Trying to get the '70 Duster with a warmed 318 sorted out. The first tests runs were kinda scary - practically had to stand on the brake pedal with both feet to get it to stop.

This was originally a /6/904/7 1/4 car. I had done the front disc conversion many moons ago and the brakes worked great before swapping in the 318.

The 318 ain't nothing radical - 600 cfm eddy on a performer intake, kb167's, lightly ported J heads shaved .030", reground factory roller cam: 210@50, .428" on both intake and exhaust.

I'm getting a steady 14" of vacuum at idle...how much do I need to be able to stop this thing without having to change my shorts after each test run?

Typically 8-9 in/hg is more than enough.. Something else is going on... Pump the brake pedal engine off till it's rock hard, then start the car with a foot on the brake.. Power assist should cause the pedal to sink noticeably... Have you verified vacuum at the booster, possibly the vacuum line is blocked..

I'm getting 14" at the end of the vacuum line where it plugs into the booster.

I've read on a few sites that 18" is generally considered the minumum required?

It's a fresh 318 with about 1/2 hour total run time, and what I think is a 42 year-old booster.

Generally Automatics need more vacumm to substain repeated braking,...a manual transmission equipped car can get by with a min of 10 inches/vac, because the engine recovers vacumm when clutched, where as an auto's idle in gear drops the intial RPM by a few hundred or so RPM, causing vacumm loss, intial/ and repeated braking consumes even more vacumm, and an auto in gear is slower to recover,....but 14 inches should be enought?,....try hooking up a gauge between the booster and motor that you can read while driving the car, and see what real world demands are when braking,...before making any decisions/modifications


PS....you are pulling vacumm from the manifold, not the carb for the booster?, correct?...

My 511 wedge idles at 18 inches in natural/park and 13 inches in gear. My power brakes work fine. You must have another problem going on.

I've got a 360 with a whiplash cam, it idles at 8.5-9" at 700-750 rpm. Yes, it's a stick truck, however, while testing the vacuum assist on the power brakes, with just the engine idleing in the garage, I would repeatedly press and release the pedal as fast as I could, and never got it to run out of vacuum assist. I could feel it get a little firmer after 2 quick pumps, but it never actually got to the point where the pedal was rock hard like when stepping on the brakes with the engine off.

are you sure you don't have some other problem? isn't there a check valve on the booster? any chance it's backwards somehow or something? leaky booster?

try taking it for a drive and actually downshift the engine so that the RPMs jump up when slowing down. with the trans locked into 2nd gear at say, 45 mph, under a closed throttle, you should have MORE than enough RPM, and probably up around 30" of vacuum. If your brake pedal is STILL hard at that point, you need to look elsewhere than the cam's vacuum at idle.

I have less than 10" vacuum and my power brakes work fine. You do get used to low vacuum and drive accordingly, but I wouldn't say it's a problem.