The CBR250Ri Turbo

Sourcing and Fitting


Weíd been looking for a small turbo for quite a while, and eventually Richard found a little IHI RHB31 at the local wreckers. The RHB31 is a water cooled turbo which is used on Suzuki Cappuccinos (600cc) and Chevy Sprints (1 litre). The compressor and turbine wheels are each only slightly larger than a 20c piece and the whole unit is only about the size of a pint glass. Itís an incredibly cute little thing, and perfect for a high-revving CBR250 engine, because the gas flows are very similar. Although the CBR only has a fraction of the displacement, car mills donít spin up to 18,000rpm without loud crunching noises and expensive repair bills.

The definition of cute. Don't you just want to hug this little compressor wheel? (Steve's life savings shown on the right for scale) ††† ††† ††† Turbine wheel and wastegate. Any smaller, you'd need a microscope.

Once we had a turbo, the next issue we had to tackle was where to mount it. If you try to bolt a turbo on the intake side of the engine and pipe the exhaust gases around the block you end up with so much lag you can measure it on a calendar. Twist the throttle now and the boost will kick in sometime next Thursday. Not only that, the heat losses from the long headers significantly decrease boost levels. So even when that boost does actually kick in, there wonít be much of it. The end result is that although you have to mark the calendar to know when to expect the boost, there isnít really enough to make it worth the ink. Add in the extra weight of the turbo and plumbing, and you can understand why the Big Four think turbo bikes are a dead end.

Thereís really only one place for a turbo to go on a bike, and thatís tucked up under the exhaust headers, right behind the front wheel. Itís nice and close to the exhaust ports, so lag is minimised and the exhaust gases are still nice and hot (heat = energy) when they hit the turbine. Yes, it can be tricky fitting the turbo in there without having the front wheel smack into it when the forks fully compress. It also means that the turbo cops all sorts of debris, road grime and spray in wet weather. Big deal Ė thatís where it works the best, so thatís where it goes. End of story.

Fortunately, Honda did us a huge favour when they designed the CBRís engine block. Thereís plenty of space for a small turbo below the exhaust ports once the stock exhaust system is removed. Richard had his beloved ďrotary hammerĒ (anglegrinder) at the ready, but we distracted him and unbolted the headers properly. He was pretty easy to distract, actually Ė we gave him the spare exhaust system to chop up. The spare is one Richard prepared earlier on a mini-roundabout. After it had gone for a high-speed slide across the roundabout and into the kerb on the other side it was pretty close to being an underseat system. The height was about right, it just needed to be pushed a tad to the left and then have the kinks removed. Instead of butchering it further there and then, Richard retired it from active duty and sourced a replacement from the wreckers.

As youíve probably gathered by now, Team Feral are in it for the love, not the money. We enjoy building all this stuff so much that we have at least six goes at everything before declaring ourselves satisfied. Theoretically, anyway. We havenít declared ourselves satisfied with anything weíve built yet, but hopefully we will eventually. Fingers crossed. Anyway, we really should have gone to the trouble of making up a custom set of tuned-length 4-1 headers. Instead, we waited until Richard had sated his frenzied bloodlust on the hapless spare exhaust system and then grabbed the useful pieces and threw the rest in the bin. The resulting stub headers were mated to a handy chunk of galvanised steel waterpipe to produce a crude manifold. A bit more bodgying with the MIG welder attached end caps and (eventually) a connector plate for the turbo, and the Feral Turbo Manifold Mk I was born. Itís not a shining example of excellent workmanship, itís most certainly not pretty, and nor is it tuned in any way shape or form. Zero outta three. Thatís not too bad for us.

According to what we vaguely remember from primary school music lessons (amazing we remember anything at all, given that primary school was a non-stop orgy of alcohol and psychedelic drugs for some of us) tuned systems should produce a ringing sound when struck. Thatís the difference between a tuning fork and an ordinary fork, isnít it? We tested the manifoldís state of tune by whacking it on the edge of a glass. The glass broke. We then tried whacking it on Steveís head (the nearest resonant cavity) and it produced a hollow thud and a few grunts of pain rather than a nice ringing tone. Even a harder hit failed to produce a clear note, although it did prompt the odd curse or two. We had a bit of a debate about whether the hollow sound was due to the hollowness of the manifold or the hollowness of Steveís head. Either way, we were disappointed that the grunts and curses werenít very tuneful. Not what we expected from someone who spent more time in music lessons than most normal people take to complete a PhD. Grade Two was the happiest seven years of Steveís life, what with all the substance abuse.

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