Oil breather

[julian64]

Does anyone know exactly where the oil breather pipe goes inside the engine.

Always been struck just how much oil is present in the airbox after a drive and wondered why.

Surely the oil breather is not below the oil level, and if its sitting high how come so much oil manages to be pumped up it.

Just trying to understand

[mikesr]

Isn't just a hole through the cam cover with a pipe bolted on the outside and thus has the cam chains whizzing around very close to it?

So lots of flying oil and the suction into the air box...

[ridds]

Bang on. Although unless your rings are letting a fair bit of gas past then you shouldn;t see too much oil.

High revs with low load will cause more oil to pass into the airbox.

[SimonSparrow]

I ran mine without the hose on briefly last week while the airbox was off, and there is a fair volume of air coming out even at tickover. Some of that could be blowby, but a lot will simply be the internal parts whizzing around. When a piston descends in the bore, all that displaced air needs to go somewhere.

[FarmyardPants]

SimonSparrow said:

When a piston descends in the bore, all that displaced air needs to go somewhere.

Hopefully most of it will be used up filling the space of another piston on its way up

[longbow]

Ooooh, an interesting topic this one. Well, I have been slightly concerned with my engine for a few months now - it seems to pop the dipstick out of its seated position after a hard drive. It isn't spitting oil out everywhere, I guess its just blowing off due to excessive pressure in the crankcase. The thing is, the breather pipe and filter isn't blocked, so I'm a bit confused. The car is driving fine, with no adverse effects.

Does anyone else's do this? I've read some race engines are prone to doing this so maybe it isn't anything to worry about.......

[SimonSparrow]

FarmyardPants said:

SimonSparrow said:

When a piston descends in the bore, all that displaced air needs to go somewhere.

Hopefully most of it will be used up filling the space of another piston on its way up

In theory this is true, but I think the crank and pistons flailing around has a asort of fan effect.

Windsor, one of my friends used an exhaust vacuum extractor for his engine. Basically a one way valve in the exhaust manifolds was used to create a partial vacuum in the crankcase, due to low pressure effect of the exhaust gases rushing past. Some race engines do it and its so effective that a holed sump sometimes can still retain the oil during the race, only to leak as soon as the engine is stopped.

[GreenV8S]

longbow said:

it seems to pop the dipstick out of its seated position after a hard drive. It isn't spitting oil out everywhere, I guess its just blowing off due to excessive pressure in the crankcase.

That suggests that under some conditions the amount of blow-by is too much for the breather system to cope with. If it isn't blocked then it's too small.

[HarryW]

You can get a garage to try measuring the blow by with a meter/gauge, what a good reading is for a AJP I do not know. I know it was one of the most important gauges used on mine when it was being run in on the dyno, too little and the rings were starting to bind, too much and the head gasket was gone (observed both ).

[ridds]

Dunno about that, a well built engine shouldn't have any problems as far as blowby is concerned. If they were gauging the build quality by blowby then I'd have been a little suspicions of how confident the people were in their ability to build the thing in the first place. ooops high blowby again, lets take here apart. If the ring gaps were sized correctly then this shouldn't be an issue surely.

For a 4.5 or 4.2 I wouldn't want to see much over 50 l/min throughout the rev range, from experience. Although this is from a system with a pressure control valve for part load.

[HarryW]

ridds said:

Dunno about that, a well built engine shouldn't have any problems as far as blowby is concerned. If they were gauging the build quality by blowby then I'd have been a little suspicions of how confident the people were in their ability to build the thing in the first place. ooops high blowby again, lets take here apart. If the ring gaps were sized correctly then this shouldn't be an issue surely.

For a 4.5 or 4.2 I wouldn't want to see much over 50 l/min throughout the rev range, from experience. Although this is from a system with a pressure control valve for part load.

Confidence and ability are nothing to do with what I'm saying, why take the opportunity to have a pop every time .
If you recall mine was being used as a test bed in which different pistons and different rings were being tested. Slightly different gaps were also being tried as were 'total seal' rings. Hence the constant monitoring.
The high blow by moment was when a new (experimental) composite head gasket let go on one of the cylinders .

[ridds]

Well I wasn't aware of any of those details.

[HarryW]

ridds said:

Well I wasn't aware of any of those details.

Fairy snuff

[Daftlad]

GreenV8S said:

longbow said:

it seems to pop the dipstick out of its seated position after a hard drive. It isn't spitting oil out everywhere, I guess its just blowing off due to excessive pressure in the crankcase.

That suggests that under some conditions the amount of blow-by is too much for the breather system to cope with. If it isn't blocked then it's too small.

Worn engine - bores, broken ring or holed piston are the likely culprits. New engine - glazed bores?

Edited by Daftlad on Friday 9th November 18:52

[ridds]

Daftlad said:

GreenV8S said:

longbow said:

it seems to pop the dipstick out of its seated position after a hard drive. It isn't spitting oil out everywhere, I guess its just blowing off due to excessive pressure in the crankcase.

That suggests that under some conditions the amount of blow-by is too much for the breather system to cope with. If it isn't blocked then it's too small.

Worn engine - bores, broken ring or holed piston are the likely culprits. New engine - glazed bores?

Edited by Daftlad on Friday 9th November 18:52

Does sound excessive, although I think longbow has had his engine apart recently. Could well be something as simple as a poorly fitting o-ring. What stick is it a early 4.2 or 4.5 type?

[Daftlad]

ridds said:

Daftlad said:

GreenV8S said:

longbow said:

it seems to pop the dipstick out of its seated position after a hard drive. It isn't spitting oil out everywhere, I guess its just blowing off due to excessive pressure in the crankcase.

That suggests that under some conditions the amount of blow-by is too much for the breather system to cope with. If it isn't blocked then it's too small.

Worn engine - bores, broken ring or holed piston are the likely culprits. New engine - glazed bores?

Edited by Daftlad on Friday 9th November 18:52

Does sound excessive, although I think longbow has had his engine apart recently. Could well be something as simple as a poorly fitting o-ring. What stick is it a early 4.2 or 4.5 type?

Bores still not bedded in - glazed??

[longbow]

Its an early 4.2 that sits at the back. I've just fitted a K&N breather filter straight onto the outlet on the timing cover. If its still blows with this in place its definitely excessive crankcase pressure.