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Old 05-07-2007, 08:41 PM
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Vacumm pressure for booster

My 350 small block is resting at 12 pounds of vacumm at idle, is this enough vacumm for a remote brake booster or do I need to get a vacumm pump

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Old 05-08-2007, 04:46 AM
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according to my booster mfg.
13" is minmum.
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Old 05-08-2007, 05:54 AM
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Vacumn measures in ...inches of mercury not pounds
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Old 05-08-2007, 06:22 AM
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12Hg is to low for a decent carb idle mix....need 14Hg plus

the booster has a one way check valve so as soon as the Hg goes up by tapping the throttle it stores the higher Hg.....a good check valve will hold the higher vacuum for up to 48 hours
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Old 05-08-2007, 11:54 AM
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booster for brakes

If I read you correctly, you are saying if a install a check valve I will be OK with a remote booster. Thanks for your reply
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Old 05-09-2007, 03:36 AM
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your welcome....but
what I'm saying is a standard normal booster already has a check valve...the engine will pull up to roughly 26Hg of vacuum on a down shift and the check valve will hold the vacuum in the booster.....when the car is turned off, a good check valve will hold the stored vacuum upto 48 hours

have no idea what you need for a "remote booster" (????)
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Old 05-09-2007, 05:51 AM
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12HG sounds too low to me, are you running a carburetor? Is this low vacuum because of a cam related issue? While the booster check valve might store up higher HG during a downhill coast for example, the first time you step on the brakes that higher HG is gone and you're back to 12hg. Seems to me you would not have consistent pedal pressure. The amount of assistance provided by a brake booster is proportional to the area of the booster diaphragm. Many aftermarket booster are 7" and with 12hg you would hardly know that you have power brakes. A larger dia booster would be somewhat better. Other factors also enter into the effort required to stop your vehicle, such as, brake pedal ratio, master cylinder bore dia., disc or shoe, etc. An electric vacuum pump would provide the higher vacuum and the most consistant pedal, IMHO
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Old 05-09-2007, 08:19 AM
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Some years ago I ran into that problem with a 389 Pontiac engine (good old days ) and I tried all sorts of tricks such as adding extra vacuum cannisters plugged into the line going to the booster and while it did help some it still would run out of vacuum quite often. I finally got a vacuum pump and that took care of it.
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Old 05-09-2007, 08:25 AM
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save you self and get a vac.pump.i had same problem this year with nova.tried every thing.got mine thru summit racing. don't take chance with panic stops! john m.
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Old 05-09-2007, 10:36 AM
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Vacuum for brake booster

Thanks for all the input Guys. The low vacuum is probably caused by the cam. .462 and .482 lift with 219 and 229 duration.
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Old 05-10-2007, 03:41 AM
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your tune up for that cam is way off....a comp 268 (close to your cam spec) is 15.5Hg at 800rpm, 17Hg at 1000rpms with Dart senior heads

http://www.compcams.com/Technical/Dy...68H-10_001.asp

(you really do need 14Hg+ at idle to get the motor to run decent or you end up spending $$$$ to work around the problem to get the motor and brakes right)

Last edited by red65mustang; 05-10-2007 at 03:48 AM.
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