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