When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I have two '56 Chevies with SBC's using a PCV valve. One Chevy has the original 265CI and the other has a 327CI, and both have modern Edelbrock carbs and intake manifold. My newly overhauled 327 in my '62 C1 will keep the original Carte AFB and OE intake manifold. Any problem in keeping the down draft tube, or should I adapt a PCV from the down draft location to the old carb? How?
Bob
Bob,
I've been studying this matter lately and here's what I think I understand distilled down to the basics.
You need to vent crankcase pressure that builds up due to piston ring blowby, especially at high rpm. Otherwise the pressure can cause seals to leak. A road draft tube or other vent like on a valve cover can do this.
If you direct crankcase gases into the intake via a PCV system, you don't dump those unburned gases into the air and its better for the environment than a simple vent. The PCV valve allows gases to flow from the crankcase but will close in event of a backfire to prevent flame from reaching the crankcase gases.
If you complete the PCV system with a filtered vent that draws clean air into the crankcase as the PCV line draws gases into the intake, it will keep the engine oil cleaner longer.
Drawing crankcase gases into the intake will affect the air/fuel mixture and will require some carburetor tuning to compensate.
Yes, I prefer to use the PCV valve, and hooking it up to the Edelbrocks was never a problem. But how do I adapt it to the old Carter and intake? I have the diagram for the hook up.
Bob
Bob,
I'm far from expert on this but here's how it looks to me. The port for the road draft tube has access to the crankcase gas, and I assume you have an oil fill tube that also has access. If you can connect the road draft port to the carburetor base or intake manifold with a PCV valve somewhere in between, and fit the oil fill tube with a filtered, vented cap you have the system. If there is no port on the carburetor or intake manifold, perhaps you can remove the intake manifold and tap a hole in it for a fitting for the PCV hose. Others far more experienced in this than me may have better ideas.
Bob,
I've been studying this matter lately and here's what I think I understand distilled down to the basics.
You need to vent crankcase pressure that builds up due to piston ring blowby, especially at high rpm. Otherwise the pressure can cause seals to leak. A road draft tube or other vent like on a valve cover can do this.
If you direct crankcase gases into the intake via a PCV system, you don't dump those unburned gases into the air and its better for the environment than a simple vent. The PCV valve allows gases to flow from the crankcase but will close in event of a backfire to prevent flame from reaching the crankcase gases.
If you complete the PCV system with a filtered vent that draws clean air into the crankcase as the PCV line draws gases into the intake, it will keep the engine oil cleaner longer.
Drawing crankcase gases into the intake will affect the air/fuel mixture and will require some carburetor tuning to compensate.
All true - if your cars have the original road draft tube at the rear of the block you can plumb in a PCV valve with no issues. Providing there is a provision in the carb base for it. Don't need to get fancy - you will want to run a vented oil filler cap...
Something like this is all you need (you can find them cheaper BTW):
All true - if your cars have the original road draft tube at the rear of the block you can plumb in a PCV valve with no issues. Providing there is a provision in the carb base for it. Don't need to get fancy - you will want to run a vented oil filler cap...
Something like this is all you need (you can find them cheaper BTW):
I had this done on my '56's. No problem because I used Edelbrock carburetors. They had the tap. I'm not where I can look at my OE Carter AFB, but I don't remember seeing a tap anywhere on it.
I remember one of my old cars had a PCV hose going from the cap on the oil filler cap to the carburetor air cleaner. Could this work. It wouldn't look nice.
Could this PCV hose be mounted from the down drat location to the air cleaner?
Bob
I had this done on my '56's. No problem because I used Edelbrock carburetors. They had the tap. I'm not where I can look at my OE Carter AFB, but I don't remember seeing a tap anywhere on it.
I remember one of my old cars had a PCV hose going from the cap on the oil filler cap to the carburetor air cleaner. Could this work. It wouldn't look nice.
Could this PCV hose be mounted from the down drat location to the air cleaner?
Bob
Bob,
The hose to the air cleaner is for the air that goes back into the crankcase. They run that line to the air cleaner so the air has been filtered betore it goes into the engine. The crankcase gases coming from the engine go to either a port at the bottom of the carburetor or the intake manifold itself. .... It your carburetor does not have a port to use, perhaps you could put a spacer plate with a port beneath it. Otherwise, you're looking at tapping the intake manifold as I described earlier.
The PCV that runs through the air cleaner is called a “closed” PCV system. The early Corvettes had an “open” system running into the carb base. The open system is an easier conversion.
I had this done on my '56's. No problem because I used Edelbrock carburetors. They had the tap. I'm not where I can look at my OE Carter AFB, but I don't remember seeing a tap anywhere on it.
I remember one of my old cars had a PCV hose going from the cap on the oil filler cap to the carburetor air cleaner. Could this work. It wouldn't look nice.
Could this PCV hose be mounted from the down drat location to the air cleaner?
Bob
I do it that way on my '57 Chev. Hose goes into bottom of the air filter. I don't use a PCV valve.
You can purchase the conversion from most vendors. Mine attaches to the base of the carb.
This is what I did. Works great. You do NOT want to run a road draft tube if you drive your car regularly. A PCV will greatly extend the life of the engine oil, gaskets, and engine itself.
OK, I'm convinced I must use a PCV return system. I don't think my OE carb has a tap for it and I don't like the idea of tapping into the intake. I'm going to try to find an adapter plate for the Carter AFB that has the port on it. I'm thinking this adapter could also help insulate the gas in the carb and keep the car from burping or stalling in heavy traffic. A problem I have with the 327 in my '56 BelAir.
Bob
If you're talking about a C1 and adapter plate could cause a hood clearance issue...
Some AFBs, like the 3721S, DO have PCV provisions in the base:
Hard to see in this pic but the fat rubber hose in front of the vacuum can is the PCV hose going to the back of this AFB..
OK, I'm convinced I must use a PCV return system. I don't think my OE carb has a tap for it and I don't like the idea of tapping into the intake. I'm going to try to find an adapter plate for the Carter AFB that has the port on it. I'm thinking this adapter could also help insulate the gas in the carb and keep the car from burping or stalling in heavy traffic. A problem I have with the 327 in my '56 BelAir.
Bob
I'm betting that you can probably buy a spacer plate with a hole big enough for a vacuum fitting ready-made. If not, simple enough to make one out of block aluminum, phenolic resin ,or even hardwood.
I'm betting that you can probably buy a spacer plate with a hole big enough for a vacuum fitting ready-made. If not, simple enough to make one out of block aluminum, phenolic resin ,or even hardwood.
Now that the shop doing the work has told me there's a fitting on the carb they can use, I don't have a need for an adaptor plate with a fitting. I still like the idea of using an adapter plate just as a cooling insulator to keep gas from reaching a boiling point which can happen when stalled in traffic.
But as Frankie said, there could be a space problem.
Bob
I ran a 1/4" phenolic spacer on my dual quad 61 setup (both carbs) without a hood clearance issue - more than that and you will absolutely have a problem IMO - I know because I tried it... Make sure they double check before closing the hood before you go that route - particularly the air cleaner stud. I crush aluminum foil into a loose ball and secure it on high spots with blue tape and then slowly close the hood and then see how flat the foil gets as a precaution...and quick check...
Make sure your shop knows to add a vented oil filler cap for your PCV system.
They'll also want to get a right hand 1963 bell housing spark plug wire loom to replace the one that gets removed when the road draft tube is removed - as its welded to the tube... The bottom center of your post #4 above - or the wires will be flopping around.
02-04-2019, 10:38 PM
Down Draft Tube or PCV Valve
