OK, I have a 350 with a .510 lift Comp Cams, Victor Jr intake, Mighty Demon 750 carb, AFR 195 ported heads pushing a t56 six speed. Currently I have one breather in my pass. side valve cover, and a pcv valve in my driver side. The pcv is split off my vacuum line for my brake booster. At idle, I have very little vacuum for my brakes. I have recently been told that if I plug the pcv into the air cleaner than my brakes will be better off at idle. Also can I tap into my 1" phenolic spacer below my carb instead of my air cleaner as it would look much cleaner? Can someone also explain what the breather and pcv function actually is? This is something that I dont understand much and I know its important. Thanks in advance!

I'll let someone else get into positive crankcase ventilation and engine breathing but you're vacuum is low because you're running a SB with a big cam.

You might want to think vacuum canister or just turn up the idle and live with it.

It is a very bad idea to use the same vacuum port for both the brake booster and the PCV. The PCV is a controlled vacuum leak, and you've got a cam that's vaccum challenged as is.

It is a bad idea to use the same vacuum port for the brake booster and ANYTHING else, as far as I'm concerned.

You DO want to keep the PCV, just make sure it's connected to another manifold vacuum source. If I had to, I'd use the carb vacuum port for the PCV, and a boss on the manifold for the booster. Less chance of screwing up mixture distribution that way.

The PCV system is the finest bit of emissions control engineering on the planet. For about $5 in parts, you eliminate about 80% of the emissions. The PCV system will help draw moisture-condensation-out of the engine, which leads to longer oil and bearing life, too. There's NO downside if the engine and PCV system operates as it should.

I've never drilled 'n' tapped phenolic. I'd guess you need to be SUPER careful because the pipe thread would tend to crack the phenolic when you tighten it. You might be better off punching a hole in the manifold under the carb mounting pad rather than in the carb spacer. Since you have a single plane manifold, a nicely centered hole below and between the secondary throttles would be OK, I'm thinking.

I do already have a vacuum canister which helps, but not enough. They make spacers that are already tapped so if I cant tap mine maybe ill just order one, just dont feel like f-ing up my victor jr. Any info on the use of breathers though?

Originally posted by seventy6speed:
I do already have a vacuum canister which helps, but not enough. They make spacers that are already tapped so if I cant tap mine maybe ill just order one, just dont feel like f-ing up my victor jr. Any info on the use of breathers though? Hey your local! I just went through the same deal. I drilled and tapped my phenolic but it's brittle as hell. Them cheapies don't flow to well either, Wilson will drill and tap one of there quality tapered spacers. I ended up pulling the manifold and drilled it, now I don't run a PCV at all.

"
RE: spacer 004110

John,
We can drill & tap for a 1/8” fitting, charge would be $30.00. Depending on what you are using the vacuum port for, this may or may not be big enough.

Wilson Manifolds"

Your carb has 3 vacumm sources,, 2 are constant, 1 ported. You need to hook up your pcv to 1 of the constant vacumm sources. As I understand this, you got the PCV and brake booster hooked into a fitting into the manifold. If this is the case, you are leaning out the one cylinder(#5 or 7) with the constant leak from the PCV. The booster isn't really a problem because it isn't a constant leak. By hooking the constant leak(PCV) into the carb port, you've entered it in a location where it effects all cylinders equally, and the carb mixture can be adjusted to compensate. This can also help a bit in idle quality and bring up manifold vacumm as you either aren't leaning out the one cylinder, or you aren't over-compensating by running the rest of the cylinders over rich to make up for that one.

What is your idle rpm? I think there is a wealth of info here. The basic problem is you have low manifold pressure/poor vacuum at low rpms. That's a no brainer. With a SBC and big cam with single plane intake it's not going to feel well at low rpms.

The advice to limit your vacuum leaks and use your vacuum appropriately are great I think.

But you can't manufacture vacuum without some rpms. You can minimize your use/loss at low rpm though.

Great stuff.

What about a Moroso vacuum pump? Can these work with or in replacment of a PCV valve? The cost difference is IMMENSELY different($400-500) but they're known to add 10-20hp as well. DO they help with a low vacuum idle as well?

I think with a vacume pump the engine needs to be sealed, and the pcv would not be hooked up so in all reality if the pcv is no longer sharing the same vacume source as the brakes the brakes should work better. ???

Greybeard, interesting info on the lean cylinder when using the big Holley vacume port for pcv, I've always run the pcv off of this port and never really paid attention to where it was getting it's signal from.

Greybeard actually I am pulling the vacuum off the larger (3/8" I think) constant off the back of the carburetor and then it is split there. The Victor JR. that i have does not have the normal vacuum port on it. I would drill and tap it, but I dont feel like pulling the intake off so that I dont get aluminum shavings into the cylinders.

I have a similar set up in my 69 Nova.
Huge Isky Solid cam, (548 lift, 264 @.050, on 108*) Victor Jr.intake, with a 650 DP Holley.
I run the PCV off the larger port on the back of the carb, with a "T" fitting to the Brake Booster.
The combo yeilds about 8" of Vacuum @ about 900 rpm. I have NO problems with the brakes at all.

BADDBOB71,

I was refferring to the use of a manifold fitting usually found on intake manifolds on the drivers side/rear that is designed for brake boosters. I've found many through the years where someone had T'd in the PVC to that fitting.
Going into the constant port at the back of the carb is much better, although, in theory at least, PCV would be better being introduced under the primary barrels.

seventy6speed,

You have the pcv in the proper port. I was seeing in my minds eye a port in the manifold being used. Don't drill the manifold for another port.

In the past 15 years I've put a Holley 80-85 jet in my PCV line and intoduced the PCV through the constant port at the front of the carb. Also, because I don't like "boxes" cluttering my engine compartment, I run ported vacumm to the distributor.