At Loring this past September, the Sportster maxed out at 86.5mph in the final pass. I'm confidant that we finally ironed out all the bugs, and any further improvements need to focus on the limitations of the stock naturally-aspirated engine. The engine is a 836cc v-twin by Kipor, rated for 20hp at 3600 rpm.
When dealing with a diesel engine, most upgrades for horsepower are pointless unless you correspondingly increase the air flow, either by super- or turbocharging. Fuel can only combust and produce horsepower in the presence of enough air. Once the intake air is used up, any extra fuel will just become smoke.
At first, I wasn't sure which method to use. There are numerous small turbo kits available, most with questionable quality and reliability. Regardless, I wasn't confidant that the engine's oil pump had spare capacity to feed a turbo as well as the crank. Alternatively, a supercharger builds lower boost pressure and consumes a bit more horsepower with the pulley system, but ultimately runs cooler and has a self-contained lubrication system. Eventually, I decided to use an AMR500 supercharger from a Japanese kei car.
I fabricated two brackets to suspend the supercharger on the left side of the bike, directly in front of the transmission primary. Then, I re-machined a pair of 4-inch V-belt pulleys from the hardware store to drive it.
The supercharger is rated at 500cc per 1 revolution (1000cc per 2 revs), and the engine displaces 836cc per 2 revolutions. Therefore, by using a 1:1 pulley ratio, the supercharger is providing 19.6% more air to the engine. Ignoring pumping and friction losses, this will allow the engine to produce about 4 more horsepower. In practice, it would probably be closer to 2.5-3hp. I will likely increase this later, but this is enough for testing purposes.
Next was the intake plenum, which I fabricated from 2inch steel piping.
The placement of the supercharge interfered with the original exhaust, so I welded a set of scrambler-style high pipes.
Finally, I had to increase the fueling to take advantage of the extra air. Information is limited for this engine, so I decided to open the governor housing to document the design and look for improvements. It's difficult to see through the small access hole, but after a few hours of tinkering, I drew this diagram:
- Lever "A" is the main pivot in the assembly. The governor pushes against it on the upper left, and it adjusts fuel on the bottom right.
- The throttle cable is attached to lever "B". Both pivot on the same shaft, and are connected by springs and direct contact as they rotate.
- The main spring to fight against the governor is attached to lever "B". There is a tiny spring attached between "A"and "B" that is too weak to have a anything to do with governing. I think it's just there to keep things held in place and prevent slapping.
- I noticed during examination that, when lever "B" is held at full throttle, lever "A" isn't held tight against it. The governor can push it back a bit before it comes to a rest against lever "B". This basically disables the upper 10-15% of throttle position at the injection pump, because there is nothing to hold it there against the governor.
- Lever "B" has a threaded hole that is missing some type of set screw. When at full throttle, without that set screw, the only spring fighting the governor for the top 15% is the tiny spring. Once lever "A" makes contact with lever "B", the main spring takes over.
To increase pump position and horsepower, I needed to purchase or fabricate a new set screw, then adjust it until there is no slop between the levers. Factory replacements are impossible to find, so I decided to make one. Unfortunately, it is a very unusual, and unavailable, thread (M14x1.0), so I had to special order threaded rod from China. Once it finally arrived (2.5 weeks of shipping!!), I made the screw.