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MG Midget and Sprite Technical - Crankcase Breathing - a Heath Robinson Approach

I've been called Bonkers on this site before and I suspect some will say this post confirms it.

My Mk IV Sprite loses rather a lot of oil via the rear main scroll. It has been leaving embarrasing stains on the tarmac in my paddock slot at hill climbs so I decided something had to be done.

When it had twin SUs there was suction for the crankcase breathing system which termintated at the carburettors after passing through the timing chest canister and a separate oil catch tank. The suction helped to reduce the crankcase pressure which kept the oil drips at tolerable level.

I fitted a Weber carburettor just over a year ago and this does not have any ports to take the crankcase breather, so the crankcase pressure has increased, causing the increased leakage.

I figured that, if the car won't provide the suction, had better have a go at doing it somehow and struck on the idea of fitting an air pump. The first 12v fish tank pump I bought was clearly too feeble but Ebay turned up a 12v vacuum pump which, it said, was suitable for medical equipment.

Not wanting to drill any holes, I mounted the pump on a plate which attached to the inner wing via a couple of existing screws. The pump is a bit noisy, but the noise of the engine running mostly drowns it out when the car's going and I can switch it off if necessary. With the engine off but the pump running, I can feel the flow of air in through the rocker cover oil filler when the cap is removed.

The pump has been in place for a couple of weeks now and has clocked up well over 50 miles of road use and a weekend of hill climbing. The vacuum pump has not eliminated the oil leakage but it has reduced it significantly and, for the first time in several years, I didn't leave any oil stains in the paddock.

I know that this is only a temporary solution but I hate taking the engine in and out and there are other priorities demanding time and money, so a permanent solution will have to wait a while.






C Mee

Interesting - whatever works is always good. but I doubt there's enough suck happening to vent enough with a bit of load happening.
I'd be tempted to drill and tap a hole for a takeoff port dead bang in the middle of your inlet manifold and run an inline PCV off a similar sized engine. Maxda MX5's have a neat little one.
William Revit

Thanks Willy.

There is a take-off spigot on one branch of the inlet manifold but it has been blocked with weld by the PO.

There is no balance pipe between the branches so I'm not sure whether tapping into just one of them would cause uneven running. I believe it is for a brake servo take-off which, I assume, just requires a near constant pressure reduction rather than a flow of air, which is what I'm after.

If you think it would not cause any problems I'll give it a try.
C Mee

Use the exhaust.

Posted this a few times when duscussing crankcase venting.

http://www.summitracing.com/parts/mor-25900




anamnesis

Thanks for that Anam.

I'll investigate further.

C
C Mee

I like the vacuum pump idea too. I think I read about that somewhere as well, so not a zany idea at all.
anamnesis

I was thinking about suggesting a vacuum pump off a diesel car as I have one off an old Escort IIRC but it's belt driven and you might not want to suffer the small power loss incurred but it'll likely pull a higher volume than the little electric one.
David Billington

I remember seeing a vacuum pump at a show, once. I think it was called ‘evacusump’ or something similar.
Dave O'Neill 2

The suction provided by the carb take off point on an SU carb is remarkably low. I measured it once with a vacuum gauge and seem to remember it was around 2 or 3 Hg with my engine at around 3500rpm. Somewhere in the archives is a photo.
The negative crankcase pressure principle relies on restricting the volume of air flow into the crankcase. For it to work as designed it is essential to use the correct restricted breather type if oil filler cap and to minimise all other entry points. The suction required then draws regulated fresh air via the oil filler cap and as long as the engine is in reasonable nick and not having excessive piston blow by, then a low volume, low suction set up will keep the crankcase pressure below ambient, encouraging oil to stay inside rather than being pushed out!

The point of this is that to use a pumped evacuation system you probably dont need a high volume pump. But you do need to seal the system and use a restricted breather type of oil filler cap.

This is why an evacuated system is incompatible with the alternative of having multiple open breather points, maximising the crankcase breathing. Both have their merits, but you cannot mix & match and expect it to work!
GuyW

From the shed, a sort of mini Shorrock. Pulley diameter is 4" for scale.




David Billington

Some very interesting comments,thank you.

According to the specifications, the pump flow rate is 12 litres a minute and can achieve -75kpa or +200kpa.

I've also delved into Vizard who gives a good description of the Evacusump solution that Anam described, so one for further consideration due to what's involved in its installation.



C Mee

Interesting point on the Moroso system that Anam linked to.

“cannot be used on vehicles with mufflers.”

Dave O'Neill 2

Guy
I overlooked that the weber manifold didn't have a balance tube,--if you were going to run with the PCV valve idea it'd mean drilling/connecting up to both manifold runners and connecting with a 'T' in the middle to get equal airflow-

Connecting to a single runner won't work-

I had a picture of one of those banana shaped weber manifolds (with a balance) in my head when suggesting to drill dead centre.

With the exhaust type venturi sucker. they work really well if mounted in the second half of the collector. but as Dave noted, i'm not sure if they'd be so good with a muffler in the system, have only ever used that style on an angry chev racer with open exhaust.

willy
William Revit

Willy,
I wasn't suggesting a pcv system, or indeed which suction or pumped system should be used. I was just pointing out that the original 1275 arrangement works at a low suction and low volume basis by limiting where air can be drawn into the crankcase void. If contriving a pumped or venturie extraction then as long as Colin also manages and restricts inflows then quite low capacities of extraction should work.
GuyW

Have I misunderstood crankcase breathing? I've always assumed that any pressure in the crankcase was due to blow-by past the piston rings meaning a top condition engine would have very little so would only need minimal breathing. I'm aware of the general test you do by removing the oil filler cap and a worn engine would be blowing smoke out but a good engine hardly anything. Is this right?
Bill B

Bill, I think that is broadly correct, though piston blow-by is a function of both engine condition and of revs. A good condition engine will still produce blow-by but at a higher rev or higher power output. As it wears that threshold comes down the rev range until a really knackered engine will blow at tick over speeds.

I demonstrated this on my engine by putting a neoprene glove over the oil filler. At low revs the extraction breather system created a negative crankcase pressure and the glove pulled down, but as revs increased (I think towards around 4000) the glove inflated and stood up, High 5'd and waved! There's a photo in the archives of this. Quite comical!

Someone once explained that the mere displacement of the moving pistons created a pressure, without any blow-by involved. I didn't really understand as when one pair of pistons rise the other falls so I would expect them to cancel out any pressure changes. But I don't know!
GuyW

Breathing and reducing pressure are related but distinct as I see it.

You need to breath/vent the crankcase to remove the impurities from blowby, but that also relieves the pressure at the same time.

As Guy said, the pcv system creates a mild 'vacuum' by drawing air down through the restriction in the oil filler cap. This reduces pressure and collects the unwanted harmful blowby gasses, feeds them into the inlet, and ultimately out through the exhaust.

A vacuum pump or exhaust scavenge would do effectively the same, but to create a low pressure (vacuum), you would still need a restriction on any aire getting in, or a completely sealed engine.

Some racers run a quite high vacuum, as it improves bhp. I assume they don't care so much about breathing, as they change oil and strip their engines more frequently.

The other approach is open breathers. That reduces pressure, but does little/less to remove harmful deposits.

anamnesis

Ok, thanks, I'm about right then! I hadn't appreciated the advantage of a restricted oil filler cap but can now see the point with respect to vacuum.

Bill B

Another bonkers query.

As the vacuum pump is, at best, a sticking plaster solution, I'm thinking more seriously about fitting a Peter May seal kit or similar,

Leakage from the rear main bearing scroll can be attributed to wear, presumably in both the bearing cap and the block.

I know nothing about the process except that it exists but would metal spraying followed by machining to finish be an option. If so, would it be the block or the crank to be sprayed? I guess sprayin the crank would be simpler provided the scroll can be re-instated.

Feasible or bonkers?

Colin
C Mee

This thread was discussed between 19/05/2025 and 17/06/2025

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