Vacuum in intake manifold

[mw85gt]

Had a new motor built over the winter. My motor builder tells me he installed an "agressive" cam. I've been anticipating that this cam wouldn't build much vacuum for my brake booster, so I hooked up a vacuum gage to the intake manifold and recorded the readings during idle and (no load) throttle transients in the garage.

As I expected, at idle, (approx 1200 RPM), the gage reads about -12psi (vacuum). Here's where the story gets interesting.........As I bump the throttle, the vacuum goes up to about -25 psi? I repeated this about 10 times with the same result.

I always thought that opening the throttle let in more atmospheric pressure and lowered intake manifold vacuum readings? Anyone care to enlighten me?

Originally Posted by mw85gt

Had a new motor built over the winter. My motor builder tells me he installed an "agressive" cam. I've been anticipating that this cam wouldn't build much vacuum for my brake booster, so I hooked up a vacuum gage to the intake manifold and recorded the readings during idle and (no load) throttle transients in the garage.

As I expected, at idle, (approx 1200 RPM), the gage reads about -12psi (vacuum). Here's where the story gets interesting.........As I bump the throttle, the vacuum goes up to about -25 psi? I repeated this about 10 times with the same result.

I always thought that opening the throttle let in more atmospheric pressure and lowered intake manifold vacuum readings? Anyone care to enlighten me?

You are correct about it dropping under load and hard throttle. However, you are just blipping the throttle which in your case is creating a little more vacuum than you had a idle because of the large cam. If you had a gage on the car while driving it.......it would still drop under hard accelleration. Depending on your brake setup, you might consider a vac canister

[jimwheeler]

I have a brand new vac canister that I used once, before going to the Willwood set up. It worked great and did away with the hard pedal, at the end of the long straights at RA. I'll let it go cheap.
wheel

[kbsmith1]

I think what you are seeing by blipping the throttle is the following:

Quick open of the throttle dumps gas and air into engine, this gives
a very slight decrease in vacuum (it's not like throttle was
wide open for long). Engine speeds up.
Now you shut throttle plate, but engine is still running say 2500 rpm,
so now you get a surge in vacuum due to engine running 2500 rpm
against closed throttle. As engine slows down due to closed throttle,
vacuum drops gradually down to where it was at idle.

[jimwheeler]

I had the perfect vacuum gauge on the first car I owned, a 1949 Plymouth, Special Deluxe. When you floored the throttle one thing happened, and it wasn't acceleration, the windshield wipers quit wiping. Needless to say, I was always driving around blind in the rain, since I never lifted.
wheel

[kbsmith1]

I had a couple of cars like that. One a 1968 343 4spd javelin,
and one a 1970 304 3spd javelin. Nothing like trying to pass
on a 2 lane highway in the rain, step on it, and no wipers .