Advance Weights

[Larry M.]

I purchaed a Mr. Gasket spring and advance weight kit (for a non-HEI distributor) to get the lightweight springs. Is there a benefit to installing the Mr. Gasket weights? I know I could call and ask Holley customer service, but I suspect they would just read to me from a pre-printed list of talking points, and I wanted to hear from anyone with actual experience.

Thanks,
Larry

[Duke W.]

Re: Advance Weights

It would help if you would specify the OE engine option and whether or not is has been modified, especially with a different camshaft.

Specify the OE spark advance map - centrifugal and vacuum advance specs.

If the dist. still has OE weights, use them. Aftermarket weights are often not hardened and the pivot holes will wear.

The Mr. Gasket 928G spring kit has three spring sets and one brass bushing. That's what you should have bought, especially if the OE weights are still installed.

Duke

[Larry M.]

Re: Advance Weights

Duke,

Thanks; the car is a 1972 LT-1 with an L82 camshaft; AIR and TCS removed. I'll be working on the map on the next few weeks, and will post the results.

The weights, as far as I know, are original. The 928G kit is springs and bushing only; I bought the 927G kit, which appears to be identical, but with the weights included. A NA PA VC1765 is installed; I see 16" of vacuum at 800 rpm.

Larry

[Richard G.]

Re: Advance Weights

This is a project that is much easier on a distributor machine.
I have my doubts there is one in my metro area which is quite small <50K.
My good friends dad had a gas station and when they closed it down to make a parking lot for a bank they couldn't sell the distributor machine they junked it. Even if I could have saved it it likely wouldn't get much use. All my friends ,with old cars, have updated the distributors to modern works.
Rick

[Thomas S.]

Re: Advance Weights

Here's some info on the MR. Gasket set that I compiled using a digital tension gauge.
The springs on my 63 were obviously not correct. I was only getting + 18 degrees from the weights instead of approx. +28. The spring end loop centerlines were way out of spec so the weight were sloppy and affecting the initial timing at just about any rpm.

[Richard G.]

Re: Advance Weights

Tom;
I started to do the same spring measuring. I never came up with a standard length to test them at. Yes, they all seemed somewhat loose as the length was just slightly longer than the originals. I expected the springs to be a little weaker. They were a lot weaker. Did you measure the force of the stock/original springs? I suspect it would be possible to end up with some initial advance, at idle, because of the extra spring length. I was going to tie the weights closed and start the car. Then check the timing to see if it's different, but have never got to it. Working over the paint isn't fun. Likely will not go there till I can get access to a distributor machine.

[Thomas S.]

Re: Advance Weights

I deal with miniature springs in some of the electro-mechanical parts we manufacture. I'm by no means an expert on this topic. As a general, more coils will allow a greater stretch distance. If you exceed the design length the spring will loose its tension and simply stretch. So what I did initially was to perform the tension test up to the point of failure on one of the springs. Once I knew that length, the subsequent tests were done short of that point of failure. That way they all would all go back to the same same starting point.

They were all measured to the same length and the resulting tension is what appeared in my chart. I didn't have a set of stock springs to work with, only the ones that were on the existing distributor that were clearly not correct.

As you already know there is no substitute for an off-vehicle testing device to bench-mark the distributor. There was a great article and plans for a relatively inexpensive home-brewed tester that used a fan blower as the turning device. It was in a recent issue of the Restorer magazine in case you didn't see it.

I have a 63 327/340 4;11, original cam and my motivation was to try an get a better idle at a reasonable RPM and to optimize the advance curve. I use 93-octane fuel. I've spoken with Duke on several occasions since he is the "go-to" or at least one of the best, on these issues. I believe at some point the advance springs were replaced to eliminate detonation because with those springs in place and the vac-can plugged, I was maxed at at around 28-degrees BTC (3,500 RPM) with an initial of 10 degrees. That's about 10 degrees short of what it should be. When I changed to springs to allow the full mechanical advance I got detonation under any type of load. The vac can (B28) should give roughly 16 degrees of additional advance and would be all in at 8". With the 4:11, I'm getting good vacuum pretty much all the time.

As it turns out, after measuring the gap between cylinder heads and block, I have just under .029". The correct setup should have been 2 x .018 (.036) which would correspond to the doubling up of gaskets on this motor which was done by Chevy to minimize detonation. This reduced the original 11:25 compression to 10:5. So I'm somewhere in-between which I believe is causing me to have detonation at points in the rpm that I shouldn't. That's likely why the heavy springs were put on by some previous owner.

There are so many variables at play. Even if you have mapped a good curve off the car, and have the correct vac-can; compression, fuel octane, altitude, temperature, etc. will all effect the final outcome. I'll be the first to admit that, in the unrealistic quest for "perfection" we tend to forget that it's impossible to achieve. What I know for sure is that in my case, if I go above 32 degrees total mechanical advance, I will get detonation.

[Richard G.]

Re: Advance Weights

With my current setup I get only mild detonation. However my 340HP was rebuild using a aftermarket composition gasket not the steel ones like original. Wish I could remember if originally my 340hp had one or two gaskets. It was surprising to me an extra steel gasket was worth .5 point of compression. I used a Felpro which was likely a .041 compression thickness. It likely dropped my compression ratio by a full point. Pistons are the original ones. I live at 20 feet above sea level. Still sorting out some issues on my motor so its not the final word by any means. Still more work to do. However, I have replaced the vacuum canister and it idles smoothly and as solid as a rock. I have to thank Duke for that.
Rick

[Duke W.]

Re: Advance Weights

It was surprising to me an extra steel gasket was worth .5 point of compression.

That's most folks impression if they haven't played around with a compression ratio calculator. Since you're dividing a relatively large number by a relatively small number, a small change in the denominator can make a significant difference in the outcome. That's why it's a good idea to play around with the compression ratio calculator in the compression ratio paper co-authored by me and John McRae, especially if you're contemplating a rebuild, to get a feel for the sensitivity of the calculation.

IIRC, on a small block increasing the head gasket thickness by about .010" reduces CR by about 0.3 and 1 cc head chamber volume is about 0.1 or so.

The onset of detonation can be very sudden, and as little as backing off total WOT advance a couple of degrees can quell it, but if you have to go below 36 then the spark advance map is sub-optimal, and the only things left are blending in higher octane fuel with the highest available pump premium or pull the heads and reinstall with a thicker head gasket.

That's why my CR recommendations are a bit conservative. You're better off not going too aggressive on CR so you can run and aggressive spark advance map.

Duke