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.
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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.