Is it too early to start the winter technical olympics? No? Well, then,
here goes:
The six cylinder manifolds, as Bill points out, are akin to household
plumbing, and although I've done a lot of work with TR6 ports, I haven't
touched a TR6 intake manifold. Maybe some of the things I've learned about
the four cylinder stuff will be of interest, though.
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.
uncle jack
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