GM HEI

I switched my GM HEI to running on manifold vacuum and plugged off the ported vacuum. It idles smoother and runs fine. Anyone else hooked up the same way? I followed the below info. It's the opposite of Rich Motts write-up as far as where to get the vacuum from the engine.




I found some interested reading about the vacuum advance in all engines while searching
TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peakcylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuumadvance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

1971 CJ5 Renegade II 225 T14 Dana 18 3.73 Saginaw PS 3 turn 11" drums

 

Interesting, I've read a similar summary but had not tested the procedure myself yet.
Did you set your initial static timing at 10 degrees like the description stated?

 

Excellent comprehensive write up. This IS the way our ignition system works. I have been down this road before with folks that do not have a good understanding of the combustion engine. mike

 

Yes my initial timing is 10*. I have it idling @ 600 rpm.

 

I always like reading these write ups. Just thinking - sometimes when you let an engine idle the plugs will foul which could indicate that it's rich at idle. But the write up indicates the mixture is lean at idle? It seems like sometimes there's more going on than just the simple explanations.
I've heard (but have no way to confirm) that GM used ported vacuum because of emissions. That may explain why they did it opposite of the so called correct way. But, that probably doesn't explain Rich Motts take on it.
I just find this an interesting subject.
Question - my HEI vacuum advance unit is bad (leaks). Which one of the hundred or so replacements should I get to replace it? Don't have room to the fan for the adjustable one. I need the hose hookup to come out the side, not the end.

 

Check on Performance Distributors.