Thread: low engine vacuum at idle?

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my motor sits at about 11 inches in Park, and 8 in Drive.

What are the main causes for low vacuum? I think I put my cam in properly, but is it possible to get low vacuum if the cam was advanced when it was put in?

I'm getting pretty close to almost tearing my whole motor down again and redoing it all just to be sure..

Thanks.

The intake closing point on the cam is what determines the amount of charge that is captured as the piston ascends on the compression stroke. It is also what governs manifold vacuum. If you close the valve too late, the piston pushes some of the charge back up the inlet tract, disrupting vacuum. Some folks think it is governed by the overlap, but I have been told by cam grinders that there is little or no positive pressure above the piston to be pushed back up the tract at that point in the cycle.

Bottom line: too much duration to give good vacuum (late intake closing point).

If you're runnin' power brakes, you may need a vacuum accumulator or a separate electrically-operated vacuum pump. The brakes probably won't operate properly with that small amount of vacuum.

brakes seem to work fine...

and the cam, is a very mild Comp Cam XE262, I didn't think this small of cam would have any negative issues?

I know of many people running XE268s and I've never heard of any issues with that cam, and mine is one step smaller...

well, since you didn't tell me the kind of car, kind of engine, static compression ratio, kind and grind of cam, whether or not it was degreed when you put it in, where you placed the centerline or just about anything else I could have used to tender an intelligent answer, I just took a shot in the dark.

Have you compression tested the motor?

Have you checked for all valves on their seats? None adjusted to tight?

83' Regal
TH350 with stock stall
3.73 rear gears with Eaton Posi

-GM Crate 350 (8.5:1 compression)
-Hedman Shortie Headers
-Comp Cam XE262 which as far as I know wasn't advanced when it was put in
-Weiand Pro Street 144ci Supercharger
-Holley 4150 700cfm Supercharger carb

I posted in the SBC forum but here seems like a better stop, it was said that the S/C would take away some of the manifold vacuum, but how much I don't know. Could it just be a lack of tuning?

Sorry for not giving you all the info from the beginning.

SUPERCHARGER????????

Well, I'm sure glad you shared THAT with me.

The job of the blower is to pressurize the manifold, so it's only natural that you'll have less manifold vacuum at idle than a naturally aspirated motor. Hook a dual-function vacuum/pressure gauge into the manifold and you'll be able to see it go from vacuum to pressure as you come up on the blower.

Try plugging your vacuum gauge into a source below the carburetor and above the blower.

You should have enough vacuum there to operate power brakes.

i have pretty much the same set up with the 350 ho motor. but i chenged the heads to dart iron eagle's. and i to have low vacuum about 11 inch pounds. the gauge is reading from below the blower. the power brakes dont work good. should i run a canister? this setup is on a 72 suburban 3/4 ton.

Originally posted by severedties
i have pretty much the same set up with the 350 ho motor. but i chenged the heads to dart iron eagle's. and i to have low vacuum about 11 inch pounds. the gauge is reading from below the blower. the power brakes dont work good. should i run a canister? this setup is on a 72 suburban 3/4 ton.

when you are idling with a blown motor you should have vac. in the manifold. most boost gauges are boost and vac. gauges, so you can see both vac. and boost. you cant run nothing off of the intake like brakes, because of the boost. the vac. should be about the same thing above or below the blower when the motor is idling. sounds like to me you have a vac. leak.

i have check for vac leaks and nothing. the gauge is a boost/vac. the vac line to the booster is on the base plate of the carb on the back.

shouldn't be showing no boost up there. the boost should be only on the bottom of the blower. i'm not sure if you ever get a good vac reading with the blower, you are having to pull a vac. on the complete blower, and thats a lot of sq.ins. if your brakes are working ok i probably wouldn't worry about it. if it idles ok and have good brakes, you have enough vac. i ran stock cams a lot of the times and i believe the vac was always low, around 13-14" do you show any boost at the carb?