Fox model airplane engines - fuel and prop recommendations

Fuel and Propeller Recommendations

What Fuel for your Fox Engine?
Fox has traditionally recommended the use of only castor oil based fuel in their engines. For an in-depth scientific explanation of the benefits and drawbacks of castor oil click here.

It is generally accepted that castor oil provides better protection than synthetics during a hot, lean run. While this may be true, the fact is, these days many Fox engines may never see castor only fuel. There are several reasons for this; First, most modelers will purchase fuel that is readily available at a local hobby shop and this will most likely contain synthetic oil or a synthetic-castor blend. Some are simply not willing to put up with the thick oil mess, varnish and carbon usually associated with castor-based fuel. Others will simply use the same fuel that seems to work well in their other brand X engines whatever the oil type or content. However, some Fox engine models continue to employ older steel liner/lapped iron piston technology that depends upon castor oil for longevity. So, as it was is the past, it is still important that at least some castor content be present if you are to expect reasonable service from these engines.

It is also important to note that the head button designs in many older, and some current, Fox RC engines tend to favor lower nitro fuel. For ease of adjustment, and best reliability, in most sport applications it is best not to exceed 10% nitro. The larger Eagle series engines may even exhibit problems on fuels with more than 5% nitro.

After extensive testing, over many years, we have determined that, with reasonable needle settings, most Fox RC engines generally run well, with acceptable durability, using the synthetic/castor blends produced by Morgan (Omega), Powermaster and Sig. However, it is likely that any quality fuel containing some castor oil in the mix will be ok. Since switching from the castor only fuel that we used decades ago, we have not noticed a significant decrease in engine longevity. Only that the engines run cleaner and without the need for relatively frequent disassembly for de-varnishing as was required in the past. But, again, we must stress this assumes reasonable needle settings. With decades of experience running model engines, and Fox engines in particular, we are confident in our ability to consistently achieve proper settings. If you are not sure of your ability to properly set the needle to avoid lean runs, you can always add a bit more castor to your fuel for added safety margin. (Sig fuel contains more castor than most blends).

We have not done any long term testing using totally synthetic fuels so, at this time, cannot comment on Fox engine durability using these fuels. We still recommend using fuel that contains at least "some" castor oil, particularly in non-ringed engines. One Fox engine to pay particular attention to is the .35 Stunt. This old design has, historically, required a higher oil content than usual to obtain dependable operation and long life. The recommended oil content is 28%, preferably all castor.

For those who wish to follow the Fox instructions precisely, Fox still manufactures a number of castor only fuels. It is interesting to note, however, that over time some formulas did change somewhat. The fuel recommended in the manual for your older Fox engine may not be exactly the same mix sold today. Going through old Fox advertisements, we've managed to come up with the following information on the most common Fox fuel mixes:

  Gold Star Dukes Fuel Missile Mist Superfuel
1970's 0% Nitromethane
20% Castor
10% Nitromethane
20% Castor
25% Nitro *
22% Castor
*Might have been partly nitroethane but we couldn't find specs.
5% Nitromethane
28% Castor
1980's 0% Nitromethane
20% Castor
2% Synthetic
10% Nitromethane
22% Castor
16% Nitromethane
8% Nitroethane
17% Castor
5% Nitromethane
29% Castor
Today 5% Nitromethane
20% Castor
10% Nitromethane
20% Castor
24% Nitromethane
17% Castor
5% Nitromethane
29% Castor

In particular, note the big drop in oil content from the original Missile Mist formula. Gold Star gained some nitro and, at one time, also had a little synthetic oil in the mix. Over the years, from time to time, some fuels were also advertised as containing propylene oxide plus detergents.

For some good general information on model fuel including oil type and quantity, nitro, storage, etc. check out this link to POWERMASTER fuels.

Our recommended propeller sizes for FOX glow engines

 Engine  Propeller Comments:
.15 Bushing  7-4, 7-5 Does not like to be lugged down with larger props.
.15BB  7-4 to 7-6
Likes to run in the 12,000-17,000 RPM range, but will also swing up to 8-6 or 9-4 in certain applications. (Like small scale models).
.25 Bushing  8-6, 9-4 We think a 9-4 is best for most applications.
.25BB  8-6, 9-4 Tends to vibrate a bit and especially so at higher RPM.
.35 Stunt 10-6 Should be set to run at a fast 4-cycle in level flight. This is not intended to be a high-speed engine.
.40 Bushing
 10-5, 11-5
9-6, 10-6
Likes 12,000+ RPM. 10-6 is best for most applications.
.45BB  10-6, 11-6 Pulls quite strong with a 10-6, but we like an 11-6 Master Airscrew cut down to 10 1/2 in.
.46ABC  10-6, 10-7
Really spins up with a 10-6 but an 11-6 may be more practical for most models.
.50BB  10-7, 11-6 We like an 11-6 Master Airscrew cut to 10 1/2 in.
.60 Eagle II-IV  11-7, 11-8
12-6, 12-7
12-6 is best for most applications, but will swing the larger props with authority.
.74 Eagle  12-6 to 12-9
Does not like high RPM. Best at 10,000-11,000 RPM but will also run well at 9,000. We like an APC 13-6.
.78  13-6, 14-6