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RE: [turbobike] Turbo horse power vs compression ratio

Doug Cutler (DCutler@Mundaca.com)
Thu, 28 Oct 1999 11:53:29 -0500

Well put, Watson !

R Douglas Cutler

 -----Original Message-----
From: 	shammar@nsk-corp.com [mailto:shammar@nsk-corp.com] 
Sent:	Thursday, October 28, 1999 7:17 AM
To:	rawant@webtv.net
Cc:	turbobike@natvideo.com
Subject:	Re: [turbobike] Turbo horse power vs compression ratio

 << File: att1.html >> 
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
affords up top, that one believes the low end power has been sacrificed.
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
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
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
unless your compressor is maxed out or beyond its "good" efficiency
Reducing the compression ratio (CR) reduces the peak cylinder pressure
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
Pressure (BMEP) is a measure of what you really have to work with over
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 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
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

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.


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.