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MG Midget and Sprite Technical - Another breather question

Rather than hijack the other thread I have a breather question too. I have one coming from the back of the rocker cover and the pipe just hangs down the back of the engine. Aint connected to anything, so if I understand correctly I have no vacuum in the crankcase?

Any harm in blocking this breather off or should I leave as is? Fit a catch tank? I have a Weber so not sure if it'll plumb into that - perhaps that's why it vents to atmosphere. I do get a bit of a drip from the back of the engine but assumed it was a TADTS situation. I have a suspicion I've seen a bill for the Peter May crank seal in the paperwork but will have to check.
Jordan Gibson

I think early midgets had a connection between the rocker cover and the air filter box (IIRC).

You could block it off without detriment and preserve your vacuum.

A
Anthony

I think there are two basic sorts of breather systems used on A series engines - with variations on each.

The original small capacity engines rely on an open system whereby the crankcase is open to the atmosphere via one or more breathers. The idea is simply to aim to keep the engine internals at or near atmospheric pressure. This works well enough on smaller engines, at lower revs and when piston seal is in good condition. Breather pipes can be just to the air, or may be connected into the induction tract, usually via the air filter. This isn't to provide suction, but simply to deal with the oil vapour.

With larger engines, higher revs etc the tendency to build up crankcase pressure becomes more critical. This pressure then pushes oil out along the crankshaft spiral "seal". The vacuum in the induction tract is used to actively draw pressure from the crankcase. The idea is not just to draw off blow-by gasses but to actually lower the crankcase pressure so that oil is not pushed out of the rear crank spiral seal. For this to work, there needs to be a restriction or limit to the amount of air that can be drawn in anywhere else, so only a small, filtered pin-hole is provided in the oil filler cap. Addition of extra breathers - unless they have a one way valve fitted - may be counter-productive as they can allow air to be pulled in and prevent the system from maintaining a less than atmospheric pressure in the crankcase.

Catch tanks can be used with either system - simply to collect the oil that condenses out of the gasses. Either to stop the oil from upsetting the carburation (it lowers the effective octane rating) or to stop oil from contaminating the engine bay.

Guy
Guy Weller

Superb - thanks Guy. When you say 'The vacuum in the induction tract is used to actively draw pressure from the crankcase' is this via the pipe from rocker cover to carb I hear the SU folks talking about?

How should a fast road engine with Weber carb handle this issue, ideally?
Jordan Gibson

Hi Jordan,
Since the rocker cover, crankcase and timing chain chest are all interconnected,they should be at much the same pressure and in theory it shouldn't matter where you draw that pressure off.
I suspect that the rocker cover and the tappet chest connections that were used on earlier engines with passive breathing might have given too direct an access to where oil was splashing around when used with an active suction system. That's why they used the oil canister thing on the timing chain cover, and stuffed brillo pads in there to prevent oil slooshing up into the pipe.

Not sure about the Weber carb set up. I know some have tried a direct connection to the manifold (as for a brake servo) but it need restricting as the suction is too great there and may draw off oil, not just vapour!

Guy
Guy Weller

The problem with using manifold vacuum is that it is at a maximum at idle and negligible at WOT so when the engine is producing the most gases requiring venting you've no suction to pull the vacuum. IIRC the vacuum takeoff on the SU is between the bridge and the throttle butterfly so always sees some vacuum. Whether a Weber has a similar takeoff I don't know.
David Billington

The earlier cars draw vacuum direct from the inlet manifold. To prevent high vaccum (e.g. on overrun) sucking the rcokers and valves (well, excess oil anyway) out of the rocker cover, they use a regulator between the timing cover and the inlet; it's a strange looking thing, like a fat biscuit (choc hobnob anyone?) with diaphram/spring arrangement to restrict the air-flow to limit the vacuum suck.

You could use one of these.

HTH

A
Anthony

Hi David

A Weber doesn't have a ported vac outlet (inlet?), so often a Weber is fitted and the vac pipe ignored for a few days until the oil loss becomes apparent; then we have a query about oil loss on the BBS... works every time!

And agreed - the ported vac is a great solution - limitng the vac at low throttle/idle whilst using the venturi to provide a vac at WOTs. Very neat!

A
Anthony

Anthony,

Maybe that was my luck in using a Dellorto rather than a Weber and not having oil peeing out everywhere.

Anyone read what Vizard has to say on the subject of crankcase venting, I haven't read it cover to cover since the late 1980s, and haven't got the time just now to see what he says on the subject although my copy is downstairs.
David Billington

http://www.webcon.co.uk/weber/45dcoe.htm

Part #48. After a quick google it seems like some have 'em and some don't in which case Anthony's approach may be taken.

Food for thought - thank you all!
Jordan Gibson

Vizard likes to vent the crankcase from every conceivable oriifice... but then he's dealing with Minis that have a crank seal, and we have Spridgets that don't...

A
Anthony

I shouldnt say this as this will upset the apple cart, but david lieb and I where talking about a year ago, and rivergate had an open vacume hose postioned just below the oil pan (not sure exactly where) and VENTED to atmospher "OH BOY!!"

Appeartly it worked and Here is why!

At idle and low revs you dont need alot of vac. (neg. pressure) to hold the oil in the engine and 1:1 atmospher is fine.

with the vacume hose postioned at the right place next to the oil pan, where there is alot of air passing over it, A vacume is created...A neg. vacume, the faster you drive the more vacume (Neg. pressure) is created

The Key to this type of neg. pressure setup, is the "POSTION" (the placement) of the vacume hose to open atmospher so as to get the maxium amount of air to flow over the open vacume hose as the car approaches faster and faster speeds


Jorden,

Im guessing this maybe the set up you have, with out seeing it....but if your engine is not a 1275 and smaller, then I have to agree with the previous post.

prop
Prop

Check this out.

http://www.jegs.com/p/JEGS/924093/10002/-1

This is the one I'm installing on my 'B'


http://www.jegs.com/p/Dynatech/750022/10002/-1
DCM McCullough

This thread was discussed between 30/09/2008 and 01/10/2008

MG Midget and Sprite Technical index

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