Vacuum Advance - Ported or Manifold

JC2+2 · Poncho Master!

Vacuum Advance - Ported or Manifold

4SPEED427 · A Poncho Legend!

Always ported. I had no idea there were specs saying to use manifold vacuum on GM's, at least in that vintage. Interesting.

Teich · Addicted!

My 62, when had the oem 283 was manifold vacuum, the new 350 with Edelbrock also has manifold both started really nice. The 62 vacuum can has a limit a peice of rubber hose on the pullrod simple but works It is limited to about 8* . 8-10 initial advance works nice. I used the old 283 dist. in 350 works very good. Was the vacuum pot changed, if so may be wrong one. Laurie

GLHS60 · Poncho Master!

JC2+2 wrote:
According to '68 GM tune up specs all SBC's used manifold vacuum to the distributor vacuum advance.

Are you sure??

Thanks

Randy

68beaumont · Guru

307 in my 68 was manifold vacuum.

Mike Ward MB · Addicted!

I've always connected ported vacuum to the distributor with the understanding that advance is required under load, not at idle.

DANO65 · Poncho Master!

Manifold. Always have. It's what Dad has always said to do and he's been a mechanic for over 50 years.
Theses guys explain why better than I can.

www.superchevy.com/how-to/additional-tech/1601-everything-you-wanted-to-know-about-vacuum-advance-and-ignition-timing/

JC2+2 · Poncho Master!

GLHS60 wrote:
JC2+2 wrote:
According to '68 GM tune up specs all SBC's used manifold vacuum to the distributor vacuum advance.
Are you sure??

Thanks
Randy

Ok Randy. I'm not positive but with initial timing at 4 BTDC and remove vacuum to adjust, I figured got to be manifold vac. I just checked an old Canadian Service Data Book and it shows spec at 8 BTDC which would be ported. The day I had the old scope hooked up I tried different ways. The engine really labors at 4 to go ported. I ended up setting it at 10 with ported which is where it was to begin with. One day next spring I'll have you over and we'll hook the scope up and set it your manifold vac way and my ported way and see which performs better. And who buys coffee.

4SPEED427 · A Poncho Legend!

I'm curious to see how this pans out. Keep us posted please.

GLHS60 · Poncho Master!

It's not as simple as one is better than the other.

Especially when mismatched carbs and distributors are involved.

Camshafts and transmissions are also to be considered.

There are many combinations of vacuum advance "cans" that are activated at different vacuum levels.

Also, some "cans" advance much more than others.

Here is a link listing the specs of many point and HEI GM "cans"

http://outintheshop.com/faq/Vac%20Adv%20Spec.pdf (Pages 3 to 7)

In your case John you you had best results running ported as your "can" activated to much timing.

You proved stock camshafts are typically happy with "around" 10 deg initial.

Some Engines with "large" camshafts often run locked timing of over 30 deg. and start fine!!

If a stock Engine runs better with manifold vacuum it's often due to too little initial advance.

You proved that changing from 4 to 10 deg.

Vacuum advance is essentially a fuel economy device, some older HI-Perf Engines didn't even have one.

When you floor the gas, there is no vacuum advance in either case.

Thanks

Randy

-- Edited by GLHS60 on Saturday 28th of October 2017 03:31:50 AM

-- Edited by GLHS60 on Saturday 28th of October 2017 03:33:20 AM

-- Edited by GLHS60 on Saturday 28th of October 2017 03:34:38 AM

dualquadpete · Canadian Poncho Superstar!

I always go manifold!! Set the 409 at 12 degree initial, with a new vac. advance "can" [ old one seized from sitting] and got 30 deg. full advance, then found out 9 ers like 14 degrees, engine came "alive" and had 34 total!!!!! Mileage improved greatly, on Hwy runs driving it "conservatively "