Turbo Twos

[jb24 at chrysler.com]

I know someone in the group has said this isn't the spot for
hot-rodding two-strokes, so I will try to keep the explanations down
and centered on the efi aspects of the project.  Speaking of which, in
the March 97 issue of Automotive Industries, it was reported that
Bimota has passed US emissions certification for their 500cc v-twin
two-stroke.  They have used low-pressure (5-8 bar) F1 fuel injectors
and used them for direct injection.  They have managed to do this
without blowing the injectors out of their seats by putting them in
ports in the cylinder wall, away from the high-pressure/temperature
combustion gasses of top dead center.  Those have to be some injectors
to fire every revolution to a 9,000 rpm redline.  If they can do it, so
can I (I hope).  Interestingly enough, I believe Bimota is using a HEGO
for their fueling, which would mean that the target voltage would be
lean and leaner depending on rpm, due to all the excess oxygen in the
exhaust when the pipe is in its tuned range.  I hope to skip all that
dyno work by using a MAF, the type I believe is called hot-film, the
successor to the hot-wire type.  The Jeep Cherokee 2.5L turbo-diesel
has both drive-by wire (no throttle cable) and a MAF for 135bhp, so I
think they might work for my application, although I am not sure
whether the MAF is analog or digital (I think it is analog).  Anybody
have any experience with Bosch MAFs?

In response to Mario's question, I will give an example.  A couple
years ago, there was a land-speed record attempt with a 500cc V4
Yamaha.  At the end of the tuned pipes, a turbo was installed.  Dyno
tests showed poor power for the turbo below the tuned range, but much
higher above.  This is primarily because of back-pressure holding the
extra charge in at higher rpms, but not letting charge into the
cylinder at lower rpms.  Curiously enough, this engine never produced
output close to current 500cc GP motorcycles(160 vs. 190bhp), which are
naturally aspirated.  Why?  Because all that extra charge was getting
out the exhaust, which no amount of porting changes will prevent.  The
pipe tuning is responsible for the power, something like 50% of the
total.  Pressure waves stuff the extra charge back into the cylinder
after the transfer ports have closed, effectively giving asymmetric
timing, but only at certain rpms.  This is one of the few instances of
unsteady flow impacting power to this great an extent, turbo lag cycle
to cycle not being one of them.

John Bucknell --- 97 Dakota 4x4, 94 Honda VFR750, 85 Dodge Omni GLH
Turbo....