Hovercraft Engine

[supastar]

Hi

I need a 250 cc 2 stroke engine for an F3 racing hovercraft.

The choice seems to be KR-1S, RGV (RS) or TZR. Any other possible engines?

A race lasts about 10 minutes, the revs run up and down violently and there are long straights at max power. There are no tuning restrictions other than a 256 cc overbore limit. I'd like an engine to last a season, about 8 races plus testing, practice etc.

Would you experts please be so kind as to advise me.

Paul

[weasley]

Don't forget the off-road/motocross bikes. Kawasaki KX, Honda CR, Yamaha YZ, KTM SX/XC, Suzuki RM etc.
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Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[Monkeywrenche...]

I think the road bikes you listed are all getting pretty rare and desirable (if they have a running engine), I'd agree that a 250 crosser engine is the way forward they need more love in the form of top end rebuilds but they'll handle the short format thrashings from the races you described.

I don't know how the power/ torque compares to road bike though.

[Kickstart]

Hi

Also consider a road race engine. A karting Rotax 250 twin might be an option

All the best

K
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[stevo as b4]

TZR 250 Parallel twins are apparently a commonly used Hovercraft race engine. You can even get a Stan Stephens tune done on them specifically for Hovercraft racing.

Does the motor need to be a bike engine though?

I'm thinking you don't really want the gearbox and ancillaries that come as std with bike motors?

Could you look at Microlight/Para glider motors, Kart engines, or outboard boat engines/PWC motors?

I guess liquid cooling is not a huge advantage if your using the motor in a hovercraft application either?

[supastar]

Many thanks, gentlemen, for your valuable comments.

The TZR250 is the sole engine used, in it's class, but why?

I'd take the gearbox off and mount a rubber coupling onto the crankshaft. Then a driveshaft, pulleys and belt.

The engine will be mounted centrally, for handling purposes, with the driver crouching over it. So water cooling is quite desirable. I'll mount the radiator inside the hull to the rear of the front lift fan. This will ensure positive cooling.

Stan Stephens has retired and his website says he'll only now work on a few specific engines.

I'll keep researching alternatives.

[stevo as b4]

If you have to use bike motor's, then assuming only a 2cyl is competitive? (a 250cc single from an MX bike makes as much power as most road bike twins at std) then I'm thinking a parallel twin configuration is a big advantage if it's going to be mounted in an enclosed space? The NSR/RGV/TZR/RS V-twins might not be as compact or easy for you to re-route the exhausts and inlets as required? All the V-twins are expensive motors to buy and tune, being from late gen last of the 250cc sportsbikes, so they are in high demand.

The older stuff like the KR1, TZR and maybe RG250 are probably more suitable for mounting in a small space. These motor's are a bit cheaper (buying a bike to get one is the cheapest way though), as they are not as sought after as the late V-twin stuff, but still aren't that cheap or easy to find these days.

There's nothing exceptional about the TZR250 in line twin motor, but it's probably always been the best combination of reliability/tractability and longetivity of all the water cooled 250's.

It has easy to re-bore linered cylinders, a simple construction and a very simple powervalve design with few moving parts. It's probably better catered for in spares than the KR1/S and almost all of the V-twins too.

[supastar]

Thank you, gentlemen, for your very useful comments and advice. After considerable research, I have opted for a cr250. It seems to be widely accepted that the 2001 version of this engine is probably the best 250cc 2 stroke ever made.

It is used in a 250 kart class so there is a lot of experience with the engine and the usage profile is very similar to a hovercraft - flat stick for most of the time. Parts are freely available and well priced. Also you only need one part as against two of everything for a twin.

The only development will be to incorporate a pull start mechanism.

Thanks again

Paul