How Do You Properly Run An Oil Catch Can?

[AK260]

And from the FSM ...

[AK260]

So if I’m reading this correctly, it’s saying that the cam cover / rocket cover works to suck blow-by gasses at full throttle through the airbox and at idle it’s done via the block breather and PCV valve.

So - let’s choke get my own thinking: does this mean that blocking the block on and just using the rocker cover outlet is sufficient if you just plumb it into a tank!? Could that be the logic behind the DSI thinking on blocking them?

As the captain pointed out, the bottom and top aren’t sealed from each other. So blocking the bottom one shouldn’t pressurise the crank case.

But does blocking the bottom one just mean more regular oil changes required as the vapours are less effectively extracted? And doesn’t a catch tank without a vacuum mean the same thing really?

Not sure about you chaps, but my oil is changed annually which equates to an average of 1000 miles per oil change anyway.

[Barefootdan]

Bumping a super other thread to life. But I'm in the process of building a catch can setup for my Z that needs to be sealed to conform to NASA regulations. I was planning to do crankcase and valve cover to a two port can. But the above option looks like a possibility as well perhaps? 

[Zed Head]

Which option above?

Whichever way you run the hoses there needs to be a connection to the intake system, if you want a sealed system.  That's where the crankcase pressure goes.  There's always blowby so there's always going to be pressure.

The simplest method would be to maintain the stock flow paths but to extend their length to run through the catch can(s).

[Barefootdan]

Thanks Zed Head. I think I was misunderstanding the NASA CCR rules for running a catch can. Now that I am rereading, it sounds like it just needs to terminate within a can, but the can itself can be vented.

 

I just assumed it needed to be a sealed system. I am running triple mikuni carbs so running a sealed system was causing a headache. 

[Racer X]

Why not run the atmospheric vent from the catch can to the exhaust downstream of the header collector, using a one way check valve, like the drag racers do? Any excess vapors are drawn into the exhaust and are burned there.

 

Edited August 26, 2023 by Racer X

[Barefootdan]

That is a good solution @Racer X....that requires more work 

[grannyknot]

On 8/24/2023 at 1:07 PM, Racer X said:

Why not run the atmospheric vent from the catch cat to the exhaust downstream of the header collector, using a one way check valve, like the drag racers do? Any excess vapors are drawn into the exhaust and are burned there.

 

Racer, I like it.

Does there need to be any kind of fresh air going into the engine or is thew blow by enough?

[Racer X]

17 hours ago, grannyknot said:

Racer, I like it.

Does there need to be any kind of fresh air going into the engine or is thew blow by enough?

Well what goes out needs a source. Otherwise a vacuum would be created. Just enough exchange of volume is needed to maintain a near neutral balance between atmospheric pressure and crankcase pressure.

There is always some combustion pressure that leaks past the pistons. If the crankcase pressure is too high or low, the movement of the pistons will be affected. Pan evacuation is one means to keep the parasitic losses to a minimum.