How do you avoid losing power brakes with large cams?
I have a friend with a 396 chevelle that barely has any brakes, and a friend with a 350 that has no power brakes.

I want a street driven powerful car... so power brakes would be nice.


Could anyone explain to me why this happens and the best way to prevent/fix it?

 

your vacuum drops and you need to add a vacuum canister.. they are available from Crane Cams and CompCams..

 

it normally will not fix the problem. The best thing to do is to convert the brake system to a self actuating caliper/piston. They do not need boosting. Or add an electrically driven vacuum pump to support the brakes. Mpbrakes.com sells them for 350. I got one off of ebay for 40bucks. They used to come in cadilac diesels.

and several things come into play. Not just total duration. lobe separation angle the biggest factor. You can always idle it up a little.

But, gm boosters normally need about 15inchs of vacuum. and if the motor is got any kind of a rough idle, it will cut into vacuum quickly.

 

Generally, any cam bigger than 230 deg @.050 will have
such low manifold vacuum at idle as to start causing problems
with power breaks. An auxillary vaccum canister will help
(provides a storage of useable vacuum) but will not completely
solve the problem. There are auxillary vacuum pumps (electric
and belt driven) that you can use ( Cadillac, citation etc) to
provide more available vacuum also. Cams with tight lobe separation tend to have less vacuum at idle (more overlap)
then another with wide lobe separation. Unless you have the
increased induction/head/exhaust flow these bigger cams won't work well on the street anyways.

 

I just swapped out my 305 for a 355 with mellings 22305 cam and used Crane Cams canister. I sometimes totally forget that I am using a canister at all. But every now and then I step on the brakes and am pleasantly reminded.

 

I have 2 of the melling cams 22303, The next slightly smaller cam, And never had a bit of problem with my brakes.

John
Well see with the comp extreme.

 

I've always wondered why my '68 Hurst/Olds can run power brakes without a canister when it has a 320° cam and .515 lift, while the Chevelles with big blocks need the canister. Is it a difference in the way the blocks generate vacumn?

 

GM did put some vacuum boosters that could get by with 12inchs of vacuum but not many. It also might have had a slightly higher stall converter that allowed a little higher idle. In many cases that will address the problem.

 

Can't really compare the GM published seat timing specs to
the more popular aftermarket specs. The w30 cam was about 232
@.050" and on 113 deg lobe centers. This cam in an olds 455
would have enough vaccum to run power brakes. That same cam grind in a 350 sbc would have less vacuum and effect the brakes more. That same cam ground on 106 deg lobe centers would have even lower vacuum at idle. (more overlap).