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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.
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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