Engine breather.....Do I need it?

[Datman]

Hey guys,

I know it seems like a dumb question, but with the dry sump set-up I have a breather off the oil tank (in the boot of the car). I have noticed, that there is still a small amount of airflow coming from the breather in the rocker cover.

Do i need to have a breather on the rocker? Would the extra gasses be able to work their way out through the dry sump system and into the breather in the boot?

The preferred option for me would be to blank off the rocker cover breather, if I can get away with it.

If the block did pressurise, what damage or side effects would it cause?

Any thoughts?

[Billzilla]

Nope, no breathers on the engine with a dry-sump setup.
Just in the scavenge tank.

[GTSBoy]

Pressurised crank cases can blow out crank seals (and other seals) resulting in the usual calamities. Whether your scavenge pump will limit how high the crankcase pressure gets is an unknown (to me anyway).

cheers

[Billzilla]

GTSBoy - The scavenge pump must be at least 150% of the volume of the pressure pump, so the engine must be in partial vacuum all the time.
From 150% to about 200% is a good size for the scavenge pump compared to the pressure pump.

[Datman]

I thought the same Bill......but as I say, there is a small amount of airflow coming from the breather at the moment.

The reason I am unsure is, the dry sump set-up uses the engines (FJ20T) standard oil pump for pressure, and a two stage scavange only pump. I would have thought, with the two stage scavange (one pressure effectively), and the obvious gearing of the pump(2:1 I think) it should run in vacuum all the time. I meant to check the amount of airflow from the breather when it was on the dyno under load and at higher revs....but I bloody forgot to do it!

Do you reckon she will be ok to blank off?

[Billzilla]

I thought the same Bill......but as I say, there is a small amount of airflow coming from the breather at the moment.

The reason I am unsure is, the dry sump set-up uses the engines (FJ20T) standard oil pump for pressure, and a two stage scavange only pump. I would have thought, with the two stage scavange (one pressure effectively), and the obvious gearing of the pump(2:1 I think) it should run in vacuum all the time. I meant to check the amount of airflow from the breather when it was on the dyno under load and at higher revs....but I bloody forgot to do it!

Do you reckon she will be ok to blank off?

That's weird ...
Yes I'd blank it off.
FWIW on the dry-sump setup on my Suzuki, it sucks in the rubber bungs in the back of the head (where they bore the camshaft bearings) unless they're in back-to-front.
Suffice to say there's plenty of suction there.


Try blocking the hole with your finger, it might turn to suction ...
(The Fraser system does)

[Datman]

OK....I'll check with the finger tonight (ooo...that sounds soooo wrong!) I have had some race tape over the hole, and it doesn't blow it off, or suck it in for the matter....doesn't seem to have anyeffect at all!

I'll put my hand over it, and give her some revs to see what happens.

Next time I hit the dyno, I will see if it makes any diffence to the power, I'll run it with and without the breather blanked off.

Thanks Bill.

[slo360]

hey datman, another thing to check, especialy if you have a gear type pump is that you run quite large return lines to the tank. On my car with a 3 stage pump running the two scavange stages into a tee reduced my pumps ability to make any real vacuum at WOT, yet running two full size lines fixed it(nominal 5-7inhg now). Also do your lines tend to be fairly long and uphill(i pressume so with a boot mounted tank) as this may also affect it to. Although if you have a gerotor style pump i cant realy think of any other reasons why it wouldnt cope, another thing to try is to plumb a vac gauge to your rocker cover, plug all other holes and go for a spin and varify that your getting some vac at wot. Also what is the pump speed relative to crank speed? is it around 50%?

[Datman]

I did a little test last night.....by puttimg my hand over the breather (sealing it), there was no change in the engine note at idle. When I revved the engine, and held the engine at higher revs, there was certainly no obvious force either way (positve or negative).

I also placed my hand over the breather...but not touching it, then revved the engine. It actually seems like the amount of air flow from the breather becomes less with revs, so perhaps as the revs rise so does the level of vacuum.....does that sound right?

[Datman]

hey datman, another thing to check, especialy if you have a gear type pump is that you run quite large return lines to the tank. On my car with a 3 stage pump running the two scavange stages into a tee reduced my pumps ability to make any real vacuum at WOT, yet running two full size lines fixed it(nominal 5-7inhg now). Also do your lines tend to be fairly long and uphill(i pressume so with a boot mounted tank) as this may also affect it to. Although if you have a gerotor style pump i cant realy think of any other reasons why it wouldnt cope, another thing to try is to plumb a vac gauge to your rocker cover, plug all other holes and go for a spin and varify that your getting some vac at wot. Also what is the pump speed relative to crank speed? is it around 50%?

Slo,

Thanks for the response. I run a single -12 line from the pump back to the tank, and yes the inlet for the tank is higher than the pump. After my little test (see above) last night, I am leaning towards sealing the breather, your idea of a vacuum gauge is agood idea though, I might look into that further.

Cheers,

Mark.

[Jim]

be aware that the volume in the crankcase that even a psi of pressure may cause leaks or other issues. sounds like the volume of the dry sump pump isn't enough....

[slo360]

hey datman, i think you realy need to try your testing at WOT, as when you free rev it the pump will be at its most effective yet blow by will be minimal, so to get a true indication you realy need to hook up a vac gauge and either find a hill to drive up or a dyno to do full load/WOT tests, and as secoh said , if you seal the crank case and have pressure bad things happen. Also what size breather line or breather outlet does it have??