Oil breather restrictions

[PeterK]

The CIS breather system on my CIS was

- 25mm ID hose from the triangle of death breather plate to the oil tank.  Mine has a corrugated hose inside, reducing the ID to 16/18 or thereabouts

- a slightly smaller ID hose from the oil tank to the inlet manifold, with a restrictor (about 6.5mm ID) where the pipe connects to the rubber airbox top

 

With my EFI conversion, I am not feeding the breather back into the engine, just into an oil catch can.  So, question - should I incorporate a restrictor ?
 

My thinking

 - If I use a restrictor, it wouldn’t matter whether it is fitted before or after the oil catch can

 - If I restrict the breather pipework, then more pressure could build up in the crankcase 

 - If I don’t restrict the breather, more oil could be carried to (& through ?) the catch can

 

I have machined an adapter to reduce the 25mm ID hose to 16mm ID to fit the catch tank. For now I have left an 8mm orifice /restriction, as I can always bore it out later, but it is beyond my welding skills to create one part way up the adapter later.  This is located between the engine breather plate and the oil catch tank.

[Phill]

I had no restrictor and very little ended up.in the catch tank. Very little of anything noticeable came out of the out pipe. Maybe @gt two can tell if anything has changed. Does much end up in the catch can now?

[PeterK]

Thx Phil

[gt two]

Hardly anything in the catch can ever but doing research this can be different car to car.

I changed mine recently actually, the bonding broke that held the old bracket to the inner arch.

I guess the heat and movement over time. The seals were also a bit leaky.

DIY special below which is lighter and shouldn’t leak. Fixed to an alu bracket.

Not pretty but functional.

 

[PeterK]

OK, thx

Guess I’m taking the adapter off and completely drill through then

[gt two]

Whatsapp me if you need anymore pics.

[PeterK]

Thx - adapter opened out & refitted

[70sThrowback]

On 11/01/2025 at 13:33, gt two said:

Hardly anything in the catch can ever but doing research this can be different car to car.

I changed mine recently actually, the bonding broke that held the old bracket to the inner arch.

I guess the heat and movement over time. The seals were also a bit leaky.

DIY special below which is lighter and shouldn’t leak. Fixed to an alu bracket.

Not pretty but functional.

 

I need something like this for mine. Where did you get it from please?

[gt two]

Hi, I made it from a karcher snow foam bottle, a hose fixing, a grommet and a K&N breather filter. I can send links if you need them.

You will need a dremel and a 28-30mm hole saw bit.