MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Crankcase ventilation and pinking

Hi all.

A relative was complaining about the high cost of the crankcase ventilation oil trap in his modern car's crankcase ventilation system.

A little investigation revealed that the trap is quite a fine filter.
Some more digging suggested that the fine filter is to ensure that heavy oily crankcase fumes don't get into the inlet manifold as they will :

1) Tend to cause combustion chamber fouling.

2) Tend to reduce the effective octane of the fuel.

Has anyone tried fitting a more sophisticated filter in a B in an attempt to help the common pinking / running on problem ?.

Don
Don

I don't know that it's more sophisticated, but my GT has a supposedly well-thought out solution (not mine) which is simply an aluminium can bolted near the servo into which run the hoses from the rocker cover and tappet chest. A large bore hose then runs to the gearbox to vent the fumes, whilst oil is trapped in the can for disposal. Can't make any claims for it, but it doesn't give any problems. I'll post a pic at http://spostins.photosite.com/GT/.
Steve Postins

Oh heck Steve, that's not strictly legal. I've done that before as well but on an engine that was well worn and really wouldn't idle properly due to excess gases. It provided a temporary cure by letting the crankcase vent to atmosphere but the solution was a rebuild. Doesn't this thing act as a flame trap as well ?

Steve, think you use your car for other than road use so may need such a device !!
Iain MacKintosh

Don,

Here's a link to someone's DIY approach to PCV filtering. I have not tried this, but it's not a bad idea.
http://bernardembden.com/xjs/pcvfilter/

Cliff
Cliff Maddox

Hi Iain,
Flame trap or firebomb?! I never thought of it like that! FWIW it is Oselli "OE" ("improved crankcase breathing"), but thinking about it, don't early B's have just the tappet chest vent that runs to air? If so, I guess it's a small step up in design.
Steve Postins

I don't think pinking is usually associated with the crankcase ventilation system, and is easy to eliminate as a cause by temporarily disconnecting it. Very little oil from the ventilation system should be burnt in the engine, high oil consumption though this cause can be difficult to eliminate. Pinking is caused by the more volatile fuels these days and can be 'cured' by retarding the ignition slightly and/or buying a higher grade of fuel or boosting additive.

Very early Bs had a pipe from the rocker cover to the air cleaner and an open pipe hanging down from the front cover or front tappet chest, replaced by the early closed-circuit system using a PCV valve in Feb 64. This valve was replaced by the much better carb breathing in Oct 68.
Paul Hunt

Maybe it is firebomb Steve. I had a quick look at your website saw the Oselli manifold and side draughts and remembered that you must have a heavy right foot (as have I)I didn't realise though that the breathing system or lack of it was OE Oselli. Paul's hit the nail on the head as well and I remember running Vauxhalls at that time and they had a pipe vented to atmosphere but like others had to route the whole thing back through the induction system.

I do get the feeling that Don's relative may have a bigger problem that is at the root the system fouling.
Iain MacKintosh

Now you've got me scratching my head again. On the early set up, which was the in and which was out? I've seen a mini K&N that I thought plugged into the open end of the tappet pipe, but also remember advice to cut the end at an angle to create a bit of vacuum from the air stream as you drive. And yes, a very heavy right foot, hard suspension, small motolita-mg-steering-wheels-and-bosses.htm>steering wheel, vast fuel bills, hands up, I admit to it all! I've yet to succumb to the pleasure of relaxed driving and anyway, they are sports cars aren't they?
Steve Postins

I love small steering wheels.

I have no other input.
--
Olly
Oliver Stephenson

Olly,
Is your crazy hybrid on the road yet?
Steve Postins

Hi all.

A little digging on the net suggests that relatively sophisticated filters are in common use in some modern engines.

I think my late model B has a simple mesh type on the tappet chest cover.

As Paul says, temporarily disconnecting the breather and plugging the inputs would be an easy way of testing the theory, though the mixture would be slightly modified.

My B is off the road for the winter, but I will try it later.

Don
Don

Disconnecting the breather won't affect the mixture significantly, as crank case gasses don't contain much oxygen - their presence in inlet gasses simply de-rates the engine. My engine has lots of blowby ('cos I thrash it) and I lose a significant amount of horsepower when the breather is attached. I just vent to atmosphere.

Steve, it was on the road for a bit but then it went off again to have new rear wings. Still waiting to get it back from the welder chap, and I'm really missing it :o)

Vroom(tssss)
--
Olly
Oliver Stephenson

Here is a good description of add on breathing systems.
http://www.teglerizer.com/triumphstuff/emissions/

This article gives the pros and cons of using crankcase vacuum for racing applications.
http://www.dragstuff.com/techarticles/vacuum-pumps.html

David Vizard discusses an "Evacumsump" crankcase reducing system on page 427 in his book "Tuning the A-Series Engine"

Clifton
Clifton Gordon

Air is drawn through the engine and into the carbs or inlet manifold, picking up any fumes along the way, so what eventually gets into the engine will contain oxygen under normal circumstances. Removing the oil filler cap weakens the mixture which changes the revs as the cap vent (for examle, rocker cover vent on emissions controlled engines) is restricted. So I would expect a small *richening* of the mixture if the vent to the carbs/manifold were blocked altogether, small being the operative word.
Paul Hunt

typically what gets blown into the crankcase is burned gases blown past the rings from the compression and power strokes. Those gases are oxygen depleted in a modern sealed engine system that does not have a crankcase intake breather. A worn engine with lots of ring leakage will sometimes overwhelm the closed breather system. Pressure will develop that will result in leaks and oil being pumped through the ventilation system into the air intake. This oil will foul plugs and the excess oxygen depleted gases will act as a NOX injection which will reduce pinging - and performance.

If the closed system is not keeping up with the blow by that is a strong indication your rings are worn and your engine has excessive blowby.

The open system ventilating the gases and oil mist under the car may be the only option for a worn engine.
In most states, such a ventilation system would not be legal under the applicable smog inspection requirements.

Barry
Barry Parkinson

"modern sealed engine system that does not have a crankcase intake breather"

But the MGB *does* have a crankcase intake breather from Feb 64 to the end of production.
Paul Hunt

Hi all.

Interesting stuff...

I would guess that the crankcase fumes in an unworn engine would have an approximately normal oxygen content but may have a significant oil mist component.

I have seen many before-and-after adverts for fuel additives etc which show photos of inlet valves with oily gunge on the back of the valve and stem, and have encountered such gunge myself.
Is this the result of valve oil seal leakage, or could it be the result of oil mist from the crankcase ?.

Don
Don

The modern engine has crankcase ventilation into the intake manifold through a PCV valve. I consider a "breather" an open flow from the crankcase into the atmosphere.

Closed crankcases with pcv valves were required of new cars in Calif in 65 or 66 as I recall. Other areas were later in imposing such requirements. Depending on where the car was sold, it may or may not have open crankcase ventilation.

Barry
Barry Parkinson

Hi all.

I understand the function of the funnel shaped PCV valve is to prevent excessive airflow (or fume flow) under high manifold vacuum conditions.

This is not needed on later UK models when the carb ports are used for the 'sucking' connection as the vacuum is never very large at this point.

Don
Don

Oily gunge on the back of the valve heads could be from moderate stem/guide leakage or significant oil from the breather. Or both.

Well, you did say "intake breather" which is what all MGBs from 64 had which for better or worse I took to mean the intake or inlet to the breathing system. And the PCV valve was only used from 64 to 68. The valve was used in all markets at that time (and none before or after) and limited and controlled flow to a relatively constant amount to prevent upsetting the mixture balance across the throttle opening range. The carb breather ports used in all markets from 68 on, unlike the earlier inlet manifold port, have a relatively constant suction and hence flow because of the 'constant depression' characteristics of the SU carb between the butterfly and the piston.
Paul Hunt

Don the PCV is a one way valve to prevent a backfire back through the carb going into the crankcase and blowing out your gaskets. that is all it does. Mark
MLS Somers

No, the gulp valve does that, the PCV valve controls flow through and negative pressure in the crankcase to limit the effect of the breathing system on mixture.
Paul Hunt

http://www.carcarecouncil.org/Engine/crankcase_vent.shtml

Paul The gulp valve takes air from the smog pump. Not the crankcase. More cut and paste below.

"Purpose: The purpose of the PCV valve is to regulate the flow of crankcase fumes into the intake manifold where they can be burned. Prior to 1963, cars had no PCV and used road draft tubes that just left the hydrocarbon emissions from the crankcase out into the open air. The PCV valve also has a secondary role as a check valve, to prevent flow back into the crankcase. This prevents potential ignition of the crankcase fumes, should the engine backfire. The PCV system is also crucial for to proper engine sealing. The system alleviates crankcase pressure, which can push out on seals and gaskets, contributing to oil leaks. "

From what I have been reading the pcv does a lot more than I thought. Prost! Mark
MLS Somers

The PCV was only used for a few years, and any prevention of a backfire from causing the ignition of fumes in the crankcase was purely incidental to its main purpose of controlling flow through the breather. The main preventer of ignition of fumes in the crankcase was the oil separator and flame trap in the front cover or tappet chest, which was a chunk of steel gauze, and this was present both with and without the PCV valve. I have to disagree with the quote about the PCV system (not just the valve) preventing the blowing out of seals, since *any* ventilation, i.e. the road draft tube system used on early MGBs, or even just a crankcase vent and a sealed oil filler cap, or vice-versa, will also prevent it.
Paul Hunt

If you have a PCV handy, blow air thru one end and then try to blow air thru from the other end. If the valve isn't stuck open, air will only flow from one side. That doesn't seem unplanned. Mark
MLS Somers

Interesting topic in that I was just about to purchase a PCV valve for my '64B. This was a post Feb '64 vehicle, so it should be there. So, in regards to the above discussion, it seems that I should go ahead and install one (prevent flow back to the crankcase, prevent ignition of crankcase fumes, and be a good non-polluting earth inhabitant). Or should I?

Brian
Brian S

Hi all.

I understand the function of a typical funnel shaped PCV valve is to prevent excessive gasflow under high manifold vacuum conditions, which will effectively weaken the mixture.
If you open one up you will find a valve attached to a diaphragm which is held open by a weak spring.
A high gasflow pulls the diaphragm down against the spring, which tends to close the valve.
I don't see how this could prevent back flow, but maybe B's had a different type to the Land-Rover ones that I am familiar with.

I understand that a modern car's PCV valve is a ball bearing and (weak) spring type valve, which could prevent back flow, I guess weakening of the mixture doesn't happen in an injected car.

Meanwhile, back at the thread topic... ;-)

I still suspect that oily crankcase fumes may be lowering the effective octane of the fuel, as well as coking the cylinder head / piston tops.
My car tends to pink over a narrow range of inlet manifold pressure (irrespective of changes in ignition timing), how does this relate to the nominally constant pressure at the carb body ports of the HIF4 ?.

Has anyone with a PCV-less car noticed lower pinking than is usual for later cars, or an increase when they reconnected the PCV to a de-PCV'd car ?.
If you do connect PCV plumbing, Brian, I would be interested to know if you notice a difference.

Don

Don

There should be very little difference between properly working PCV valve and later non-PCV valve systems as they are both designed to give a relatively constant low level of flow through the crankcase. One problem that I have seen attributed to later non-valve systems is significant burning of oil through the breather, liquid oil bubbling up the front tappet chest cover even when the breather hose was disconnected. In one case the only cure was to replace the front tappet chest cover, even though the gauze was present in both. This oil tends to foul the plugs which is more apparent than coking of the pistons and cylinders. However hot-spots of coke or shards of metal can cause pinking but are usually associated with running-on. How have you eliminated timing as a cause? Disabled both vacuum and centrifugal advance mechanisms? How have you determined oil vapour is the cause, unless by disconnecting/blocking the breather, which changes other characteristics anyway. The 'curves' provided by the distributor are actually very crude in relation to the ideal requirements of the engine so it is quite possible to have pinking from timing causes over a narrow range of conditions. And after 30 years our engines are nothing if not individual.
Paul Hunt

Hi.

I accept that I am being anorak-ish about this minor problem, as Paul says a retardating the timing solves the problem. I just find it interesting.

In my tinkerings I have changed the distributor springs and disconnected the vac advance in various combinations.

I did have a running on problem when I bought the car, but cooling system changes have cured this, the main one being relocating the front number plate.

I don't know that crankcase oil vapour in the cylinders is a problem at all on a B, but some performance car ppl seem to think it is worth taking some care over, as do some modern car manufacturers. Maybe filtering any crankcase oil mist out would lead to the engine accepting a fraction more advance, which may offer a useful performance boost.

As I said earlier "As Paul says, temporarily disconnecting the breather and plugging the inputs would be an easy way of testing the theory, though the mixture would be slightly modified.
My B is off the road for the winter, but I will try it later."

Thank you to all who have contributed so far.

Don
Don

I feel a need to add to the PCV debate.

I think PCV is a wonderful idea, eliminates at least one source of oil drops on the driveway and burns some nasty gasses as well as reducing the concentration of those same gasses in the oil/crankcase.

Some confusion is apparent. The stock '64-'68 (ish) PCV valve was intended to limit induction of crankcase fumes at high vaccum. Nearly all other cars PCV valves are only to limit blow-back potential (as indicated by their function). Yes, they are both PCV valves but they are very different in design and function because they serve different purposes.

Excess oil mist inducted into the engine will hasten the formation of deposits in the combustion chamber that can trap heat and lead to pre-ignition. It would be an engine disaster, indeed, if you were inducting so much oil that the compression of each charge was affected enough to cause pinking!

The Telgerizer site was most informative to me and leads me to avoid the vaccum-pump drag race solution like the plague!

Boy, that guy with all the hardware store stuff was ambitious, eh? Why not just get a K&N breather and have done with it?????

Mike!
mike!

This thread was discussed between 10/02/2005 and 21/02/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.