Engine Vacuum and power brakes ?

[rustytoolss]

I know that all of the after market brake companys say that you need at least 18" of vacuum to operate a power brake booster.
But in the real world of of cars that have some modification (mine is stock) , how much /or how little vacuum is really needed to operate a power brake booster (my 318 poly idles in netural at 16-17 " of vacuum.

 

[Dana67Dart]

There are vacuum pumps for low vacuum engines.

If you have a stock engine you have more than enough for a booster

 

[Bewy]

The area of the booster diaphragm is also important. Whatever the vacuum amount is, it acts against this area. Some boosters are dual diaphragm, which increases diaphragm area & the force the booster can produce.
I have seen cars with 12" of idle vac have satisfactory booster performance. Bear in mind also when you initially hit the brakes, rpm will be above idle & vac will be higher than idle vac.

 

[RustyRatRod]

It will help tremendously if you have your timing optimized. Advanced timing will greatly help the vacuum signal and give better assist with power brakes. I think the vacuum signal you've expressed having is enough to do the job. More is always better.

 

[jdsduster]

i have 10" and brakes are borderline driveable.i would not think you could operate with any less without assist

 

[rustytoolss]

The area of the booster diaphragm is also important. Whatever the vacuum amount is, it acts against this area. Some boosters are dual diaphragm, which increases diaphragm area & the force the booster can produce.
I have seen cars with 12" of idle vac have satisfactory booster performance. Bear in mind also when you initially hit the brakes, rpm will be above idle & vac will be higher than idle vac.

I have an 8" dual booster.

 

[Dana67Dart]

There are also vacuum resivours you can add from newer cars, but i doubt you will need it.

You could also ditch the power brakes, it is not hard to stop our darts without power brakes. I have stock KH front disc brakes, stock no booster, no problem!

 

[rustytoolss]

There are also vacuum resivours you can add from newer cars, but i doubt you will need it.

You could also ditch the power brakes, it is not hard to stop our darts without power brakes. I have stock KH front disc brakes, stock no booster, no problem!

Thinking that I may have to go back to manual brakes also (thanks for your input.)

 

[Dana67Dart]

Someone mentioned earlier that the vacuum goes high when you first let off the gas (ie higher RPM, closed throttle) the vacuum booster then gets "charged" to whatever amount of vacuum is available. Assuming you check valve is functioning correctly and rhe booster does not have any leaks, you now have enough vacuume to apply the brakes with boost.

Same woukd happen if you shut off the engine while driving. Yoj should have enough vacuume for 1 assisted brake.

Now stop and go is a diferent story!