Classic Mustangs List Archive

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Mail From: davet (email redacted)

Can anyone give me an idea of what is considered normal vacuum reading on a
302 engine with a high-lift cam and how much fluctuation is acceptable. I
am looking at a 67 fastback and the idle vacuum bounces from about 11 to 13.
If you rev the engine and let off it goes to about 20 as the engine slows.



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This read-only message was archived from a public mail list.
Mail From: Keven D. Coates (email redacted)


> From: "davet" <(email redacted)>, on 5/18/98 10:21 PM:
> Can anyone give me an idea of what is considered normal vacuum reading on a
> 302 engine with a high-lift cam and how much fluctuation is acceptable.

I have a vacuum gauge in my dash (where the clock was), and so I know vacuum in
all situations. My engine will run between 8 (when you first start the engine
with all the accessories running) and 15 (when it's good and warm). Usually it
will idle around 13. My cam is 226 degrees at .050 with .493 lift. Vacuum
varies drastically with camshaft type. Longer duration will usually give you
less vacuum at lower speeds. Normal 302 engines with stock (not
Hi-Po)camshafts might run 20 inches at idle.

I don't know what you're asking about acceptable fluctuation exactly. I don't
have much control over what vacuum my engine idles at, except to change the
timing and idle speed. To me, as long as it's idling and not dying, then it's
fine.

If you are concerned about wear, and using vacuum as a relative indication of
wear, then you'd be better off doing a leak down or compression test. This is
a much more accurate indicator that is not affected by carb settings, etc.

I hope this information is helpful. Good luck!

Best Regards,
Keven Coates
DSP Datacomm

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