Fun71

That totally depends upon which distributor you are talking about. The GM HEI does not use a bushing; its advance is controlled by the contours of the mating surfaces on center bar and weights. The factory points distributor uses a bushing, as do some aftermarket HEI style distributors.

It's the opposite - no vacuum advance at idle speeds / low gears.

distributorguy

You can add a bushing (typically a thin piece of vacuum hose) to an HEI. When I rebuild them I actually weld in the hole. It allows me to adjust phasing and limit timing as well as pre-load the springs more at idle.

Macadoo

Oh yeah, I already did that per my original post. Derp! Looks like I have 14* vacuum. So the point of vacuum advance is to get more advance at idle and cruise without adding it to wide open throttle? Is that about right?

Macadoo

Oh yeah, I already did that per my original post. Derp! Looks like I have 14* vacuum. So the point of vacuum advance is to get more advance at idle and cruise without adding it to wide open throttle? Is that about right?

oldcutlass

When the carb is wide open vacuum drops off, only from idle to part throttle.

Macadoo

This seems like a legit article. It states:

"The trick is; for performance use you want as much advance as you can get on the crank and the least amount from the mechanical weights in the distributor, hence why race distributors have no mechanical advance at all. To really make a trick street set-up you should only have about 10 degrees or so of mechanical advance in the distributor and about 24 - 26 degrees of initial timing on the crank to obtain your total of 34 - 36 degrees of total timing."

http://www.badasscars.com/index.cfm/...prod/prd76.htm

Opinions?

oldcutlass

Well they give you just enough info to get you in trouble. It really depends on the engine. The problem with running more initial advance is that the engine will get hard to start and kick back. When this happens things tend to break after a while. There are hundreds of tuning articles and opinions running around. The only thing I suggest is to find out what your engine likes. The numbers I give are very conservative and a good starting point to fine tune from there.

Macadoo

Right. I ran into the hard starting issue when I was playing with the initial. I'll keep reading.

distributorguy

That article does not apply to you. Good race distributors DO have an advance curve - and I build a couple hundred of them a year. My customers win. Locked distributors introduce an array of unexpected issues that people seem to be blind to because they want simplicity and don't want to learn. Timing needs to be what it needs to be. Trying to cheat the advance curve and altering the timing away from the ideal setting means slower acceleration and less power. You already know that to some extent by advancing your timing and seeing how the car runs.

Last weekend I installed a gauge cluster in my car including a vacuum gauge. It runs around 18-20" of vacuum, so I suspect you still have a major tuning issue if you're only making 15" of vacuum at idle. Does your brake booster leak? If you plug all the manifold ports does vacuum go up? Try it, and make sure all your rubber vac lines are not hardened, and they fit very tight.

Macadoo

It's more like 17Hg but I did notice the other day that my idle changed slightly when I hit the brake pedal

jpc647

The 17" HG is about normal with a stockish engine and cam. Mine idles around 18", so 1" could easily be gauge variation.

oldcutlass

The main thing to look for is that the gauge needle is steady.

distributorguy

2" lost with a steady needle could easily be a brake diaphragm leak. Leaks don't always pulsate unless they have a reason to surge. It could be as simple as the hose grommet on the booster being old. Or one of many short pieces of vac line being hard. Its most likely something simple.

Macadoo

I did a couple of tests on the brake booster and it seems fine. I think the reason my gauge reads a little low is the line to the gauge is a little stretched. In other words, my vacuum gauge has a vacuum leak, lol.

Macadoo

So how much mechanical would a mild 350 want?

distributorguy

As much as it will take. Experiment. You'll know when you go too far.

Macadoo

I'm sorry, I didn't ask that quite right. I guess I'm asking about the balance equation. Let's say I want 40* all in (not including vac), what percentage of that might I want as centrifugal in the distributor and at what all-in-RPM should I shoot for? Seems to me that if one were to set it at, say, 15* centrifugal, 25 initial, all in by 2500 RPM, it might be a jack rabbit off the line but a slug at cruise, and vice-versa. Finding the right balance between the two would be the key. Or, should some of the centrifugal be reserved for acceleration? Cruising at 70mph, I'm running about 2100 with the trans locked (200-4r), unlocking brings it to about 2300-2400. If all my centrifugal is used up at that point, I would have nothing left for acceleration, yes? Or am I thinking of it wrong?

distributorguy

If one setting gives you good bottom end performance and a different setting gives you better top end performance, then you use that data to have the distributor set up properly. All-in at 2500 is a bad idea. If you can make your engine ping, it WILL happen between 2700-3000 rpm.