RE: Fuel for HPI Firestorm??
Are we talking oil content or nitro content? If its oil, I break-in on 14-16% oil and drop down to 12% for normal running. If its nitro, I run 20% all the time. The more nitro you have the wider the tuning window (sweet spot) it will have. Less nitro narrows your tuning window so the perfect tune is a little trickier to get depending on the engine. Less nitro is more efficient, since the needle is leaner. More nitro needs a fatter needle to compensate for the extra oxygen in the fuel (nitro = full of oxygen molecules)
Most engines will run just fine on 10% nitro, but most folks want a little more power so stepping up the nitro a tad will give more power. It comes down to how much you're willing to spend. The engine isnt going to care much what you run in it as long as it has some methanol and oil in it. If you're a good tuner, you could run it on 0-5% nitro too with a few easy modifications.