Crankcase breather, blowby, windage

Competition engines and 'live' projects only. Good photos to illustrate your post are expected.
Marc
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Crankcase breather, blowby, windage

Post by Marc »

Hello Guy,

Is there a benefit to having the breather hose attached to the filtration system? Does the vacuum created by the intake help relieve crankcase pressure as speed rises compared with just venting to atmosphere?
Julian
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Post by Julian »

There were some fairly serious engines built in the 80s and 90s that relied on the crankcase being in a partial vacuum - what the actual benefits were is something I would certainly love to find out!

I strongly suspect there is an awful lot more to it than just evacuating the crankcase though - the diffuse oil spray in the crank would behave differently for a start and even that depends on what kind of oil control is being used. Some engines need the spray, some work better without it.
Guy Croft
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Post by Guy Croft »

With a good filter there will be only be a very small depression in the air cleaner. The breather is routinely connected into the filter pan to stop emission of oily fumes and hydrocarbons.

There is an argument for venting it into the exhaust header via a special valve (Canton make them), which creates low pressure in the breather pipe, I myself have never dynoed that setup so I just cannot comment on how well it really works. I don't know anyone else who has done it either - so maybe it doesn't work.

Beyond dry-sumping, which some people loosely suggest can create low crankcase pressure and thus more power, I am uncertain about the virtue of running crankcase depression on these engines (to get more power) and certainly beyond a very low level of depression the engine seals would not suitable for it.

GC
Ken H
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Post by Ken H »

I can vouch that a dry sump can generate a significant negative pressure, having measured this on my Thema 16V turbo engine with a 3 stage Peterson dry sump (I've since switched back to wet sump). Whether or not there was a power gain, I can't say.

Ken H.
130tc
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Crankcase Breather Filter

Post by 130tc »

Hello,

I have just changed my carbs on my 130 abarth and had the same questions,
i was advised to vent it through a small filter.

Now the question i have is how much emission of oily fumes would you expect.
When engine is at opperating temp (85 Deg) you can see the fumes. Is this normal or am I loosing compression in to the crankcase ?

Thanks
Steve

PS. Best fourm on the net by far. Keep up the good work
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Crankcase breather filter
Crankcase breather filter
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Filter and manifold
Filter and manifold
Filter.JPG (110.8 KiB) Viewed 11566 times
Guy Croft
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Post by Guy Croft »

Hello Steve

first, thanks for your kind remarks. I am grateful to all members for upholding the ethics I built into the site on day one and I hope it stays that way.

As for the breather, don't use that small filter (K&N), it really is too small and will soon clog with oil mist and create crankcase pressure, oil leaks. I know, I tried it years ago. If you don't want to run the breather into the air cleaner buy an aluminium 'catchtank' - nothing fancy, all race cars use them. In the UK any number of alloy race accessory firms offer them, Brise, Pace. I am sure a quick internet search will turn one up.

There are ways to measure blow-by but they all need quite costly machines (they're a bit like domestic gas supply meters) and anyhow an amount (indeterminate) of gas and vapour coming out is quite normal on wet-sump motors, the gas comes from blowby past the rings of course. A compression test is as good as anything:

1. Hot engine, all plugs out.
2. Wide open throttle
3. Compression gauge in plug hole, crank motor for a few seconds.

Sealey do a very good and inexpensive compression tester.

Look for 205 psi plus on a really good engine and strong battery. A really tire engine may have as low as 120psi cranking compression, sure, worn rings and 800 mile to the pint of engine oil, but it may still be powerful enough when it comes on full firing load and the rings seal up better. If there is a high variance between cylinders (more than 10%) you've got a valve leakage problem (or a ring problem), but read the compression gauge instructions because there are ways of determining ring-or-valve leakage.

Let us know how you get on.

GC
Marc
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K&N on the breather..

Post by Marc »

I have had the same results as Guy, the thing fell apart from the pressure build-up.
A Greddy catch can works well, they can be found on the internet.
romilos
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Post by romilos »

I just saw that topic.
I have the same question.

Some say that with the hose in the inlet manifold, you just make the engine dirtier!

So i removed the hose and i have it free down the road, somewhere in the engine bay.
I did it after my rebuild, so to keep my engine as clean as possible!

If i understood correct, Guy you recommend not to put it in the intake manifold...??
1NRO
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Post by 1NRO »

I've read that increasing the vacuum within the crankcase improves the sealing of the rings in the bore by pulling them tight to the lands but thats just something I've read not proven.

Nik
romilos
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Post by romilos »

I haven t seen a race car with the hose in the intake manifold.
If in the crankcase we have above atmo pressure then when we enter the hose in the atmoshere he have "movement" of the burned oil...


Some other say that the factory puts them in the inlet so the engine has "an easy death" cause of the not clean air of the mixture!

Otherside, the burned oil increases the temp of the inlet air .
I think that does not "helps" the power gain..
Guy Croft
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Post by Guy Croft »

Production engine breathers are routed into the intake to reduce emissions to atmosphere (of whatever comes out) and quite right too. Venting to atmosphere under the bonnet can also be obnoxious - the heater can pull in the fumes. A catchtank should have an exit hose vented under the vehicle into the airstream.

I don't say routing to the air cleaner is a bad thing, on a clean engine with low oil content in the crankcase gas it would likely only have a small effect on power - if indeed any at all, from contamination of the incoming charge.

We know that the breather is needed to prevent overpressurising the crankcase, at least if we have read this whole thread we'll know that...
is it possible to generate crankcase depression via routing to the inlet tract - well, truth is I don't know, but I doubt it.

You would need, I suppose, a great deal more depression than you're going to see anywhere in the inlet tract and naturally the only real vacuum is between throttle plate and valve(s), this more-or-less disappears when the throttle is opened. How much vacuum anywhere else in the inlet tract? Some in the choke in the carb? What about F1? No choke at all.. In the air filter? Hopefully none, or the air filter is blocked.
The gas flows into the air cleaner not because there is high vacuum - it should be close to atmospheric pressure and the breather gas is higher-than-atmos.

And do we want low crankcase pressure on our engines? Again - read above. I'm not sure. Yes, depression will help a firing cylinder because the piston is forced down by the pressure differential between combustion region (high P) and crankcase (much lower P), and a piston on the intake stroke. But what about a piston that is trying to evacuate the cylinder on ex stroke, or compress on the firing stroke. Will it hold it back? Well, it would tend to wouldn't it?

Yes, F1 engines use quite high crankcase depression, I've seen it at work, and each piston pair (on common crankpin, they are all Vee engines) is virtually sealed off from the adjacent ones, they have special crank seals individual-to-piston-pair roots-type scavenge pumps, plus incredibly complicated dry-sump tank/hose systems - but beyond that I have no idea how it all really works to enhance the power.

Good rings will not generate significant blow-by, the small ring gap should 'choke' under high pressure and not flow at all. Engines tend to breather more at low load. If you want very low blow-by all the time, use Total Seal rings.


GC
romilos
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Post by romilos »

Oh that s a complete answer!!
Venting to atmosphere under the bonnet can also be obnoxious - the heater can pull in the fumes
I think you are right with that!
I must put a hose with more length, or a catchtank, as you say!
130tc
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Filter

Post by 130tc »

Thank you for the better than expected reply.
I have ordered a catch can, it should arrive any day now.
Also I have charged the battery and booked a compression test and tune for next week, I will post results - then hopefully all will be good!

Regards

Stephen
cheitane2003
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Post by cheitane2003 »

Hello Guy:
It is said that is important the crankcase breather connected in the air cleanner to protect the upper piston zone.Specialy with new or rebuit motors in running in periods.
These micro oil particles in that air makes an 2 cycle motor effect which oils a bit the cyliders over the piston without ignition problems (always assuming a motor in good condition).That‚´s another reason for their existence together with polutions questions.
Is that true in your opinion?Can be dangerous for the motor to stop this oil feed leaving apart polutions problems?

Thank you very much.

Siro,
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bigmac
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Post by bigmac »

Guy, what about 'Plan B' - a catchtank vented to the airbox?

I am about to start drilling a new backplate and have been asking myself whether to drop the breather inlet. But dont like the idea of venting to atmosphere.

What about using a labyrinth catchtank (oil separator= catchtank stuffed with scouring pads) receiving inlets from crankcase PCV and rocker cover, with its outlet plumbed to the backplate? This should keep oil out of the throttlebodies and get the fumes back into the engine....

thank you, Colin
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