My (carbureted, old) race bike works just fine on Shell 91. It has mildly more compression than stock (the head has been milled approx 0.5mm).
Once upon a time, someone gave me the leftover contents of a can of C12, so I gave it a try. I won't dispute that it did give crisper throttle response. But IF there was a difference in total power output, it was too small for the butt dyno to notice.
Don't forget the down sides:
- It cannot be left in your gas tank or in your float bowls because it goes bad quickly, which means you have to go through an exercise of draining the system and purging it out with standard fuel every weekend.
- It eats gaskets and seals in the long term.
- Many race fuels contain lead. Lead is bad for you. While motorcycle racing itself can also be bad for you in a much shorter timeframe, why add an extra long term health factor.
If you have a built engine with so much more compression than stock that you cannot run it on standard pump fuel, *that* is where you need to use race fuel and *that* is where the big performance gains can be.
There are low-octane racing fuels that are meant to work with standard engines (the C12 that I tried is not one of them) and I won't argue their benefits ... but Shell 91 costs $1.35 per litre and MR12 costs a lot more AND you have to deal with the headaches. Not worth it for me.