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This is something I came across recently and I
thought I'd share it.
This is a competing brand of piston, a very
well-known brand, and in fact, at least one of our
competitors ships this brand of piston in their
engine kits. This piston is designated by the
manufacturer as being a match for Thunderstorm
heads.
As you can see, the dome angle is 10 degrees.
However, the squish band shelf in the Thunderstorm
head is 15 degrees, and in fact the factory
Thunderstorm piston has a matching 15 degree
dome angle.
Why is this a problem? Well, ideally you'd like the
squish clearance between the piston and the head to
be in the .030" range; this promotes good chamber
turbulence and efficiency because the fuel gets
squeezed out of the squish band while the piston
passes through TDC. We talk about squish clearance
and how to optimize it in the
General Instruction Sheet that we provide with
all engine kits.
However, the shallower 10 degree dome angle of the
piston shown above will not not allow a proper
squish clearance when used with a standard 15 degree
Thunderstorm head. The squish band and the benefits
thereof will not be realized if you use this piston
under a stock Thunderstorm chamber.
When I asked the manufacturer of this piston about
the 10 degree dome angle in a 15 degree chamber,
they told me that by making it 10 degrees, they
could use the same piston under the 10 degree
Lightning head (Lightning heads came stock on
XL1200S models as well as some 96-98 Buell models).
And it was better to have too shallow of an angle
under the Thunderstorm head than too steep of an
angle under the Lightning head (which of course is
true, but kind of misses the point).
So basically, they're offering something that really
doesn't work well so that they can minimize the
number of different part numbers in their book and
pistons on their shelf.
As someone who works really hard to find every last
horsepower for his customers, I just can't imagine
that. It's outside my scope of understanding.
There's another problem with trying to double up
that application, too. The Thunderstorm chamber is
67cc, the Lightning chamber is 62cc. Using the same
piston for both applications is either going to
result in a high CR under a Lightning head or a low
CR under the Thunderstorm head, depending on which
chamber the dome is sized for. I'm not sure if they
advertise two different compression ratios depending
on the heads or not, I didn't ask.
Here is a Hurricane piston designed for a
Thunderstorm head:
As you can see, it has the proper 15 degree dome
angle, which matches the standard Thunderstorm
chamber squish shelf. The dome volume of this piston
will give a 10.5:1 compression ratio under a 67cc
Thunderstorm chamber.
Likewise, if you buy a set of Hurricane
pistons for a Lightning head, you get a 10 degree
dome angle to match the 10 degree squish shelf angle
of the Lightning head, and a dome volume that gives
10.5:1 compression under the Lightning head's 62cc
chamber volume.
We also have 30 degree pistons for higher
performance applications. The 30 degree dome angle
has advantages, but
it needs more dome height (and thus tend to give
higher CR) for a given amount of squish area. And of
course, fitment requires machining the combustion chamber
out to 30 degrees to match, which we can handle, we
do it all the time.
The moral of the story here is to be careful what
you buy. Don't let the cost saving measures of your
vendors compromise your result.
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