[XNTRCI-T]

Do PCV valves come in different sizes? I think I need a larger one than I have. I have a 6-71-huffed 355 sbc, and when I get on it pretty good I blow oil mist out the breather. I'm running one of these...

I tried removing the PCV valve from the housing to reduce the restriction. That cured the mist-out-the-breather problem, but I was sucking so much oil through that unrestricted line that it fouled my plugs, so I put the valve back in.

I thought about going to an exhaust-scavenged set-up, but I thought I'd try a larger valve first to see if that solves the problem.

Thanks in advance for any help! :thumbup:

 

[BogiesAnnex1]

Do PCV valves come in different sizes? I think I need a larger one than I have. I have a 6-71-huffed 355 sbc, and when I get on it pretty good I blow oil mist out the breather. I'm running one of these...

I tried removing the PCV valve from the housing to reduce the restriction. That cured the mist-out-the-breather problem, but I was sucking so much oil through that unrestricted line that it fouled my plugs, so I put the valve back in.

I thought about going to an exhaust-scavenged set-up, but I thought I'd try a larger valve first to see if that solves the problem.

Thanks in advance for any help! :thumbup:

A huffed engine cannot use a PCV valve when the manifold pressure goes positive, the valve only functions with manifold vacuum (pressure less than atmospheric) is present. When the blower puts pressure on the intake side of the valve it closes, then the crankcase pressurizes and you see the oil mist from the breathers since there's no where else for the vapor to go, although some valves will open to the effect of crankcase pressure, their ability to pass enough vapor is restricet to a point where they can't handle the amount of blowby a blower motor can make.

There is no real easy solution:

a) One can buy a bag of rags and just clean up the mess the vented oil makes.

b) Another is to tube the breathers up to the intake of the blower, but this can put a lot of oil into the mixture which screws up detonation resistance, something a blower motor really doesn't need even more so than a normal motor.

c) Routing the breathers to a catch tank to trap and condense the vapors is a good solution, you can even put a drain back to the valley or a valve cover so that condensed oil can return to the engine. These can finally breath overboard (i.e. road tube) or back to the blower inlet, or into the exhaust. I like a one way valve on such installations though there are arguments against this, but my favor is to prevent a backfire from lighting off the vapors in the crankcase.

d) If you're not using catalytic converters you can take (c) above and route the vents into the exhaust system as long as the crankcase pressure is higher than the exhaust back pressure. Again one way valves are a good idea as to keep exhaust gases out of the crankcase when there is no intake boost pressure. You can tee into the collector or the exhaust pipes with an appropriately sized tube for the hose size your using at a 45 to 30 degree angle with the direction of the exhaust flow. This will help keep a negative pressure on the evacuation system which aids flow out of the crankcase.

Bogie

 

[XNTRCI-T]

Thanks, Bogie. I always enjoy reading your well-thought-out, detailed responses in these forums. Here's a little more info...

Here's my hot rod...

As you can imagine, I'd rather not clutter up the motor or firewall (polished stainless) with a bunch of hoses or cans.

My PCV hose is plumbed to the PCV nipple at the base of the rear (primary) carb. I pull 15" vacuum there at idle and normally see 8"-10" there at cruise throttle, so I'm getting good suckage on the PCV line.

I don't seem to be getting a lot of oil through the PCV now, but I got a ton when I took the valve out of the line. I had basically an open 3/8" tube with 15" of vacuum on it, so I assume I was moving a lot of air through there and pulling a lot of oil with it. Also had the effect of a huge vacuum leak!! :nono:

When I took the valve out I cleaned it with brake cleaner, blew it dry with the air hose and sucked on it with my mouth to see what kind of flow I could get. I was surprised at how restrictive it is.

I have one of these left over from my experimental aircraft days...

It's an air/oil separator used on airplane motors. I'm thinking I might try to adapt it into my PCV system. It's all aluminum and could be polished nicely.

My last choice would be be your suggestion d). That is probably the best technical solution, but in my case it has a couple of negatives. First I would have to weld a bung or tube into my ceramic-coated header that hangs out in front of God and everybody, and secondly I will be blowing burnt oil out my side-pipe onto my rear tire and whatever else is nearby. ain:

 

[American Muscle]

how many horses you pushing? that is a nice ride. Wanna race my 305 HAHAHA

 

[XNTRCI-T]

how many horses you pushing? that is a nice ride. Wanna race my 305 HAHAHA

The guy that built my short block (race motor shop) estimates 500-525. All I know is it's way more than I can get to the ground. The blower is more for the cool factor than performance. I'm only running 1:1 for about six pounds of boost. I've got a 5600 pill in the MSD-6AL and I've only been on the chip once in four years! ain:

 

[American Muscle]

I just had a 355 to crap out on me(poorly built). What are your engine specs cam, heads, etc.

 

[XNTRCI-T]

Short block by Sehr Performance, Sioux Falls, SD...
- 1979 350 4-bolt 010 020 block out of a C-10 chassis
- checked for core shift & magnafluxed
- align bored crank & cam
- parallel decked
- bored .030 over, torque plate honed
- oil passages dressed & oil galley holes threaded & pipe plugged
- GMPP crank & rods (don't have exact specs, stock stroke)
- Sealed Power dished "blower" pistons
- Comp roller hydraulic "blower" cam - 224/236 @ .050, 112 lsa, .438 lift
- Comp timing set

Build-up by me...
- Dart Iron Eagle S/S 76cc heads (8.2:1 cr with the dished pistons)
- Comp full roller lifters (Pro Magnum)
- Blower Drive Service 6-71 blower & drive kit @ 1:1 for 6# at 5000 rpm
- 2X Edelbrock 1405s (600 cfm) re-worked for the blower per Edel specs
- MSD billet mag-pickup distributor, 6AL box & Blaster II coil
- initial timing 18, total 34 all in by 2800, no vac advance, no boost retard

 

[BogiesAnnex1]

Thanks, Bogie. I always enjoy reading your well-thought-out, detailed responses in these forums. Here's a little more info...

Here's my hot rod...

As you can imagine, I'd rather not clutter up the motor or firewall (polished stainless) with a bunch of hoses or cans.

My PCV hose is plumbed to the PCV nipple at the base of the rear (primary) carb. I pull 15" vacuum there at idle and normally see 8"-10" there at cruise throttle, so I'm getting good suckage on the PCV line.

I don't seem to be getting a lot of oil through the PCV now, but I got a ton when I took the valve out of the line. I had basically an open 3/8" tube with 15" of vacuum on it, so I assume I was moving a lot of air through there and pulling a lot of oil with it. Also had the effect of a huge vacuum leak!! :nono:

When I took the valve out I cleaned it with brake cleaner, blew it dry with the air hose and sucked on it with my mouth to see what kind of flow I could get. I was surprised at how restrictive it is.

I have one of these left over from my experimental aircraft days...

It's an air/oil separator used on airplane motors. I'm thinking I might try to adapt it into my PCV system. It's all aluminum and could be polished nicely.

My last choice would be be your suggestion d). That is probably the best technical solution, but in my case it has a couple of negatives. First I would have to weld a bung or tube into my ceramic-coated header that hangs out in front of God and everybody, and secondly I will be blowing burnt oil out my side-pipe onto my rear tire and whatever else is nearby. ain:

Looks good, I like airplane parts on cars, bikes, boats, and of course airplanes.

Don't forget a bag of red rags is always an option, LOL!

Bogie

 

[BogiesAnnex1]

Short block by Sehr Performance, Sioux Falls, SD...
- 1979 350 4-bolt 010 020 block out of a C-10 chassis
- checked for core shift & magnafluxed
- align bored crank & cam
- parallel decked
- bored .030 over, torque plate honed
- oil passages dressed & oil galley holes threaded & pipe plugged
- GMPP crank & rods (don't have exact specs, stock stroke)
- Sealed Power dished "blower" pistons
- Comp roller hydraulic "blower" cam - 224/236 @ .050, 112 lsa, .438 lift
- Comp timing set

Build-up by me...
- Dart Iron Eagle S/S 76cc heads (8.2:1 cr with the dished pistons)
- Comp full roller lifters (Pro Magnum)
- Blower Drive Service 6-71 blower & drive kit @ 1:1 for 6# at 5000 rpm
- 2X Edelbrock 1405s (600 cfm) re-worked for the blower per Edel specs
- MSD billet mag-pickup distributor, 6AL box & Blaster II coil
- initial timing 18, total 34 all in by 2800, no vac advance, no boost retard

I'd be a bit cautious about no pressure retard, as the density of the mixture trapped in the cylinders goes up, as happens with a blower, the burn speed increases which reduces the need for as much advance as would be required if the engine wasn't huffed. 6 pounds of boost is modest and should be safe, but to be sure keep and eye on the plug insulators. If they're getting toward toasty white, perhaps taking some advance out with a pressure retard would be a good idea.

Bogie

 

[XNTRCI-T]

...perhaps taking some advance out with a pressure retard would be a good idea. Bogie

I was worried about that when I was putting the motor together, so I talked to Scott Sehr (the short block builder and my tech support for the engine build), Blower Drive Service and MSD. The general consensus was with my relatively modest build, my driving style (very conservative, no racing) and my 1900 pound car, boost retard was probably not a must have. I pulled the heads off last winter and there were absolutely no signs of detonation... motor looked brand new inside (only 2000 miles ince new!). As I said above, I've only stood on it long enough to get on the 5600 chip one time, and that scared the bejeezus out of me!