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Re: [turbobike] Water injection

Fri, 1 Oct 1999 08:12:28 -0400

You're not that far off, really.  I ended up with a bit more compression than I
wanted on my FJ, and found that when fully heat soaked on long summer rides, 6-7
psi was all it would tolerate on 93 octane.  By adding a super cheap and simple
single stage water injection unit I was able to run 10-11 psi.  Then there's
always good gas when you want to run more boost.  Still, this only goes so far.

The one advantage to actually doing a top-end specifically for turbocharging is
that you have a few options you wouldn't otherwise have.  For one, normally
aspirated engines like a very tight squish band to improve mixture turbulence.
The smaller the piston-to-head clearance at operating temperature the better.
This is required because combustion pressure is low and flame front travel speed
is low as well.  In a turbocharged or nitrous application this is not the case.
Since flame speed is proportional to combustion pressure, and the pressure,
specifically the BMEP (Brake Mean Effective Pressure) is greater over the entire
cycle in these cases, the added turbulence of the tight squish is not needed and
is even detrimental to smooth combustion.  Similarly the added peak combustion
pressure of a high compression ratio is not needed either.  The effects of these
parameters becomes more significant as specific power levels rise.  The point to
all of this is that when you build the top end specifically for you turbo
application, you can both reduce the overall compression ratio and open up the
squish band by using a very thick base gasket.  For extremely high performance
applications you might even run the piston squish band 0.080" below the cylinder
deck height at TDC.

It's something to consider anyway.  Doing a motor like that wouldn't necessarily
mean you'd have to buy new pistons, although I would recommend good, hard chrome
wrist pins with a heavy wall.  I wouldn't think you'd want to change the rods,
either, if it's just going to be street ridden at less than 15 psi.  Wrist pin
problems are often misdiagnosed as rod problems since wimpy pins seize in the
small end and then throw the rod as a result.


"Bob Buehler" <ribicf@hotbot.com> on 10/01/99 12:04:30 AM

To:   "Turbo Bike Mailing List" <turbobike@natvideo.com>
cc:    (bcc: Bob Shammas/EQA/Nsk-Corp)
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Subject:  [turbobike] Water injection

Hi Gang:

I need some advice on water injection. I was told or read somewhere that water
injection was relitively simple and inexpensive and would alow for the use
compression ratios of 9 or 10 to 1 as is found with stock pistons.  I believe it
also was supposed to inhibit detonation using stock compression ratios even at
boost pressures as high as 12 or 14 psi.

This all sounds to good to be true.  It it is true, it sounds like I could
substitute low compression pistons and high dollar rods for a water injection
system. This would also save a lot of motor disassembly work.

Maybe I'm dreaming but could I go from low boost (5 to 8 psi) on a stock motor
to 12 to 14 psi by just adding a water injection system.

Any input is always greatly appreciated.

Bob Buehler
Twin Turbo CBX or Bust

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