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Old 03-07-2011, 07:26 AM
mw85gt mw85gt is offline
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Join Date: Feb 2008
Posts: 111
Question Vacuum in intake manifold

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?
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