The fuel used is critical to making power in the Comer C51. Check your local track for what is allowed. At first you might think about getting the high octane race fuel but that is the worst thing you can do. These race fuels typically are 110 octane, which is not good in making power in this little 50cc motor. High octane fuels burn much slower than low octane 87 pump gas. In simple terms, the explosion of the 87 octane has a much larger and faster burn rate. The higher 110 octane fuel you will see dribbling out of the exhaust as unburned fuel. On the dyno it is very clear that as the RPM builds you make less and less power since the engine just can’t burn the fuel. The engine will make as much as 1500 RPM less peak RPM with 110 octane. The downside to pump gas is ethanol. Ethanol absorbs moisture into the fuel mixture. To combat that we use fresh 87 octane fuel as often as possible. We also like to get fuel from the same name brand gas station each fill up as there are differences in blends, detergents, etc. Race fuels are more consistently made which reduces these issues but the high octane is much more detrimental to power than these additives. Another option is non-ethanol fuel found at marinas or in some farming communities. It has an octane of 91.
Fuel at Races:
Since fuel is so critical to engine performance it will be checked at regional and national races. Some series will take the easy route and mandate the same race gas across all classes. They use a meter to check the fuel and it needs to be calibrated each weekend. Ethanol, octane and additives all affect the meter readings and make their job difficult. Another national series is letting the kid karts use low octane fuel which they supply. In this method the series purchases ethanol free fuel for you to use. This levels the playing field for the competitors.
Oil at Races:
Oil type and amount is still a variable at 90% of the races. A couple tracks are keeping it really simple by supplying the fuel with oil they mixed. This takes out the option to run different brands, types and amounts of oil, which substantially affects performance. Some races have you come to the grid with an empty tank and they will fill it up with a race approved fuel/oil mixture.
There are a lot of options when it comes to oil. You will need to check your race series rules to find out what brands are legal. The reasoning behind this for ease of fuel inspection in tech and to keep the competitors on a level playing field. For example, if the series allows 3 different oils you often will see 3 small samples of oil at the tech station. When checking, they may ask you what oil you are using since their samples are the baseline. In addition, the amount of oil you are using will affect their meter so most series are allowing a 6 – 8 oz. per gallon range.
Castor 927 is a type of oil we like to use for racing and it comes with positives and negatives. The benefit is we see about 200 peak RPM more on the dyno. There is an old saying that “castor is faster” and we have found that to be very true with the Comer C51. The downside is castor oil is derived from castor beans and it does not burn as clean as synthetic oil. This creates carbon build up inside the motor, which again has its pros and cons. As carbon builds up you will see your motor get faster because the compression is getting higher. This is why race technical inspectors series check the cc volume above the piston at top dead center. The rules state an 8.3cc minimum of cylinder volume. Less than 8.3cc you may make more power but will also get disqualified. So at national level races we check the volume as needed. Running castor we can see over .1cc of change per practice day. So you might have a perfect 8.4cc going into the weekend and be at 8.2cc on Sunday which will result in getting you disqualified. This is best left to your engine builder if they are at the track. They may just scrape the piston top and remove the carbon, play with base gaskets or remove carbon from the cylinder head dome. Which they choose will be based on your engines performance.
Motul 800 and Redline 2-Stroke Synthetic are synthetic oils that are man made and run cleaner with much less carbon build up. This is very convenient, as cc checks are not needed very often. The downside is we lose that high RPM we gained from castor oil. Some people run a blend of castor and synthetic to get the benefits of both oils.
Amount of Oil:
The most common oil ratio is 8 oz. per gallon, which is 16 parts fuel to 1 part oil also known as 16:1. If you mix 6 oz. per gallon you are now at 21 parts fuel and 1 part oil. The 16:1 ratio has less fuel in it than the 21:1 ratio. So when at 16:1 you may need a larger jet than when you run the 21:1 ratio. So you can see that oil amount greatly affects the jet you will need. We believe the largest jet you can get is best as more fuel and air makes more power. Therefore, we typically run an oil amount that gets us good performance and needs a big legal jet like .0245” to .0257”. Typically that puts us at about 9 oz. of oil with full castor but watch out for the effects of carbon build up as discussed above. We get the best dyno and track results with this mixture and have won many races. If you go much above 9 oz. you tend to foul spark plugs and much lower than 6 oz. you have to run too lean of a jet.