The air box. I could just install the Frozen Boost reducing coupling, 2"to 3", and a Boondocker filter and be done, but I am first going to see how I can improve the stock box and test it that way. Air is the most important factor in our engines performance. The more air, the more hp. All we have to work with is a normally aspirated engine. We aren't going to push air with a turbo or pack it in with a supercharger! Ha Every engine mod, 2 stroke or 4 stroke, head, cylinder, or exhaust is ALL for air flow. We might forget that installing a $1000 exhaust, fuel controller, ported head job, etc won't get us the performance we are expecting if massive amounts of air cannot "freely" enter the engine. ANY resistance to free flowing air will reduce the engines performance. I have tested several intake mods using an AFR probe in the pipe and observed the results. By just installing a performance exhaust system AND a free flowing intake system, the AFR's would make a big jump, "lean". That is why there are disclaimers with exhaust and intake mod parts. You have the ability to greatly increase airflow and lean the engine by doing so. You can even end up in a situation where you may need to "add" fuel. I have seen this happen. Anyway, I felt it is super important to address the air issue. It is the single biggest factor in engine performance.
I removed the foam filter, snowmobiles don't have one.... for a reason. If MX bikes weren't in dirt, dust, sand, etc... they wouldn't come with one. Then I removed the back-fire screen. Next I found some memory wire. I drilled 8 holes around the plastic cage and 8 holes around a fender washer to install on the center post. I wired the 8 memory wires into their respective holes making a cage. The memory wire will not bend. It also has a benefit of being "springy" and "live" and helps keep the filter clean. Last, I made a plastic frame, two sides, to rivet in a frog skin window in the air box near the strut rod. To keep snow out of the box, I used Ski-Doo air box foam and outer-wears fabric and made two plugs to plug the side air inlets. The filters flow air freely but will not allow snow to pass. They are held in place by sewn velcro on the plug and 3M sticky back velcro on the inside of the side panel.
This should give the engine a lot of air. Might even be a little on the lean side down low but good at elevation? I will let you know.
