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

SJ 40.old version

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Old 08-26-2008, 06:19 PM
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zero-cool
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Default SJ 40.old version

I picked up a scat cat with sport jett 40.I think its the old version.needle is on the carb.can i run a standard apc 9x8?would that work well?what rpm should I be taching on the ground with this engine.I dont want to over or under prop it.also id like to use standard apc if i could.im going for top end speed only!
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Old 08-27-2008, 07:21 AM
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Default RE: SJ 40.old version

If you can post a photo of the engine that would help.

Is this a sport jett 40 or an SS40 engine ? You can tell the different by the muffler and carb.

The sport jett has two needles (high and low end mixture) and a straight barrel muffler.

The SS40 muffler has a bulge in the middle, and the carb only has a high speed needle in the carb.

The primary difference is the tuned rpm for the muffler. The SJ-40 you want to prop to turn over 16,000 peak ground rpm. The SS-40 you want to prop for no less than 17,500 peak ground rpm.

In any case, for a stock blade prop, stick with an APC 9x7 at first. On a scat cat, its going to be pleanty fast. Find peak rpm (briefly), back down 600-800 rpm from there for flight.

There is no way to under prop it. You will not hurt the engine.

Overprop is possible - you want to hit the rpm numbers with your prop selection.

Bob
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Old 08-27-2008, 06:44 PM
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Default RE: SJ 40.old version

my old Jett SS40 on an 8x8 APC runs like a raped ape!

yes it has a newer piston and liner, and dont worry about the single needle carb it works great!
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Old 08-27-2008, 09:33 PM
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Default RE: SJ 40.old version

bob,it only has high speed needle.the muffler does have a bulge in the middle.so what prop would you use if you were going for top end speed?
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Old 08-28-2008, 08:15 AM
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Default RE: SJ 40.old version

You have an SS engine ..... does it have the standard glow plug (SEMPRA engine) or a Nelson plug in it (CAPS engine).

Since you are up north, you may have a CAPS engine (used the nelson plug, otherwise its pretty much the same engine used down south).

Remember, RPM is important. RPM is your friend. You want the engine to be "Happy". Look for 17,500 rpm or more at ground peak. Back off 600-800 rpm for launch.

Props...

FAST: 8.5x7, 8.5x7.25 (APC D-1 racing series). We used these for racing. Turns way up there (20K rpm and better).

Next best: 8.75x7.75, 8.75x8W, 8.8x8 (APC D-1 racing series) Look for somewhere around 17,500-18,000 ground rpm

Pretty darn close: 9x7 , 9x8 APC standard blade. Use a 9x7 to get use to the engine. If you want to try one, make sure the 9x8 gets up over 17K rpm on the ground and easily unloads in flight, if not drop back to the 9x7.

The D-1 props are best here because they are designed for the rpm and flight loads involved here. The APC 9x7 and 9x8 are ok in that respect too.

I suggest not using a standard 8" blade APC (c-2 series). The blade root and hub are rather thin. They can and have thrown a blade under the right (wrong?) conditions. You can sometimes get away with an 8x8, but definately nothing smaller than this.

VERY important here if you are using a standard fuel tank, make sure it is 100% wrapped in foam and is not touching the airframe. Sometimes this is tough in the scat cat. If it touches the airframe, the fuel will foam, and that is nothing but problems.

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Old 08-28-2008, 07:29 PM
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Default RE: SJ 40.old version

bob,thanks for all the great info
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Old 04-13-2010, 11:55 AM
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Default RE: SJ 40.old version

Bob,

I've got what appears to be an SS engine due to the carb (single needle), but the pipe does not have a bulge in it. I bought this engine used, so don't know what it really is. Without the bulged pipe, what rpms can I expect with the same props?
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Old 04-14-2010, 10:34 AM
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Default RE: SJ 40.old version

Disregard my post above, found that I do indeed have the "bulging muffler".
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