O.K. this is probaby a silly question not asked much but I'm not sure myself so, just to make sure what exactly does positive crankcase pressure mean (a vaccum that pulls air out of the engine) is what I'm kinda thinking with a check valve so backfiring won't mess stuff. Kinda a round about answer I know but I do know It's how your engine breaths. Anyhow here's the question when you've got a built engine that is camed up (Insky with 280 duration, 106 centerline, and 510 lift) it only pulls about 5-7 pounds of vaccum at an idle, so do I need a PCV valve or do I not pull enough vaccum to operate it correctly. Or should I put a breather into the valve cover like the other side. Another thing is I haven't deceided on which carborator to go with yet I'm probably gonna have to buy a new one but I'm not really a fan of holleys but that's what kinda intake I have and don't have the clearence for an adapter plate, plus intake's ported and polished to match my heads and I would hate to change intakes so what to do. (Quadrajet would be my first choice if it wasn't for above)
Just wandering what suggestions are out there and how much cfm exactly I need and maybe even how to fine tune it to my combination would be great!! This is what I'm working with it's a 350 that's .040 over and has 10:1 pistons in it, along with a set of old double hump heads (casting 492 I believe there the 375 horse heads, stainless steel 2.02, 1.60 valves) thats been ported and polished along with my intake ( the guy who ported my heads is great even though I've never meet him ( a buddy deal if ya know what I mean) I say that because they look great and he even stamped the cc's into each port and there all within 3 tenths of each other so I was impressed ( maybe you will be to, maybe not) anywho It's also got a performer rpm intake and hedman headers with enlarged pipe and a flowmaster, along with an HEI distributor and Accel super coil and a module. Well what do ya think? any suggestions are greatly appricated!!!!!!!!
its in a 91 camaro with a 5-speed too! just thought ya might be curious. Thanks

There are 2 vacuum sources on an engine and both deal with the carb. Ported and unported. The 5-7" of vacuum you measured is coming from inside the intake manifold. Unless you need vacuum for power brakes, this only shows how much vacuum the pistons are creating while pulling in an air/fuel mixture. If there isn't enough to operate power brakes you can switch to maunal brakes, install a vacuum reservoir or install an electric vacuum pump assist.

Positive crankcase pressure is something different. It's the pressure inside the crankcase/oil pan. When a piston moves up and down, there's an equal amount of air under the piston as the is on top of the piston. Extra pressure can be created from blowby past the rings. When the crankcase is pressurized, the pistons have to compress the air when going down. Pressurized air also forces oil past seals and gaskets.

A simple PCV valve can help evacuate this excess pressure but as you stated, with low manifold pressure, it's not very effective although 5-7 is still enough. I use a header evac system that uses the exhaust to pull crankcase pressure out. Other racers have installed electric or belt driven vacuum pumps to do the same thing more efficently.

Pulling the crankcase into a vacuum allows the pistons less resistance on their downward stroke which can increase hp however there are a few drawbacks. If the crankcase is pulled into too deep a vacuum, lower than 12", it's possible to actually suck the oil out of the wrist pins. An engine that pulls this much vacuum should also use low tension rings. Setting up an engine that constantlt uses an efficent vacuum pump should also have the rear seal installed backwards. Since the crankcase is in a vacuum, it will be pulling air in past the seal instead of preventing oil from going out. Since the crankcase is in a vacuum, there's hardly and seal/gasket leaks.

If you don't want to switch intake manifolds, and I have no idea why you want to run a factory type manifold, you can still use a performance spreadbore carb by using an Edelbroke performer carb. It's a spreadbore that's based on the old Carter AFB carbs. Stock Eliminator racers are allowed to use these carbs becasue the old Quadrajets are getting hard to find. As for quarebore carbs, there's nothing wrong with Holley carbs. Chances are you've never used a new one. Most people keep fiddling with old carbs with no luck and are unimpressed. Buy a new HP series or buy a Demon carb. Make sure it's a double pumper since a vacuum secondary carb should only be used for street use.

Well thanks for your responses, I think I' m understanding the vaccum thing a little better now, my buddy has the exhaust port or whatever ya call it on his headers that hookes up to his pcv valve but my headers don't have the port, was wondering if I need a pump or something, I already have a vaccum canister for my power brakes. well anywho I have actually bought 2 new holleys but never a demon one and no actually still haven't been impressed even though they worked a lot better than the other senerio you described which I've done also which is tuning on old holleys or rebuilding them anyway how much cfm do you recomend I figure about 650 would be sufficent enough if it were a mechanical secondaries (I don't really want to depend on vaccum to open my secondaries) and do you have any jetting tips I've never tweaked out a carb to get the max out of it but would like to. anyway thanks for your help