With my new motor , my vacumn is about 10". Have factory power disc on a 71 and they are lousy. What are my alternatives? I put an extra vacumn can on and that didn"t make any difference. Don"t like the price on the hydro-boost. Should I be able to get by with 10" of vacumn on the factory power brake booster? Any in-put would be appreciated. Thanks, Jim

I had same problem when i installed longer duration cam. I discovered the bigger cam liked more initial ign. timing 18 degrees and liked to idle about 850 in gear, having said that the pedal is still a little stiff at times.

Just as a follow up to previous post engine had 12" lost 2" now has 10" with that timing and at that rpm. Only you can determine what you are comfortable with when it comes to brakes.:yes:

Jim what base/initial & total timing are you running ?

What are your cam specs and motor size?

If your running an aftermarket perf cam they generally like approx 18 deg base timing.

So if your running a perf cam and are running less then 18 deg base timing try 18 deg btdc base timing.

Make sure idle is low like 650 rpm to ensure mech adv in dist isnt partially activated when setting base timing or it willnotbe correct and most lke will be retartded. Also unhookand plug the vac avd too when checking & setting base/total timing.

If your running the base timing retarded that will makethe idle/low engine speed vac considerabley lower. so if forex youhave 10" vac with retartded base timing then when you advance it the vac could easily increase to 12-13" or even more depending on just how much the base timing is retarded. and inc the vac to 12-13 could be just enough to get your power brakes working ahlf way decent,been there done that mult times in past 37yr's working on these motors/cars.

Scott

When I tune my car for it's annual smog test I cut initial from it's usual 16º down to 6º and my vacuum drops from about 12" to 10" but my power brakes still work fine.

My distributor is set up to run 12 degrees initial timing . I guess I would have to see about redoing the distributor to put that much more timing into it and keep from pinging. The motor is a 396 with a comp 270H cam.

The motor is a 396 with a comp 270H cam.

That combo should have plenty of vacuum for power brakes. You'll need to do some tuning...
Like others have mentioned, give it more initial. :yes: At least 16 degrees to start...

My distributor was set up to run 12 degrees initial with 36 total so I will have to re -do it . That could be my problem. What about too lean a mixture? Will vacuum come up if I fatten it up a little? Thanks for the in -put guys.

The very retarded 10 deg base/initial timing is definately your problem,as stated the cam you running should have considerably more idle vacuum and be able to easily run (pull enough vacuum ) for the power brake booster.

This can be very easy to fix,pull the dist and take to a shop or send it out to have 18 deg mech installed all in by 2800-3k rpm.

Then simply run 18 deg base timing which you cam will like ,then you will have 18 base + 18 mech in dist = 36 deg total in by 2800-3k rpm,perfect you your setup.

PM me if you need to get info on shops to send dist out for recurve.

Scott

Jim,
Several years ago I went through this.
Most factory Chevy distributors from the late 60s give 14-18 degs mechanical adv.
So with 18 initial +18 mechanical....I should have been golden.
Wrong!
My distributor had roughly 23 degs in it.....
So when I cranked up the initial so the Vette ran great and idled strong...., my total was outa control...41...
I had to retrard the timing back to 12 degs, throttle blades not centered on the idle transfer slots....whole nasty ball of wax....
This was with a limiter bushing pressed onto the adv rod.
I took the beast apart again and peened the outer end of the slot...this deduced the mech adv down to 18.....we're talking only a few thou here of travel.....

Going to the shop is a one-shot/one-kill deal but the home garage remedy works too.
You can also have the open end of the slot brazed then gradually file the slot open.

Thanks for the advise. I am going to change timing and see what happens. If it works, I'll re-do dist.