13"-14" Vacuum with 220/230 @ .050 Duration Cam? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 05-01-2006, 09:22 PM
acr acr is offline
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13"-14" Vacuum with 220/230 @ .050 Duration Cam?

New 350 SBC motor,runs great.But the PB pedal seemed a little hard. I checked the vacuum at the PB fitting in the back of the carb.& I'm pulling 13-14" at idle. Does that sound right?

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Old 05-01-2006, 10:16 PM
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You are in the ballpark, although a lot of details missing. No matter, that amount of vacuum should be enough to run power brakes. Your vacuum is going to drop down a tad when you put it in gear and the RPM drops so that's really the number you want to see. 12" of vacuum seems to work OK with power brakes but getting below 10 gets dicey.
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Old 05-02-2006, 08:56 AM
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brakes

I just had this same discussion with a customer last week. He claims that the brake people are telling him that they need 15 hg of vacuum to work correct???

The engine i built him has 13hg at idle and they are having brake issues like you are also.

I suggest ,as i did with him to put a can on it....


Keith
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Old 05-02-2006, 10:45 AM
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Thanks for the replys. Most all of the folks who sell brake stuff say you need a minimum of 18", then somewhere in the small print they'll usually say at least 16", some on this forum whose opinions I respect say 12". So who knows!! And yes I'm going to try a storage canister, although all it does is give you more vacuum, not more inches of hG. If that doesen't work I'll get an electric pump. FWIW, I'm replacing my master cylinder with one that has a 1" bore since I have a disc/drum combo.

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Old 05-02-2006, 11:42 AM
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13"-14" Vacuum with 220/230 @ .050 Duration Cam

Fred

I saw your post and thought I would reply. I'm using a 7" dual diaphragm power booster with the 1" bore master cylinder in my 40 pickup and it has a real easy pedal. Like we talked before, a friend of mine used the same pedal and booster setup that you have in the 36 with the only exception is the master cylinder. His engine is a stock type crate engine so I know it has a lot of vacuum. In my 40 I'm running just a little less cam than yours and I have around 18" of vacuum so that may be why I have an easy pedal. I would try the master cylinder first, but a dual diaphragm booster might be another option.

Donnie
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Old 05-02-2006, 03:13 PM
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Quote:
Originally Posted by acr
Thanks for the replys. Most all of the folks who sell brake stuff say you need a minimum of 18", then somewhere in the small print they'll usually say at least 16", some on this forum whose opinions I respect say 12". So who knows!! And yes I'm going to try a storage canister, although all it does is give you more vacuum, not more inches of hG. If that doesen't work I'll get an electric pump. FWIW, I'm replacing my master cylinder with one that has a 1" bore since I have a disc/drum combo.
Actually you will be able to store more vacuum than your engine pulls at idle because there are certainly instances while you drive that your vacuum will be higher than that, like when you get off of the throttle, etc. Your can will hold this higher vacuum for those times when you need it like when you've done an acceleration test in front of hi-po and need to haul it down asap!!!!
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Old 05-02-2006, 03:55 PM
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Yes, a dual diaphragm booster from MP Brakes will solve that problem.
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Old 05-02-2006, 04:14 PM
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acr,
adding to nairbs post?,
the booster has a one way check valve so it pulls and holds whatever the max Hg (25 on a downshift?) of the motor is from normal driving....

a good check valve and booster will hold that max Hg for a day or so....

rev the motor above idle, Hg goes up, stored Hg goes up
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Old 05-02-2006, 09:06 PM
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Thanks to all, good info. Donnie, I have the master cylinder in my possession & plan on installing it this Saturday. If that doesen't work, the vacuum can should be here Fri. & will go on right after the MC. I'll post the results. Red,NAIRB good point about the can storing the higher vacuum.
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