MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

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 Midget and Sprite Technical - Draught tube crankcase ventilation -- enhancement.

Draught tube crankcase ventilation -- enhancement. Is it possible?

Does anyone have any suggestions, as to how the draught tube could be made more effective/aggressive?

Some systems use the flow of exhaust gasses at the bottom of the manifold, to suck crankcase gasses out. But that means welding the exhaust pipe.

The draught tube relies soley on the forward movement of the car. Does anyone know if there are any factors, such as tube length or end shape, that might enhnace the effect?

How about simulating the effect of the exhaust vent system, with a fan sucking on the draught tube? The touble is, that might clog with oil.

Or how about perhaps using a Y piece in the draught tube to connect a fan, as shown in my childrens age technical drawing? ;)

Nope, not on a Spridget, on a Ford pre-crossflow. But the early Spridgets use draught tube evacuation too, so it could be useful here.

Does anyone have any printable solutions, that don't involve a rude word ? :).

Lawrence Slater

To be most effective, the end of the draught tube would need to be in a low pressure area. I think as standard it just goes straight down into the airstream. Without a "splitter" front bumper dam I believe the space under the car is generally a high pressure area as the airflow is squashed between car and road surface. Maybe a tube to the side could find a low pressure area beside the front wing, although spraying oil as you pass pedestrians might be deemed anti-social.

Unless of course they were just immigrants walking the hard shoulder. ;-)
Guy W

Lawrence,

It would probably be most effective if you connected the input of the fan to the end of the draught tube and vent the output under the car somewhere. Easy to set-up a U tube manometer to check for pressure drop or otherwise.
David Billington

Ahha. The old, 'kill 2 birds with one stone' ploy Guy. I like that about your 'final solution'. lol.

Yup, makes sense. What do you think of my less cruel fan ploy?

But wouldn't that clog the fan with oil David? It was my first thought, but I ruled it out.
Lawrence Slater

I had an idea several years ago, but never followed up on it

My thought was to cut an splice a T connection into the crankncase vent tube then run a hose off the straight side to a funnel that would be in the 4inch holes at the front of the car that holds the heat tube to the heater core, then on the back.side of the T connection just run a long hose thats cut off level.at the ground with a one way pvc valve so no air can be sucked up iinside

The air forces into the funnel as you drive it pushes past the T section, creating a low pressure spot thus pulling vacume and air out of the crank case and its vented out the backside towards the ground...(giving the car more push for an extra 3hp...haha)

Prop
Prop and the Blackhole Midget

<<vented out the backside>> A truly Prop quality statement!
Guy W

Lawrence,

The vent should be dealing with oil vapour and stuff rather than liquid oil, if liquid oil then you have a bigger problem with your vent system, but then I know your oil sucking thread so maybe you do. Over time you might get a build-up of crap on the fan but then treat it as a service item to be checked and cleaned occasionally. If I was doing this I would use something like a squirrel cage fan so the motor was external and not subject to any vapour. Regarding high pressure under the car, I'm not too sure in the case of a spridget, but as the car behaves like an airfoil the longer path over the car leads to a low pressure region on top and lift potentially but that doesn't necessarily mean the underside is at a higher pressure than ambient, the lift is caused by the difference in pressure.
David Billington

Guy W, Cumbria, United Kingdom


<<vented out the backside>> A truly Prop quality statement!


Haha... and I wasnt even trying

Prop
Prop and the Blackhole Midget

<<vented out the backside>> A truly Prop quality statement!?>>

LMAO Guy cheers!

I wondered about this during Paul Noeth's thread recently. I had an idea.

A series engines in USA used to be fitted with an air pump for emission control. Could one of those be rigged to draw from - say - a tee piece in the dipstick tube to pull air from the crankcase and just vent it under the car? The hardware to mount and drive the punp already exists, so would it be simple just to replumb it?

Just an idle thought.
Greybeard

"The air forces into the funnel as you drive it pushes past the T section --"

I like it Prop. no fan needed. But maybe keep it for when stationary.

David, it's my Capri. It's not excessivley smokey. But it's in the nature of draught tubes that they drip, and you can smell them.

Greybeard, no need to re-plumb, just need to force evacuation, just as the PCV does on the later systems.

I'm working on getting a 1300 xflow pcv valve and trying that plumbed into the weber manifold, but I'd like to try this out too.

Lawrence Slater

Lawrence, the thing that makes them work as standard is because the bottom of the tube is cut at an angle. I cant remember off the top of my head what that angle is but I wonder if changing the angle may increase extraction. Also your picture shows the tube at an angle whereas the spridget ones were vertical which may also help.

Trev
Trevor Mason

I think it's vertical on the Capri Trevor, but it doesn't hang down very far below the sump. I'll have to measure it. I think it's cut sq too. My sketch was only to prompt discussion, as I don't have a clue how to make a draught tube more effective by altering the angle.

As David said, a manometer might be a useful tool to employ.
Lawrence Slater

It is definately the angle on the end that makes it work. The longest end needing to be at the front as the air then flows round the tube creating a difference in pressure. That is why I wonder if changing the angle could improve things. From memory I dont think they are normally much lower than the sump.

Trev
Trevor Mason

OK ta. Sun out today, so I'll take a peek.
Lawrence Slater

I remember an aftermarket device for the end of your exhaust pipe that worked on the same principal as Props idea. Just shows you that "vented out the backside" is not a new idea!!!!!!!

Rob
Rob aka MG Moneypit

I've been reading the history of venting.

It was ww2 and the need for tanks to go through deep water the spurred the move to PCV over draft tube.

Then in 1952, Professor A. J. Haagen-Smit, of the California Institute of Technology at Pasadena, linked unburned hydrocarbons to smog, and PCV was later introduced on all road cars.

But prior to that road tube was considered ok albeit with the shortcoming that it simply doesn't work unless air moves over the end of the tube fast enough.

So I still reckon a fan to keep the air moving is a good idea, but better still is the connection to the exhaust system. Howevr, I don't want the hassle and expense of having a tube and trap welded into my exhaust.

I need an inline fan, or a fan at the front in the trumpet, that takes over the ram function, when the car is stationary.
Lawrence Slater

Sorry Lawrence, I just assumed you were talking about Spridgets, having momentarily forgotten your Ford.

Trevor is quite right about the angle at the end of the tube, but the optimal angle is relative to the airspeed over it, so it's a compromise.

It is essentially an airfoil section, like any other. Wings, sails etc. which leads me to wonder if the drag of the device (in other words the draught generated) could be significantly enhanced by a better design of foil.

Nothing fancy of course. I don't know how much space you have to play with in the Ford's engine room, but I imagine there's a fair bit. If the foil were in the shape of rectangular box with one side cut away so the open bottom of the box was at a steepish angle - say 45 degrees for starters - and the deeper side faced forward but not completely vertically; maybe 10 degrees off vertical so that the airflow over the box was directed positively towards the bottom edge I'd expect it to generate a pretty good draught. Depending on it's size of course, but I wouldn't think it would need to be more than a few inches wide.

If then the closed top of the box structure were connected via a hose or hoses to your crankcase it should pull the smoke and vapour out pretty well. As long as the car is moving of course, so the fan idea for the stationary car would be of benefit. As David suggests it should be treated as a service item to clean it of condensed oil-mist residues, or even replaced periodically. 12vDC computer fans for example are small and cheap. I just saw some selling at £1.68 a pop. But I also liked the squirrel cage fan idea.

On the other hand it would be much of a stretch to include a trap to catch the condensate.

I've sailed all my life so I have a fair handle on how effective foils are and what I've described is exactly how some wind-powered ventilators work to draw hot air out of boat interiors in warm climates. Even in light breezes they can be very effective.

I love a ramble on a Sunday :-)
Greybeard

I've decided to convert it to PCV, via suction from the inlet manifold.

Ford Escort Mk1 PCV valve. I've bought a NOS item. Picture enclosed. I've already got an oil separator on the engine, connected to the draft tube. So I'll connect it up and see how it goes.



Lawrence Slater

Amazing.

Bought this oil separator from a Ford 1600 xflow engine, for £15.00, and it came complete with the PCV valve.

Such a simple device.

I pulled out the original oil separator that was plugged into the block and was connected to the draft tube.

Then I pushed this one into the block in place of the original. I extended the length of pipe, and connected it into the weber inlet manifold, in the mixing chamber below the carb, where the vacuum take off is for a servo connection.

Started it up, and it's perfect.

Pre-xflow engine designed for a draft tube, and it runs perfectly well with a PCV system for a later x-flow engine. Why does anyone stick to a draft tube?

The only downside is that I already bought a new PCV valve that I don't need now. The one that came with the oil separator is servicable too, so I'll keep that and try to sell the brand new one.



Lawrence Slater

P.S.

I think this valve would work perfectly well to replace the mushroom valve on a Spridget too.
Lawrence Slater

This thread was discussed between 05/09/2015 and 13/09/2015

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now