I have a 71 big block 4 sp with a cam that is a little too aggressive to develop much vacuum (4 to 5" at idle). Not quite enough vacuum for the power brakes. The brakes work, not very well, but I don't trust them at all. What pump have you guys used and where did you mount it.

Thanks,

Ken

 

OK, just did mine this afternoon. I got a Comp Cam unit from a guy on the forum. I have a SB, mounted the pump on the passengers side behind the AC compressor. Also installed a reservoir tank which I glassed the bracket to the AC chamber on the front of the firewall. Now have 22 in. vac All the units look the same to me in their ads.

 

Very few engines will generate that little vacuum. To be sure, disconnect ALL vacuum-operated lines/systems from the engine...except for the distributor advance can; plug off all open ports. [With engine running, you can also disconnect the advance can to verify that it is not leaking vacuum, as well.]

Fire up the engine and measure the vacuum again. If significantly higher than previously measured, you have a big vacuum leak somewhere in the 'stuff' you disconnected. You can also alter the timing to see if you can obtain higher vacuum level with your engine.

If you can get 7-10" Hg vacuum from the engine at idle, you can run the vacuum operated systems with what you have. You will need a vacuum reservoir tank and a check valve to maintain the idle vacuum level for those systems when the engine is being used.

 

Also, Captain Nitpicker says: A "vacuum leak" isn't really "leaking vacuum". It's not like anybody ever said "I've got a vacuum leak that is leaking vacuum all over my driveway! What's a good product for cleaning up vacuum stains on concrete?" What's actually going on here is that air is leaking in, causing the vacuum system to fail to hold a vacuum. Kind of like if your shopvac had a big hole in the hose, it wouldn't be able to suck up any dirt at the nozzle, because air is coming in through the hole. I just thought I'd throw that out there, because a lot of folks don't really seem to understand how vacuum works. (Technically, it's not even right to call it vacuum, it's just a low pressure area, but now we're really picking nits!)

Anyway, yeah, check for vacuum leaks, (with the way the vacuum system works on these old C3 'vettes, you can have a vacuum leak as far away as up near the headlights!) play with timing, (typically, big cams will develop more vacuum at idle if you give them more timing) and, have you adjusted the idle mixture screws for highest vacuum at idle? BTW, how are the headlights working? Oh, and one more thing: You may need to get a new vacuum advance canister that will let your vacuum advance be stable at your idle vacuum. Are you even using vacuum advance, and, if so, are you hokking it up to ported or manifold vacuum?

Scott

 

http://forums.corvetteforum.com/c3-t...p-install.html

 

This is a new engine that I did not put together and I did not have vacuum leaks before the engine change. So the first thing I did was plugged all vacuum lines and vacuum measurement was still the same. No vacuum leaks. I then got out my fire extinguisher in case it was needed and with car running squirted starting fluid at base of carb and intake manifold gaskets and no leak found. So far so good. I then called the cam manufacturer and asked how much vacuum should I expect at idle. He said with the overlap of this cam "no vacuum at idle". The engine builder said this was the largest cam you could run and have power brakes. Well they work but not well enough for me.

Has anybody run a vacuum pump off of the fan or alternator belt like newer cars or should I just get an electric driven pump.

 

Are the cam specs a secret?

Scott

 

The camshaft is a new elgin industries e1143p and with a true cloyes double roller chain set. This camshaft is a hydraulic lifter grind .576 lift 246 duration @.050 on both the intake and exhaust and it is ground on 108 degree lobe seperation. This camshaft has a distinct lopy idle and is designed to make maximum power in the 2500-6500 rpm range.

 

Thanks Mike80. I know my vapor canister is behind that vent. I will take the vent off and see if I have room for it.

 

Thanks for the info 7T1vette. Found no leaks. I have the reservoir tank and check valve hooked up. I have the distributor vacuum advance hooked up to ported vacuum right now. I will try non ported.

 

Hmmm. I'm certainly no expert, but I would imagine a cam that size in a big block would be able to generate more vacuum than that. Back in the day, I had a cam about that size in a 440, and it idled a little funky, but it operated vacuum powered accessories just fine. I think it had a 110 lobe separation angle, and only about .501 lift at the valve, but the duration was about the same. Anyway, good luck with the vacuum pump.

Scott