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Re: [turbobike] Carb Question

Mon, 22 Nov 1999 10:29:04 -0500

Hi Jeff,

You'll want a lot more than 50% of the engine displacement for the plenum.  You
want the volume so large that the draw from each individual cylinder is
insignificant compared to the plenum's total volume.  Look at Kawasaki's
original ZX-11 - it had a 15 liter airbox that breathed through a little opening
in the front of the fairing.  That opening was (almost) enough to supply the
average air requirements when flowing continuously.  The instantaneous air
requirements of each cylinder however require a very high flow rate since the
intake event is limited in duration.  The airbox still wasn't large enough to
reduce pressure fluctuations entirely, but did a good job.
Remember that all of the piping from the turbo, through the intercooler, etc is
part of the plenum volume, helping to reduce surge.  Of course, to really supply
the cylinder with what it needs, the large cavity needs to be right next to the
throttle body or carb.  The higher the boost level, the more critical these
things become.

Another analogy is breathing through a straw.  While a straw may flow enough air
continuously to meet your requirements, you need to take a big breath all at
once every 5 seconds or so.  If you  could keep the air flowing into a big
plastic bag the whole time via a small pump, then when you needed it you could
take that breath unimpeded by the straw's diameter (as long as the bag is big

I like your carburetion solution, but it doesn't address the reason for the
plenum volume requirement.


"Jeffrey K. Churchill" <jcperformance@juno.com> on 11/22/99 09:39:44 AM

To:   Bob Shammas/EQA/Nsk-Corp@Nsk-Corp
cc:   turbobike@natvideo.com
Fax to:
Subject:  Re: [turbobike] Carb Question

  I think you are dead-on,....the formula is something like 50% of engine
displacement on a four stroke,..right ??
  Funny thing is,..I didn't do it on my Buell turbo project bike,...it
has practically
no plenum volume,...but responds and runs great.  The .63 turbine  A/R
is a little
big for a street bike,....I haven't fully tested the .48  hotside
yet,...but I found that the
key was making the intermediate (carb)  circuit lean enough for good
while adding an additional main fuel circuit for adequate high rpm
  It seems that the overall fuel needs are so broad that a regular carb
compensate properly.  Of course  F.I.  solves this nicely.
  I'm sure my solution is not unique,...but the method and result was
eye-opening.  Using a cv carb allows good velocity everywhere,  and MAY
why I am getting away with no plenum.  The extra fuel circuit doesn't cut
in until
the slide lifts over 3/4,...allowing a smaller regular main jet and
needle combo.
  Have you seen something like this before ??

Jeff Churchill

J.C. Performance    617-06 Bicycle Path   Pt Jeff Sta  NY  11776   USA
techline: 631-928-9863    Fax:  631 928-6679
website:   www.jcperformance.com

On Mon, 22 Nov 1999 07:49:13 -0500 shammar@nsk-corp.com writes:
> Realistically you'll never get too much plenum volume.  The need for
> plenum
> volume is all but forgotten on four cylinder engines since the
> intake events of
> the cylinders provide an almost uninteruppted flow out of the
> compressor.
> However, a larger accumulator volume reduces manifold pressure
> fluctuations
> between intake events.  On a twin, you'll run into surge much more
> easily and
> must have a huge plenum.  Having a large plenum is akin to being
> able to blow up
> a balloon with full lungs the whole time.  You don't want empty
> lungs before the
> intake valve event has finished.
> Turbine response (how fast it makes boost) will be governed by the
> exhaust event
> and turbocharger matching.
> That's just my $0.02.  I've built a few bikes but that doesn't mean
> anything
> either.
> Bob
> Bryan <bgoodwin@iquest.net> on 11/21/99 01:37:26 PM
> To:   FFDennis@aol.com, turbobike@natvideo.com
> cc:    (bcc: Bob Shammas/EQA/Nsk-Corp)
> Fax to:
> Subject:  Re: [turbobike] Carb Question
> I think the only problem you would run into there is that the volume
> of the
> airbox would be too large. This might cause the turbo to boost
> later.
> At 10:38 AM 11/21/99 EST, FFDennis@aol.com wrote:
> >In a message dated 11/19/1999 23:56:32 Central Standard Time,
> ABNZX9@aol.com
> >writes:
> >
> ><< Now for another question how about intake plenum and attaching
> it to CV
> > carbs?  Should I buy a set of velocity stack and machine them in
> order to
> >fit
> > an intake plenum to the carbs that way?   I have a feeling that
> the intake
> > plenum I am currently using is robbing me of effieciency.
> >  >>
> >
> >You raise another interesting possibility.....If you can
> encapsulate the
> >entire carburetor within a plenum, and then pressurize the entire
> plenum,
> you
> >eliminate the need for separate pressurization lines for bowl
> vents, CV
> slide
> >vents, etc., as you would have equal pressure operating on all
> sides of the
> >crab.  The pressure in the venturi would be lower than the other
> areas,
> which
> >is how a carb works.  Your next problem is then to seal the
> throttle cables
> >and fuel lines where they pass into the plenum.  I've seen ram air
> setups
> >like this.  It seems to me that a turbo plenum could get away with
> being
> less
> >aerodynamic internally due to the mechanical pressurization.
> >
> >Just my .02.........lots of book knowledge and reading, I've never
> built a
> >turbo bike (but I have a KZ 900 motor in pieces, bead blasted, for
> a future
> >project......)
> >
> >Dennis
> >'99 M2 Cyclone
> >'87 KLR 650 Urban Assault Vehicle
> >'78 KZ900 bike in boxes
> >
> >
> bgoodwin@iquest.net
> FZR1000 turbo EFI
> (aka The Money Pit)