Long duration cam and vacum ?

[Josh]

My newly completed project has soft brakes at idle and decent at speed. A basic GM disc drum set up with vacum booster. I assume this has to do with the long duration cam. I notice with taking vacum readings that at idle in Park I get 20 mmHg, but as soon as the idle slows down some when putting it in gear it plummets to 10mmHg or less. Is this noraml for a long duration cam? The LSA of the cam is 114 so I thought that it would help, but the 262/ 269 at .050" may be the issue (540 BBC)? Is the only solution raising the idle to 1200+ RPM? It is about 800 RPM now when in gear.
Thanks

[PRRC]

You can use a vacuum canister to help store vacuum or switch to hydra boost system or go to a manual brake set up. Manual brake set up correctly will feel about the same a as a power disc set up. with better feed back.
Tim

[MCB Matt]

Low vacuum will cause a ROCK HARD pedal and it would fee like the car is never going to stop.....soft pedal could be quite a few other issues....

[Josh]

Well when is goes soft, it is rock hard at the bottom and is a bit scary. Hell of a quad workout!

[pitts64]

First thing, I'd get rid of that choke. Nothing worse then sending raw gas down the cyls of a cold running motor washing the oil away. I'll bet you have the intake heat blocked too! Chokes were designed for women trying to pull out onto the interstate with a stone cold motor...

I think you need to do some tuning. Is that a roller cam?
Do you have the vacuum advance connected to full vacuum? Did you set the timing with the vac advance connected?

If all that doesn't help your braking, switch to manual brakes... Vacuum cans are a waste of money in my experience..

[ProdigyCustoms]

I have had zero luck with vacuum cans too. If it is dropping 400RPM when you put it in gear with that cam, your converter is wrong first of all. Second, 1000RPM idle with that cam at that cubic inches is fine. I happen to have done a few 540s in projects with cams almost exactly like that one.

Now of the pedal is soft, I would try to rebleed, or look at the master cylinder.

And i agree with pitts, that choke gotta go!

[Josh]

Hey Frank-
Any ballpark figure of what total timing I should shoot for on this motor? Mind you this is in a heavy 1979 full size Chevy..Thanks

[ssealeycpa]

I run 36-38 degrees of total timing on my big block (454 .030 over). (Which should be fine for a normally aspirated big block) Once you go over 36 degrees, you need to keep a heads up for detonation. Your initial timing at ~1,000 rpms will probably be about 20-25 degrees. I always set my total timing with the engine running at full advance (~3,500 rpms)

[Josh]

I had the initial at about 16 and the total at 35 and it was detonating a little going uphill, stabbing the throttle. I backed off the initial to 14 to get the total down to 33 so I would not have the popping. I thought I would be able to get 35-36 total out of it but for some reason I cannot. Engine temp is good at 180 and the heads are AFR.

[Skip Fix]

Bump the idle up. Running a direct manifold source for vacuum advance can help some too, when I race NMCA classes that measured vacuum at idle everyone ran a vacuum advance.

Brake booster design is also a factor. My TA's 4WD dual diaphragm booster works fine until idle starts getting below 9.

A looser convertor also will make a big difference as it is lugging the motor down in gear.

[Neil B]

I've got '69 Z with a 302 that makes 7" of vacuum at idle (1000rpm) with a stock brake booster and the power brakes work perfect.