacuum advance is setting timing at 33 deg at idle
Hi everybody, i’m new here i hope i’m posting this in the right spot. i have a 1957 F600 with a freshly rebuilt stock motor (rebuilt by previous owner) i’ve been fixing it up as much as i can and i’ve hit a wall. it has the 272 with a 2 bbl holley 2300 and a new hei distributor. i got the hei because a friend of mine had one and never used it it was worth $50. and when i set the initial timing of 10 my truck shop manual says. and when i plug it in (to manifold there are no ports on the base of carb) it shoots the timing to 33 deg at idle is this safe? common sense tells me no but it does run well i’m just afraid it’s ready to detonate or something. the distributor on it was original and had no vacuum advance. i’m stuck i don’t want to run it anymore until i know it’s safe. do i just keep the vacuum unplugged? i’m at a loss with this i do not know engines well enough to figure this out. can anyone help? thanks,
and when i set the initial timing of 10 my truck shop manual says. and when i plug it in (to manifold there are no ports on the base of carb) it shoots the timing to 33 deg at idle is this safe? common sense tells me no but it does run well i’m just afraid it’s ready to detonate or something.
Yes it is safe because there is no load on an engine at idle. Not a cause for concern. It is also very efficient, and the engine will run cooler in summer and stop and go driving. Anytime a load is applied as when under acceleration on the road any additional timing advance will instantly be pulled back out, reverting to just the initial timing + centrifugal.
Even when connected to a timed or ported source of vacuum you'll often see lots of advance at idle as well, because many people set the idle a little bit higher than the factory 450 - 550 RPM anyway, it runs smoother because the vacuum advance is just starting to tip in just off idle.
And I want to stay out of the ignition vacuum source and brake valving theory/debate(s).
This engine is not a PASS CAR ENGINE but is a HEAVY TRUCK ENGINE (of the period) and does not need all of that timing lead. It was issued with a mechanical distributor only. Simply refer to it's specs and you will see it is not built for high RPM usage and most likely that the cam/valve-train will not allow it to go much further. It was designed that way to prevent over rev as it is a LOW TORQUE design.
And I want to stay out of the ignition vacuum source and brake valving theory/debate(s).
This engine is not a PASS CAR ENGINE but is a HEAVY TRUCK ENGINE (of the period) and does not need all of that timing lead.
It won't matter one whit at idle, and to your point, the timing lead will be unaffected at part throttle and cruise conditions.
Simply refer to its specs and you will see it is not built for high RPM usage and most likely that the cam/valve-train will not allow it to go much further. It was designed that way to prevent over rev as it is a LOW TORQUE design.
Well OK, but again engine RPM doesn't have anything to do with vacuum advance. In fact, that's the whole point of vacuum advance. The one thing I wonder about is the 10° initial advance spec referenced. Usually the cars and trucks of the era had numbers like 3° initial.
It won't matter one whit at idle, and to your point, the timing lead will be unaffected at part throttle and cruise conditions.
You will have full manifold vacuum advance @ idle. With ported you will not.
Well OK, but again engine RPM doesn't have anything to do with vacuum advance. In fact, that's the whole point of vacuum advance.
That all depends on how the distributor curve(s) (mechanical and vacuum) are set. This engine originally had no vacuum advance feature by design and purpose.
FOMOCO did not use manifold vacuum as a primary source except for the LOM (and that was a regulated/modified signal) and later IMCO (which was a mixture of signals but operated mainly on ported)
OK, enough. the OP should have the info he asked for..
I dont see that 33 degrees vac advance timing at idle is a problem either. But what I would be concerned about is the high gear cruise timing at part throttle. There you would be adding the mechanical timing and the vac advance timing together and depending on your RPMs it could be advancing too high getting you into detonation. What I would do is to simply disconnect the vac advance for now and plug the exposed ports. Look at getting the new distributor curved to match the old one. Then if you want to experiment with adding vac advance reconnect it to a port above the throttle plates and slowly add more vac advance by adjusting the vac advance can port (some are not adjustable) a half turn of the hex key at a time until you do get into detonation. Then back off til it goes away. With it set up properly you could get 10 to 20% better fuel mileage and you will notice your engine will run a little quieter.
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But what I would be concerned about is the high gear cruise timing at part throttle. There you would be adding the mechanical timing and the vac advance timing together...
And... That's exactly how it's supposed to work. Have you actually tried this? I have. It works fine. It doesn't require any contortions or retuning or anything. What it does do is make everybody's eyes bug out when they look at the timing light at idle. But, every modern computer controlled engine does exactly the same thing. It caused trouble in the 1970s EPA smog era, because the most efficient combustion happens to spike NOX emissions relative to HC and CO. So at the time, they crippled the motors by killing performance, reducing compression, reducing fuel economy, etc etc, but it did reduce NOX.
10-31-2018, 06:28 PM
