2+2GT, Distributor Curve Questions??

horseballz · Nov 4, 2011

2+2GT,
I didn't want to hijack any other threads so I'm starting this one. How does one determine the optimum advance curve (mechanical & vacuum) for a given engine setup? Here is my engine at a glance:
>99-ish Explorer 5.0 with stock hypereutectic pistons
>GT40-P heads
>Performer RPM intake
>B303 Ford roller cam - specs: Ford Racing Performance Parts
(cam installed advanced 4 crankshaft degrees)
>1.6/1 roller rocker arms
>Quick Fuel Technology 580 vacuum secondary carb
>Try-Y headers/2 1/2" exhaust
>T5 transmission
>Mallory "57" series magnetic breakerless vacuum advance distributor

The distributor in "out of the box" form gives maximum 24 deg mechanical advance at +/- 3000 rpm, approx. 13 degrees at 1500 rpm and no mech advance below 950-1000 rpm. The vacuum can is adjustable from o to about 16 degrees.
I have seen you post stock Ford advance specs in the past, however it seems that these specs are not necessarily applicable with the different vacuum characteristics of an upgraded/performance cam. When those specs are listed, they state at distributor rpm, can I assume that they mean distributor rpm being half of the engine rpm. Please school me and/or point me to some informative articles/tutorials concerning distributors.
Thank You Much,
Gene

2+2GT · Nov 4, 2011

First thing I'd do with that is baseline the distributor at BOSS 302 specs. The BOSS was a pretty aggressive smallblock engine, and had a dual advance distributor. Once you're on those curves, you'll be in the game, and then you can tweak for best performance. You can ignore the vacuum retard, unless you have fitted a Distributor Vacuum Control Valve.

brianj5600 · Nov 4, 2011

If it was mine I would shoot for 16* initial, 18* mechanical and 10* vacuum. 24* mechanical is too much. It limits your initial to around 10*. Extra initial will make the motor more responsive.

blown65 · Nov 4, 2011

brianj5600 said:
If it was mine I would shoot for 16* initial, 18* mechanical and 10* vacuum. 24* mechanical is too much. It limits your initial to around 10*. Extra initial will make the motor more responsive.

X2. Even the stockers I've had in the past still liked more advance and were much more responsive. On top of that run cooler at an idle.

dennis112 · Nov 5, 2011

X3. Lots of great benefits--as long as the motor doesn't go into detonation from too much initial.

2+2GT · Nov 5, 2011

Well, there's the problem. That's why I suggested the BOSS specs as a starting point.

And don't forget, total advance is but one specification describing the curve, or rate, that eventually results in that total. I did a distributor one time, the C5OF-E on a 67 GT350, some fool had put lightweight springs in it, it went from 0° advance at idle to 14° at 800 rpm (1600 on the engine). Car ran like crap. I replaced the springs, adjusted the baseline to Ford spec, and that thing just oozed power after that.

This stuff is more critical than most folks realized. I became a believer when a guy brought me his distributor, when I suggested it might be the problem. He'd tried everything else- Custom carb, 3.90 diff, you name it. 351C 4V was just a dog. I set the distributor to scock specs, and now he can smoke both rear tires anytime he stomps the gas pedal. I'd only done a few back then, and I wasn't as familiar with the results. I've even seen 200's with the vacuum-only Loadomatic distributor perk up, and most people will tell you these can't be adjusted.

horseballz · Nov 5, 2011

To elaborate on/reiterate part of my original questions:
1 - The Boss 302 specs listed are at distributor rpm - is this correct?
2 - In lieu of a distributor test stand/machine I should be able to get close to those specs with a 40 degree BTDC scale on my balancer and reading the specs as double what the distributor RPM specs are on that spec sheet?
3 - If I am reading that spec sheet (and between the lines) correctly, maximum advance should be 17-22 degrees at 4000 engine (crankshaft) RPM, 15 1/2- 20 degrees at 3000 RPM, etc?

I'm surprised that maximum advance doesn't occur until 4000, but if that's what I should be looking for, I'll try it. Please correct me if I'm interpreting this information in error.
Thank You,
Gene

2+2GT · Nov 5, 2011

With a timing light equipped with a degree knob, or using the scale on the damper (with initial timing temporarily set to TDC) you can do a pretty decent, if noisy, job on the centrifugal advance. The vacuum advance, however, requires a controllable vacuum source.

This used to be a pretty routine part of engine set-up back in the day, try to find a local shop that can do this. Any decent speed shop should be able to. There's a guy I know near here who has a chassis dyno built into the floor of his shop, he can wind a car up to 75 mph and adjust the engine settings standing on the floor.

2+2GT, Distributor Curve Questions??

horseballz

2+2GT

brianj5600

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blown65

dennis112

2+2GT

horseballz

2+2GT

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