Originally Posted by Gregski (Post 7610713)

Thank you for that, however let me share with you an interesting observation, one that I have validated with a number of tests. The point that I will be making is that it is not that clear cut, Base Timing + Mechanical Timing, or in other words so far I have not seen such a thing as pure Base Timing, allow me to explain

It appears that Initial Timing (what you call Base Timing) always includes some Mechanical Timining aka advance. In my case my original Mechanical Advance Curve starts at around 800 RPM, so if my idle RPM is 850 I am already relying on mechanical advance in the initial 16*. This could explain why in my original configuration 16 Initial plus 24 Mechanical = 32* instead of 40* (that's because the initial 16 already has some of the 24 mechanical in it) hope I didn't just confuse the hiebie jibies out of somebody

If you look at the curve for the new weights, (pic above in the thread) it starts even earlier at 600 RPM, so for sure at 850 RPM idle we are using the mechanical advance. Now what muddies the waters a bit is crappy aftermarket inaccurate parts. Which I blame for 10 initial plus 8 mechanical = 18 (which really shows that their curve does not start at 600 RPM, lol)

Now looking at the GM HEI distributor part number 93440806 which I will be purchasing tomorrow (aka Pay Day) it's Mechanical Advance curve don't start till 1600 RPM - so we shall see how that all adds up

Originally Posted by hatzie (Post 7610908)

Advance can be adversely affected by one more setting that's usually ignored...
What's the end play on the distributor gear. http://www.chevytalk.org/fusionbb/showpost.php?post/1257140/Measuring HEI gear end play.
It should be 0.010" +0.004" - 0.000" but most are set up way too loose. Every 0.0128" is 1° of advance as the gear lifts so the closer you can get to 0.010" the better.

It's cheap and easy to fix...

Originally Posted by rich weyand (Post 7611045)

To answer some earlier questions. I am running 16-17 base timing. That is, timing without the vacuum advance hooked up and with the idle slowed to about 600 rpm.

The GM HEI adds 20* mechanical advance, so I am running 36-37 degrees total timing.

The AR23 vacuum advance adds another 15* (spec is 7.5*, which is camshaft degrees), to make 51* all in.

51* is what the advance is at cruise, 2500+ rpm, with the throttle plate mostly closed (not accelerating), so the mechanical and vacuum advance are both in.

I have seen 52* given as an absolute maximum for cruise "all in" timing.

Check this: http://67-72chevytrucks.com/vboard/s...d.php?t=689321

Originally Posted by Gregski (Post 7611539)

thanks I just checked mine and its at .065 and that's with one shim already in there

I also checked my old original OEM points distributor and it has about the same at .062

keep in mind a gasket will take up some of that space, I think at least half

Originally Posted by Gregski (Post 7611540)

thanks my original setup was very similar to yours, now as I am getting more comfortable with this stuff I am just trying to tune it in a bit more precisely, can you tell me at what RPM your mechanical advance starts ie kicks in please

Originally Posted by Gregski (Post 7613366)

by the way do any of youz know if the cad plated "gold" bolts are better than stainless in terms of rust resistance?

Originally Posted by rich weyand (Post 7613485)

Stainless steel is more electropositive than carbon steel. You will get bimetallic corrosion. That is to say, the stainless steel will not rust, but it will promote and accelerate rust in carbon steel to which it is attached, compared to using carbon steel to attach to carbon steel.

Grade 8 hardware has 50% more tensile strength than stainless steel hardware. Grade 5 hardware has 20% more tensile strength than stainless steel hardware. No comparison.

Stainless steel hardware sounds good at first blush, but you probably don't want to go there.

Originally Posted by rich weyand (Post 7613485)

Stainless steel is more electropositive than carbon steel. You will get bimetallic corrosion. That is to say, the stainless steel will not rust, but it will promote and accelerate rust in carbon steel to which it is attached, compared to using carbon steel to attach to carbon steel.

Grade 8 hardware has 50% more tensile strength than stainless steel hardware. Grade 5 hardware has 20% more tensile strength than stainless steel hardware. No comparison.

Stainless steel hardware sounds good at first blush, but you probably don't want to go there.

Originally Posted by enaberif (Post 7614268)

This is why when using stainless hardware in places that isn't stainless you should use a anti-size coating of some sort to prevent seizing or corrosion.

Originally Posted by Gregski (Post 7619543)

... an inline fuel pressure gauge ... aint much use,