Sidneyhop454

My understand is the VAC can on this distributor fully kicks in at 15"hg vacuum. I run 11 to 12 at idle. How should I set this distributor up timing-wise?

MelWff

the instructions for the distributor refer to the vacuum timing coming in during partial throttle driving not idle. So are you asking what to set your initial timing to? Using the instructions concerning the bushings and springs you want a total of 36, so black bushing would require an initial of 18 and the blue an initial of 15. Spring wise silver and blue appear to be the best combination.
https://www.jegs.com/InstallationIns...1/121-8572.pdf

stingr69

As a general rule, the vacuum can needs to be fully deployed at least 2" less than your idle vacuum if you want to run it hooked up to manifold vacuum. EXAMPLE: If you have 12" manifold vacuum at idle, you want a canister that is fully deployed at 10" of vacuum so that the timing will not dither around when idling.

You could also run a can set up to ported vacuum but can selection is not as critical.

Sidneyhop454

Thanks for the replies. So the options are to have the vac timing all in at idle or not engaged at all at idle?
Considering the 8572 kicks in fully at 15 lbs of manifold vac and I run 12 I should shoot for it to kick in after idle. Does anyone know how many lbs it starts to kick in at?
Also my understanding having a vac advance at idle will also improve vacuum. Will it improve 3lbs worth?

stingr69

Do no use that can hooked up to manifold vacuum on your setup or the idle RPM will become unstable. Feel free to try it and report back.

calwldlife

no
the idea is to have full vac adv at idle.
to assure this, it should take 2 inches LESS than
your engine puts out at idle.
so if you have 13, you need a can that moves all the way at 11in.
otherwise the vac will surge and cause the advance to change
and cause unstable rpm.

once you get the right vac can then you work on TOTAL
advance travel.
this is the amount of advance the vac adds.
so take initial, centrifugal, and vac travel to get whatever
degrees you want and the engine needs.

Sidneyhop454

Ok, running from manifold vac is out considering my vacuum.
I'll set the initial timing at 15-16° with the vac can plugged. Use the blue mechanical advance limiter bushing which is set to 21°, and connect to a ported vac source on the carb for the vac can for another 10° totaling 46-47° max timing advance. Sound about right?

stingr69

Start there and see how you like it.

calwldlife

think about it.

ported will solve the idle problems but how can
the problems go away when at speed the same thing applies?

you need solid workings by knowing what vac inches
move it and then solid amount of travel.

Sidneyhop454

so the timing will be erratic when at speed and vacuum drops below 15 is what you're saying?

Sky65

Change the vac can. That's what I did on my 8572. Take a look at this post.

https://www.corvetteforum.com/forums...m-advance.html

Tom

lars

The MSD distributors use the "B26" vacuum advance. This is a unit intended for a modest performance application, and is generally a good selection. In your case, simply replace the "B26" with a "B28" unit, with one of my advance limiters installed (e-mail me for info), and you'll have a great setup. The B28 is available through Rock Auto under Airtex part number 4V1053 for $11.41. This part number cross reference is outlined in my paper, available from me via e-mail request.

https://www.rockauto.com/en/parts/ai...m+advance,7232



Lars
V8FastCars@msn.com

Sidneyhop454

Looks like I'll be picking up a different can. Thanks for the specifics!
Interesting how MSD lists their vac advance of 10° all in at 15" but the B26 lists it 8° all in at 11-13".