the g-force loads), but is also strong and durable
enough to sustain these enormous cylinder pres-
sure loads. How much force are we talking about?
A typical small-block Chevy with a 400 gram
piston at 7,000 RPM could easily be subjected to
g-forces of 3,400 g’s – or just shy of 3,000 pounds
of force. Think about that. As RPM increases, so
does the g-forces because the piston must travel
the same distance (the stroke) in a shorter period
of time. Of course, if we make the piston lighter
or shorten the stroke, the forces will be reduced.
Reducing Mass and Friction
One area where reduced mass improves almost
all areas of piston design is JE’s asymmetrical
piston skirt technique. All reciprocating inter-
nal combustion engines create what is known as
thrust loads on a given side of the piston. These
are lateral, or side loads imparted through the
piston into the cylinder wall based on the direc-
tion of rotation and a combination of crankshaft
stroke and rod angle, among other factors. On
a typical V8 engine that rotates clockwise, the
loaded thrust side is the inboard (intake) side of
the left bank and the outboard (exhaust) wall of
the right bank.
Most pistons are designed with symmetrical
skirts for both the major and minor thrust sides
of each piston. But during the creation of JE’s
forged side relief (FSR) design, enlightenment
took a step forward by reducing the skirt area
on the non-thrust side of the piston. This re-
duces the mass while also lowering friction which
further diminishes the g-load on the piston. All
68 | D r a g
I l l u s t r a t e d | DragIllustrated.com
These two illustrations (top and bottom left)
reveal the asymmetrical design of the major
thrust piston side (A) compared to the minor
thrust piston skirt (B). Using FEA, the designer
can reduce the skirt area without increasing load
which reduces friction and weight. This FEA
illustration (top right) of a JE piston reveals the
load on the major thrust side of the piston. The
colors indicate concentration of load exerted on
the piston. The dark green areas indicated the
highest load, which are still far below the piston’s
ultimate tensile strength. Pistons are subjected to
a wide variety of stress loads and each one can
be evaluated individually. In this case (bottom
right), the major thrust side can be evaluated for
contact point stress. In this case, you can see that
two small points in red toward the bottom of the
skirt indicated where the highest stress points
are located. The points are intentionally low on
the skirt to help stabilize the piston in the bore –
especially across bottom dead center (BDC).
of this was first created using SolidWorks and
then perfected with FEA. That’s what makes this
technology so efficient.
The accompanying photos and illustrations
will help make these descriptions a little more
clear, but they also demand some explanation. For
example, FEA results use a simple color-coded
depiction of load concentrations with light green
as the least stressed portions and moving toward
warmer colors for increased load with red areas
reveal the greatest load. Ideally, the designer
would prefer that in every test case that the FEA
tests would reveal low stress concentrations, but
that only happens in science fiction novels. In the
real world, high cylinder pressures or extreme
engine RPM applications will produce high stress
levels that will be indicated in red.
If you look closely, the chart is delineated in a
scale called the von Mises scale. Essentially the
scale can be set by the engineer to any level he
desires. Often, he may want to see the stresses
across a tighter scale to get an idea of exactly
where the stresses are the greatest and where
these can be relieved or at least spread over a
larger area to reduce the concentration of the
load. You can see that in the FEA analysis of the
pressure loading on the piston (Photo 02) that
the top of the wrist pin boss area show the high-
est load concentration. This is to be expected and
as long as the stress does not exceed the piston’s
capabilities.
The images of both sides of the FSR piston
showing the major and minor thrust sides of
the piston reveal how even the major thrust side
shows acceptable loading in the center of the
skirt. Plus, the low thrust side, even with its much
smaller contact area, is still well within accept-
able levels. This means that even with its smaller
contact area, the piston can accommodate this
load and be completely durable.
There’s far more to the FEA story than we can
jam into this short introduction, but ultimately
these computer-aided design advantages allow JE
to produce a superior product while still making
the pistons affordable. And that’s a great combi-
nation. - JEFF SMITH
Issue 120
PARTNER SPOTLIGHT