In a message dated 12/13/2003 11:27:29 PM Eastern
Standard Time, canarder@frontiernet.net writes:
Lynn,
With as many 13Bs operating in what seems to be exquisite
comfort at 5500 rpm, is
there any compelling reason not to go to 7000 or
so now that Tracy has a 2.7 PSRU
coming online? Do you think that additional
1500 RPM would have a dramatic effect on
engine life? That
is yet to be determined. There are rotary powered generator sets that operate
for several thousand hours between overhauls without problems. Suppose
the engine would last for 1,500 hours at 6,500 RPM. but only 1,000 hours
at 7,000 RPM. The average pilot fly's less than 100 hours per year. So
you will have the engine off every ten years, instead of 15 years.
By 38 mm chokes do you mean the primary and secondary
induction butterflies? The choke is the
smallest diameter of the throat above the butterfly. We use Webbers that
have removable chokes, so we can change the size. The choke (as the name
implies) determines the total air flow capacity of the carb. The rules
specify the choke size we must use. We use a Webber 48 IDA. dual throat
carb. The bores at the butterflies are 48 MM in diameter.. The chokes are
38 MM in diameter. The engine can use a lot more induction than we are
permitted.
The figures you provide indicate a very nearly linear
relationship between RPM and
HP. If that is so, it would seem that a street
ported 13B operated at 7000 rpm would
yield about 177 HP and peripheral ports at the same
7000 rpm would yield about 237
hp. Does that sound right to you? There
are a bunch of items to cover there, but yes is the correct answer.
What kind of "epoxy" would one use to block up the
side ports if one installed peripheral ports? Devcon
Plastic Steel. Or similar.
With all that power (presumably from NA rotor housing,
what's the exhaust like? As
bad (hard to tame) as a street ported Turbo housing? If
you are starting from scratch, or have a choice, use the NA rotor housings.
They have a splitter in the exhaust port that cuts down the noise quite
a bit. The intake system porting has little affect on the exhaust note.
Without the splitter, there is a very pronounced noise problem.
I've seen pictures of bridge ports, and they look
hideously difficult to do since you
have to leave "corner" of the housing there (presumably
to prolong apex seal life)?You do the ports
with a die grinder and a drill press. Practice on damaged housings before
do a good piece of cast iron. Not that hard to do. But like anything, there
is a learning period. The "Bridge" in bridge porting must be left in place
to keep the corner seal from falling out. It also keeps the apex seal from
sliding into the port. Use only 2 the piece apex seals. How
much fuss and bother would peripheral ports be to machine? Do you
have to
penetrate the water jacket or anything radical?
It must be really difficult or have
a LARGE downside because I haven't heard from anyone
who has them. http://yawpower.com/ Above
is Paul Yaw's web page. He has great pictures of his home made periphery
port. The factory periphery housings are about $1,700 each. I don't know
how much Paul charges for his. But if you do it yourself it is much cheaper.
And you can start with used housings. I used exhaust tubing and Devcon
to make mine. No problem. You go right through the water jacket. The section
is filled with epoxy after you press the tubing through the two holes.
It is on the cold side of the engine and the fuel air mixture keeps it
cool. No problems at all, and there is no skill at all involved in producing
monster HP. Again practice on some junk housings. A drill press is helpful
but not required. A die grinder is required. Air or electric.
Is it safe to presume that the ~ 0.5 BSFC will hold
pretty constant across the
various port configurations?
Inquiring minds need to know :o) ... Jim S. I
have seen similar numbers around, but it is more likely from an unported,
or mild street ported engine. The bridge ported 12A is .645 at 244 HP at
9,400 RPM. A street port would do much better. My 12A has 173 HP at 6,500
RPM BSFC is (the devils own .666). torque is 139.6. At 7,000 RPM 218 HP,
torque is 147.6 at .671. At 7,500 RPM 218 HP, torque is 152.6 at .673.
So if you have a 3:1 PSRU the torque at the prop flange would be 457.8
foot pounds. Look up the HP rating of the "real" airplane engine that has
that much torque. Lynn E. Hanover
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