I just installed a vacuum gauge on my 68. I am running a rebuilt (600 mi) 396 .30+ 375 hp BB w/stock manifold and Holley 750. I am getting a consistant reading of 10hg on idle. Now, apparently 15 to 20 is normal. The question I have is that this motor runs sooo damn good, I hate to think it's in the valves or anywhere else. The vacuum uses the same manifold fitting as my power brakes. Help would be appreciated.

The vacuum reading you are getting is probably lower than stock because you put in some kind of a performance cam. What cam are you using? Vacuum is a function of the internals of the motor, usually dependent upon the cam. Cams with more overlap tend to 'bleed' off more vacuum and therefore your vacuum readings will be lower. The 15 to 20 you say is normal is just an average range for a stock motor with a wimpy cam designed for comfort and economy versus performance. Does your motor seem to have more 'lump' to it now at idle? If so, that is again due to the more performance oriented cam. There is no problem with a reading of 10, you just might find that any accessories that are vacuum operated don't work quite as good. You mentioned the power brakes, and that is a common problem when switching to a better cam. If you like the motor like it is, you might consider either adding an electric vacuum pump for the brakes, or switch to manual brakes. Sounds like a pretty hot setup, but not having brakes is BAD! Power brakes without the 'power boost' part are really poor at stopping so be careful and fix it soon. www.mpbrakes.com (http://www.mpbrakes.com) has the electric vacuum pumps if you want to go that route.

Bill C.

Thanks Bill, now tht you mention it, after the rebuild (and the performance cam) my brakes did feel a little less resistant. They work fine now, and stop the car just fine. It does have some lope to it, even at operating temperture. I guess I'll just have ti live with it and use the 10hg as my guide in the future.

To add to the post from BC which is right on, you could also have a simple problem of incorrect ignition timing. From the sounds of it, this isn't likely the problem but it might help a little. Try adjusting the timing with the vac gauge on it and see if you can get it higher by changing the timing.

I agree with the rest that it's probably because of the cam maybe timing.If you want some good info on reading vacum gauges check this out http://www.centuryperformance.com/vacuum.htm

Ever heard a factory 396/375 idle? Had a pretty nasty solid-lifter cam in it to begin with; I wouldn't worry about it.

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JohnZ
'69 Z28 Fathom Green