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How much vacuum does it take to run a boost on a brake system?
The issue for power brakes is making sure they function "ALL" the time. That means Panic Stops, mountain driving and long steep downhill grades. That said I would like to see a minimum of 12hg at idle on the gauge for safe operation under all conditions. 14hg would be better. That equates to around 60* of cam overlap in a sbc. This depends on the motor and how efficiently it runs but it is a good ball park figure in overlap to shoot for. Less overlap, more Hg. More overlap and the Hg falls off more quickly. My 383 has 58* of cam overlap and I have 13 - 14hg at 650 - 700 RPM idle. Jack
 

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Discussion Starter #3
Jackel, Very good explanation Thank you. I am getting 10psi at 1000rpms no matter what I do. I have both mixture screws out about a 1/4 turn for max vac. any more and vacuum drops off. I may have multiple issues. I am on my way out to the garage to tweak things. I will update.
 

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Jackel, Very good explanation Thank you. I am getting 10psi at 1000rpms no matter what I do. I have both mixture screws out about a 1/4 turn for max vac. any more and vacuum drops off. I may have multiple issues. I am on my way out to the garage to tweak things. I will update.
1/4 turn from dead seat? If so, something IS wrong. What cam are you using?
 

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Discussion Starter #5
I think this carb needs to be replaced. I set timing, changed springs, made sure the curve was right. That is when I noticed none of the vacuum ports on the carb are progressive. They are all constant. A friend of mine had a bin full of carbs and asked if I wanted any of them. After looking through them I found a Holley 750 vac sec. I had it in my garage for about 6 years and when I started this project I took this carb and did a rebuild on it. Some thing may have been wrong with the carb before i got it. Another friend has a carb he is going to lend me to try out to make sure I am going in the right direction before i purchase a new one. As for the cam specs I do not know. I picked the motor up complete with everything except the carb(ie pulleys,belts,fan, starter, dist. wire and plugs) on a everything must go sale. There is no lash on the rockers so i am assuming it is hydraulic. I also am assuming it is one of the 520/540 lift hyd cams alot of people use. Because see sounds loppy. Also after setting the timing I tried to set the screws to max vac and at 800rpm I was a 5psi. and 1/4 turn was all I could get. Monday I am picking up the carb, then I will update.
 

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For a power booster to function properly you will need at least 18" of vacuum. Anything lower will give you a hard pedal. You can get away with 16" if you run a reserve canister.

Source: http://www.piratejack.net/index.php/tech-support/parts-faqs/brake-booster-faq

Power brake upgrades, be sure to check for a minimum of 18 in. of vacuum prior to performing the upgrade. Vehicles with tall heads or superchargers most likely will not have sufficient vacuum. If you do not have sufficient vacuum, check your intake manifold for clogging, vacuum fittings and for collapsed vacuum lines. Also ask us about our 12 volt vacuum pump.

Source: http://www.classicperform.com/TechBook/BrakeTroubleshoot.htm
 

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Discussion Starter #9
For a power booster to function properly you will need at least 18" of vacuum. Anything lower will give you a hard pedal. You can get away with 16" if you run a reserve canister.

Source: http://www.piratejack.net/index.php/tech-support/parts-faqs/brake-booster-faq

Power brake upgrades, be sure to check for a minimum of 18 in. of vacuum prior to performing the upgrade. Vehicles with tall heads or superchargers most likely will not have sufficient vacuum. If you do not have sufficient vacuum, check your intake manifold for clogging, vacuum fittings and for collapsed vacuum lines. Also ask us about our 12 volt vacuum pump.

Source: http://www.classicperform.com/TechBook/BrakeTroubleshoot.htm

Great info. Thanks. Once I work out my carb situation I am moving to the booster. May have to get one of those vacuum pumps.
 
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