Pulse minus PCV = Performance?

Minimichael

Well-Known Member
#1
So I wanna talk about little Briggs flatheads (the 2 and 3hp variety) and how the ways I've vented the crankcase may or may not be affecting performance in a good way.

The first engine, a model 8B, has a Keihin slide carburetor mod-fitted to it. The carb is fed by a small vacuum operated fuel pump (rectangular Mikuni clone - df44?) because it's mounted higher than the the gas tank. The pump is driven by the air pressure pulse coming from the crankcase. I draw the pulse through a fitting I've tapped through the valve cover -- not the little rubber outlet hose coming from the valve cover, because that is a one-way check valve that only allows air to flow outward, and I don't think the pump will work right if it can't feel both the push and pull of the air pressure pulse. Anyway, both the tapped line and the out-only check tube feed first into an oil catch can, and then out to the pump via a single line. That line runs the pump. This line also T's off and continues on to exhaust whatever extra pressure it may want to release; and this last part is specifically to serve the purpose of the original valve cover check valve, because it wants to be able to release the pressure that would otherwise build up in the crankcase. (In truth, this last leg of the exhaust line shows no perceivable pressure, tho I only tested this at idle rpm). All this being said, my little rebuilt 8B runs as it should.

The second engine is a 6BS. It is more or less unmodified; running the original tank and carb arrangement. But the one change I also tried here involves the valve cover's rubber outlet tube. Keeping in mind that in this case I didn't modify the valve cover so it is only pumping air outward. And of course the air pulse from the 6BS is strong enough that you can definitely feel it against your hand, because it doesn't have all the extra hoses and oil can of the first engine.

Anyway, with each of these engines, I've run the last leg of the hose into the air intake -- tapping each between air filter and carb.

Both engines run great, and I felt all warm and fuzzy because I'm also helping the environment by routing hot crankcase exhaust gases right back into the machine itself. I even made sure I told my kids about this space-age engine design feature. After all, I'd always heard about Positive Crankcase Ventilation and how it's something you don't want to be without. And I mean, isn't PCV a plus because you're feeding hot petroleum-ish air right into the carb itself?

Well, I've come to find out that if I fit the second engine's hose (the old 6BS) with a T fitting at the intake point, allowing it to flow not only into the intake (if it wants) but also instead head for the exit and leave the system entirely, out of a final stretch of open hose, it immediately starts running stronger. It's subtle, and I only have observed it at around idle rpms, but it definitely peps up with escape route opened. It almost sounds like a burden's been lifted, and the engine starts sounding like it's breathing easier. Yep, opening the system with just a little 3/16" hole does that.

I haven't noticed this with the 8B engine (at idle rpms anyway) but I also question whether this one has any pulse left to exhaust after the pump, hoses, can, etc..

So, I think my problem may be in expecting that hot air gassy mix to mix with gasoline and just be recombusted over and over. Fire needs fresh air. Still, I would think the fuel mix process should be helped at least a little bit. It has a lot going for it: it's warm; it's gassy; and it's getting shoved into the intake. But no, I'm thinking the gases being fed back into the 6BS stock carb via the stock rubber air filter mount w/PCV inlet, are dampening the air/fuel mix quality; it's affecting the carb mixture's combustibility.

Now, this engine's crankcase gas may be particularly heavy with blow-by exhaust forced down from the combustion chamber. It's a little plugger of an engine, but does need a rebuild. It may also be that the dampening will be negligible at higher rpms because so much fresh air is coming in and outflowing the pulse gas. I haven't been able to tell the difference when I'm riding. Maybe also the pulse gas is no match for the pressure created by the air rushing into the intake, and so the pulse can't even enter the mix at higher rpms.

This may be rudimentary stuff for some of the mechanics and racing people around here, but I kinda just wanted to write it all down!

I'm also wondering what other people know...

Is PCV helping engine performance? Is it an all around performance killer? Snake oil? Or is there a happy medium?
 
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#2
Nice write up!
I have always been led to believe cool air is more dense equates to better combustion which equals more power... hence exhausting the hotter air out, adding in the cooler air it would all make sense to me.

Unless you put a small turbo on it, or better yet a miniature Blower! Hmm thots to Ponder...;)
.
However I know nuttin honey and I have the receipts to prove it! :confused:

Now with that said I'm gonna follow along and see what the smart guys have to say...
 

SAS289

Well-Known Member
#3
Did you see the Coleman BT 200 build in "Projects and rides"? He's using an open crankcase vent off the rocker cover. His engine will breathe well.

I just use the stock rocker cover that has a PCV valve. The output (blow by) just goes into the atmosphere.

Crankcase ventilation is interesting.
 

Minimichael

Well-Known Member
#4
Did you see the Coleman BT 200 build in "Projects and rides"? He's using an open crankcase vent off the rocker cover. His engine will breathe well.

I just use the stock rocker cover that has a PCV valve. The output (blow by) just goes into the atmosphere.

Crankcase ventilation is interesting.
I think so, and thanks, I'll take a closer look! Interesting indeed.

Btw, I'm also thinking about if I let the case breathe too easily then my pump may not get enough of a pulse. I've read conflicting info on getting the pulse from the carb manifold, and just see a reliable pulse in the case.
 
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