when old is new again

Researching the air-cooled Honda XR650 L engine I came across the acronym RVFC cast into the valve head. RVFC or Radial Four Valve Combustion was a proprietary valve arrangement created by Honda to make a four valve head flow more like a hemi head.



Honda RVFC or Radial Four Valve Combustion, splays the valves radially away from the bore's axis and creating a hemispherical combustion chamber when viewed from the engine's profile. The splayed valves arrangement allows quicker flame propagation across the combustion chamber, allowing the engine to produce more power at a given RPM. Additional benefits include better cooling and the engine's timing can be more radically advanced.

MV Agusta's new 750cc inline-four uses a radial valve configuration because the engine utilizes a small stroke relative to its bore. A smaller stroke will allow an engine to rev to higher RPMs and produce large amounts of horsepower but when we reduce an engine's stroke it will produce less torque because smaller connecting rod has less leverage on a similar sized crankshaft. This isn't a concern for a race bike that will be ridden at the upper echelon of power but for use mortals the revvy bike will ride like a two-stroke. Additionally, when we reduce stroke we give the engine less time to fill a cylinder with air-fuel mixture, ignite and expel the exhaust before the next cycle.

At 82mm, the XR 650 engine had a longish bore and plenty of torque but the Honda RVFC format proved that better flow means quicker combustion and a more efficient transfer of chemical to kinetic energy. Here is a synopsis on the benefits of radial valve technology relating to the MV Agusta 750cc:

"With short- stroke designs, there is little inertial mixture flow to aid in the induction of the mixture beyond the BDC point of the stroke. Even with longer duration of the cam profiles, it hasn't enough time to keep drawing much after BDC. So it all must flow in on the initial draw of the descending piston. Anything that can be done to improve this initial mixture flow will do a lot to help the power output. Having the added benefit of better chamber shape, with less valve area being shrouded by the chamber walls and cylinder walls gives us the second benefit of getting the best burn out of the additional mixture that has been inducted into the cylinders. Then the radial layout of the exhaust valves has a similar effect in getting the exhaust gases out of the cylinder. I am not certain, but I believe this was first done in some Ferrari F1 engines."

-- Tom Lyons (twlyons@juno.com), March 10, 2003.
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