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
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.
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
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
powered slide rule.
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
that there was not a chance of the manifold overlaping the port
Good theory, but dosen't get the job done for a racing engine.
think about calculating the gas speed therough the ports as a
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
gradual taper to the valve so that the column of fuel loaded
increasing in speed up to the valve. Though this was not a mind
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
in braking and save all the hassle.
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
here in Ohio. Dave Tabor was the owner and all round everything.
they had an extremely high percentage of SCCA run-off's winners
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".
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