Vacuum Problem

1172 Views 4 Replies 3 Participants Last post by cdahl383, Sep 16, 2012

chui1980

Discussion Starter · Sep 15, 2012

To my understanding the bigger, loppy cam you get the more vacuum you loose. The more vacuum you loose the more you compromise tranny shifting and distributor advance.
What would be the only option to increase vacuum without removing and replacing the cam.
I heard by installing a vacuum canister.

I just need to know before installing the new 350 engine Iam building.
Right now my 307 is at 12psi at idle and 7psi in gear. To me that is way to low.

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cdahl383

· #2 · Sep 15, 2012

Those vacuum readings seem like they're pointing to a vacuum leak somewhere unless you have a ridiculously lopey cam or something. I'm running a 234/238 @ .050" 112 lsa cam and I get 16" in park and 10-11" in gear at 800 RPM.

What kind of cam are you running? Is this engine stock or did you modify it? If its stock you should be seeing over 20" or so I'd imagine.

Common areas for vacuum leaks that cause big drops like that are at the intake manifold or around the carb base gasket. You can spray carb cleaner around the area while running to see if idle RPM rises or not.

Let us know some specs on your motor. Those vacuum readings could be right for a highly modified engine with a cam with a lot of duration, but if its stock or close to stock I'd suspect a vacuum leak somewhere.

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sbc1320

· #3 · Sep 15, 2012

cdahl383 said:
Those vacuum readings seem like they're pointing to a vacuum leak somewhere unless you have a ridiculously lopey cam or something. I'm running a 234/238 @ .050" 112 lsa cam and I get 16" in park and 10-11" in gear at 800 RPM.

What kind of cam are you running? Is this engine stock or did you modify it? If its stock you should be seeing over 20" or so I'd imagine.

Common areas for vacuum leaks that cause big drops like that are at the intake manifold or around the carb base gasket. You can spray carb cleaner around the area while running to see if idle RPM rises or not.

Let us know some specs on your motor. Those vacuum readings could be right for a highly modified engine with a cam with a lot of duration, but if its stock or close to stock I'd suspect a vacuum leak somewhere.

I think he has a Thumper Cam - Comp Cams?

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chui1980

Discussion Starter · #4 · Sep 16, 2012

Yes it is motha thumper cam from comp cams.

I think it is a Model #12-600-4

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cdahl383

· #5 · Sep 16, 2012

Looks like its 227/241 @ .050" w/ 107 lsa. Is this in your 307 or 350? You have a 307 in the car right now but are looking to drop in a 350 right?

Thats some pretty good duration for a 307. My cam is around that with duration but has a 112 lsa. Between the different lsa and more cubes that I have it would produce more vacuum than in a 307 I would think.

That could be right for vacuum numbers with that cam. It still seems like you could get a little more though. I didnt see any vacuum numbers listed with that cam. My cam states I should be able to produce at least 12" of vacuum at idle which I'm close to that.

What kind of timing settings are you running? The more initial you run the more vacuum you're engine will produce. With a cam like that I would think you could run at least 14-18 initial and keep your total around 34-36. If you're only running 8-10 initial then you'll definitely have some crummy vacuum readings.

Make sure you are running as much initial as you can without having starting issues or going way over on your total timing and then see what kind of vacuum readings you produce.

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