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

Timing on a Jett or Nelson

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Old 12-04-2007, 11:47 PM
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5.0Rick
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Default Timing on a Jett or Nelson

Hello,

Does anyone know what the timings and compression ratio are on a Jett or Nelson 40 to 50 size?

Rick
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Old 12-05-2007, 02:52 AM
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Default RE: Timing on a Jett or Nelson

me too I would like to see the ports shape/size.

Anyone has pics of the insides of a Jett?
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Old 12-06-2007, 12:35 PM
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Default RE: Timing on a Jett or Nelson

Which engine ?
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Old 12-06-2007, 02:08 PM
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Default RE: Timing on a Jett or Nelson

Bob, 90LX would be nice!
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Old 12-06-2007, 05:40 PM
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Default RE: Timing on a Jett or Nelson

Hi Bob,

Actually I am looking for specs on the SJ 46 or 50.

Regards,

Rick
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Old 12-07-2007, 04:28 AM
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Default RE: Timing on a Jett or Nelson

Sorry Rick, i was talking about some pics inside of the engine. Didn't mean to high-jack the thread.

Peace-out,
Vasek
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Old 12-07-2007, 11:10 AM
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Default RE: Timing on a Jett or Nelson

Ok... here ya go...... no giant secrets

Note the radical port designs [&:]

Timing is pretty basic:

165 on the exhaust, and 130 on the transfers. Shown is a SJ-50 ABC on the right and a SJ-90LAAC on the left. Ports on the 60LX and 90LX are similar ... just timed a few degrees higher to trade the torque for more rpm.

The newer, (cheaper?), import engines have lower timing. The AX for instance. This gives them more torque to run the oversize props, but prevents them from unloading in the air. Trade there is slower, quieter and very user friendly - but at the sake of power. And the better engine designs include smaller carbs that draw fuel and throttle well with the larger props at lower rpms. That is where the standard consumer market is, so they are designing the right product for the market. And for most pilots, these are good engines to select from for many aircraft projects.

Nice thing with the sport jett engines --- you get power, the added rpm, and they are STILL user friendly. The performance comes from care in design, material selection, fab and fit and finish, crankshaft design, and the exhaust system.

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Old 12-07-2007, 06:35 PM
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Default RE: Timing on a Jett or Nelson

Thanks Bob, that is exactly what I was looking for. Do you know what the compression ratios are?

Regards,

Rick
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Old 12-07-2007, 07:34 PM
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Default RE: Timing on a Jett or Nelson

Thanks Bob, that is exactly what I was looking for. Do you know what the compression ratios are?
Compression ratios are generally misunderstood by most people.

There are two types of compression ratio. There is geometric compression ratio and actual compression ratio. Geometric compression ratio is determined by the stroke of the engine, BDC to TDC. Actual compression ratio is determined by the distance the piston travels from the closing of the exhaust port (usually the highest port) to TDC. If there are any doubts ask this question. How can there be any compression if the exhaust port is still open. Which ratio do you want.

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Old 12-07-2007, 07:51 PM
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Default RE: Timing on a Jett or Nelson

ORIGINAL: Ed Smith

Thanks Bob, that is exactly what I was looking for. Do you know what the compression ratios are?
Compression ratios are generally misunderstood by most people.

There are two types of compression ratio. There is geometric compression ratio and actual compression ratio. Geometric compression ratio is determined by the stroke of the engine, BDC to TDC. Actual compression ratio is determined by the distance the piston travels from the closing of the exhaust port (usually the highest port) to TDC. If there are any doubts ask this question. How can there be any compression if the exhaust port is still open. Which ratio do you want.

Ed S

Thanks for the explanation Ed. I am looking for the actual compression ratio (from the time the piston moves above the exhaust port). I find it interesting that many folks mention compression ratio and shims and nitro % without really discussing the actual compresion ratio. I am also an automotive enthusiast and compression ratio and fuel selection go hand in hand in that space. Thing is, in that space most people measure their compression ratio and make decisions on fuel octane based on imperical data and not on inconclusive data. I guess I would like to take the same approach. I took all of the measurements from my Rossi 53 to calculate the compression ratio and plugged the numbers into a compression calculator and came up with 8.94:1 Here is the calc that I used http://www.rbracing-rsr.com/compstaticcalc.html

It would be nice to find some info on what the recommended fuel is (Nitro%) for a given compression ratio. Do you know of any such source of info?

Regards,

Rick
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Old 12-08-2007, 05:18 AM
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Default RE: Timing on a Jett or Nelson


ORIGINAL: bob27s

Ok... here ya go...... no giant secrets

Note the radical port designs [&:]
Yeah Bob, i was hoping for something more spicy [>:]

at least is the piston rod a bit "profiled" ?

posting pics of what i was hoping for (in a less radical rendering of course)

Peace-out,
Vasek
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Old 12-09-2007, 05:12 PM
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Default RE: Timing on a Jett or Nelson

Nah... nothing that goofy looking inside....

Its just normal lookin parts.

Dub told me a store some time ago about his early speed engines.
He bought a Fox .29 .... went 130 mph

After a few weeks of porting and polishing and airfoiling... it went 129.

Just because it looks pretty doesnt mean it runs better
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Old 12-09-2007, 06:11 PM
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Default RE: Timing on a Jett or Nelson

Vasek- what's with the channel below the port in the first pic there?
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Old 12-10-2007, 03:57 AM
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Default RE: Timing on a Jett or Nelson

Duck- the channel allows for more mixture to make it's way to the chamber (keeps it away from the exhaust port where it just "exits" unexploded )see pic 1. More or less the tune pipe also "redirects" the exiting not yet exploded mixture back to the chamber. Thus the power surge.
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Old 12-10-2007, 05:04 AM
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Default RE: Timing on a Jett or Nelson


ORIGINAL: bob27s

He bought a Fox .29 .... went 130 mph

After a few weeks of porting and polishing and airfoiling... it went 129.
I know what you're saying Bob, but that's NOT how the West was won! If you get my drift
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