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MG MGB Technical - Airbox and Crank Vent with DCOE

Further to the thread on “Crankcase Ventilation with DCOE” started me thinking ‘again’ about an air box for my DCOE. Basically a ‘port’ can be added to the air-box to connect the breathers. The air box will provide a negative pressure and will recirculate any nasty blow-by gases. I was toying with the idea of making one, but as usual finding the time is a problem at the moment, so that leads me to the question, does anyone know of a ‘off-the-shelf’ air box that will fit without touching the inner wheel arch?
MG Mike

Could you just run a pipe through the back of a large K&N?
Steve Postins

Good point Steve, I neglected to mention that my ‘plan’ was to use a cone filter in front of the radiator (behind the grill) and duct cool air into the air box. I’m convinced that the K&N is fed with warm air at best from both the radiator and the exhaust.
MG Mike

I think you're right. My DCOE certainly prefers cooler wheather. I've put in a fabricated heatshield and a duct from the front based on those pictured in the works Le Mans MGBs. I went without the box as those cars didn't have them, and the consensus seemed to be the ram-air effect was going to upset the flow and mix too much.
Steve Postins

Here's something I've thought about in this same vein...

I'm running a EMPI filter on my DCOE. I believe it's intended to be used on an air-cooled VW with Webers - and it only cost about $16. It clears the brake master...not my much, but it clears. This filter is simply a base plate attached to the carb and a top plate with a filter trapped between. Two wingnuts hold the top cover and filter to the base plate.

It would be very simple to fabricate a simple air box to replace the filter. It's not really's straight on the top and bottom with rounded ends. Just bend some aluminum around the filter to make it match the curve. Install this in place of the filter (with proper sealing) - with a hole on the bottom to mate with some flexible ducting and run that forward to a cone filter. The finished product would look nice - and would be very inexpensive. You'd save enough money to buy a pair of short velocity stacks to fit inside the housing!
Joe Reed

Regarding the cone filter in front of the radiator - isn't that a positive pressure area? If so, you'd lose the negative pressure in the airbox.

TWM sells an airbox. The thing is ridiculously expensive though. Something over $200 if I recall.

I've thought of using a piece of aluminum rain gutter downspout - the rectangular section stuff. But I'm not great at fabricating and don't have the right tools for it. So that project will never get off the ground.

Matt Kulka

Using a pipe in an air-box is very hit-and-miss, that is what the original non-positive ventilation system used. You may get negative pressure in the air-box if the air inlet is letting in less air than the carb is demanding, but then depending on the position of the other end of the breather system there may be more suction on *that* when the car is moving i.e. if the open end is pointing down at the road with passing air blown across it. Whichever way the ventilaton moves it will be passing unfiltered air containing dust and moisture through the crankcase, not a good idea, which is one reason why the positive and filtered systems were introduced, and when a hot engine cools down it will be drawing in muck from both ends. Another reason is that the hit-and-miss factor means that it isn't properly venting the crankcase of condensation anyway, and fumes are more likely to come into the cabin.
Paul Hunt 2

This is all good information; it is truly great when the forum brings together some good debate. Firstly the positive pressure at the front of the car is actually about 12-18 inches in front of the grill and not behind it (the air here is turbulent and chaotic) and then this ‘pressure’ is a magnitude away from the negative pressure provided by the positive displacement pump that is the engine. ( from Bernoulli's Equation where Pressure diff (Pa) = 0.5 x r x v^2, where r=1.255kg/m^3 and v is in m/s) all of that adds up to a few psi even at 90mph about 18inch in front of the grill. Whereas the suction for an 1800cc engine at 4000rpm is enough to remove your fillings! SO, the air box will ALWAYS have a negative pressure and be sufficient to vent the crankcase. You could be lucky and hit a positive point behind the grill, but this would not be constant and only worth a couple of psi at best at high speed. I like the idea of using the standard K&N filter housing and replacing the element with an alloy case, this way you can feed the box from a remote cone filter at the front of the car, the greater advantage coming from the cool air as apposed to the warm under bonnet air. Should be relatively easy to make too!
MG Mike

Results eagerly awaited - good luck!
Steve Postins


So are you planning to modify your grill à la Works MGBs?

Alexander M

This thread was discussed between 22/09/2005 and 23/09/2005

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