Crankcase EVAC to header, VS just a breather.

[thefarmboy21]

I'm thinking about building an EVAC setup for my Hemi-predator, instead of just using a breather that does nothing. Anyone have any experience with this on a Kart? We always ran them on our V8 race engines and I would assume the same theory applies here, but I never see them on any Karts, even race karts, but I don't know why.

The concept of the PCV system is to draw out the vapors and fumes from inside the crankcase, but with the aftermarket parts setups, most people just put a breather on there....which other than being a vent, doesn't really do anything.

The race version of that, would be the EVAC system....which on top of acting like a PCV, the evac system is piped into the exhaust which creates a bigger vacuum pull. In turn pulling more vapor from inside the engine AND they claim it creates a vacuum of sorts on the piston....which they say helps "suck" the piston down faster and help gain higher RPM's.

Thoughts?

 

[Noseoil]

If it's "pulling" the piston down, which means it's creating a vacuum for the up-stroke to have to work against. I just use a catch pipe & breather for my crankcase vapors. Works for me.....

 

[thefarmboy21]

You would think that, but all the racers in basically every sport use them, sooo......

 

[itsid]

Hm,
on a multi cylinder engine, that kinda makes sense to have a low pressure crank case and keep it at that.
pistons are synced in a way that the inside volume remains constant.
And since the "straw" setup can only create so much of a low pressure... it makes sense
(does not apply to two cylinder boxers )

Anyways.. a single cylinder is a different breed though;
the crankcase volume changes during piston strokes, since there is no second piston to counteract that.
Also the exhaust speed -which you need to create the underpressure- is ONLY happening at the exhaust stroke (which by design is the UPWARD movement of the piston)
the one thing you are doing is trying to restrict said upward movement of the piston.
frankly, I doubt the effect will have measurable values tbh..
but the simple fact remains, when the piston is travelling downwards, the output valve is closed (no suction); and when the output valve is open to create the underpressure, the piston is travelling up.
(that might create an overall underpressure in the crankcase at best, which resides in there to kinda pull the piston down in the next cycle.. but frankly... it helps less later than it hinders first IMHO)

So yeah.. on multi cylinder engines, that might make sense (not always, not much even, but certainly at times a bit)
on single cylinder engines that sounds actually like a stupid idea.

'sid