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Re: [turbobike] Turbo horse power vs compression ratio

Fri, 29 Oct 1999 08:02:00 -0400

You're definetely right on the exhaust valve timing.  Advancing the exhaust cam
will make it boost sooner but it'll usually sign off sooner too.  I'm about to
try this on my ZX-11.

The timing thing works because you can get the mixture to actually burn in the
exhaust, creating big pressure.  They make impressive boost, but don't make any
power that way.  Under sustained conditions, you'll burn up the turbine that
way.  I once screwed up on a dyna-S conversion on my FJ.  I think I only had
about 15 degrees of advance.  I saw an instant 23 psi boost when I twisted it,
but I think a 250 Ninja could've passed me at that point.

I believe that a higher CR would make boost sooner, since the pressure is higher
during blowdown, providing more potential energy to the turbine.  The magnitude
of the effect will depend largely on valve timing.  You'll also make more power
earlier with the higher CR, so the transition won't be as abrupt, and the bike
will feel much stronger earlier.


"John Williamson" <john.williamson5@virgin.net> on 10/29/99 07:48:58 AM

To:   AbeFM@dis.member.org, Bob Shammas/EQA/Nsk-Corp@Nsk-Corp, "RAW ANT"
cc:   turbobike@natvideo.com
Fax to:
Subject:  Re: [turbobike] Turbo horse power vs compression ratio

An exhaust valve that opens a little earlier should do, and help get the
bigger volume of gas out at high rpm as it opens when pressure is still
'high', retarded ignition at low rpm also should make boost earlier for the
same reason.
I dont think compression ratio would make much difference.

----- Original Message -----
From: Abraham Mara <umaraa00@umail.ucsb.edu>
To: <shammar@nsk-corp.com>; RAW ANT <rawant@webtv.net>
Cc: <turbobike@natvideo.com>
Sent: Friday, October 29, 1999 7:08 AM
Subject: RE: [turbobike] Turbo horse power vs compression ratio

> Does a higher compression ratio help build boost faster?  Spark advance
> doesn't seem like it would.....
> -Abe.
> -----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
> turbo
> 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
> done.
> Do you have dyno curves before and after?  Have you ridden a stock one
> 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 range.
> Reducing the compression ratio (CR) reduces the peak cylinder pressure
> (PCP),
> helping to reduce preignition/detonation and reducing the maximum stress
> all
> of the components.  One might believe that this would reduce power, and
> is
> theoretically correct, assuming all other things are equal.   But PCP
> 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
> 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
> 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
> timing,
> not to mention matching the proper turbo to the application.  More
> displacement
> will always help fill in the bottom and make boost sooner.
> Later,
> Bob