fbekholden.com

Hey there,

I have just upgraded my cam in the engine and now i have 2 concerns. 1 being that since the upgrade my disc brakes/brakes in general have gone funny and seem to be hard especially when cold in the morning almost like i have no booster. 2nd is that my tacho seems to have gone hay wire one minute it works perfect next it's reading 3.5 grand at idle. The cam i through in the ol 186 is a street/race cam from crow cams and is the second largest for hydralic tappets working between 3 to 6 grand if thats any help...

thanks Pete

If you go up in the cam you can lose manifold vacuum which directly affects your booster.

bootlegger wrote:If you go up in the cam you can lose manifold vacuum which directly affects your booster.

I don't doubt you Dave, but why does it reduce the vacuum?

Is that because the inlet valve is open for longer meaning the same volume of air now has longer to enter the cylinder = less velocity = less vacuum?

Dare I say it, a long gentle suck instead of short, sharp one?

I cant answer that one. All i know is my chev has a pretty big cam in it. There is hardly any vacuum and the brakes are crap on it. Ive bought a smaller cam for it just havent gotten around to changing it to see if it solves tge problem.

i have a 186 and i have just put bigger cam in crow 161, 450 lift
my brakes are the same hard
i only have 4 vac in manifold standed they have about 18
you do loose a lot of vac

Seems like a trap for young players taking their new lumpy-cammed motor out for a test drive...

Thanks all just seen a picture today of a LJ running triples and he has the vacum running off 3 spots of the manifold may have to look into this. Once the car has been running awhile it gets better than just after or the first application. As for the tacho i think i have found it I'm suffering a voltage drop issue and a battery thats charging but not enough with me driving night so i have to sus out connections and possible lead size might need to be changed..

Lumpy cam = more valve overlap. Inlet valve on one cylinder is open whilst the exhaust valve on another cylinder is also open. Insteading of sucking down the inlet manifold (and making vacuum to drive the brake booster), the "inlet open" cylinder sucks from the other cylinder. The bigger the valves in the head, the worse the effect (big exhaust valve = nice big open hole when it is open, which looks more appetising to that sucking cylinder than trying to draw past the restrictive throttle butterfly(s)). Car runs better with overlap from the lumpy cam, but can be a pig at idle (remember that the same vacuum is driving all your carby circuits).

Loss of vacuum after a lumpy cam install can be handled by:
a) toughing it out - the brakes still work, you just have to put more effort into the pedal. You can end up with thighs like Zena, Warrior Princess on a circuit car though ,
b) fitting a vacuum resevoir (a pressure vessel and non-return valve) that stores vacuum and lets you use a little each time you brake, or
c) fitting an vacuum pump (electrical or mechanical).

Cheers,
Harv.