This was a chassis dyno.

My mechanical is only throwing out ~ 13 degrees maximum. The motor made
the best peak power with 37 full advance. We tried 36 and 38 with an advance
timing light (no tape needed).

When we compared the baseline curve from 2000 to 5000, the motor made
the best curve when the timing was set to advance from 1200 to 3000.

I only had 3 sets of springs. We ended up mixing a mid-weight with a light
weight spring to get the best numbers.

This may not be an optimum curve, but it's the best I can do with an OEM
distributor.

Is there something I'm missing with the mechanical advance? You seem to
think I'm able to grab more advance from this circuit? Any tricks to getting
more so that I can tune down the base timing?

 

My distributor is factory 1980 Z28, as well. I've since changed out springs, centerplate and weights, to get what I'm looking for. If anything, the typical 1980 stuff had too much mechanical advance, forcing a really low initial setting.

What are the numbers on the center plate and weights in the distributor? 13* is DEFINITELY not stock 1980 stuff. I trust that everything moves freely and easily? Rotor snaps right back when you grab it and turn/release?

If you get a chance, plot out your current curve. Get someone to help you, and use the timing light to plot the curve, every 250rpm from idle to 3000 or so (wherever it's fully advanced). Do this with the vacuum disconnected, of course.

 

I beleive he said he had installed the Moroso or Mr. Gasket stuff in the centrifugal advance mechanism. I have used that kit before and I don't like it. I play mix-n-match with stock GM weights and centerplates to get what I want, just like Todd. However, I have found that the 20* of centrifugal advance that many/most stock HEI centerplates/weights gives is just about perfect, if matched with the right springs. My timing curves usually look something like this:

16* intial + 20* centrifugal (all in by 2800) = 36* total.

Obviously, within about 1* of what you want.

I also agree with Todd about the vacuum advance. Stock cans give way WAY WAY WAY too much once you have the initial/centrifugal dialed in for best power. You can either modify yours to limit it to about 12* maximum advance (requires some welding and some trial-and-error) or you can buy a Crane adjustable vacuum advance canister. It comes with a lock-out cam that can limit the maximum amount of vacuum advance and it's range is enough that you can get it down to about 12* maximum if you limit it as far as it will go. Then just play with the RATE it comes in (how much vacuum it requires to bring it all in) and see what works best for your combo- the Crane unit is adjustable for both ammount and rate.

37* centrifugal & initial + 12* vacuum advance = 49* max advance.

Most small blocks will take about 50* maximum advance under light throttle/light load conditions, jsut to get you in the ballpark. That's why vacuum advance was invented in the first place.

 

Reading comprehension owns me these days.

In general, the aftermarket curve kits are teh suxorz, as the kids like to say. The weights aren't very well done, fit and finish is weak, and they often bind. I stick with the mix and match factory stuff (in fact, Damon was one person that I went to for plate numbers).

 

Some good info on vacuum advance and vacuum cans to better suit your combo.


http://thirdgen.org/techbb2/showthre...hreadid=263250

 

Using a frequency counter on the MSD tach output and my cheapo $100.00
advance light, here is my curve (engine no load, mechanical advance + initial of 24 degrees):

_900 RPM: 24
1500 RPM: 26
2000 RPM: 29
2500 RPM: 34
3000 RPM: 37
3500 RPM: 37