Engine vent termination point

Hello again,

What is the proper location for the engine/cam vent to terminate? Currently mine runs left and down from the top front of the engine and vents under the front left fender? It seems if this is not properly placed the fumes could find their way into cabin vents.

Thank you,


To the left and down is correct. It is clipped to the radiator brace rod and continues down beside the chassis.
I have added an extension hose to carry the fumes back a bit further.


Looks just like mine, I think I will do the same.

thank you,



It varies a bit per version. Don’t know what XK version you have but e.g. the XK 140 FHC has a different routing and length.

Bob K.

I’m no expert but I would think that a proper road draft tube should enter the slipstream between the car and the road at a right angle–perpendicular to the road. That would maximize the suction. Similarly, the main jet orifice of a carb is positioned at a right angle to the venturi air flow. IMHO.

On the '36-'51 pushrod engine models, the road draft tube is indeed perpendicular to the road, with an angled cut on the end.
From the OP’s description I assumed he has a 120 like mine.
If fumes were a significant problem, one could of course route the tube into the intake through a hose connected to the inside of the air cleaner element, as was indeed how it was done on my '74 XJ12. This was known as positive crankcase ventilation, an early step in air pollution control found on most cars beginning in the 1960s.
My 1950 Mark V has both, a road draft tube, and PCV directly from the rocker cover into the air cleaner.

I’m not sure the road draft tube was designed to work as some kind of suction device by means of the venturi effect. I think it was more to let oil vapour condense and drip onto the road (or garage floor…)

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I agree. The phenomenon that would apply would be the Bernoulli Effect, constant energy across a stream line, which varies in proportion to the square of the fluid velocity, so 1/2 the speed would produce only 1/4 the effect. Although engine speed will vary across each gear selection, any Bernoulli effect would only be evident at higher vehicle speeds.

my original vent on 140 ots was a baalfle with 2 holes and a standard cover exiting to left and under ex headers to terminate by the dipstick facing slightly down but i don’ t think out. hose was paper with heavy corrigated aluminum cover. very cheap then? jj

Perhaps. But it seems odd that they would call it a road draft tube if its operation had nothing to do with the road draft.

The purpose is to let out unburned fuel vapor which sneaks past the piston rings on the compression stroke. Otherwise it would build up in the oil and dilute it, though the heat of the oil helps boil it off.
Plus of course the total volume within the crankcase changes as the pistons move up and down. If it was sealed it would act like a compressor and expander and put a drag on the speed. You need it to be at atmospheric pressure or the performance would be affected. Probably easiest to visualize on a one cylinder engine.
My '38 SS has one which is straight cut, not angled.
So I suppose the air stream effect would be something like blowing on the top of a bottle.
Later on they realized it was contributing to air pollution, so they routed it back through the air intake system.

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Judging from your previous posts you have an XK120?

My OTS had a clip to attach the cabling and terminated here:

Clip detail:

Clip was black, btw.


in my experience (part of accident warranty inspection) the vent must exist in some way and most engines will have a terminal explosion and blow out large pieces of bloc and shrapnel, along with a large fireball or plasma ball . hundreds of people have been killed in marine industry and ships lost.

Hello, Thank you for the photos.

So it appears yours is actually terminating under the hood, not under the fender or else where under the chassis, correct?

Thank you,


I think part of Tadek’s was missing, it was originally longer to hang over the outside of the chassis.