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Posted for "Tom Kendall" <Tom.Kendall@att.net>:
----- Original Message -----
After lurking on the list for a while I see most folks
are using some combination of the lower intake
manifold with custom fabricated runners. It seems
everyone is using metal and not composites for the
intakes. Are there a particular reasons for this?
Like heat, vibration, etc.?
Can't speak for everybody else, but you nailed the reasons for metal on my
choice of material: the intake is very close to the exhaust manifold (and,
in my case, the intended turbo-charger). I don't like the idea of plastic
of any kind as the only thing between a pressurized air/fuel mixture and a
1,000+ degree exhaust pipe or turbo!
Aluminum is a tried & proven material for intake manifolds, and considering
bolt stress concentrations, thermal growth considerations, vibration, etc,
it seems to be appropriate for the application. Some people have used
fiberglass or carbon composites for the intake runners, and these seem to
work fine. But again, I'm concerned about the bonding or sealing of the two
different materials together and the potentially disastrous effects of an
in-flight failure there. So it'll be brazed or welded metal for the intake
runners and everything back until I'm upstream of my fuel injectors and well
away from the heat.
Dynamic charging is certainly feasible, especially since in an aircraft
application you can optimize it to a single RPM for maximum climb out power
/ performance. Dynamic charging is to maximize horsepower by getting a
little extra boost. Doesn't make sense to tune for anything but the high
end since you could do the same by simply opening the throttle wider. So it
won't buy you anything for cruise performance.
Intake runner length is the critical tuning parameter for dynamic charging;
what is happening is that a pressure pulse is created at the end of the
intake stroke when the intake port suddenly closes and the air starts
"piling up" behind the now shut port. This pulse moves back up stream
through the runner until it hits a big change in diameter (i.e. the intake
plenum). At that point the runner has a higher pressure/density than the
intake plenum. It is "supercharged". But now things start to unwind: the
air in the runner starts to move back into the lower pressure plenum
(especially since the port on the other rotor can be opening up right about
now and dropping plenum pressure even lower).
If you can make it work out so that the time it takes that pressure pulse to
travel to the plenum is the same time as it takes the rotor to move to the
next intake cycle, then you have a pre-charged runner that dumps into the
combustion chamber as the port opens up. Clearly, the length of the runner
is critical and depends on the speed of the engine. The faster the engine
is turning, the shorter the runner can be. A longer runner is fine. Just
not a shorter one.
IIRC, the critical length at 6,000 rpm is about 20". Could be way wrong on
that, and others on this page will know better.
Runner diameter is not as critical, though common sense will tell you that
the larger the diameter, the more air can be packed into it and the less
resistance there will be to flow (i.e. less pressure drop). There is a
limit though: Too big, and the velocity is so low during the intake stroke
that you won't develop any significant charging pressure pulse. The
pressure pulse height is inversely proportional to the area of the runner.
Another trade-off. But if you keep the area close to the area of the stock
manifold you can't go far wrong.
One way to alleviate all of *this* concern is to use a turbo charger. Of
course that brings a whole new load of questions and considerations....
Tom Kendall
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