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bob27s -> RE: Quickie JETT on FAI fuel? (10/25/2004 4:29:21 PM)
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Hey there......
Thanks for writing......
With the quickie jett, you should have no direct issues running the FAI fuel. Sounds like you are getting good performance, but you are correct..... at 19K peak on the 9W, that is 500-800 rpm below what the engine typically turns on 15% fuel.
My guess is the guys with the nelson engines see the same rpm drop. If that is the case, I'm surprised they are running those props.
The key here is to keep the RPM up. The engine runs best when you can get a ground peak in the mid-high 19K's, and a launch RPM somewhere over 19K....... around 19.2K to 19.4K. Only one real way to get there.....
Drop a prop size. Experiment a bit with the above numbers in mind.
The 9W is about as large as you would want to go based on what you described, but may be almost too big. You may notice that it is hard to set the needle... the engine is trying to trickle down off the pipe anything below 18.8K rpm which may give you a false peak.
Tip here - as an experiment..... stick with the 9W...however... take a spare 9W, clip 7 or 8 mm off of each tip. That prop should easily exceed 19K. Use that to set the needle. Find peak rpm, back down 500 rpm. Remember the needle position. Then reinstall the full-blade prop. When racing, turn the needle out 1/2 turn..start the engine....... wait 10 seconds......turn it back to where it was.....then throttle back a bit or have your launcher/caller hold the nose of the plane down, until its time to launch (or the engine will seriously overheat). Go to full throttle just before the flag drops. As long as the engine can unload over 19.5K in the air, it should run great [8D]
Although the others are using the 8.8x8.75, try running an 8.8x8.5 if you can get ahold of one. Its a very quick prop, and the engine should turn it up into the RPM range we are talking about here. One of my other favorites on the 'early edition' QJ-40 was the APC 8.75x8.25 - its an amazing prop. Another option is the 8.75x9NN.
Do you know what shim stack us under the head right now ??
Let me check with the boss. Might be worth dropping the head a bit. This will help it unload in the air, but it will not do a great deal for ground RPM. It might get you a couple of hundred rpm to get over that 19K "hump", in which case it may be useful. Prop selection is still most important... let the muffler do its job.
Bob
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