Internal Flow Improvement

 

Traditionally most “Hop-Up” efforts have centered on improving the flow through the engine by reducing the pressure drop.  Looking at each area from the  venturi inlet through the transfer port in the cylinder the following can be performed.


Venturi - The maximum beneficial diameter of the venturi or throat is approximately the same diameter as the bore of the crankshaft.  For consistent performance with a large diameter the engine needs to be run with a pressure fuel system to eliminate fuel starvation during launch.  In many cases this only involves removal of an intake restrictor and often installation of a less restrictive Needle Valve Assembly (NVA). The three piece K&B design provides a minimum restriction for a venturi mounted NVA.  For a remote NVA a fuel nipple is mounted in one side of the venturi with any hole in the opposite side plugged.  A few engines benefit from boring out the existing venturi portion of the case.  When a new venturi is required, as is the case for an R/C conversion, the throat diameter is matched to the crankshaft bore diameter unless restricted by the venturi boss on the case.  If the venturi becomes too large on a pressure system the fuel spray into the inlet may tend to wander and at times spray out the inlet until the engine reaches operating rpm.  This condition makes the engine difficult to start and needle reliably.

Crankshaft Inlet -  The crankshaft inlet on a shaft induction engine should blend into the bore of the shaft with minimal restriction.  This is aided by radiusing the edges and aft lips of the inlet blending the port surfaces to the bore of the shaft and removal of large flats often remaining after machining by the manufacturer.  Care must be taken to avoid sharp edges which result in stress risers which can lead to shaft failure.  An additional improvement can be made by adding a fillet to the front of the port helping turn the flow from the vertical to the horizontal bore in the shaft. 

Crankshaft Bore - Enlarging the crankshaft bore is used by manufacturers  of modern schnurle ported engines to improve performance at high rpm.  However on existing designs significantly enlarging the bore can weaken the shaft leading to failure.  Such failures are common on Torp Greenheads where the bore has been enlarged and the crank inlet edges have been sharpened leading to shaft failure at the aft end of the inlet or where the crank disk joins the shaft.  What can be done on most crankshafts is some moderate internal grinding to remove often present circumferential machining groves and moderate polishing.  This will remove stress concentrations in the shaft potentially improving the strength of the shaft. 


Another modification is to add a bit of taper from the inlet to the exit of the bore followed be a more generous radius at the exit.  This helps reduce the pressure losses as the flow decelerates from the relatively small flow area of the shaft to the large area within the crankcase to the transfer passages.  It is performed only on cranks whih have been proven adequately robust.


Piston/Liner Cutouts - Addition of cutouts in the transfer port side of the piston have been used to improve flow.  If improvement result they are small in my experience and have the potential to result in the piston going out-of-round.  Due to this less-than-desirable potential and questionable benefit this rework is not performed unless specifically requested.  Cutouts in the piston do however reduce the weight improving the balance as discussed under “Crankshaft Rework

Rod Streamlining - Streamlining the cross section of the rod has the potential of reducing the aerodynamic drag of the rod as well as reducing flow restriction.  A very conservative approach is taken and is only performed on engines with known strong rods.

Cylinder Transfer Port Taper - An almost standard rework procedure from the nostalgia era is to add some taper on the bottom edge of the transfer port.  Although the benefit of this modification is somewhat questionable it certainly does no harm and is done as part of a complete engine rework.