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airraptor -> RE: Shrike 40 with thinner airfoil (10/5/2012 12:30 AM)
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quote:
ORIGINAL: combatpigg
quote:
ORIGINAL: SpeedBoy
If this engine doesn't has the power Im looking for , Im willing to do some timing job ( if someone tells me ecxactly how to do it )
The first thing you need is a fine tooth rotary file tool. You can use a Dremel motor or a die grinder to power the tool. The fine tooth gives you maximum control. Think of yourself as a dentist and the engine is your patient that you are operating on.
The tool in the picture has a 1/8'' diameter shank to fit directly into a Dremel motor.
I also show a 1/4'' adapter for when I use a full size die grinder with a 1/4'' collet.
You don't need a degree wheel to set the exhaust for 180 degrees because the prop drive washer can be scribe marked where it passes the casting marks on the crankcase. If your engine doesn't have distinct marks on the case that you can use for your 180 degree reference, then you can place your own marks on the case next to the drive washer..or make your own degree wheel.
It is nice to have a strong LED flashlight to shine into the cylinder so that you will have no doubt when the exhaust port is opened and closed. Take a reading to see where the stock timing is at, then estimate how much material you need to remove from the upper edge of the exhaust port, but only remove a fraction of what your estimate is...then check the timing again. Make sure to deburr your cut before reassembly. You do not need to work the entire upper edge of the port, just cut it to form a ''D'' shape laying on it's side. You can grind the port slowly and recheck your progress as many times as you want. If you are afraid of accidentally going past 180 degrees, you can stop at 170-175 and then raise the liner with a shim to get the last few degrees. Piped engines can benefit from a slight lowering of compression anyway so it shouldn't slow the engine down if you raise the liner .002-.004'' I like doing it like this because the liner height is another tuning tool to play with.
There is most likely a good video about setting up a degree wheel on You Tube. It helps to have a metal bar that is bolted across the top of the liner. In the middle of the bar, thread it for a machine screw. Lower the machine screw just enough so that the piston taps it at TDC and this is your ''positive stop'' that helps you set the pointer on the degree wheel perfectly.
Even a brand new engine has 5 degrees or so of ''dwell'' when the piston is at TDC or BDC because of the built in clearances in all the moving parts. This is why the ''positive stop'' bar is needed...it allows you to split the difference of the piston's dwell when you raise the piston up to the bar clockwise and counter-clockwise. These words will make more sense once you get your hands into the engine and see for yourself what is going on. If you enjoy learning new tricks, this is a rewarding project but it might be smart to practice on a less expensive engine that needs to have raised exhaust timing to really hit the pipe like a speed engine is supposed to.
This mod kills torque, so you will never be able to turn sport sized props very well..you will be commited to little speed props and the engine simply will act like a tired dog if you cross the fine line of how much load it can handle. It is magical to see the engine get happy after shaving/scraping the blades, or trimming off some diameter, whatever it takes to find the invisible line that the engine can tolerate. Reducing the thickness of the blade's airfoil sometimes works, but remember that the airfoil provides strength...so be careful.
As much as pig and i dont get along my engine guy and myself use much of the same techniques for modding engines. i think your 17,000 with the 10x7 is low and should be able to pull a bit more out with some work. the last engine i did was a Aviastar 53 from sig for 60 dollars. i set it up to run on the jett muffler and it would pull the 10x7 APC at the same 17,000. it took a bit of work to get it there. it took three engines to get it to this point. I would think you would have a a bit better flying plane with a jett 56LX and a 9x8 or 9x9 prop with the same speed as the 60 picco.
open up the carb, put two chamfers on the case where the crank hole is near the carb, square off the corners of the crank fuel mix entry to help get more air flow, raise the exhaust timing slowly (5 degrees or .005") at a time. also look at widening the exhaust port also if narrow, look to get the squish down to .016-.022 range. the transfers like to be around .050 for every 10mm of stroke difference than the exhaust (lower) to match up well on the jett mufflers. now if you cant reach that number by raising the exhaust and going to go past 180 degrees then you need to lower the sleeve. do this by machining the sleeve down some of machine the case down. now with all of these cuts and machining you need to make sure you Do Not get any sub porting very bad on a tuned muffler/pipe set up. use a bright LED flashlight to check this as you go. a quick way to check crank timing is to use the four back plate bolts as a guide. The bolt holes are 180 out and usually the crank opens and closes on the bolt hole at 7 o clock and 1 o clock while looking at the crank pin. with any cut or machining you doing do it in small amounts then run the engine again to see where your at. DO NOT MAKE A BUNCH OF CUTS then run it until you gain experience. i am still new and modding engines only 5 yrs and still make mistakes lol. one thing on compression look at your oil color on the plane/test stand. Clear oil means the compression or nitro can be raised, light brown to brown is just about right. Brown to dark brown is to high or lower nitro. black is tearing the engine up. lol
also there is 500-1000 rpm to be had in most of the grey APC props. look hard at the props under a good light and feel the "edges" with your fingers and you will see where to improve them. [;)]
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