I was all for a Go Kart engine before the electronics specialist on the team ruled it out because LiPo fire + Petrol = death.

That being said, they're pretty good for power density, and if you know your way around engines, 50Hp isn't out of the question. There are pretty much two bulletproof engines out there - the Yamaha KT100 and the Rotax 125.

I'll start with the Rotax 125 - mainly because I've tuned/built a few. It's a lovely engine, mechanically speaking. Terrible to work on, fiddly as hell, but once its working, they've been those engines that still run to peak power after a crash. With a little work on the cylinder and piston assembly, you can get it to 11.5:1 compression, and if you buy a larger fuel delivery system (I was using .065'' jets the last time I worked on one), grind out the heads (or buy larger ones) it will be very inefficient (and very, very loud!) but will last the 3 minutes of the fight, with around 50Hp at peak power, which would probably be in the 12000ish RPM range. Torque is OK - stock I think it's 20Nm at peak.

The Yamaha KT100 is both lighter and slightly cheaper than the Rotax. It's also got a lot less power - around 15Hp at stock, but that can rise to around 35Hp if you bolt on the right speed parts. It's also a little more torquey than the Rotax, and it's been used in literally everything - Go Karts, Ultralights, trail bikes, kids hovercrafts - it's been around since the 70's, IIRC.

Allocate 30ish KG for the engine/fuel and you should be OK.


Three things you're going to have to watch out for if you do use a Go Kart engine

1 - Heat. The exhausts on these get hot. 250C isn't out of the question. They harder you run, the more likely you are to get some extreme head off of the header. The exhaust will need to be positioned well away from your batteries and your removable link.

2 - Inverted running. These engines pretty much exclusively use carburettors, so you'll either need to upgrade to fuel injection (which I honestly don't know if they do) or use a float reserve carburettor - same as they did on the Spitfire, but you'll have the same problem as the Spitfire, in that if you're upside down for too long or pull some negative Gs (read: getting flipped) your engine will starve for fuel.

3 - Torque delivery. You'll need a hell of a clutch to deliver the full torque of the engine to a large spinning bar or disc. Plus, the engine doesn't deliver full torque at idle - the peak torque on both of these engines are around the 8-9k mark.


A note on modifying these engines - especially the compression side of things. More compression makes it harder to start, but delivers better horsepower. The drag car that I do engine work for runs at 16.4:1, and requires 3 truck batteries to start it, and even then you only get 2 chances. Most street engines run at between 8:1 and 10:1