Here's a treatise on intake manifolds - more than you could possibly
want to know, but it's a rainy Saturday morning and not much else to
do. And please recognize that somebody else may have come up with a
better mousetrap than mine.
Best flow - Weber type manifold
next best - properly prepped long manifold
next best - properly prepped short manifold
next best - stock long manifold
worst - unprepared short manifold
Like any tube or pipe that is conducting a gas, the highest flow is
obtained if the tube converges slightly. The lowest flow occurs if
the tube diverges or gets bigger at the discharge end. What this
means for our intake manifolds is that if you just open up the
discharge end of the manifold runner, you actually decrease flow. I
worked on one engine where the head had been prepped by a drag racing
guy. The head flowed the highest I had seen. Then when I put his
intake manifold on and measured the flow of the whole carb / manifold
/ head, it flowed worse than stock. This was all because he had
opened up just the discharge end of the manifold runners.
The difficulty with prepping a long branch intake manifold so that
the runners are straight or converge rather than diverge, is that
there is not enough aluminum on the casting to allow that. In order
to make the runner a consistent or slightly convergent diameter is to
add aluminum to the exterior of the manifold and then do the porting.
This was illegal in the olden SCCA days, but we can get away with it
in vintage racing by adding the aluminum and then giving it a surface
finish that looks stock, so people aren't upset by appearance.
I prepare a few of these manifolds a year, reinforcing so they don't
crack and porting so they flow for max horsepower. It's a
time-consuming and nasty job that costs $350. Nasty because it takes
a lot of time to whittle out all that aluminum.
uncle jack
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