RE: jett fire 40 prop selection
Harlan,
RPM and pipe length are about right. Is this a pre-tuned pipe system from Dub? The engine will turn what the pipe is set for. You have to work with that. Dub would have supplied instructions on the RPM range, and recommended props with the engine.
Very likely the pipe is set up for about 17.5K rpm.
Sounds like the engine is peforming very well. The standard blade 9x8 is sorta the baseline for a DD/Jett50 combination. The RPM you noted is right in the ballpark.
Taking it to the next level for speed...
If its not misbehaving with the current pipe setting (needles well and fairly easily) try an APC 9x9.
If you try the 9x9 and its not well behaved.... might get a bit goofy on the needle, hard to set..... thats ok. The engine may only want to turn around 16,800 or so peak. Thats ok. Just do not get it lean. Find peak, back off immediately. This means its just critical on the pipe, but close.
Set-up proceedure in this event : First run with the 9x8. Set the needle with the 9x8... find peak..back it down 800 rpm off peak. Install the 9x9 without touching the needle. Launch it that way. It will jump on the pipe the moment the engine unloads in the air.
Give it a try. If it wont immediately jump on the pipe after launch, try clipping the prop down .13" on each tip.
Other suggestions.......
Utilize an APC 8.8x8.75 prop, 8.75x9W or 8.8x9.25. Make sure in each case the engine peaks over 17K on the ground. On the 9.5, if needed, trip about .25" off of each tip to make sure it gets over 17.0K peak on the ground (you might not have to). You always want to launch 500-800 rpm down from peak.
These are all Q-500 racing props. They will run very well on the dust. Experiment a little to see what sort of speed you get. I much prefer the D-1 series props simply because I am 100% confident they can handle turning 18K rpm on a regular basis.
You can try the 8x9 APC standard blade too. Be careful. You are much better off trimming down an APC 9x9 or D-1 series prop than using a stock 8" diameter. Just be sure to balance it if trimmed.
Let me know how you guys make out.
If I can be of any help, drop me an email or PM
Bob