Boris:

Stock is red, new is blue. Old peak HP is 41, new is 68. Old peak torque is 31, new is 41.

Bore: 98

Stroke: 72

Cc: 543

Intake and exhaust valves are Kibblewhite 1mm oversize. Throats are opened up and ports reshaped. Five-angle valve cut. Cam is from HotCams.

Exhaust is polished and matched to the new spigot I made for the exhaust.

Also tapered the guides:

Exhaust is a collection of U bends from Summit, and some bad welding. Used a Cherry Bomb muffler under the engine, and a bit of tail pipe to put the exhaust into the low-pressure zone at the rear of the bike. Header merges the two ports gradually at about 120mm, and the length (including internal part of muffler) is about 15% shorter than the Big Gun that came with the bike. The Cherry Bomb cost me US$18 retail, weighs within a couple grams of an Arrow, and is 6db quieter. No measurable backpressure either. A big exhaust box can capture the pulse and quiet it better and more efficiently than an absorbtion-type silencer.

With the intake, I was trying to get a two degree convergence thru the whole thing. So there is a pretty constant taper from the throttle butterfly to the valves. Wanted to open it all up as well, since the velocities are pretty high on a single. A longer pressurized intake tract would also help smooth the pulses.

Intake used a lot of epoxy, and quite a bit of time with porting tools. Filled the back and floor of the port to make a smoother lift-and-drop rather than a go-straight-hit-the-wall-and-maybe-go-in. Finish is concentric 100-grit grooving to improve laminar flow.

Epoxied an aluminum ring around the intake spigot to give more meat, then opened it up to 48mm. Throttle body is a 52mm (stolen from a TL-R) that has been filled and tapered down to 48mm where it meets the intake spigot. Injectors blow upward 45deg at 75psi to assist the charge lift and push things toward the valve end.

Injection system is based on MegaSquirt (www.msefi.com). And does a great job. Cheap, great support tools, and reliable – what’s not to love? Been running on my other racebike for over a year with no problems. Will also be converting over to run ignition, but want to get EFI part working first. One thing at a time.

Presently using the Innovative Motorsports LC-1 wideband oxygen sensor to monitor exhaust and help tuning from the datalog.

Throttle body has a large V-stack connected to an airbox of almost 8litres. Airbox has a spigot at the front that connects via a hose to a ram-air collector box in the nose that also houses the filter.

Piston is from JE, 98mm 12.5:1 compression. Combined mass (piston, pin, rings, clips) is 4gm lighter than stock. Doesn’t sound like much savings, but at redline that is over 19kgm.

Had Charlie Brown at Superior Sleeve make an iron wet liner and he also did the boring and assembly. The total cost of sleeve and machining was $215, pretty quick turnaround. The bore was cut back enough and the liner thick enough that I can go to a 100mm bore if I want to later.

Went iron since the wall is thin. And it is easily replaced without buying a new cylinder and starting over.

Radiator is the stock RS-250 unit, with a splitter on the output to hit both spigots on the KTM cylinder. Sitting on the grid the water temp reaches 194 degF, and stays below 130 during the race. The temp sensor is right after the water pump, so it is at the hottest part of the cooling circuit.

Bottom end is pretty much stock. Should be fine, since I am not stroked and the reciprocating mass is a bit lower. Put on a lightened flywheel from Trail Tech to give the crank a break, and increase the alternator output for the fuel pump and injectors.

Gearbox is setup with close ratios, all KTM parts. Plugged the hole and removed the gear for kickstart, since it won’t work with the frame.

First race this weekend was red flagged. On the way back to the pits, several folks were pointing at my bike like there was something wrong. Thought that maybe I caused the red flag? Finally, one person pointed at their silencer, then at my bike. I realized they thought my silencer had fallen off!

So I shook my head, and pointed at the bellypan, which confused them even more!

When the race finally started, I got a bit of a bad start since the midrange had not been fully tuned yet. But I worked my way up to second place, missed first by about 2metres. Second race I got a bit of a better start, but only managed second again. Two of us were dicing hard thru traffic, and I am not a very aggressive passer.

I really also need to gear higher. I was hitting the rev limit about 2/3 down the straight, and was in the wrong gear for most corners. At the limiter, I was seeing about 120mph indicated.

Once I get the midrange running better, watch out!

Complete install:

How the motor fits in the chassis:

Good side view showing underslung exhaust: