Home-Brewed Supercharger Installation

...or how I learned to stop worrying and love the bomb.

This Bomb

In my never-ending quest to squeeze every last fraction of power out of knock-off diesel engine (and tempt the conrod to make a break for freedom through the case), I decided that forced induction was my only way forward. With a diesel engine, there's really no reason to throw extra fuel at it without more air, because that fuel will just get blown out the back as smoke. Good for driving mosquitoes away, but not much else.

Unfortunately, this engine is not very well-suited to a turbocharger. It has a single cylinder, so the exhaust gases come out in discrete pulses rather than a mostly constant stream. This can reduce the effectiveness of the turbo by preventing it from spooling up quickly or maintaining a strong and steady boost. Also, the oil pump in the is engine is....somewhat pathetic. I wouldn't trust it to maintain pressure to the turbo bearings. I decided to try supercharging.

Supercharging on a tight budget, at least where smaller engines are concerned, is a much simpler prospect because of the temperatures involved. The supercharger takes its power from a pulley or chain, so the hot exhaust gases never go near it. Because it runs so much cooler, any appropriately-sized air pump can be used, as long as it has the necessary flow rate and pressure rating. I was lucky enough to source an industrial roots-type air pump from Ebay on the cheap which met all the requirements.

It's design is basically identical to a typical supercharger, except it is manufactured from cast iron rather than aluminum and has a primitive oil-lubrication system that requires frequent refilling. I fabricated some brackets and mounted the pump to the forward engine lugs on the frame. It is spun by a chain and sprockets from the crankshaft, and the gearing ratio overdrives the pump so that its volumetric flow at every rpm level is always greater than the input needed by the engine. This forces more air into the cylinder than could normally be accomplished by natural aspiration, which in turn allows more fuel to be injected per power stroke.

The Meat Grinder

Originally, the supercharger's output was connected to the engine input with a length of steel pipe. This pipe was also equipped with a liquid-filled pressure gauge to measure the boost level. Peak boost has been between 5 and 6 psi, depending on the ambient temperature and elevation. I have had the bike dyno tested to verify that everything works correctly and provides a measurable power increase. The original engine power curve was as shown:

The power output at the rear was measured both with and without the supercharger installed. The red line below shows the output while naturally aspirated, and the blue line (supercharged) shows a consistent power gain of about 1hp across the whole range.

I ran into another snag when the weather started to heat up in the spring. I had originally attached the supercharger with a plenum made from a length of steel pipe, but my boost level drastically dropped as the temperature went up, to the point that any day above 80F would result in a meager 1psi of boost. This occurs because according to the Ideal Gas Law, when a gas is compressed, work is done to it and it heats up. However, heating the air has the effect of lowering the boost pressure and reducing the beneficial power increase in the engine.

To alleviate this, I commissioned a custom intercooler from Bell intercoolers to replace the steel plenum. I highly recommend this company. Their parts are high quality, built to order, have a fast turnaround (less than 3 weeks from initial design to my doorstep!), and the prices are excellent, considering the amount of custom engineering done.

I installed this unit on the bike, and have not had any problems since. No matter what the ambient temperature is, it keeps the intake air cool and maintains the boost pressure.

The Complete Installation

Racing Exhaust and Custom Baffles

I wanted to equip my 450/500T racer with a pair of megaphone pipes, but I also wanted to make it suitable for road use without filling my shelf with alternate mufflers and exhaust bits. I purchased a pair of EMGO megaphones and totally gutted them, including knocking out the mounting plate at the back opening.

Then, I made an aluminum exhaust tip that would fit into the large rear opening. The inner surface of this tip is tapered so that it acts like a reverse megaphone, but it also has a mounting surface to support a perforated pipe.

Inside this perforated tube is a removable plug:

Pinned plug. Its location is adjustable

All of this allows me to configure my current pipes in multiple ways, depending on the particular requirements at the time. For racing, I can run them as full open megaphones for top speed, or I can add the reverse cone aluminum slug to give better midrange performance. On the road, I can add the perforated pipe and wrap it in fiberglass as a glasspack pipe, or I can add the steel slug and have a baffled pipe.

Installed on the bike

Offerings to the God of Speed - Building (and Breaking) a Homemade Transmission

The most challenging part of designing a diesel race bike was building a compact transmission that could simply bolt to the side of the engine block. Because of the size of the KZ400's frame, I could not use a separate gearbox or a CVT transmission. Either one of them would have required cutting and lengthening the frame. To minimize size and length, I decided to build an upgraded copy of a 2-speed minibike transmission. These transmissions were common on Rupp minibikes in the 1960's but had fallen out of popularity when CVT's became widespread. As a result, they are somewhat hard to come by now, and wouldn't really be capable of taking the beating I had planned.

Most of the parts are easy to come by. All that is required is two centrifugal clutches with the correct rpm engagement settings, a countershaft, and a sprocket with a built-in sprag clutch. One clutch and sprocket are mounted on the crankshaft, and a second clutch and the sprag sprocket are mounted on the countershaft. When the crankshaft (first gear) clutch engages at low speed, it drive the sprag clutch sprocket, which locks and drives the wheel. At the same time, the sprocket mounted on the crankshaft is spinning at a different gear ratio and driving the outer shell of the countershaft (second gear) clutch. When the countershaft reaches the correct rpm, the second-gear clutch engages. This outpaces the input from first gear, so the sprag clutch disengages and freewheels for the remainder of the speed range. 

The Mk1 sprocket used an off-the-shelf sprag bearing to shift gears. This quickly failed due to the shock loads involved in the gearchange, which greatly exceeded the rated load.

Lasted barely 3 miles

The early design with the failed sprocket

 Because I could not locate a suitable mass-produced option, I decided to design and manufacture my own clutch. The Mk2 sprocket used the same material as before, but was instead mounted on a normal bearing. A steel collar was mounted to the shaft next to it, inside the sprocket's flange. This collar contained a ratcheting pawl which would engage in a series of machined notches in the interior of the sprocket, and disengage in the opposite direction.

Machining the notches

The finished product

Broken ratchet

Mk2 failed in a fairly spectacular fashion, with chunks of steel flying past my thigh and my wheel locking up at 40 mph! The sintered steel sprockets that I used, while cheap and easy to machine, proved to be extremely brittle and the flange shattered while shifting gear.

Boom

Mk3 was virtually identical to the previous model, but used a carbon steel sprocket with a reinforcing ring press fit around the edge for strength. This version proved to be somewhat reliable, but still required frequent rebuilding due to the extreme wear experienced by the ratchet pawl.

Reinforced

 With Mk4, I did away with the ratchet design and substituted a ramping ball bearing system. The mounted collar has four ramps which contain hardened individual bearing balls, and flange of the sprocket has a series of small notches. When spun in the freewheel direction, the balls simply stay in the deep end of the ramp. When spun the opposite way, they wedge between the ramp and the notches in the sprocket.

This design improved the mechanical reliability by a factor of 40. The previous version could only last about 20 miles between rebuild, whereas Mk4 didn't fail until it had been in service for over 850 miles.

Final version

Strengthened and improved

When the sprocket finally wore out, I decided to simply build a strengthened copy of the same design. It is virtually identical, but the flange on the sprocket has been thickened to prevent the mushrooming seen on the worn out part above.

Final Design

Making A Cheap Oxy-Acetylene Cart

Over the weekend I whipped up a cart to move my oxy-acetylene torch around.

Sizing up the junked hand-truck

Cutting the truck to size

Adding pipe inserts for strength

Brazing the joints

Brazing completed and wire-brushed

Add new wheels and grip tape to the handle

The finished product for only $20