-7 psi at idle? - Page 2 - Fuelly Forums

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Old 11-10-2008, 09:27 AM   #11
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Remember that differing cam profiles can cause different amounts of idle vacuum. Pro Street cams can have enough overlap that at idle the intake manifold only has 7 inches of vacuum. Everything is fine, it's just an aggressive cam.

I'm sure a 650 turbo 4 has a slightly more aggressive cam than your 88 escort does. Could easily make for 17 vs 22.
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Old 11-10-2008, 11:45 AM   #12
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Keep in mind also that the turbo if spinning is forcing air into the manifold plus if you have short runners there is not much smoothing of the intake pulses and that can affect vacuum gauge accuracy and impact the amount of vacuum in the intake. Heck if the intake valve doesn't close at bdc then the air charge could be blowing back out the cylinder until the intake valve does close if the amount of air is small enough.
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Old 11-10-2008, 01:15 PM   #13
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Does it really (anti)matter at all?

Why I like diesels, Reason 18: No throttle plate to generate vacuum. When there is vacuum there is a dirty air filter.
Not that vacuum is bad, but it's too confusing to me. I mean if -7 psi is "a little low" (Erik), or "pretty low for idle" (kamesama980), isn't -9 or -10 psi lower yet? But it's described as an increase!
Yaahhhh! Increasing decrease! More vacuum is less pressure! Lower vacuum is higher pressure! (dizzy, dizzy, thud!)
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Old 11-10-2008, 06:56 PM   #14
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It's all relative . . . .







To atmospheric pressure if it is negative then it is less than atmospheric if it is positive then it is greater than atmospheric. How about a turbo diesel with 40 psi boost - friend has one in his boat. You should see the size of the turbo and the 6 inch exhaust pipe and yet it idles at about 0.17 gph . . . it's a really big 4 cylinder.

Higher vacuum at idle means that it doesn't need a lot of air and fuel to run . . . which is good.
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