Dennis, I'm quite certain your aware of the shortcomings of the Fox .35. The rod eggs on the bottom end which in turn hammers the crank pin. Eventually regardless of nitro, the crankpin breaks off which takes a lot of time but it will fail. This becomes very audible when the engine is pushed as I've noticed this many times using the Fox for Foxberg for example. I run a 9x6-9x7 for the event. Depending on where the event is being held, some require 10% and some it's your choice. The next failure point is the crank window. The stock crank breaks or even cracks but still works along the square edge of the window. Seeing your using the Tiger needle, your down on power due to the volume of choke area being taken up. I have no idea of your elevation but running 10% works fine for most and usually is well received by the engine. Once you start into 15% nitro I feel your taking some risk. The Fox is very low compression and I've used 35% with no appreciable difference over 15%. I was using it on a combat wing and it ran just fine.  Using the 9x6 is a light load but the Fox prefers a load therefore I can see it going into a two cycle.  Running 10%-12% nitro is modest by far for the Fox. I exclusively use 10% and have done so since the 80's. 

               I wasn't aware of a high compression head as I own pretty much all the other heads that were made commercially available. I never found any other head that I purchased make a difference in performance other than hard starting. The stock head always seems to work better in regards to quicker and more reliable starting. It seems that all the hemi heads I ever used proved to be problematic during restarts. This would be the Muggleton, the L&J version and the Fox. These heads seems to reduce compression.  Certainly upping the nitro would offer the quickest results. To change a head only takes a few minutes and is not a irreversible change so trying it would be my choice.

                  I guess the important thing here is how many squares are you trying to fly using a 9x6? Your really leaning on the engine if your trying to fly something over 400 square inches.

HI
The  first thing you should do is replace the  ST NVA  or at least file the bar down, on the sides at the part where it is inside the venturi of the FOX, that bar is 27 thou  larger diameter and cuts the power of the motor.  If you do this, make sure you file flats on the sides with the feed hole pointing down, This will open up the area , and will give you a power boost, You can put a stock FOX  NVA in or  use an  OS 15FP  NVA , with 1 thin washer on either side.

Randy

I have seen people recommend a 10-5, too.  It should unload the engine a bit, let you run it richer for more cooling.

Do what Randy said: either file down the spray bar or get something narrower.
With the ST assembly you are basically running at part throttle.
Upping the Nitro  to 15% will not really make more power, but will let the motor run cooler.

Guys,
Finally got my Fox 35 happy with a 9x6 prop on open uniflow tank, 7% N, 27% (50/50) oil. I am doing 5.0 lap times on 60' C to C lines. I want to get a little more speed to around 4.85 ish (to handle the wind). I am using an ST NVA. My choice to pick up say 400 rpm without going full 2 cycle seem to be - bump the nitro to 10 -12%, change the NVA to reduce the spraybar to 0.130" or go to the high compression head. The easiest is to try the nitro first, I am concerned that to much nitro could cause a problem with the shaft breaking. Assuming a lean ish 4 cycle, how far can you push the nitro before you get into shaft breakage range with the light load 9x6?

Seems that to get to the rpm needed (which it does easily in a medium 2 cycle) a certain BMP is needed no matter what method is used but is the nitro more of a hammer blow then just more fuel/air from the larger venturi? If the HC head is used does this also have the same effect as the higher nitro?

What has been  the experience of the group?

Best,   DennisT

   If you are quite certain that the engine is "happy" and it is running consistently like you want it, why mess with the engine any further and just make up a set of lines that are a foot or so shorter? That will get you a faster lap time for the same engine speed, and is what guys have done over the years when flying in the wind.
   Type at you later,
    Dan McEntee

Ken,
Interesting suggestion, I think I might have one in the prop box. The El D has around 400 ish sq inches of area.

Dan,
The shorter lines is another option. If the nitro doesn't do it I might just go that way. Down here in FL the wind always comes up for second round. I like to set things up to handle wind around 12 mph ish. Above that its more of leas just make it though.

Best,   DennisT

I like 10/29 with a APC  10.6/6. Awesome!   TS