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Triumph TR3 FAQ page

TR four cyl intake flow

I've had fun putting combinations of manifolds and carburetors on TR4 heads and running them on the flowbench. I've used TR3 manifolds both stock and modified, I've tried the TR4 manifolds both stock and modified, and I've run the Webers with the appropriate manifold. I've also made flow bench runs with just the manifolds and no carburetors. In addition, I've seen some gosh-awful attempts to modify these manifolds on engines I've taken apart to prep for racing.

Keeping in mind that this is flow bench stuff, and not dyno data, here's how various manifolds and carbs rank at .500 lift, best to worst, all on the same competition-prepared head, same flowbench, same day:

#1 - Weber carbs and manifold =100% (assume this to be the standard)
#2 - long manifold, my mods, HS6 SU's = 95%
#3 - short manifold, my mods, SU's =85%
#4 - long manifold, stock, SU's = 85%
#5 - short manifold, stock, SU's =75%

This all leads to some rather fun observations:

#1 - Webers: Besides being much better at mixing the fuel and air, the Weber setup provides the best overall flow, due in good part to superior manifold design. I think, but I cannot prove, that the additional venturi area of the Webers doesn't provide that much of an advantage, because the bottleneck in the whole system is the valve and seat.

#2 - modified long manifold: I can get pretty close to Weber manifold flow with my mods, but boy, does it take a lot of work! The problem is that any port/manifold passage must constantly decrease in cross section from the beginning of the runner down to the valve pocket (or, at best, be constant diameter). We can do that in the head, but in the long TR4 runners, we run out of metal so it takes a bit of skulduggery to do it.

#3 - short manifold: can be modified to match the flow of the stock long manifold. That doesn't necessarily mean that driveability will be the same because of velocity vs. torque considerations, but I haven't had a chance to check that out on a dyno. Nevertheless, TR3 owners, there is hope for you.

The most amusing head / manifold combination I've taken apart was one where the head had been prepared by a drag race mechanic. Boy, were those ports BIG! And flow was very high, too. Then I put his manifold and carbs on the head and the whole combination flowed LESS than a stock setup!! Inquiring minds ask "Why?" Well, to match the manifold to the head, he had just cut a big chamfer in the manifold ports to match the size of the head ports. This violated the "constantly decreasing area" dictum and destroyed the effect he had achieved in the head. Of course, those monstrous ports also didn't have much velocity, so I don't know how the engine would do with Webers.

Jack Drews


Just for the books, in the ancient past I remember the so called "optimum gas speed" for best flow was 325 fps. Anyone remember differently? Interesting enough this might explain a little why lots of times increasing
the size of the inlet valves does NOTHING for the power and may even be a deterant. This is so much fun I may have to unlimber my turbocharged solar powered slide rule.

Kas Kastner


Cylinder head flow

Machining the outside port edge and inserting a sleeve works very well for the early heads. Those heads (early ) were machined with a ball mill so that there was not a chance of the manifold overlaping the port entrance. Good theory, but dosen't get the job done for a racing engine. You might think about calculating the gas speed therough the ports as a better method of determining the shape. The speed through the valve is the fastest due to the restriction of the valve and from this I have in the past recommended a gradual taper to the valve so that the column of fuel loaded air is increasing in speed up to the valve. Though this was not a mind boggler in the flow bench it proved to be the best in the dyno. OR, you can just smooth out the port some, do a careful valve grind and then go a half marker deeper in braking and save all the hassle.

kas kastner


John, sorry to be late in replying but you are absolutely correct. One of the best small engine builders/cylinder head shops was a company called Comptune here in Ohio. Dave Tabor was the owner and all round everything. For years they had an extremely high percentage of SCCA run-off's winners - everything from MGB's to B and C sedan Datsun's. Dave said he alway got more power from the 1 3/4" SU's (HS6) than the 2" . Problem was the cars that were eligible for 2" were not allowed to use the 1 3/4".

Ted Schumacher

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