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Rocker Cover Breather


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My GT6 Mk2 is running with triple dellorto carbs , any recommendations with regards to the breather from the rocker cover , when i purchased the GT6 it had a piece of foam tied on with a cable tie . Would a  breather pipe running to an oil catch pot be a good idea with a air filter fitted I have heard that the engine runs with a slight vacuum .  

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The breather from the rocker cover is to deal with crankcase pressure.      "Blow-by" gas escapes around the piston rings even in well sealed bores, and must be vented, else pressure will build up in the crankcase and the connected - and less well sealed! - rocker cover.     Originally, engines used to just let it escape below the car, but in an early move towards emissions control, the vent would be connected to the carbs, so that the oil vapour in blow-by gas was burnt in the engine.  Some engines put the connector  into the air filter, but this fouls the filter medium; some downstream of the filter, but that fouls the  carbs.    The ultimate solution was to vent downstream of the inlet control butterfly, into the intake manifold, where engine heat kept the oil as vapour, all the way into the combustion chambers.

But this raised another problem.   The vacuum in the inlet manifold is maximal when the butterfly is closed, and excess gas would be drawn in from the crank case.   Triumph fitted a "pressure control valve" in the line, that kept the flow from the rocker cover almost constant. whatever the vacuum in the inlet.     That valve isn't necessary if you use a catch can, but then you have the unpleasant problem of emptying the can, of the ghastly mess inside, condensed oil and water, as frequently as required.    

If you are using an electric fuel pump then another solution is available.    The port for the mechanical pump may be used as a crankcase vent, and if the catch can is mounted above on the bulkhead so that the hose between runs at a steep angle, then oil vapour will condense out in the hose and run back into the engine.     Water vapour condenses at a lower temp and will get to the catch can and escape, so that the can never needs emptying!    A plastic can may help as the metal ones often recommended have more thermal conductivity and will run cooler.

John

 

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and the more power you uprate needs even better breathing 

is the smiths valve still fitted ??  i guess not 

 and the triple D's wont have a breather take off on the filters so Johns suggestion of a catch tank seems the best way 

you might need a slim plumbing fitting in the cover to increase the hose size from the rocker cover ??

Pete

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Many forget that there should also be a fire trap in there, this is the late mk3 GT6, the lump on top with the pipe feeding into the carbs. On this one the lump contains interleaved plates, earlier mk's have wire mesh. The alloy covers have no fire trap and the Smiths valve should be fitted to also act as a fire trap.

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Doug

 

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This is SoS.  The plastic catch can - red circle - has two inlets and a vented cap.   The upper inlets is connected to the rocker cover, with an accessory filter vent, the lower has  a large hose that goes down to the fuel pump port.   I never have to empty it!

John

PS is that a 'fire trap' inside the rocker cover?    I think it's more of a oil droplet trap, to discourage oil from getting into the hose in the first place.    In old, neglected engines the wire wool in the trap can be solid with the Black Gunge, blocking the vent!

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I have a catch can mounted next to the clutch master cylinder and plumbed from the rocker into the pvc. I took the can off for the first time yesterday and it had about 10mm of oily water in it and some white gloop in the bottom. I mounted the can as high as I could but with only short journeys it's probably not getting hot enough. 

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4 hours ago, Iain T said:

I have a catch can mounted next to the clutch master cylinder and plumbed from the rocker into the pvc. I took the can off for the first time yesterday and it had about 10mm of oily water in it and some white gloop in the bottom. I mounted the can as high as I could but with only short journeys it's probably not getting hot enough. 

And no slope  to drain back into the engine.   See above.

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16 minutes ago, JohnD said:

And no slope  to drain back into the engine.   See above.

It is higher but only by a few inches. I could use the old petrol pump hole but then with only a two port can the rocker vent/pcv hoses would have to stay as original. Is there any advantage in doing so? 

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12 hours ago, Iain T said:

It is higher but only by a few inches. I could use the old petrol pump hole but then with only a two port can the rocker vent/pcv hoses would have to stay as original. Is there any advantage in doing so? 

As John points out, a longer, upward hose/pipe is helpful. On my Zetec/ST engine in my spit I use the OEM breather port at the bottom of the block (I have cut one open, it is a convoluted series of chambers) which originally had a PCV attached. However, as I run throttle bodies, there is no sensible place to connect one. So my pipe from the block goes up, and has a T to join the rocker box breather. That feeds into a sealed 2 port catch tank. The other port feeds into my airbox to provide a little vacuum.(I have a remote air filter, and yes, there is a little bit of oily residue inside the airbox, but a wipe every year is sufficient to keep things happy)

With this setup I collect almost zero in the tank, and have not had to empty it in 15K, which includes several track days (ie continious high RPM, regularly 7000) which is when the old setup, just using the rocker box breather, had struggled with unpleasant results.

I am guessing teh triple dellort setup will be using regular high rpm and doing trackdays or similar. So needs a well designed breather system. I have seen a race "TR5" that had a 32mm rocker box breather fitted. That is the same size as a rad hose. But pparently it was needed. Certainly a 3/4" rocker box breather is a sensible move, but utilising the puel pump oriface is even better.

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Clive, good point I could use a T piece to connect the rocker and 'pump' breather hoses into the catch can then out to the pcv. Easy to try, I'll need to make the outlet and reposition the Huco. 

Iain 

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Thanks for the replies and a great help , JohnD would it be possible for you to post more pictures with regards the connections at filter vent and the fuel pump port as i would like to replicate your set up . Again a great help thanks .

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