I’ve acquired a nice SU carb and manifold assembly from a '65 E that was scrapped a couple of decades ago. I’ll rebuild the carbs to replace the ones I now have installed, which work ok but are not optimum. The passages in the manifold retain their rough surfaces from sand casting, this shot taken at a carb flange:
Nick, I’d do it just for the fun, I had a spare XJ40 inlet manifold and noticed that the ports were quite a different shape vis-a-vis the head so I matched them up.
Never tested to see if there was any advantage but enjoyed the work.
I cleaned up my steel headers inside, gotta be better than welding lumps, you will feel like its making more power, kind of like decals make your car faster
Here’s my 2 cents worth, not worth the effort (based on my limited knowledge) but heck if your bored, go for it.
If you really want to get into it Desmond Hammel discusses how to do it.
Cheers,
LLynn
I expect you’re all right. 3500 rpm times 4.2 litres divided by two divided by six and consulting Google to find out what the heck cubic litres translates out to CFM and the answer is a hair over 43. At redline it’s 62 CFM. Across a 2" diametre conduit, not much room for turbulence to make a difference.
Nick, it’s not like you to be so mathematical. I’m not sure now that I know you at all.
But I don’t understand. Divide by six to get the displaced volume per cylinder? Divide by two because the intake cycle is only half of the four cycles? But each 2 inch throat supplies two cylinders, not one, non?
And whatever the CFM, how do you get that there is no turbulence at that CFM? Probably right, but how do you know?
You’re right. I should have divided by four, not two. The pistons suck air through the carbs into the cylinders only once in four rotations of the engine, so the CFM numbers I’ve posted above are too high by a half.
It’s intuitive. My HVLP turbine paint equipment pushes well over 100 CFMas guaged 25’ away at the end of a 1" collapsible hose, also at low psi. By logical extension air flow through a 2" diametre conduit that’s maybe 3" long is not going to be much affected by surface conditions, especially when considering the fairly crude fuel:air metering of the carburetors themselves.
when I built my car, I did all the parts to the best level that I could muster. weather they could be seen or not. smooth that manifold and you will be glad you did, even if you cant feel the HP increase. Times a wastin Nick get er done
I did some comparisons between a cleaned up and smoothed 4.2 triple intake manifold and a stock one on a dyno years ago when I was racing. It produced slightly less power - not enough that you would notice seat of the pants. Years ago people used to polish heads and manifolds - that’s not done anymore. The accepted theory today is that roughness in the walls aids turbulence and assists fuel mixing with the air, which is much more important for power.
Fettle. You won’t gain many ponies by reworking the inside of the manifold, but if you break all the edges on the outside and remove anything that a cloth will catch on you’ll enjoy working on, or cleaning, the engine more. And you’ll save on bandaids.
Nick
A four stroke engine breathes its displacement every two revolutions. Your 4.2 litre engine inhales 4.2x6000rpm/2=12,600 litres/min = 210 litres/sec.=0.21cubic metres/sec. A 50mm diameter pipe has an area of 3.14/4x0.05x0.05=0.0020 square metres. Divide 0.21cubic metre/sec by 0.0020 square metres and you get 105 metres/sec distributed over 3 inlets = 35 metres/sec gas velocity=115 feet per second!!! Now engine does not run at 100% volumetric efficiency…a measure of the “breathing efficiency” of the machine…but maybe 80 % so the flow rate will be about 80% of 35 metres/sec which is 28 metres/sec or 92 feet per second.!!!
At those gas velocities the roughness of the flow passages WILL have an effect on the pressure loss through the pipes. The factor is “relative roughness” which is a measure of surface roughness divided by diameter…and this factor is used in the calculation of pressure loss…its moderately complicated but surface roughness and velocity are contributors. And…another important thing…any pressure loss that is avoided provides an increase in flow that is a “squared” relationship to the pressure loss. This is a bit dependant on the system but it is not a “linear” relationship…
I suspect that this is why porting and polishing was widely used in the past to achieve reasonable gains…and especially so for use at high engine/gas speeds. Modern casting methods have probably improved quality of castings in terms of accuracy and surface finish so the improvements may not be there…but your photos looked like there was room for improvement in shape and surface finish.
I’d give it a shot!!
Regards
Matt
sozfingers
(Keith. P. series two roadster,,,UK)
15
DEs Hammil ported my head and married up the manifold to the head and doweled it for precise location, I think Peter Crespin closes up the holes for the brake vacuum, you can also radius the pistons each side of the bridge, it all adds up…
I don’t totally close them, but the cross-sectional area of the three take-off holes connecting to the cast-in air rail is much larger than the area of the single vac take-off to the reservoir/servo. Plus there is no significant continuous ‘flow’ into or out of the piping unless there’s a bad leak somewhere.
So I put a small straw or similar into thte air hole at the upper part of the three bifurcations and fill the whole upper ‘blunt’ area with epoxy to restore the knife-edge the same as the lower part of the bifurcation. You can then pull the straw out. This is the first one I did and the holes were still bigger than needed, but a lot better than stock.
The other thing to do is match the head to the manifold using the gasket as a template. Ideally you’d used a metal mask dowelled in situ but the head gasket is good enough.
I can drink to that! I haven’t tested, but the golf ball etc. what I believe they call nowadays “micro turbulence” can indeed assist flow. I am sure Jaguar tested these things a lot, engines in a dyno and Normie did millions of miles on test cars, documenting all the things found.
Can’t you polish something visible, Nick? You already did that, huh?
Cheers,
Pekka T.
Fin.
sozfingers
(Keith. P. series two roadster,,,UK)
18
Yes, didn’t make that quite clear, saw some dyno stuff on a Pinto engine some years ago where they increased horse power by allowing the carb gasket to project a millimetre into the airflow, and look at the successful inlet manifold on the nippy little Triumph Dolomite sprint , ninety degree bends, rough cast alloy, one got on my tail in the early seventies when I was bowling along at ninety , couldn’t shake him !!!
It would be interesting to know how Jaguar treated their racing C’s and D’s
in terms of manifold porting. Surely that would determine if it’s a good
idea or not for the XK engine? Can’t imagine Lyons paying for expensive
labor if it wasn’t required.
Had one of those (Dolly Sprint) for 20 years before the E. They suffered from poor quality components/castings and benefited hugely from simply being fettled. Story was that the first six engines off the production line were dyno’d, producing from 127 to 140 bhp. the differences were all down to component quality, matching ports, cam and crank synchronisig
