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The technology of PWC's, like all modern
technologies, is moving forward at a rapid pace. At Group K, attempt to maintain
the most current and accurate technical information, based on our testing and
experience. However, we are not under the illusion that any of our technical
information is permanent or perfect. Technical information is subject to
difference perspectives, based on different experiences, by different people, in
different places.
It sometimes happens that we get letters from other industry technicians who
have a technical perspective that differs from our own. We respect the opinions
of these technicians, and feel that many of their thoughts deserve posting
space.
This document will show, what we consider to be, some of the very credible
"disagreements" with our tech information. We will post these letters
unedited (as we received them). In many cases we will also post our response to
the sender(s).
We invite technicians, who sell modification parts and/or services to the
general pubic, to submit letters relevant to specific technical subjects. The
letters accepted for posting, like our responses, will contain no slanderous or
vulgar language...this will be a venue for technical perspectives...not ego
wars.
Harry Klemm - Group K
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LETTER TO US:
December 15, 1996
Harry Klemm - Owner Dear Harry:
First I would like to thank you for your kind words regarding our Sea Doo 785
Spec 2 Exhaust system and let you know how much I look forward to your technical
postings on the net. I do however want to say that I disagree with some of your
conclusions as to the relationship of high rpm and the need to run race gas.
I must first acknowledge that I do not have the practical experience with
modified 785 XP engines as you. Most of our testing has been done on stock
engines (save for six weeks we spent testing with "Pops" Fischetti at
the start of the 1996 tour). Still, I must disagree with your conclusion that higher revving engines have
a greater tendency to detonate and to avoid this you can run race gas. First, a
couple of underlying facts (or at least I think they're facts) I base my beliefs
on:
1. With a given engine, more cylinder pressure = more torque, which is to say
Pressure x Area x Leverage (stroke) = Torque.
2. High cylinder pressure is an important element of detonation.
3. The presence or absence of detonation is determined by a race between the
end gases (those at the outside perimeter of the piston) burning because of the
spark ignited flame front reaching them or excessive heat/pressure burning them
first.
4. Given the above, the closer you run an engine to peak torque the greater
the likely hood of detonation.
5. The main reason race gas would help an engine live longer is its ability
to resist detonation.
It seems that a 785-XP engine revving at 7,500 rpm is further away from peak
torque than the same engine revving at 7,400. Revving an engine past peak torque
not only reduces peak cylinder pressure it also leaves the end gasses captured
at this high pressure for less time. If this is true revving the engine higher
is tougher on connecting rods, cranks, main bearings etc., but should not be
more likely to detonate, indeed the higher revving scenario should detonate
less.
I invite you to run one of your engine packages on our dyno, we could look at
fuel flow, cylinder pressure etc. and possibly find an answer to the mysterious
high speed engine seizures.
Respectfully, Ross H. Liberty
Owner - The Factory Crew
"Horsepower is a Beautiful Thing" http://www.factorypipe.com -----------------------------------------------------------------------------------------------------------
OUR RESPONSE:
Ross Liberty Ross,
The text of your letter caused us to closely review our own literature
documents about the Sea Doo laydown rave motors. We did not recall ever writing
that "detonation related" overheating was the reasoning behind our
rpm/octane recommendations. After reviewing our documents, we found that we were
right...(sort of).
Our documents stated that "Our test boats exhibited the (performance)
losses associated with overheating "on 92 octane" whenever we exceeded
170 psi compression and/or 7100 rpms. We can understand how this inference could
have been assumed to mean, performance losses caused by detonation..."but
our documents didn't say that".
The greater truth is that we have no "specific" idea what is
causing the peak rpm losses we observed during our testing. It bears noting that
not all of our tests with high compression, or high rpm, (on 92 octane) resulted
in piston seizure. The test results that "turned us away", were tests
that showed immediate and/or progressive losses in "hot tachometer
numbers" that we witnessed during on water tests. When the "hot"
numbers showed a loss of 100 - 200 rpm against the "cold" numbers, we
considered those losses to be unacceptable. The only way we reduced that loss,
(on water), was to reduce the compression, the peak rpm, or to increase the fuel
octane. This was the basis of our recommendations.
We acknowledge that you may be correct on some, or all, of the assertions
made in your letter about detonation. We also agree that dyno testing time could
possibly bring to light some of the "true" temperature
gremlin(s)".
We appreciate the dyno time offer...and we hope to take you up on it. In the
meantime, while our customers are seeking our best opinions for safe rpm vs
octane recommendations, we intend to stick to our guns. Is it possible that we
are making our octane choices on the conservative side of things?...your darn
right it's possible. However, with the majority of our customers interested in
"longevity first", we prefer to be on the conservative side of
potential piston failure.
Regards
Harry Klemm, Owner
Group K
Group K
4597 Calle De Media
Ft Mojave AZ 86426
Factory Pipe Produc