Swan or chimney?

[silloyd]

I am just about to change the head on my 1098 for a 12g940.

I currently have a chimney / cannister shaped tappet cover breather pipe but it looks like it's going to get in the way of fitting the new (second hand) LCB exhaust manifold.

Amongst my spares I have a swan-neck tappet chest breather that would be less obtrusive. So, the question is whether I can swap them over without problems or whether one is better than the other?

If it helps, I have a rocker with a breather that will be fed to the (HIF 38) carb and a solid oil filler cap.

Over to the experts...

[PSL184]

Just use the rocker breather into the carb and replace the tappet chest cover with a non breather type cover.....

[silloyd]

Err, from other posts I thought the side need a breather?

[PSL184]

Some will say you need as many breather as possible, which I wouldn't disagree with, but if your engine is in "good" condition then the rocker cover breather is the most efficient.... My 948 doesn't have a tappet chest breather.....

[silloyd]

I guess the net effect of a plain front tappet chest cover is to revert the engine to a closed breather system (as it was when I got the car) but with suction from via the rocker cover rather than the tappet cover.

Originally I had a rocker cover with no breather and a pipe connected from the chimney style tappet cover to the carb.

[JOWETTJAVELIN]

The engine shouldn't really be modified... The tappet chest breather on mine does throw out some crud now and again and also some smoke when revving, so I think you definitely need it.

[silloyd]

JOWETTJAVELIN,

If the engine was intended to have a closed breather and all I'm doing is shifting the suction from the tappet cover to the rocker cover, what material difference is that likely to make to the running of the engine?

[bmcecosse]

KEEP a tappet chest breather! Obviously the swan type is more prone to dropping a bit of oil, the can keeps it in. Just bash the can if necessary - that's what Mini owners do!

[silloyd]

BMC, I see your point. I assume you mean keep the tappet breather open to the elements and not connected to the carb(?)

I'd like to understand the logic but I've read several posts on this subject and, due to the varying opinions, am still not undecided on what the best solution is.

From what I gather the purpose of the crankcase breather is to relieve air pressure build up and/or pulses caused by the reciprocating pistons acting in effect as an air compressor. Furthermore connecting the crankcase air space to the carb induces a degree of negative internal pressure that assists the not-ever-so-efficient scroll oil seal on the rear of the crank to reduce/eliminate oil loss at the back of the engine. I'm not aware of any other reasons.

I don't really understand why connecting the carb to the rocker cover, which I guess draws air up past the push rods, is preferable to connecting directly to the tappet cover. (I note that some mini engines have a breather on the timing chain cover). There doesn't seem to be, much difference in either method to my (novice) view but on this forum it definitely seems to be a strong preference to connect to the rocker cover.

Whichever way you connect the carb, leaving either the tappet cover or the filler cap (filter type) open to the elements would seem to only act to reduce the crankcase pressure reduction effect, i.e. because the air will always flow along the path of least resistance, which I would have thought is not desirable.


As ever the views of the collective forum brain are appreciated.

Yours,
Confused of Cardiff.

[bmcecosse]

I always connect one breather to carb (or manifold) and leave one open. Thus allowing fresh air to flush through - or any excess pressure at time of great stress, to escape. No mayo and no leaks....... it works.

[54woody]

There's a good description of how the crankcase ventilation on the early B series engine works on the mga guru site (see http://www.mgaguru.com/mgtech/engine/cv101.htm). Yes, it is a B series engine that he's talking about, but I imagine the A series engine is essentially the same in this regard.

[silloyd]

That's really helpful, thank you and just the sort of thing I was looking for!