Re: Timing....which is the better indicator?
Actually there are 2 advance curves to consider. One is the Power curve. The faster the engine turns, the more advance needed. There is the "ideal" curve - easy to measure with a dyno. You simply experiment at RPM increments to find the most power output at each RPM.
Then you curve the "mechanical" part of the distributor to follow that curve - the springs and flyweights.
In reality, you want maximun cylinder pressure at about 37 - 39 degrees AFTER TDC for max power. Remember it takes awhile to get max pressure after you light it. To get that, the faster it turns, the earlier you have to light it.
But - max power is not necessarily max efficiency as far as mileage goes. You can get a bit more MILEAGE out of it if you light it slightly earlier. Thus the vacuum control - It ADDS to the mechanical curve. Usually lighting it sooner at speed by about 12-15 degrees really makes a difference. It's much harder to measure that on a dyno, so we usually shoot fr about 12 degrees.
At cruise - light or partial throttle ported vacuum is at it's greatest. Manifold is somwehat below it's peak, but not all that far below. Using ported will give full vacuum advance, manifold may not - depending.
As you increase throttle opening - obvioulsy you are wanting more power to accelerate - ported vacuum starts to drop, as the throttle blade is farther away from the port hole, venturi effect drops. Manifold does the same, it lowers a tad under load. Good! Either under that condition is fine too. The idea is get it closer to the power curve. Not dropping enough results in detonation - ping - death to an engine. Some ping is inaudable. If you hear it, believe it's doing damage!
Now lets give it a hard punch - ported goes away completely, manifold does too - OK, acceptable. But as the R's come up, manifold returns faster than ported - possible ping and damage. Who cares about mileage, you need to get out of the way of that truck! You want it on the power curve!
Now close the throttle - ported goes to 0, manifold goes sky high! Hmm, that often causes backfire. You want the advance to drop, not go up during decel!
Now, at idle, manifold is at max, so advance is too. At idle you are around 15 plus initial, say 8 - that equals 23 - too high to idle smoothly, causes overheating, etc.
As you accelerate, ported offers a smooth slowly increasing advance, where manifold stays fully advanced. Ping again. Way too much advance, and no curve if it's already at max. At max you are waaaaay above the preferred power curve.
Much above 1/2 throttle ported drops it back to the power curve, manifold does not.
The Howell - several guys I've worked with that had them switched from manifold to ported and eliminated their ping problem.
Use a timing light - look for this:
Idle - 8-10 initial
Vacuum disconnected, mechanical starts advancing about 1200 RPM.
2500 RPM mechanical is all the way in - about a 15 degree change above initial, but it came in SMOOTHLY AND EVENLY as the R's increased. Not jumped in!
Vacuum connected - idle - no change from initial.
vacuum starts advncing about 1200 RPM.
Remember, there is no way to disable the mechanical, so what you now see is CUMULATIVE - both vac and mechanical.
The advance should slowly go up, evenly, no jumping, until you reach about 2500-3000 RPM, where it no longer advances. The total when it stops going up should read about 35 degrees.
You don't want much more than 35 or you'll get excessive engine wear.
Remember - the TOTAL is the sum of the initial, plus mechanical, plus the vacuum.
Close to that should give great performance without ping.
Remember, max power is curved by the mechanical, the vacuum is only to increase mileage SAFELY.
If you do use the EST - electronically controlled timing feature - and it's a very good way to go - be sure you use the knock sensor - it's an integral part of the EST.
If your distributor is vacuum advancing wrong, too fast, to little etc, then you'll need an adjustable diaphram. If it's advancig too much, make a little bump bracket inside the distributor to limit it.
The problem we encounter often - the distributor vacuum curve was designed for something else, a carb etc. The vacuum source's curve is different.