Iron_dog

Hello everybody,
I would like to know which are the correct specs for the vacuum advance can for my stock L48 350. I've already recurved stock distributor as Lars paper. I've also removed smog stuff... but no cam job. For specs I mean "Start @ ""Hg" and "Max advance".

thank you

hugie82

I think it's 12Hg vac. For full advance. Did you get the adjustable vac advance canister?

MelWff

Lars has a paper on the correct vacuum advance, email him.

Torkman

http://www.lbfun.com/warehouse/tech_...ance_Specs.pdf

Iron_dog

Thank you guys I already have Lars paper but I can't find the right specs for my application... also I'm thinking about buying an adjustable one, any advice? thank you

Torkman

What exact specs are you looking for? Provide both the distributor advance or the crank advance (crank is twice the distro advance).

Iron_dog

In Lars PDF there are many cans listed, I would like to understand which is the best for my application. Distributor mech advance is 18 @idle and 36@ from 2500 RPM. Thanks

Torkman

So, if I understand you right, your initial "set" timing is 18 (assuming vac advance disconnected) and by 2500 you are all in at 36. If so, that is very good. That means your centrifugal advance is 18 and the springs are working about right to get to 36 by 2500. So, if you want to get to around 48-50 of total advance (adding in the vacuum advance), you will need another 12-14 advance from the vac can. When you look at the specs Lars has on the paper, look at the HEI section (assuming you have an HEI and not a points system) and look at the max adv number. You will need to double that number in that it is dist advance not crank advance. So, if you need 12-14, look at the AR12, AR15, or AR23. The AR12 I have had trouble finding, but the 15 and 23 can be ordeerd from any parts store (use the VC part number). Only real diff between the 15 and 23 is the start time...I matched mine to what was originally on the motor and put on the 15. (I had an AR10 but it was pushing adv of 19 which gave me a mechanical of 36 + 19 vac = 55 which is too much. Now with the 15, I have 36 + 13 = 49...just about right. These cans are only about 15 bucks...get both and install one at a time (10 minute job) and see what can provides you with the best numbers and driving performance. Remember, as Lars said, this is fine tuning only and does not offer a ton of increase, but if you go too far with the timing, it can really degrade performance.

Lars, chime in here if any of this sounds wrong.

Iron_dog

Thank you for the long answer full of info! I have a points (converted to Pertronix III) distributor, how does the situation changes? thanks

gcusmano74

Your '74 will have a old style vacuum canister. The HEI systems were not installed on Corvettes 'till the 1975 model year.

Iron_dog

Understood, so which "old style" can is good for my setup? thanks

Peterbuilt

If your stock engine produces 18 Hg at idle then a VC1802 is the can you need.

Torkman

16 vac adv is about the least he can do with the available vac cans. This would put him up around 52 which might be too high. I would try the 1802 and road test it to see if you get any pinging. Back off in 2 degree increments until she stops pinging and see what you have. You will be very close to top performance. Remember to see what the vac disconnected at idle initial timing is once you are all done so you know what to set it to for during future tuneups.

Iron_dog

I've about 22 Hg at idle!

lars

You probably don't have 22 inches in "drive." A good, general performance application unit, used by MSD on all their distributors for mild performance engines, is the B26 (VC1765). This will assure that the vacuum is fully pulled in at idle in "drive." The curve, though, is a little but long, with most of these units pulling 17 degrees of vacuum advance. To limit this, go to a hobby store and buy a piece of brass tubing with an inner diameter matching the diameter of the vacuum advance connector rod. Cut the tubing to about a 1/4" section and press it onto the rod end. This will slightly reduce the "stroke" of the vacuum advance to get it down into the 14-degree range without needing to weld the slot. This will be a perfect combo for your application, and it will run very well.

Lars

Iron_dog

Lars thank you! is this the same if I have 4 spd?

Iron_dog

I realized I've a B26... is it ok???