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Cobra Fox35 setup

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Dennis Toth:
Eddy,
Did Steve run the Fox 35 in a 4-2-4 or a rich 2? From the description in article he lowered the compression, put a bigger diameter spraybar, but punched the nitro up to 20%. I guess he wanted the extra fuel draw with the larger spraybar and a softer break (if any) but added nitro to get some additional power back. Maybe to run the 20% nitro with a stunt size prop he needed to back off the compression to avoid pre-ignition (that crackling sound when they get to hot). Seems like there is lots of ways to get the old Fox to run.

Best,    DennisT

Brett Buck:

--- Quote from: Dennis Toth on March 21, 2024, 01:47:06 PM ---Eddy,
Did Steve run the Fox 35 in a 4-2-4 or a rich 2? From the description in article he lowered the compression, put a bigger diameter spraybar, but punched the nitro up to 20%. I guess he wanted the extra fuel draw with the larger spraybar and a softer break (if any) but added nitro to get some additional power back. Maybe to run the 20% nitro with a stunt size prop he needed to back off the compression to avoid pre-ignition (that crackling sound when they get to hot). Seems like there is lots of ways to get the old Fox to run.

--- End quote ---

  After some probing, because a lot of the people from that era were reticent to tell us what they did, lots of people, or most competitive pilots in the Fox 35 era, were pouring as much nitro in it that the crankshaft could stand. 20. 25. 30% or more. The difference just going from 5% Fox Superfuel to 15% is astonishing as far as power goes. I would also caution it is astonishing how much louder/harsher the exhaust is, too, and since they were typically running open exhaust, that would be a serious consideration now.

    It's a great idea as long as you have a bunch of spare cranks. My observation is that above 15%, the chances of breaking a crank go up dramatically. %5, I would guess that the failure rate is nearly 0, I always ran Superfuel or some equivalent and never broke one. Above 15, from what I have seen, it is just a matter of time. If you were using it for competition thats just the cost of doing business, but that cost was only about $4 a piece.

    Brett

EddyR:
 Dennis I do not remember from that long ago how anyone person ran there motor. Since I was flying a lot in those days I do know not many people ran slow 4-2-4. Also most ran 10-5 not 10-6 props. A lot of people ran 9-6 at very fast rpm. I always built light 500 SQ" models that weighted around 30 ounces.  Jack Sheeks  told me most of his models weighted 50 ounces. I used a early OS Max and Johnson motors and I always seemed to have more power than other people. The Johnson I had back in the early 1960;s was the Orwick Johnson.
 They ran a steady flat 2cycle the entire flight. That very same motor I used for old time stunt and won OTS at the KOI five times in a row. I gave that motor to Bill Little about 15 years ago.
EddyR

Dennis Toth:
Brett,
I understand running the FOX 35 on the high nitro blends stresses the crank. I think that Steve used the larger spraybar (I assume it is like using an ST NVA) to get stronger fuel drawn, then he used the nitro to compensate but got some pre-igniting or crackling and need to reduced compression (to save the crank). Kinda like flying at Tuson with the thin air.

I have had my Fox's crackle when running a muffler and full 10x6 prop. I solved it by dropping diameter to 9 1/2" to reduce the load (even down to 9" on really hot days) which works well. I like the idea of the stronger fuel draw so might be worth playing with Steve's setup.

Best,     DennisT

Dennis Toth:
Steve's setup is an interesting approach to use the larger ST size spraybar with the high nitro fuel to make up for the restricted air intake flow. Many guys have use the ST NVA in FOX's an said that it doesn't lose much power. However, that was in the days of open exhaust. With a muffler it takes a double hit. The muffler seems to knock off about 200 - 300 rpm at full tilt but maybe 50 ish at stunt settings.

Has anyone check what the rpm reduction is on the FOX between a reduced size sparybar (like the original or a Randy Smith PA Fox NVA) and the standard ST NVA with a muffler? Then how much nitro (if the base is 5%) is needed to boost rpm back to the rpm with the smaller diameter sparybar? Maybe Steve had something here!

Best,    DennisT

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