Best explanation of positive crankcase ventilation (PCV) I've ever seen...
- Thread starter Lee_in_KC
- Start date
Screaming Metal
Active Member
Here, Here, Here................:clap:
Ted Brown
Member
Lee, You have a BLOWN engine, there is plenty of pressure inside your block and oil pan, best vented into your header system, a special valve is used for this, a one way valve mounted at an angle into the exhaust sustem, the faster you go the more it draws... I myself, HATE vacuum leaks, PCV is a giant vac leak as far as what the engine thinks... Don't plug up your breathers.. 
Lee_in_KC
Active Member
Yeah Ted, that's how I ran across that aritcle. I've been researching PCV recently because I'm getting a lot of blow-by and sludging up my oil pretty quickly. If I get on the motor much at all the PCV can't keep up and I blow oil mist out the breather. I tried removing the PCV valve and just running a straight-open line and that helped with the blow-by but it fouled my plugs with oil. I was able to run with that "giant vac leak" because I was already running the carbs pretty rich. I am now back to using the PCV valve, so I still have the "mist out the breather" issue and my fresh oil with less than 500 miles on it is already turning brown.
I think my problem is the PCV valve I have is too restrictive. Does anybody know if there are different sized valves? Seems to me the larger your displacement, the larger a valve you would need.
I was aware of the exhaust evacuation system, but my understanding is that this is really a racer set-up and is only effective at higher RPMs. Anybody know different?
I think my problem is the PCV valve I have is too restrictive. Does anybody know if there are different sized valves? Seems to me the larger your displacement, the larger a valve you would need.
I was aware of the exhaust evacuation system, but my understanding is that this is really a racer set-up and is only effective at higher RPMs. Anybody know different?
Wild Mango
Member
Lee, IMHO sounds to me as though you better leak that baby down if she's oiling plugs and misting oil out the breather. They all breathe a bit but yours sounds excessive. The best SBC breather systems are the ones that come out of the front of the covers and up to a couple of outlets over the thermostat housing, with filters to catch the oil mist that makes it up that far. Not with a blower, though.
Blow motors usually run a 1" line from each cover to a catch can. Or you can vent it into the header collector as long as the dump pipe is pointed away from the car cause it'll speckle tarry stuff on the body work and make one sticky mess. Don't vent it into the motor intake.
With a normally aspirated engine it's manifold vacuum that sucks oil up past the rings and fouls the chambers diluting the fuel charge. Way worse with alcohol which won't tolerate oil at all. With our Midget engine we run low tension/friction rings with piston gas ports on the top ring and pull a vacuum on the crankcase with a Moroso vacuum pump to counter the vacuum in the intake system. About 10"WG per 1000rpm works a treat.
Your blow motor probably won't pull any manifold vacuum to speak of so a vacuum scavenge on the crankcase won't help, nor will a PCV valve which needs vacuum to work. Free breathers will get the job done for you, long as the ring seal is doing it's job.
Blow motors usually run a 1" line from each cover to a catch can. Or you can vent it into the header collector as long as the dump pipe is pointed away from the car cause it'll speckle tarry stuff on the body work and make one sticky mess. Don't vent it into the motor intake.
With a normally aspirated engine it's manifold vacuum that sucks oil up past the rings and fouls the chambers diluting the fuel charge. Way worse with alcohol which won't tolerate oil at all. With our Midget engine we run low tension/friction rings with piston gas ports on the top ring and pull a vacuum on the crankcase with a Moroso vacuum pump to counter the vacuum in the intake system. About 10"WG per 1000rpm works a treat.
Your blow motor probably won't pull any manifold vacuum to speak of so a vacuum scavenge on the crankcase won't help, nor will a PCV valve which needs vacuum to work. Free breathers will get the job done for you, long as the ring seal is doing it's job.
Lee_in_KC
Active Member
Thanks for the info, Mango. Part of my problem, I think, is that I don't have good ring seating. Motor only has about 2,000 miles on it since newly built. Short block was built by a reputable racing motor builder who race-prepped the block... align bored (crank and cam), parallel decked, bored .030 and torque plate honed. I pulled the heads a couple of months ago to look for signs of detonation (none) and check the cylinder walls. The cross-hatching looked very fresh to me (I'm not a motor guru). I didn't do anything special to break the motor in (roller cam), I just started driving. I'll see if someone in our car club has a leak-down rig and give 'er a look-see.
By the way, I think the plugs got oiled because without the PCV valve in the line from the valve cover to the base of the carb, I was pulling some serious air volume through that line which had to be carrying some oil with it, even though I do have baffles in the covers. I put the PCV valve back in and I don't have oily plugs any more.
By the way, I think the plugs got oiled because without the PCV valve in the line from the valve cover to the base of the carb, I was pulling some serious air volume through that line which had to be carrying some oil with it, even though I do have baffles in the covers. I put the PCV valve back in and I don't have oily plugs any more.
Lee_in_KC said:
Hey Lee , I have the same problem. I've been running just breathers on both valve covers and if I get on it hard, I get a little oil out of the breathers. I'm about to switch valve covers and put a PCV in one side and a breather in the other to see if it helps any.
Scavengers are the way to go. they will do a fine job of venting the engine at low rpm( it doesn't really take that much) It sounds like your big problem is at full tilt boogie anyway. Thats when the scavengers really go to work. I run them on my drag rail and have no misting, plug fouling, no trouble at all. good luck.
Ted Brown
Member
When using the stock type inlet/outlet breathers, I always run a catch can type of unit, with an oil return line back to the engine, that can I make using a 2 inch tube with 45 degree angle cuts in about 3/4 of the way through on two opposite sides, to catch the oil before it gets to the top and out a hose to where ever you exhaust it, that way there is very little oil at all getting out... even my Big Block needed that under hard pedal... Ride safe...
Wild Mango
Member
Lee, I think I would leak her down just to eliminate that as a cause. All SBC's tend to breathe a bit of oil because of the crankcase design. A pan with a windage tray helps but its still a nuisance. In your case being blown your cylinder pressures are that much higher and just makes it worse.
Compounding the problem is that crankcase fume tends to stop oil draining back into the pan because the drain passages in the valley are limited, and oil collects in the valve covers for the same reason. I think its mostly the oil in the valve covers that gets blown out by the escaping fume, and so Ted's fix of traps in the breathers and plenty of breather capacity is what works, we know that.(the original SBC's had an oil filler/breather 1 1/2" pipe direct into the valley space through the inlet manifold)
Speedway sell a kit to make your own "Ted" type breathers and I always use tall valve covers for the same reason.
IMHO I would fit 1" hose connect type breathers to each valve cover and plumb them back to a vented catch can. Use braided hose and the engine looks more and more like it means serious business and really belongs on the strip. Cool....................
Compounding the problem is that crankcase fume tends to stop oil draining back into the pan because the drain passages in the valley are limited, and oil collects in the valve covers for the same reason. I think its mostly the oil in the valve covers that gets blown out by the escaping fume, and so Ted's fix of traps in the breathers and plenty of breather capacity is what works, we know that.(the original SBC's had an oil filler/breather 1 1/2" pipe direct into the valley space through the inlet manifold)
Speedway sell a kit to make your own "Ted" type breathers and I always use tall valve covers for the same reason.
IMHO I would fit 1" hose connect type breathers to each valve cover and plumb them back to a vented catch can. Use braided hose and the engine looks more and more like it means serious business and really belongs on the strip. Cool....................
Wild Mango
Member
Yep Lee thats the idea although in a road car tipping the catch can out now and again is no hardship, a healthy engine doesn't blow that much out.
I know it goes everywhere on a fuselage, I am an EAA man myself - another topic altogether!!!!
I know it goes everywhere on a fuselage, I am an EAA man myself - another topic altogether!!!!
Screaming Metal
Active Member
OK....I'm back.......got a blown motor and it spitting out oil or just misting slightly??????
Do what Ted suggested and Mango....they got ya on the right track.....on the outside chance things are a messing up with blowby........this grates my nerves but sometimes you gotta do this.............call the guys that built your engine........ask if they've put it together with cromemoly rings just on the top OR on both top and bottom compression rings.....
If its both..........you'll have to run a 200 dergree thermostate FOR a couple of weeks, if you drive it a little.....if your on a long weekend cruise (800 to 1000 miles), you could change back to your lower temp. AFTER your rings are run in. Both being chrome will put a lot of wear on your bores.........
On hot racing engine thats gonna see duty on the street........run a chrome moly top and a cast iron second.....they'll seat fairly fast and wear well, both........
Running exhaust scavaging system will keep ring flutter down and also help with ring sealing.........................................
Do what Ted suggested and Mango....they got ya on the right track.....on the outside chance things are a messing up with blowby........this grates my nerves but sometimes you gotta do this.............call the guys that built your engine........ask if they've put it together with cromemoly rings just on the top OR on both top and bottom compression rings.....
If its both..........you'll have to run a 200 dergree thermostate FOR a couple of weeks, if you drive it a little.....if your on a long weekend cruise (800 to 1000 miles), you could change back to your lower temp. AFTER your rings are run in. Both being chrome will put a lot of wear on your bores.........
On hot racing engine thats gonna see duty on the street........run a chrome moly top and a cast iron second.....they'll seat fairly fast and wear well, both........
Running exhaust scavaging system will keep ring flutter down and also help with ring sealing.........................................
Screaming Metal
Active Member
Anytime you have a healthy engine with a serious knotstick in it, wether its a mushroom tappet or rollercam........run stud girdles and this is gonna get you into the tall valvecovers automatically since they're the only ones that'll clear. Get yours with the the baffles..............it'll help......then you can do your stuff on trying to keep the slippery stuff inside the engine block......................
de the engine block......................
