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Old 09-09-2002, 06:21 PM
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Post Cam vs. Vacuum

Should one expect the amount of vacuum an engine produces at idle to be lower if it has a "big" cam?

I'm wondering how I can compare my vacuum readings to those in the Chilton manuals.

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Old 09-09-2002, 08:39 PM
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What's your reading? If it is running well, plugs are colored nicely, responds instantly to throttle application and generally runs strong your vacuum reading will give you an idea of the "size" of cam. Long duration cams will lead to lower readings with 10 in/Hg not unusual. Your actual reading is not that important, tune to get the highest you can.

Your driving a hot rod there son and your Chilton manual does not apply.
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Old 09-09-2002, 08:50 PM
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[quote]Your driving a hot rod there son and your Chilton manual does not apply.<hr></blockquote>

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Old 09-10-2002, 04:19 AM
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That's what I figured. I'm at about 10 in/Hg at "idle" (I have it set to idle at about 1,000 RPM). I thought maybe because I've got it set to idle higher, that would change the vacuum reading as well. (I set the idle higher by tightening the screw on the throttle arm - - more throttle, lower vacuum, I assume).

Thanks for the info. In addition to the Chilton, I've got several automotive mechanics "textbooks" (I don't want to bug you guys too much! ). These texts also give you these tables that say "vacuum at this, problem is likely this..." I was starting to worry a little... <img src="graemlins/pain.gif" border="0" alt="[pain]" />
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Old 09-10-2002, 05:29 AM
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Whenever you go to a cam that is "hotter" than stock, you are generally going to have more degrees of overlap. This plays havoc with vacuum readings, therefore you cannot use vacuum gauge to diagnose problems as with a stock engine.
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Old 09-10-2002, 10:57 AM
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You should still tune to get the highest vacuum reading you can at very low idle speeds. A good tuning trick I use all the time is to see how low I can maintain a very low idle speed. Try lowering your idle speed until the car just barely idles and adjust your idle mixture and timing at that point. You might be able to raise your total vacuum reading by up to 4-6 in/Hg. You might also find there is some power to be made by recurving your distributor to get the highest vacuum reading at every rpm. Detonation is your only restriction.

Vacuum is unimportant except for the fact that you want the highest possible at all rpm, this indicates efficiency. Tune for it.
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Old 09-10-2002, 04:39 PM
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I'm glad you mentioned knocking, 4Jaw - - I'm always worried about this!

I've got the distributor fairly far advanced - - to the point where the starter struggles a bit to start the car.

As far as I know, there is no knocking. I certainly can't hear anything that resembles a "knock" or a "ping". But I don't know if I've ever actually heard a knock on any car! (though I think my dad's '85 Corolla used to ping when I floored it! It was more of a "rattle" when I floored it at lower rpms) <img src="graemlins/spank.gif" border="0" alt="[spank]" />
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Old 09-10-2002, 07:00 PM
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Hi durration cams and associated long valve overlaps will produce low engine vacumn. You have already received good advice but remember hi- preformance profiles esp. racing cams will not preform well at low rpm.
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