I have a aggressive solid liter flat tapper cam. Unfortunately don’t know any details. 750 Holley on top of single plane tall ram intake. Set valve lash and idle vacuum is around 7”. Was higher when lash was loose (about 008 too loose). Seems like vacuum is too low but I cannot find any vacuum leaks. Does 7” seem ok?


David Finley #3659

Last edited by George P
Original Post

Seven inches of vacuum is normal for a big cam. My last engine would only idle at about five or six inches of vacuum at idle at 4100 feet above sea level. It would go up a bit as the altitude dropped, which is normal.  My new engine only idles at about eight or nine inches of vacuum with the idle set at 900 rpm. 

No real answer except a dyno test with a concurrent tuneup. Your combination certainly needs a power valve a number or so below the vacuum gauge reading, or it will always run rich (more so than one would expect running a race cam on the street). Power brakes suffer from such combinations, enough that there are electric vacuum pump systems sold to restore safe braking on the street.

In my experience, a "high-performance" aftermarket cam is generally going to give you around 14. A stock cam is going to give you 17 to 20.

That is basically necessary to operate accessories like power brakes and air conditioning.

The Pantera power brake booster will work down to about 14 or 15 inches at idle. You will notice the pedal getting a little harder even at that. At 12, you have lost your brake assist and the simplest solution is to go to a vacuum pump.

In my opinion a cam generating only 7 inches at idle is a full race cam and doesn't belong on a street car. There are other issues with it besides just low idle.

It has too much timing overlap. I'm not going to debate how it got to that combination. That is for a cam designer to explain.





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