PROPWASH
8
October 2015
The hole in the upper right on the piston
is designed to allow flow from the upper
part of the piston into the transfers. This
can be done a lot more as the next picture
shows.
Neither of these modifications are
common. They don’t seem to help the
power and the piston is weakened by the
large slots. The final piston modification
involves cutting a flat on the piston. This
both lightens the piston and improves the
combustion chamber shape. The minor
increase in volume doesn’t affect the
compression ratio much. If you want to run
a really close squish clearance, the bottom
of the cylinder, the combustion chamber,
and the piston crown all need to be
machined. That was done to the piston
below. It’s not really needed until the
squish clearance is less than
.010” (.25 mm). Most tuners also remove
around .002” (.05 mm) from the ring area
to prevent seizures.
All that’s left is to pick a pipe among
many excellent choices. My April 2012
Propwash article “
Tuned Pipe Design Factors”
gives an idea of what to look for in a
basic design.
If class rules require a stock engine, a pipe that generates a strong low pressure
(suction) pulse is needed to help overcome the restricted intake tract. A pipe I
developed for that is illustrated below.
It has a large diameter and a diffuser section designed to expand the exhaust to that
diameter, producing a strong low pressure pulse. It has good low end but poor over rev
power. See the comparison to an M&D pipe in the graph below. That’s fine for heavy
hulls like thunderboats, but bad for riggers. This pipe was designed to be fully
adjustable to test what the various lengths do. In the graphs on the following page the
diffuser length was held constant and the baffle cone was adjusted to give various band
widths. The notation is: the diffuser length from the exhaust flange to the start of the
band/band width. Note that this also changes the tuned length of the pipe. The wide
band pipes had a longer tuned length. Somewhere between a 55 and 75 mm band
width was the best compromise.
Zenoah Modifications Revisited
(Continued from page 7)
This increase can give a time area
allowing around 6 ½ to 7 hp. Be sure the
lump on the top edge covers the ring gap
at bottom dead center. If it doesn’t the
ring ends will catch on the port edge,
break, and score the cylinder.
A modified carburetor will also be
needed to match the increased flow from
the new ports. The stock .500” (12.7
mm) venturi on a Walbro WT 257 carb
can be increased to .535” ( 13.6 mm) or
a little more. Because of the butterfly a
bigger bore isn’t much help.
Streamlining the butterfly shaft helps,
but the WYK barrel style carb is a better
solution. Quickdraw has modified this
carb for really impressive flow. It may
be overkill on a modified Zenoah.
See their modifications at
Page_43.html
Most tuners do piston modifications
as well. These are mostly designed to
lighten the piston and improve transfer
flow. Lightening is done by removing
material from the inside of the piston.
The basic flow improvement comes from
widening and radiusing the transfer cut
out area.
Inside a Lightened Piston
Flow Improvements
Giant Flow Slots
Piston Dome Modifications
The drawing of the stock pipe
The stock pipe build
