head rear vent hole

PostPost by: mini64 » Thu Jun 02, 2011 4:15 pm

What do you guys do with the head's vent hole at the rear - I've always taken the time to pipe it over to my vent bottle, but I see photos of cars on here that have it open. Does oil not come out?
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PostPost by: billwill » Thu Jun 02, 2011 4:56 pm

mini64 wrote:What do you guys do with the head's vent hole at the rear - I've always taken the time to pipe it over to my vent bottle, but I see photos of cars on here that have it open. Does oil not come out?




On the original 2-seater Elan it had a right-angle rubber grommet & a pipe that vented down near the gearbox.

On later ones a straight pipe goes to the carb input box so any oil fumes get burnt to avoid pollution. Well that is the theory, in practice it often means that the rear spark plug oils up, doesn't fire so well & you get petrol fume pollution instead.
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PostPost by: tvacc » Thu Jun 02, 2011 5:43 pm

Here in the states we used to call the tube that went down by the gear box the "road tube"

I would plumb it to a catch bottle if you no longer have an airbox that you can connect it to.

You could put the catch bottle right under the coil in that little area the always catches things when you drop them.
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PostPost by: Robbie693 » Fri Jun 03, 2011 8:19 am

I remember from my old MkII Cortina 1500 that the vent system had a condenser trap attached to the engine which was a box with a baffle in it and an outlet for the road tube. Presumably this enabled the condensed oil to run back into the sump rather than running down to the road along with the oil mist. I think it worked as I don't remember too much oil coming out of the tube.

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PostPost by: Dag-Henning » Tue Jun 07, 2011 10:49 am

- Suggest you leave it open. Nothing comes out, not even on a race engine.

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PostPost by: billwill » Tue Jun 07, 2011 11:21 pm

tvacc wrote:Here in the states we used to call the tube that went down by the gear box the "road tube"

I would plumb it to a catch bottle if you no longer have an airbox that you can connect it to.

You could put the catch bottle right under the coil in that little area the always catches things when you drop them.



You can't easily get to that void in a RHD car over here. The brakes & clutch master cylinders are in the way.

I'm having a go at using an aluminium bicyclist drinking bottle as a catch bottle & I've made a brass strap to fasten it onto the cover plate that covers up where my mechanical petrol pump used to be. I will need to use something like a turkey baster to occasionally suck up any caught oil to save me taking the carbs off to empty it.

I won't know until I get a new head of course and I am a little worried about whether having oil vapour at that place might be a fire risk.
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PostPost by: CBUEB1771 » Wed Jun 08, 2011 12:29 am

mini64 wrote:What do you guys do with the head's vent hole at the rear - I've always taken the time to pipe it over to my vent bottle, but I see photos of cars on here that have it open. Does oil not come out?


The vent hole is there to prevent the crankcase from becoming pressurized too much. Just inside the vent hole is a roughly cubical chamber whose function is to reduce the speed of gas coming up from the crankcase so that oil mist has a chance to separate and drain back. This is why Dag says no oil comes out. It may be true that little oil comes out in a ten lap event but it is a different story in a road car where a fair amount of oil would come out over long hours of running. I would stick with either venting into the carburettor air box or vent through a bottle as you are suggesting. It is no longer the "done thing" to dump oil onto the road.
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