need more stopping power. vacuum issue?

[blackbeaSSt]

for the first time this morning i was scared ****less when stopping. person in front of me slammed on their brakes and i was a decent ways away but going pretty fast. i started to hit the brake and realized she was stopping faster then me! i came within a foot of hitting her and i was standing on my brakes.

the guy i bought the truck from said that the cam is pretty much taking up all the vacuum and thus the brakes aren't all there.

any ideas on what i can do to test or fix the problem before its too late

[Adams74Chevy]

I don't know much about the vaccum part but in general bleed the air in the brake lines, check brake fluid level, adjust the rear drums are all easy things to check to make sure your brakes are in good shape. Another thing to look in to is upgrading to slotted or crossdrilled rotors, my father did that for his truck since he usually has a trailer hooked up or a load in the bed, it helped, but they will eat pads quicker.

[motornut]

check for vac leaks too

[baclay9]

You would have to have a pretty huge cam IMO. My buddy is running a custom grind that has a duration of 300/310 and he has good brakes while driving.
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[454HO]

I think you need at least 10" of vacuum to operate power brakes, although 12" would be safer. How much vacuum does your engine pull at idle now? It's quite possible a little tuning can net you 3-4" over what you currently have if tuning has not been optimized. What is your initial timing set at? Do you have vacuum advance hooked up, and if so to what port? What is your idle speed set to? Have you set your idle mixture screws? Lots of variables can affect this.

[LONGHAIR]

Originally Posted by 454HO

What is your initial timing set at? Do you have vacuum advance hooked up, and if so to what port?

The port where your vacuum advance is connected has absolutely nothing to do with the operation of the brakes....unless of course there is a leak.

As far as where it is connected concerning the performance of the engine, yes......but it's not a simple as "just switching" the line to the other fitting. There is a calibration issue here. The advance curves are different as well as the amount of advance that the weights can even provide. The initial setting would be different too.

[454HO]

Originally Posted by LONGHAIR

The port where your vacuum advance is connected has absolutely nothing to do with the operation of the brakes....unless of course there is a leak.

Not necessarily true. Usually a performance cam likes the timing to be a bit more advanced compared to a stock cam in order to to idle smoothly. This can greatly increase vacuum at idle. So if the engine is set up per factory specs at say 4 degrees initial and ported vacuum advance, then I can imagine that being a MAJOR issue in why the vacuum is low. Such a setup would probably require excessive throttle opening (further reducing vacuum) just to get the engine to run and it would not be running efficiently.

Even if the initial timing were set to something more appropriate for a performance cam, say 14 degrees, the engine still may not run smoothly with ported vacuum advance. If this were the case, moving to manifold vacuum advance could provide the additional timing the engine wants and result in a smoother idle with higher vacuum for the power brakes.

Does this mean the distrubutor might need some work? Of course. And that is why I was asking the questions. But since the original poster has not provided any details on the engine or how it is tuned, all we can do is speculate...

[LONGHAIR]

Originally Posted by 454HO

Even if the initial timing were set to something more appropriate for a performance cam, say 14 degrees, the engine still may not run smoothly with ported vacuum advance. If this were the case, moving to manifold vacuum advance could provide the additional timing the engine wants and result in a smoother idle with higher vacuum for the power brakes.

Does this mean the distrubutor might need some work? Of course. And that is why I was asking the questions. But since the original poster has not provided any details on the engine or how it is tuned, all we can do is speculate...

You can't "just move the vacuum line connection" w/o recalibrating the distributor. Even if that move would increase your vacuum at idle, you would probably get into a pinging situation because of it. If the vacuum really is that low, the advance cannister my not be functioning properly. That will get you an erratic idle, which compounds the problem.

The whole thing seems to me like "too much cam" w/o having the surrounding systems "adjusted" to compensate for it. This could also include the torque convertor. If it is too tight, other issues get involved. He also may be lacking the compression needed to make the cam function too...

My point about switching the vacuum line was more about the fact that it will not "use up" the available vacuum than the technicalities that get over many people's heads.

[3r!c84]

ive got the same problem with my engine i just put in. 400sbc with 500lift cam. hard pedal !!!! im going to watch this thread

[72lb4x4]

Have you
Checked the brakes at all 4 corners and made sure they're working right?
Evaluated the system to make sure its all working right? For example if it was converted to disk the prop valve and/or MC could be wrong.
Measured the vacuum?
Bled/flushed to make sure no air is in there?

Just throwing things out there...

[3r!c84]

the system worked fine with the 305. nice good pedal not hard at all. working very good. i didnt check vac ill get a gauge and try that

[menace121978]

there is 1 fix i think you guys might be interested in... you could always do away with the booster. after i had bought my set-up i was talking to a custom car builder and wilwood dealer and he told me that if you go with the 7/8th bore size master cylinder you can do away with the booster and have a better feeling pedal with better stopping.(and a cleaner engine bay i would have gone this route if i had known and i could have given a better 100% honest opinion but hind sight is 20/20. i plan to do my next build this way but it wont be for awhile yet.

anyone else hear of doing this??

[3r!c84]

that sounds like too much like work haha i want to keep the stock system.

[baclay9]

What about just adding a vacuum canister under the hood to store vacuum? I have heard of people doing that before with good results.
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[3r!c84]

Originally Posted by baclay9

What about just adding a vacuum canister under the hood to store vacuum? I have heard of people doing that before with good results.
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ive got one under the hood. its not hooked up tho. never was..

[454HO]

Originally Posted by 3r!c84

ive got one under the hood. its not hooked up tho. never was..

If you are talking about the black plastic ball on the driver's side, on my '86 truck that ball was plumbed to the HVAC controls. There was a one-way check valve between the manifold vacuum port and the line that went from the ball to inside the cab. This would have absolutely on effect on the brakes. It would however allow you to switch the airflow from the dash vents to the floor vents without the engine running though.

If your engine is incapable of creating enough vacuum to operate the brake booster, then adding a vacuum can will not help. If your engine can create enough vacuum, but only under such conditions as during engine braking or highway cruise at light throttle, then a vacuum cam might help you. But it will only store enough vacuum for one or two additional brake pedal applications. If you drive in stop and go traffic, then you will quickly run out of power brake boost.

[smokedat]

some cams with radical lobes will affect vaccum. For a power booster to function properly you will need at least 18" of vacuum at idle in park. Anything lower will give you a hard pedal. The solution is change cam or add a electric vacuum pump or perhaps you can get away with a vaccum canister. But from personal experience my stock brakes sukd i had disc in front and drum in rear, you also got to keep im mind these brakes are from the 70's so you cant expect much out of them

[baclay9]

So is it the duration or lift that determines how much vacuum you make? Anyone know? The lift on the cam I bought is 552/555. 87lowrider is running the same cam and said he really doesn't notice any problems with the brakes. I am talking about BB though so there could be a difference there I guess.
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[smokedat]

Check your numbers they are wrong cams dont go that high 300+ is max on car. 500 is only found if motorcycles

Originally Posted by baclay9

So is it the duration or lift that determines how much vacuum you make? Anyone know? The lift on the cam I bought is 552/555. 87lowrider is running the same cam and said he really doesn't notice any problems with the brakes. I am talking about BB though so there could be a difference there I guess.
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[3r!c84]

what do you mean. ive got a .503/.510 cam?


but i hooked up the vacuum canister. i will see if it helps tomorrow

[baclay9]

Originally Posted by 3r!c84

but i hooked up the vacuum canister. i will see if it helps tomorrow

Let us know if it helps. I am interested to know.

[clinebarger]

Hydro-Boost!

[baclay9]

I was talking about lift not duration
Valve Lift is .552/.555
Advertised Duration is 274/286
Duration @ .050 is 230/236

[smokedat]

valve lift is on the high side. what is youre LCA Lobe Center Angle the lower it is the less vaccum thus choppier idle, this is a good thing defenite sound and performance of having a cam, minus the breakin issues

Originally Posted by baclay9

I was talking about lift not duration
Valve Lift is .552/.555
Advertised Duration is 274/286
Duration @ .050 is 230/236

[baclay9]

Originally Posted by smokedat

valve lift is on the high side. what is youre LCA Lobe Center Angle the lower it is the less vaccum thus choppier idle, this is a good thing defenite sound and performance of having a cam, minus the breakin issues

The LCA is 110

Here is the cam card of the one I will be running.

http://www.compcams.com/Company/CC/c...?csid=396&sb=2