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NEGATIVE CRANKCASE PRESSURE

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M

MikeH

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Hi All, During a discussion on another forum, the subject of the dreaded rear oil seal leak came up. One suggestion by a respected member was to install a negative crankcase pressure system, which in his opinion, stopped almost all of his oil leaking; from wherever leaks were occuring. My question is, what the heck is a negative pressure system, and how does it work? Thanks, Mike

63 TR4 Surrey Top
 
PCV (positive crankcase ventilation) provides a negative pressure inside the crankcase. A PCV valve allows the engine crankcase fumes to vent into the intake system to be burned during the combustion cycle. The road draft tube on the earlier engines provided a similar function, but the car had to be in forward motion for it create enough of a vacuum to draw out the fumes...which, of course are dumped into the atmosphere. A well designed PCV system is much better at keeping crankcase pressures from building up and trying to force oil out of any orifice it can find. /ubbthreads/images/graemlins/eek.gif
 
Enough of this negativity, LBCs are supposed to be a positive experience. /ubbthreads/images/graemlins/smile.gif
The negative pressure thing is all that lawnmower engines used for oil control, in my experience. The timed breathers (flapper valves) were designed as to maintain a few inches of vacuum on the crancase without which the oil would get past the valve guides in a hurry as there were no stem seals used. The seals on the crankshaft did not themselves keep the oil in, but were for maintaining this vacuum which is what kept the oil in.
 
In addition to the common PCV systems, there are also venturi systems tied into the exhaust pipes on some cars to create a negative pressure at all times. The details escape me at the moment but I remember them being discussed in David Vizard's books. While his LBC texts are focused on the BMC A-series engines, the techniques apply to all. I can look up some of the details if you want. A quicker response may be to post this on the Spridget forum as some of those guys may already implement the method.
 
Thanks for the responses, the exhaust venturi system sounds interesting. I believe that David Vizard's books are available on line, I'll see what I can find.
Mike

63 TR4 Surrey Top
 
If you don't find it let me know and I'll see if I can locate the text at home.
 
I can vouge for PCV in my TR250, The DPO had is disconected & I would blow oil all around the valve cover gasket & front seal. I replaced the "gulp" PCV valve & all is well in the world. Saved a lot of time on my behalf. /ubbthreads/images/graemlins/cool.gif
 
3 common way to lower the pressure inside the crankcase that I know of are the pvc valve system, many drag racer install a angled pipe with a valve attached into the open header collecter that works like a venturi ( speed shops sell the setup but I don't think they work very with a full exhaust system), and the old smog pumps, many racers install updated versions of the smog pump to help ring sealing in hi HP motors.
 
My 1500 racer pushed about 3/4 quart out in a 30 minute session. I tried various tricks including a second vent via the fuel pump blanking plate.
Eventually, I left only a vent in the valve cover with a 1/2" ID hose attached to a 2" PVC plastic pipe spacer between the air filter and carb (I have a single carb). After I did this, the oil seal stopped leaking and I have run a 4 hour enduro with no oil lost.
I also have a threaded dip stick plug (I was losing oil out of that area as well).
Near as I can tell, the crankcase is under a mild vacuum. Keep in mind that my car is mostly at full throttle and high RPMs. I'm not sure if this would work on a street car.
I do not have a pcv valve in the system. The catch can I show is for "the rules"....it never has any oil in it.

A schematic of my crankcase system.
carb-vent-1500.jpg
 
HI,
i plan to put a 1.5" to 2" hose from my valve cover to a catch can in place of my battery,the size of a battery with a LARGE k&n air filter on it( probably flat rectange ON TOP AS LARGE AS POSSIBLE). run the crankcase vent into this same box. a very small amount of baffling, drain hole, and i'm done. battery goes in the boot(trunk).
ROB
 
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