Air box - worth making air intake runners?

I have a 4.2 in a Mk2, it’s a 9:1 motor
It’s got a straight port head, 420 log manifold, and 2x HD8s

Due to the very little space between carbs and engine compartment, and the Mk2’s propensity to heat soak, I built an angled air box connected to a flexible hose to an air filter under the fender. The system made a big difference in running condition, but over the years the face of the box has warped (I used too thin steel for the mating surface with the carbs).

I’m remaking the front of the box, and my question is: Right now the carbs are sucking air from an open space. I know runners improve air smoothness etc. I could fit definitely 1/2" of tube, perhaps 1" if I’m creative, inside the box.

Is this worth doing?

I doubt if would make a hill of beans difference.

I would look for more improvement by putting a radius type bell mouth inside the air box.

I can tell you Datsun thought it was worth it when they designed the air cleaner system for their 19671/2 to 1970 2000cc Roadster…don’t remember if they kept the design for the “Z” cars.

They didn’t.

A stack that short, on a relatively short intake tract, may have had an effect: that is not quite the same case, with the Jag.

It may well, however, make YUUUGE gains on the mental dyno!


Thanks for the comments. I think I agree that in such a small space it won’t make a difference. If I could fit longer / larger runners than maybe but in this case not worth the effort.

If your carb flanges are bolted to a flat face with a 90 degree corner and no rounding, it reduces the maximum flow - by about 18%, from memory. Could be 8% but it’s significant anyway, although your twin 2” may be plenty of carb until close to WOT.

The ram pipe effect is non-existent at such short lengths, but adding a throat radius well worthwhile. Jag did the same on their pressed steel plenums on the XJ and S2 E. On the 12 cyl E the used flat sheet but a bolt-on radius casting that doubled as breather. I’d machine a couple of 180-degree ‘half bagels’ with studs, to clamp the air box to the carb flanges. Drill for the vent of course.

I’d been meaning to have a look at my spare (unmodified) Mark X airbox, and just remembered to do. These are pretty large, so I suppose Jaguar could have done whatever they wanted to the passages.

It has opposite each carb, coming in from the back, a vertical “fin” that separates the air stream into two halves (the fin is part of the casing, about 1/8 inch thick). I suppose this would accelerate the air, like a venturi. I have no idea of the rationale. It’s going into a single carb throat, although after the carb the flow is diverted left and right into two intake runners.

Yes, nice casting, but the carb inlets are offset slightly below the main body of the plenum, which receives air via the 3” hose and is probably quite turbulent . I expect the ribs were designed to deflect incoming flow down into the carb throats?

And continuing on the Mk10 plenum: the bix flex pipe ends up with an air filter inside the front corner of the right wing, where there are two 12” flex tubes attached to the wall through which the air enters (many owners will know exactly what I mean). Whatvis the purpose of those 2 flex tubes???