Tuesday, December 3, 2013

Airhead alterations

The usual route to "performance" with BMW's airhead engines is the typical American hot-rodder's approach, big valves, big ports, big compression, big cam, dual plugs, etc.  Bigger valves can flow more air, but, combined with bigger ports this increase in flow generally comes at the cost of velocity.
  Intake charge velocity is a prime contributor to combustion chamber turbulence and given the volume of an airhead's featureless, hemi combustion chamber, anything that can help to stir things up should be preserved or enhanced.  While those big ports and valves might flow some impressive numbers on the flowbench (which itself is an abstraction as no engine experiences steady-state flow), those big numbers generally will only provide an improvement at higher RPM, if at all.
  There are those that assume that since the larger airhead engines are oversquare, they must be designed to operate at high RPM.  Even if this were true, who would want a street-ridden engine that only makes power while you're wringing its neck?

 Let's briefly take a look at the BMW opposed twin and why it is not suited to high RPM operation.
- Heavy valvetrain?  Check.
- Foot-long pushrods? Check.  Ever heard of Leonhard Euler?
- Crank supported only at the ends?  Check.  Yes, it will flex at high RPM.
- Poor crankcase rigidity?  Check.  The crankcases will flex at high RPM. Even though the engine has a 180-degree crank and the cylinders are opposed at 180-degrees, the engine is effectively a 360-degree twin.  Both pistons reach TDC at the same time and they reach BDC at the same time, just like a British twin.  And just like a British twin, the crankcase volume changes by an amount equal to the engine's displacement every time the crank reaches BDC.  Udo Gietl discovered how much the cases will flex while building the Superbike championship engines to survive sustained high-RPM operation.
- Large, slow-burning hemi chambers?  Check.

The last item on the list is what I intend to address.  The accepted orthodoxy is to simply add a second spark plug opposite the original, back the timing off about 2-4 degrees and call it done.
This is a half-measure at best.  I think a better way would be to redesign the combustion chamber to gain turbulence which will accomplish a faster, more complete burn of the intake charge than can be had by simply lighting the original, lazy mixture at both ends.  Unfortunately there has been a lack of cylinder head development for these engines, but there have been many advances (both factory and aftermarket) in cylinder heads for the airhead's closest cousin, the American V-twin.  Yes, I said it.  Be intellectually honest and realize that, except for the cylinder angle, the engines are very similar.
  The knowledgeable V-twin tuners long ago abandoned the dual-plug, giant valves, giant ports, giant cams dogma.
Look at the chambers of any performance V-twin head, you'll look for a long time to find an open, hemispherical chamber.

To that end, here is the modified chamber that I propose.  While it may appear simple, I assure you that many hours went into coming up with this form.  The heads will be welded and reshaped to duplicate this pattern.

The ultimate goal is an engine with improved performance across its entire RPM range that will run cooler, with much less ignition advance and be ping-free on pump gas.
To those who would say "Yeah, V-twins make a ton of torque but I want horsepower" I say, ponder this:  Horsepower does not exist and can not be directly measured, it is the sum of an equation.  That equation is TORQUE x RPM, divided by a constant, = horsepower.  Torque is an actual force that exists and can be measured, horsepower is a concept that is expressed as the sum of an equation.
There are two ways to make that sum larger, increase RPM (ill-suited to an airhead) or increase torque output (the goal).

No comments:

Post a Comment