What Engine Vaccuum should I have?

Shelbytexas · #1 (**permalink**) 06-21-2005, 03:23 PM

GT500 4 speed with KC 427 w/ 428 Crank; .575 lift, 290 duration Hyd Roller. Makes about 550 HP at the Crank, 450 on the Dyno. Engine produces only 10 lbs of vaccuum at idle (idles nicely at 800 rpm). I put a vaccuum canister on it with no improvement. A few friends figure Vaccuum should be higher. Any ideas? Brakes fade badly at low engine speeds.
Thanks for any advice!

Excaliber · #2 (**permalink**) 06-21-2005, 08:34 PM

10 to 11" of vacuum is about all your going to get at idle AND cruise rpm with those specs. It will go lower when you step on it, higher when you lift off on it.

Vacuum reservoir cannister HOOKED UP CORRECTLY should help. The larger it is the more vacuum it will hold, but it will still only be 10 to 11" and that may not be enough for your brakes. Thats why some "hot rodders" use "juice brakes". Hydraulic motor to pressurize the brake system, Buick is well known for it.

Kicking up the idle speed and BASE TIMING rpm will also help. I wouldn't expect it to ever be much better than 12, 13" MAX!

Coloradocobra · #3 (**permalink**) 06-22-2005, 01:48 AM

Do you have a check-valve in-line with the line to your tank/booster and is it installed in the right direction? This helps store vacuum at higher levels, during decceleration, for example, so that you have it available at idle. I run a similar cam and usually have no problem unless I have to use the brakes several times in a row at low idle.

I was told to keep the idle up at 1000 to 1100 to "keep oil up on the cam" so you may want to run a little faster idle, which will probably also give you a little more vacuum.

Low idle speeds are a can and should. You can idle it at 800 but you probably shouldn't. Of course, others will dispute this so take it for what its worth. Of course, now that I re-read, I see that you have a hyd roller, which eliminates the need for oil as opposed to a flat-tappet cam. The higher idle will still help with vacuum, though.

Gary

Shelbytexas · #4 (**permalink**) 06-24-2005, 03:23 PM

Thanks, both of you!

SCOBRAC · #5 (**permalink**) 06-24-2005, 05:54 PM

290 duration and you have 10" of vacuum... You are a lucky man. I'd set my idle closer to 950-1000.



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