Manifold vs port advance

[Matty]

Yes I am wanting ti start a fight. Ha jk.

I did the manifold vacuum today and it really brought my car to life performance wise. The driveability is so much better. I complete understand why this is the case now. I also have an adjustable vacuum canister also. Basically engines don't like to run at 12 degrees of timing. The only reason they set the timing that low is for ease of starting and it burns cleaner for emissions. They resorted to that for the convience of the everyday driver. Perfromance wise the car needs the timing advanced to operate properly.

What I experienced was basically the same as putting in lighter mechanical advance springs. My problem was that I had already tried just that and mine was advancing at idle giving inconsistent idle and performance. I couldn't find a combo on mine via weights and springs.

As for the running cooler I am not for sure.
I will say I wouldn't think this would work if you had a poorly tuned car or inconsistent vacuum. I have a .500 lift cam on a 112 Lsa.

[Comechero65]

No arguement just sharing info.

I would never run full manifold vac to the dist as i doubt I could get my engine to idle slow enough. Right now it idles at about 800 to 900 and that is with the throttle plates all the way closed.
How I set mine up I used the lightest springs I could find for the mechanical advance in a stock dist. that gives me about 21 deg of mechanical advance. Timing is set at 14 deg. That and 21 deg of advance yields about 35 deg mechanical advance and that peaks at about 2700 rpm. Add to that a max of about 12 - 14 deg vac advance makes about 47 - 49 deg total advance. For me runs fine and no ping ever.

This is in my 5.0 GT40 roller motor with explorer cam, 4100 1.08 carb.
Ron

[Rocket989]

Sometimes it's one heavy and one light spring and it depends on who made 'em!! Some engines run better w/o vacuum advance period (rare though from what I've seen on a lot of street motors). It's strictly a no to light load function for idle/ cruise quality and fuel economy anyway. If it works for you...run it!!

[Comechero65]

I used a set of springs for an HEI dist kit. Using the two lightest springs and adjusting the anchor points yielded the curve i wanted. Should the vac advance be too much I could shim it with washers to reduce the max amount.
Ron

[Matty]

I didn't have any issues getting mine to idle correctly. It actually starts and idle better now. It's idling at about 21 degrees and like that way better on cold and warm starts this far. I compelety agree on setting your mechanical first but I tried on mine and on my very cheap distributor it doesn't work. I tried mismatching springs also.

[Matty]

Base timing with vacuum unhooked is at 11 so it fires right of and immediately jumps to 21. Seems to like it!

[vicegrip]

Matty,you are correct that ported vacuum advance was an emmissions deal,keeping timing advance down at idle to raise combustion temps.Other advantages are that it stabilizes idle speeds better than manifold and helps cruising speed fuel economy somewhat.On the otherhand,manifold vacuum is more responsive to engine needs at the moment.My junk is all manifold because it mostly eliminates low speed pinging.If you have an aggressive cam,sometimes manifold vacuum varies causing an unstable idle.
Vacuum advance with either method is completely out of play at WOT,only initial and mechanical.Cruising timing can be as high as the middle 50's BTDC with either,depending on your setup.

Ron.if your throttle plates are completely closed at idle,I would say you must have a vacuum leak somewhere. It should choke out and die.Both springs being light,your mechanical is probably in play at 800,900 idle.I know your motor runs good but you should be able to set your idle anywhere you want with the cam you have.

[Comechero65]

Ron.if your throttle plates are completely closed at idle,I would say you must have a vacuum leak somewhere. It should choke out and die.Both springs being light,your mechanical is probably in play at 800,900 idle.I know your motor runs good but you should be able to set your idle anywhere you want with the cam you have.

They were using vac advance before there was any concern about emmisions.
At wide open throttle you will never have max vacuum but there still should be some. I don't drive at wide open throttle so not an issue. Vacuum and vacuum advance will peak at somewhere in the middle of the rpm range or at least somewhere before wide open throttle.

My throttle plates appear to be closed and that is my lowest idle. I have tested it for leaks and have found none. And no there is no mechanical advance happening at that rpm. Starts to increase just around 1000 rpm.

Have thought about delving into that more but it still runs good so not in a rush. Just enjoying it too much.
Ron

[poboyjo65]

Autolites are design to idle with the throttle plates slightly open, so they never close all the way.

[Comechero65]

Never had an autolite carb that wouldn't let the plates close which was new to me. don't have any leaks I can find but other than the high idle runs just fine.
Don't know why, maybe one day I'll swap carbs as a test to see if that changes. I have couple others I could try, just lazy I guess and in no hurry.
Ron

[Rocket989]

Very little...like less than 2, maybe 1-2" of vacuum at WOT or there's a restriction which is usually the carb being too small.

[redhotcomet]

+1 for manifold vacuum. I tried ported, but it didn't like it.

[poboyjo65]

Autolites are design to idle with the throttle plates slightly open, so they never close all the way.

maybe it isn't the primaries but the secondaries that are held slightly open.

[Comechero65]

That is a possibility i have thought of. there is an adjustment screw on the bottom of the carb is supposed to be set to slightly crack the secondary plate just slightly so the plate isn't vacuum locked and can open easily when called for. It's very touchy to set and i thought i had set it about right. but you never know, I might have opened it too much which could cause a vacuum leak. Would have pull the carb to check it.
Ron

[poboyjo65]

Also the throttle shafts & journals can be wore & are prone to, on these old carbs, which will cause vac leaks. but they can be re-bushed.

I think ported & manifold vac are just different tools on your belt. some motors like one,some like the other, -like vicegrip said, different cams like different ones & the way you have your timing set up or curved can be a factor as to which seems better. [JMO !!]