Engine and gearing question

[Razzar]

I am trying to decide what way to go. It would be great to hear some of the pro's and cons of each.

My kart is going to be used mostly off road.
Weight-170lbs with the engine
Tires- Rear 18x8, front 16x6

The options i am looking at are:

6.5hp or 8hp engine (Will be running a torque converter)

13hp engine (Will be running a torque converter)

I would need good amount of torque in the low end since it is an off road kart but would like to see it be able to due a decent top end too.Gearing is crucial i know but i am unsure what gears ratio would work best to get the good bottom end torque and decent top end. It will be mostly driven by kids 13 and under . Would a 6.5 or 8hp engine fit that roll good or is the bigger 13hp needed.Some suggestions on gearing ratios for each motor with an estimate on how fast they may go would help out.

Thanks

 

[Kaptain Krunch]

Motorcycle engine.

My kart has a KZ400 engine, 36hp stock, 6spd manual, 10k redline, sounds awesome too. She'll spin the tires all day long in first, and still has a theoretical top speed of 90mph in 6th.

 

[modelengineer]

Another vote for motorcycle engines.

Mine has ~50hp, and I've had it up around 130kph (80mph); and that was with a completely clogged airfilter so it wouldn't rev much over about 7,000 rpm (it revs to 11,000 normally).

For your situation I'd go with a 110c-150cc semi-auto bike engine to make it easier to drive for the kids. One accel pedal, one brake pedal, and a gear lever. It will be so much more fun that a CVT.