mongoose87

Have a general question about using vacuum ports on the carb vs the intake:
I saw a show recently that showed the difference in air/fuel ratio between the different cylinders when using a single carb, and it got me thinking about the vacuum port on the intake that the transmission and accessories (wiper cowl, headlight doors, cabin HVAC) use.
I would think the intake vacuum port would create larger differences in A/f between cylinders.

I have to think that the ol' General designed it this way for a reason, but it would make sense to me to cap off the intake vacuum port and pull all vacuum from the carb.
Has anyone ever done this and tested cylinder to cylinder A/F ratios?

MelWff

It's a vacuum source not a vacuum leak, I feel it would make no difference.

Jebbysan

If it is not leaking then no change at all.
Cylinder to cylinder A/F differentiation is caused by a number of factors but the most obvious one on a small block is the terrible cast iron four barrel Q-Jet manifold.
On a Big Block.....cylinder to cylinder variance is usually cause by the Big Block's "good" and "bad" intake ports. The Left port is "bad" and the Right port is "good". But even so...a million race and street engines have been built using regular big block heads and terrible Q-Jet cast iron intakes and run just fine.
Checking A/F distribution between cylinders with a single carburetor is splitting hairs........

Jebby

mongoose87

So are aluminum intakes better? Is that why you never see an aftermarket cast iron intake?
Are you saying distribution is better with a square bore intake?
I thought the 4 barrel spread-bore intakes are supposed to be better compared to square bore when it comes to distribution.

HeadsU.P.

The Vac port off the back of the carb gets all eight cylinders helping out. Where as the Intakes Vac port fitting only gets cyl # 8 I believe.
Most of the vacuum needs come off the carb and only a couple off of the Intake.
The needs:
!. Dist.
2. HVAC
3. Headlites
4. PCV
Some Trans
Some wiper doors
Vapor canister
P.B.
Cruise Cntr?

First four are a must.
Last five are options.

mongoose87

Maybe I'm cutting myself too much slack, but when I put my mityvac to the main vacuum line feeding the headlight doors, wiper cowl, HVAC, etc. it doesn't hold the vacuum 100%.
My headlights do come up, my wiper cowl does go up and down all the way, and the HVAC line is capped.
I can imagine I'm not the only one running their "driver" quality cars this way, but in this case, it seems like it would be better to move the vacuum lines to the carb, no?

derekderek

All of those vacuum needs are virtually no actual vacuum flow. With the exception of headlights. And the tank recharges quickly. Now if you hit the headlight switch just as you are passing 5500 rpm in second gear you could create a lean condition at the worst possible time. Most everybody does headlights while sitting still at idle or worst rolling along in traffic at 25 mph.

HeadsU.P.

PCV and dizzy are constant flow. Might say vapor canister too. And what holds the heater doors open / closed?
Booster, if equipped, would have constant vacuum.

Jebbysan

Aluminum is 6/10ths the weight, distributes heat 7 times the rate of cast iron and is 10 times easier to port or modify/weld/cut.
I am saying the stock manifold is very poor compared to just about any aftermarket manifold made......(There are some stinkers out there though, like the ancient Edelbrock SP2P)....even some that GM made themselves.....any dual plane manifold is going to have 4 cylinders that are happier than the other 4.......has nothing to do with square vs. spread bore.....although you will almost never see a spread-bore single plane intake designed for perfomance.....for obvious reasons, just stare straight down it and it is no where near being equal.
If one is measuring cylinder to cylinder variance on a street engine.....they are bored.

Also......vacuum can be constant......but unless it is leaking it means squat.

Jebby

7T1vette

A vacuum system has very little flow of air thru the system.....IF it is sealed well and components don't leak. When the vacuum system sustains a leak, it WILL supply that amount of 'sucked in air' to the intake charge and affect the A/F mixture negatively. Not so with a vacuum system in good condition.

MelWff

The carb ports do not get their vacuum from the carburetor venturi's it comes from the holes on the bottom of carburetor which are exposed to manifold vacuum. Scroll to picture 12, front of carburetor base plate has all those openings exposed to manifold vacuum.
https://www.hemmings.com/blog/articl...the-quadrajet/