Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Header length question

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Old 08-25-2004, 04:31 PM
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Ernie Misner
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Default Header length question

Greetings!

On the Mac's Products website, they claim that getting max rpms by finding the right length of the header pipe is the way to go. (before adding expansion chamber, mousse can, etc.)

How can that be, when the expansion chamber part of the tuned pipe has many variables in it's design, and then rpm figures must be checked again when using the whole tuned pipe system, not just the header??

Actually I am a sport flier that will be attaching a mousse can setup to the header, but I thought you guys would have the most experience.

Thanks!

Ernie
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Old 08-25-2004, 09:31 PM
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Default RE: Header length question

Quote from Mac's website:

Next, run the engine at full speed, open-face (with nothing attached to the exhaust, although this may not be possible with some engines whose carburators require muffler pressure, in which case we have actually hand-squeezed a sealed fuel tank). Next, put the full-length header and tuned pipe on the engine and run it. If the RPMs are less than the open-face reading, cut 1" off the header (1.5" for engines over 1 cubic inch) until the RPMs are above the open-face reading. Then cut the header .25" until maximum RPMs. Or, when the next cut causes a decrease in RPMs, this is the best tuning length for static and vertical performance (fun flying, aerobatics and hanging it on the prop).

Notice the part, "Next, put the full-length header and tuned pipe on the engine and run it."
After each cut of the header, debur and clean. Re-install header and pipe. Repeat the last 3 steps until max RPM is met.
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Old 08-28-2004, 12:26 AM
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Ernie Misner
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Default RE: Header length question

Thanks! I've been reading that paragraph wrong.

Next question..... "when the next cut causes a decrease in rpms, this would be the best length for vertical, fun flying, etc."

With this configuration, the header would be just a bit on the short side. I thought that being on the short side would be best for racing (peaky but powerful), whereas being a bit on the long side would give a broader and more linear power curve, best for fun flying.

Which is it? Slightly short (below peak rpms) header, or slightly long (below peak rpms again) header for a wide and useful powerband for funflying?

Thanks again,

Ernie
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Old 08-31-2004, 06:34 PM
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ZAGNUT
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Default RE: Header length question

more RPM=shorter header...

so with a normal sport engine/prop combo you're still going to unload a bit in the air and will need that shorter length

with a speed prop you do a lot more unloading and need an even shorter header in order for the engine to stay on the pipe in the air


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Old 09-01-2004, 03:00 PM
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Default RE: Header length question

Makes sense Dave, thanks!

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