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MG MGA - Speaking of drip pans and such.....

Maybe someone can answer this....

In my other thread about drip pans, you can see a large gap below my engine plate , on the side of the pan.....I have run a hose from the side-cover pipe into this gap, to catch , what I think, is one of the largest sources of oil mess on the bottom of the car....
The hose has a 5/8" I.D., so it should still vent very well.
Is there any reason why I shouldn't do this?....Seems logical, but logic doesn't always apply.
Thanks
Edward
Edward Wesson 60MGA

Edward

I think Barney told us a while back that the bottom of that pipe is so shaped to provide a negative pressure in the pipe when the car is in motion. This negative pressure sucks the 'dirty' air out of the cylinder block. You have negated some of this ventilation by ducting the pipe.

Hope I am correct. Barney will tell us.

Steve
Steve Gyles

Edward

To add a little more history to this. Quite a few years ago I removed that down pipe and turned the plate the other way up. I then attached a rubber pipe to the stub pipe and joined it to the rocker box pipe via a Y joint to the front carb filter box. Barney picked me up on this and explained how the system is designed to work. I have to admit that one bad side effect of my supposedly clever idea was a build up of that creamy sludge inside the rocker box cap etc. Once I reverted to the original design it all went away. The crankcase was once again ventilating itself properly.

Steve
Steve Gyles

Steve
Doesn't the vent hose from the the valve cover to the front carb do most of the crankcase venting? The carb is creating a negative pressure....

I can understand why running the pipe up instead of down , would create problems.....

Well, I guess I need more research. Thanks.
Edward
Edward Wesson 60MGA

Edward

That was what I thought but I was convinced otherwise during quite a long thread on the subject. I seem to recall I put up a long defence but lost!

Steve
Steve Gyles

Sorry guys, the rocker cover vent does not get any suction whatever from the air filter, it can't unless its blocked-up. Any gas/vapour coming from the rockers is blown out by any exhaust pressure from the valve guides and crankcase and drawn in from the air filter for burning up as it passes through the engine. Same thing with the large crankcase breather pipe, its to get rid of oil vapour and any gas pressure to reduce seal leakage and who wants pressure below the pistons anyway!
Pete
P N Tipping

P.N.
Your argument makes sense, with the possible exception of the air filter connection....

I believe that carbs do produce a negative pressure as they pull air through the filters....(Otherwise the filters would never get dirty)...
This helps reduce crankcase pressures through the oil drain holes in the head and block....(as the engine burns some of these carbon products).

I will yield to your argument about the large breather pipe...It makes good sense....

It's too bad, as I would love to figure out a way to eliminate the oil from this pipe, from spreading all over the bottom of the car.

Edward

Edward Wesson 60MGA

Peter

Air goes from the front carb filter box up the pipe to the rocker cover into the crankcase and is vented out through tappet cover pipe.

This is part of Barney's description back in 2009:

...."Suction inside the air cleaner (pressure drop across the filter element) is very small compared to the vacuum level created by the draft tube. With the draft tube in place and the car moving, air goes in through the air cleaner, then through the vent hose to the valve cover, through the crankcase, and out the draft tube. This is amazingly effective at road speed moving a lot of ventilation air through the crankcase."

Steve
Steve Gyles

Steve
If pressure is forcing the air through the draft tube, then why would it make any difference if there was a large hose on the end, going into the drip pan?...Air would still move freely though the tube, wouldn't it?
Edward
Edward Wesson 60MGA

Edward

My interpretation of Barney's words is that it is the higher suction from the draft tube that causes the air to sucked up the rocker pipe. Not air being forced up the tube.

Steve
Steve Gyles

"Pressure" is a relative term. Atmospheric air pressure is 14.7-psi at sea level (standard definition of one atmosphere). Anything below atmospheric pressure is commonly referred to as a vacuum. You have to specify the magnitude of the vacuum (like manifold vacuum) the same as you specify magnitude of pressure (like tire pressure). Absolute Pressure will be 14.7 PSI higher than Gauge Pressure. So 25 PSIG in a tire is actually 39.7 PSIA, while -9 PSIG in the intake manifold is actually 5.7 PSIA.

With engine running, pressure drop across the air filter is very small, perhaps 0.1 PSI. The is, there is only a very slight "depression" (low level vacuum relative to atmospheric pressure) inside the air filter, so the carburetor does not normally "suck" air from the valve cover. The draft pipe on the other hand serves as a venturi to create a greater depression (slightly higher vacuum)) in that pipe when the car is in motion. Vacuum in the draft pipe is relatively small, but it significantly stronger than vacuum inside the air cleaner. From the viewpoint of absolute pressure, pressure inside the air cleaner is pushing air through the vent hose in the valve cover then through the crankcase and out the draft pipe.

When the car is standing still, any small pressure in the crankcase will push the fumes out both directions, about 1/2 through the draft pipe and 1/2 to the air cleaner. This is when you may get a small amount of oil in the air cleaner from the vented crankcase fumes. That is normal. Likewise a small amount of drip from the draft pipe is also normal, as most of the crankcase venting goes in that direction. It is just the nature of the beast with the open vented crankcase.

If you want to stop the drip from the draft tube, and eliminate oiling of the air cleaner, here are ways to duct all of the crankcase fumes into the intake manifold. When doing so you can also create a small vacuum in the crankcase which will help to stem oil leakage (drips) from other places (like crankshaft seals for instance). But you have to be careful to
(1) retain some form of fresh air ventilation through the crankcase to keep in engine clean and dry inside,
(2) be sure not to ever create positive pressure in the crankcase, and
(3) keep the crankcase vacuum at a low level, not more than about 2 psi depression, so it doesn't suck dirty air in through possible leak points (like crankshaft seals).

This is all about crankcase ventilation, which must always be retained. There are a few different suitable forms of crankcase ventilation described on my web site. Defeating the draft pipe venturi action is not one of them.
Barney Gaylord

Thanks for that Barney. We had a long conversation about this back in 2009. I remember disageeing with you for a while but after many threads I put my hands up and put my car back to the correct ventilation system. Almost immediately the sludge disappeared from the rocker box and the idle rpm alone increased by 250. I would say to others to accept any small drips from the draft pipe for the overall engine efficiency.

Steve
Steve Gyles

Barney
I did read the article, and still have a few questions....
How much of an angle on the tip of the draft tube, and what direction does the opening face?

My draft tube doesn't look like it has any angle at all, but is just cut square....Maybe a replacement.
Thanks
Edward
Edward Wesson 60MGA

Angle on bottom end of the draft pipe is about 30 degrees with the open side facing rear.

Illustrations in factory literature and parts diagrams are often incorrect, showing a flat end or angle forward or toward oil sump.
Barney Gaylord

Thanks, Barney....Looks like the grinder is coming out....
Edward
Edward Wesson 60MGA

Interesting thinking there Barney, all good reasoning once again. Have to admit I hadn't heard that argument before. I'm going to have to think up a way of proving to myself which way the air is moving in that rocker vent pipe... At least we can agree the carbs don't suck gases from the rocker box!
Pete
P N Tipping

This thread was discussed between 27/01/2014 and 29/01/2014

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