vacuum w/ 2bbl carb change

[Anonymous]

I am wondering if there is a vacuum decrease in the intake manifold if I change to a 2bbl or 4bbl carb in the 170 that I have. It only stands to reason that with more cfm's with the same displacement would mean a drop in vacuum.
I use a vacuum wiper system and any drop in vacuum translates into even slower working wipers. I would like to keep the vacuum system as it is original. I get about 18 inches of vacuum with the single barrel carb now.

 

[Anonymous]

It only stands to reason that with more cfm's with the same displacement would mean a drop in vacuum.

Stop standing....your reasoning is all false.

CFM is measured at FULL THROTTLE, not idle.
Your vacuum won't change at all.

 

[rust collector]

If you keep out of the throttle, vacuum shouldn´t change...
How about going to a junkyard and pick up a diesel alternator?
Some of those have a vacuum pump on them?

 

[Anonymous]

Another cheapo junkyard gem is a vacuum cannister/reservoir. They come in all shapes and sizes. You may need a check valve with some...others will have a check valve built in.

That way, your wipers will still work when you floor it to get onto the freeway!

 

[fordconvert]

Early 80's cadillacs with 4100's had a belt driven vac pump, may have even had a chrome cover on it IIRC. Otherwise like others said look for diesels. If you want to try and hide it look at the high performance places many of them sell electric pumps to run things like brakes with engines with those 1500 rpm idle cams.

 

[66 Fastback]

As Linc mentioned, the vacuum that your engine develops at idle will not change. But as far adrivability, if you are over-carbureted, once you open the throttle plates rapidly, you will have huge bog and the engine will be sluggish untill rpm's build up enough to start generating some vacuum. That is why if you go to a 4 bbl or multi barrel setup, it is advisable to use a progressive throttle linkage that will not open all of the bbl's at once.
Doug

 

[Anonymous]

But as far adrivability, if you are over-carbureted, once you open the throttle plates rapidly, you will have huge bog and the engine will be sluggish untill rpm's build up enough to start generating some vacuum.

also why holley 500 2-bbl's come with a 50cc accelerator pump!