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Re: [turbobike] Turbo horse power vs compression ratio

Thu, 28 Oct 1999 08:16:34 -0400

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That's an interesting and often observed phenomenon.  The real question is, is
it an illusion?  Often one's perception is so blinded by the big hit the turbo
affords up top, that one believes the low end power has been sacrificed.  We
usually find that the power curve never drops below stock unless something is
wrong.  Maybe there is more tweaking in the carburetion that needs to be done.
Do you have dyno curves before and after?  Have you ridden a stock one again for
comparison?  This situation is a lot like when you put nitrous on something.
After getting used to the big hit, it kind of feels like something's wrong with
the bike when you twist it without nitrous.  Then you realize it was that slow
all along.

You can generally make more peak power with less compression and more boost
unless your compressor is maxed out or beyond its "good" efficiency range.
Reducing the compression ratio (CR) reduces the peak cylinder pressure (PCP),
helping to reduce preignition/detonation and reducing the maximum stress on all
of the components.  One might believe that this would reduce power, and this is
theoretically correct, assuming all other things are equal.   But PCP occurs at
only one slice in time over the power stroke event.  The Brake Mean Effective
Pressure (BMEP) is a measure of what you really have to work with over the
entire event.  By using a reduced CR and packing more mixture into the cylinder
(via more boost) you can increase the BMEP above what is was before you reduced
the CR, and end up with a similar PCP.  This way the maximum stress stays the
same but you make more peak power.  Of course, this is where you do lose low end
(off boost) power since there is no additional charge when you're off boost.
For true street use, less boost and more CR is usually better and results in a
more linear power curve (if there is such a thing on a turbocharged engine).

In practice, getting it right is a delicate balance between chamber
shape/design, boost level, intake temperature, CR, squish, and ignition timing,
not to mention matching the proper turbo to the application.  More displacement
will always help fill in the bottom and make boost sooner.


rawant@webtv.net (RAW ANT) on 10/27/99 08:42:22 PM

To:   turbobike@natvideo.com
cc:    (bcc: Bob Shammas/EQA/Nsk-Corp)
Fax to:
Subject:  [turbobike] Turbo horse power vs compression ratio

This question is for anyone that can help.
Well my turbo system is complete. I notice that I have a lot less low
end power. Could this be do to the turbos back pressure. The second
question is, if I lower my compression and run more boost will this
actually afford me more horse power or is it just easier on the motor.
THANX everyone.

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