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MG MGA - Road draft tube

Here's a good one for you: which way does the open angle of the draft tube face? I'm guessing towards the block, but I've been wrong before.

I was going to make one to fit to my 1800 which has the baffled tappet cover and upturned tube. I've been running without a properly ventilated crankcase for a while now. Tsk tsk. A thought just occurred to me: will the baffles in the cover prevent the draft tube from creating the necessary vaccuum at speed? I can easily switch to an MGA tappet cover and tube, I would just be missing the tapped hole in the block to mount the pipe clamp.

Mark
Mark J Michalak

Mark
Mine faces towards the rear of the car which will create a vacume at road speed.
Kris
Kris Sorensen

Mine is also as Kris stated.
Andy Bounsall

Yes, definitely to the rear, to create a vacuum at speed. It is that way on both my 948 midget and early 3 main MGB engine.

Tore
Tore

Thank you for the info.

As to my second question, does anyone think the baffles in the later style 1800 tappet cover would affect the vacuum at all?

Mark
Mark J Michalak

OK guys, I have no idea what you are talking about. Can you explain what the draft tube is please?

Andres
Andres Losin

The draft tube is also called a downdraft tube, coming off the engine to vent it. It goes down below the engine, and at speed, a vacuum is created at the tip of the tube, which forces internal engine gases to evacuate from the engine. At idle, as there is no vacuum on the tube, you will frequently see smoke emanating from the bottom of the car, accompanied by a definite oil smell. I don't think that the baffles will affect the vacuum although they would affect the amount of oil entrained in the exhausting gases.
mike parker

Interesting, but I find some early photos of MGA engines show the angle of the tube facing the front! See attached image.

A good project for those with time on their hands, to research as many old photos as possible and come up with a result. Factory photos if possible.

Mick


Mick Anderson

Hi Mick that engine must have come from the "famous reverse gear option only model " because they thought the streamlining was better when going backwards lol

gordon
g c pugh

Is this tube not just a simple crank case ventilation opening.
I don’t believe that the direction of the chamfered end of the pipe would in any way effect the vacuum draw on the pipe. Besides has anybody ever tested to see if the area at the pipe end even has a negative pressure? As we all know an MGA engine bay is a high (positive) pressure area, hence why the cars tend to run hot rather than run cool, which was a reason the Twin Cam cars had the louvered panels in the engine bay.

I believe the air would move through the pipe, due to expansion and contraction as the gases inside the engine block heated up and cooled, plus any difference between the top vent and side vent pressures.

One thing I can tell you though, those pipes are dangerous when you are putting the block on the ground. I once helped a friend remove an MGB block and it landed on the ground a bit quicker than it should have done and the vent pipe went through his shoe. Luckily he was OK.

Anyway that is my 0.02c worth. I look forward to other people’s opinions.

David.
DJ Lake

We had a good discussion on crankcase ventilation a year ago. Barney took me to task on a modification I incorporated and I ended up returning the system to original (with positive results). In the following link you can read what Barney has to say about crankcase ventilation and the breather pipe etc:

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=6&subjectar=6&thread=200612281420095446

Steve
Steve Gyles

I went to a technical school in 1962 (long ago) and studied automive technology. We were taught that the road draft tube created a slight vacuum while the car was moving. The oil filler cap on cars with draft tubes was always a vented cap with filtering material inside the cap. Part of maintinance was washing the filter cap in solvent and blowing it dry. The cap tended to oil up from blowby when the car was not moving. When positive crankcase ventalation came along (PCV) the oil filler caps became a sealed cap, and a fresh air hose was installed from the inside of the air cleaner to one of the valve covers. The PCV valve was in line in a hose from the crankcase to the intake manifold.


Ed Bell

The "picture" Mick posted appears to me to be a drawing of a generic BMC B block engine. That illustration shows up in Graham Robsons book MGA MGB MGC on page 24 and on page 50 of Robert Vitrikas MGA book. There is no heat shield shown and the carb vents are run wrong in it. The oil cap and front crank nut are also different than an MGA.
This picture shows the vent pointing in the right direction. Page 48 of Vitrikas book.


R J Brown

Seems to me that if the opening faced forward, it would blow air right into the crankcase as speed. Is that a photo or an illustration? The Moss illustration seems to show the opening facing the block, and the SF illustration is inconclusive.

In any case, rearward seems to be the way to go. Perhaps this would explain why my oil gets black so quickly. Speaking of which, what's the general consensus on flushing oil?

Mark
Mark J Michalak

If the vent were angled backwards the air and dirt could be forced up the tube and into the engine. With the tube angled as designed it creates a suction (low pressure) on the end of the tube While Moving. Standing still it has no effect. Here is another picture from Vitrikas book page 105 showing the proper angle.
DJ the purpose is to provide a vent to blow-by. The gasses that flow past the rings that would pressurise the crankcase if no vent existed.


R J Brown

I believe that the tube ideally will be cut at an angle parallel to the engine alignment, i.e. opening sidewise toward the left side of the car.
mike parker

I just took pictures of 1) 62 KM11 2) a 58 1500 coupe 3) a 60 with early 1800 and 4) the original tube from the 60 1600's original motor. All 4 are shaped the same. Highest point of cutout to the back and longest point to the front. Pictured below is 58 1500.


R J Brown

Boy did I learn a thing or two. I always thought the pipe was that long to ensure the oil didnt drip on the chassis and the angle of the end cut was a design decision. The link noted above by Steve, but written mostly by Barney holds a wealth of knowledge.
Cheers.
DJ Lake

This thread was discussed between 11/03/2008 and 13/03/2008

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