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RE: [turbobike] Turbo horse power vs compression ratio

Abraham Mara (AbeFM@dis.member.org)
Thu, 28 Oct 1999 23:08:55 -0700

Does a higher compression ratio help build boost faster?  Spark advance
doesn't seem like it would.....

-----Original Message-----

That's an interesting and often observed phenomenon.  The real question is,
it an illusion?  Often one's perception is so blinded by the big hit the
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
wrong.  Maybe there is more tweaking in the carburetion that needs to be
Do you have dyno curves before and after?  Have you ridden a stock one again
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
the bike when you twist it without nitrous.  Then you realize it was that
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
helping to reduce preignition/detonation and reducing the maximum stress on
of the components.  One might believe that this would reduce power, and this
theoretically correct, assuming all other things are equal.   But PCP occurs
only one slice in time over the power stroke event.  The Brake Mean
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
(via more boost) you can increase the BMEP above what is was before you
the CR, and end up with a similar PCP.  This way the maximum stress stays
same but you make more peak power.  Of course, this is where you do lose low
(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
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
not to mention matching the proper turbo to the application.  More
will always help fill in the bottom and make boost sooner.