Thursday, 12 April 2012


I have eventually started this project so its as good a time as any to start this blog. Before getting into the details, I suppose its better to give a bit of background on the project.

The bike in question is a 1996 Honda CBR250RR mc22. I have had the bike for over 3 years and pretty much learned to ride on it. Too much money and time has been spent on it to justify selling it so instead I'm using that as justification to spend even more money on it!

The idea of an EFI conversion started a few years ago through conversations with friends but was always put off as I didnt have the money or the mechanical knowledge. Now though I've come a long way with the mechanics and I have the money also. The single biggest driver for the project though was the challenge. For anyone who isn't familiar with these bikes, the engine is a 250cc inline-four, 4-stroke which puts out 40-45bhp in standard tune depending on the model. It manages this output by revving to a 19,000RPM redline. As standard, fuelling is provided through four 29mm carburettors. While the standard carburettors are more than adequate to provide fuel, the attraction of EFI on the bike is to have total control over the bikes fuelling curve just by plugging in a laptop and also to potentially have a much quicker throttle response due to not having to wait for the vacuum slides to react to flow changes through the venturi.

The price of these benefits though are the complications involved in the conversion.
  • Cost. ECUs are not cheap. Since no model of this bike was ever fuel injected, I have to borrow components from other bikes and fit them together to make up a working system. Since I want total control over the fuelling, I couldnt just use an ECU off a newer bike either. The ECU needed to be an aftermarket, programmable unit. I decided to use the Microsquirt ECU by Bowling & Grippo. The reason being it is an opensource ECU so all firmware upgrades and tuning software are free and there is plenty of support available through forums.
  • Injector sizing. The injectors need to have a high enough flow rate to supply the required amount of fuel needed at high RPM within the limited time available. At 19,000RPM, there is only 0.0063 seconds available to inject the fuel. This is the time taken for the engine to cover 720 degrees of rotation so it assumes the injectors are constantly open (100% duty cycle). It is recommended that injectors should not be run at more than 80% duty cycle so that actually leaves only 0.00505 seconds to inject the fuel for each engine cycle. The saving grace with the engine though is its small cylinder size. With a capacity of only 63cc per cylinder, the required fuel is not too high. Taking the above into account and making some assumptions regarding AF ratio and volumetric efficiency, I calculated the injectors should flow 160-170 cc/min. If the injectors are too big, there can be issues tuning at idle and low RPM as the injectors could possibly provide too much fuel during only the opening and closing stages. 
  • Space constraints. The mc22 is a small bike and the majority of space underneath the tank is already taken up with other systems. Fitting all the components for an EFI conversion would be a challenge.
  • Throttle bodies. Most existing banks of throttle bodies are designed for much bigger engines from 600cc upwards. The problems with these are the bore size is too big so adapters would need to be made to fit the throttle bodies to the cylinder inlet stubs. The other problem with too large throttle bodies is that the engine would reach a state of effective 100% throttle very early and so slow speed and off-idle response would be very snatchy and make the bike almost impossible to ride at anything other than full throttle. Another problem is the width of the bigger throttle bodies just would not fit between the frame rails of the CBR250. The throttle bodies need to be under 300mm wide to fit the CBR.
  • Electrical power. The CBR250 does not have much leftover power being generated at idle which is a potential problem when running a high pressure fuel pump required for fuel injection. Fuel pumps typically draw 3-5 Amps. This current draw would put a strain on the battery and charging system if the bike were to be left running at idle for any length of time. To help solve the problem, I installed LED tail lights in place of bulbs and also LED dash lights to save that bit more power for the pump.
 The throttle body issue seemed to be the biggest issue to overcome so I made it the first. After a lot of research, I eventually came to the conclusion that I would use a set of throttle bodies from a Suzuki GSR400. These were chosen because they had the same diameter intake stubs as the stock carburettors and so could be fitted directly to the standard intake rubbers given the correct spacing. I bought the throttle bodies from a motorcycle breakers in Japan and had them shipped over. They look like they will do the job perfectly although they will need some modification to get the spacing right for the CBR250. The two castings will need to be machined and new throttle plate rods will need to be made to suit the new spacing.

Given the amount of work and the expense in modifying the throttle bodies, I decided to first test the EFI by mounting a bank of injectors inside the stock airbox and spraying the fuel down into the carburettor velocity stacks. I plan to remove the slide springs and hold them open so that the carburettors act as much like throttle bodies as possible. Once I can verify that the EFI works I will go ahead and modify the GSR400 throttle bodies and make the setup more permanent.

I was lucky enough to find a secondary injector rail from a 2003 CBR600RR for pretty cheap on ebay in the US. This was good for a few things. First is the design of the secondary injector rail is such that it will be easy to install it in the airbox with as little modification as possible. Second is the flow rate of the injectors on the rail are only about 180 cc/min and so are almost perfect to use on the project. An added bonus is that the injectors fit perfectly into the GSR400 throttle bodies.

I also happened across a free set of throttle bodies from a 2001 GSXR750! While the throttle bodies and injectors are useless for the project, they will be very useful as they came with MAP (Manifold Absolute Pressure) sensor, TPS (Throttle Position Sensor), fuel pressure regulator and fuel rail that should be easily modified to fit the GSR400 throttle bodies.

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