Fuelly Badge


Saturday, October 30, 2010

Unicam > SOHC

I've read a few remarks in forums and comments sections of moto sites about Honda's choice to use a "cheap inferior" SOHC valve layout on the VFR1200FA. I want to explain the differences here and clear the air.

Technically,  the Unicam design is a Single Overhead Camshaft layout. There is one camshaft (per cylinder bank) placed overhead of the valves. But this design, which is unique to Honda, has some key differences and advantages.

A conventional SOHC cylinder head has a camshaft mounted down the middle of the head with a rocker shaft on either side. The valves are opened with rocker arms. It's simple and effective, but there are some disadvantages compared to a DOHC design... The main disadvantage is the added reciprocating weight from the rocker arms. The added weight limits the engine's rev ceiling. Another drawback might be the compromise of the combustion chamber shape. The spark plugs and ignition coils cannot be placed in their most efficient location: top center of the combustion chamber. The camshaft sits in the way.

On a DOHC cylinder head, the camshafts are placed directly over the intake and exhaust valves on opposite sides of the head. There are no rocker arms or shafts and the ignition coil and spark plug sit directly over the center of the combustion chamber. It can rev higher than a SOHC but it requires a bit more maintenance and it's bulkier.

Honda's Unicam is a hybrid of both designs. There is only one camshaft, but instead of sitting down the middle of the head, it is located directly above the intake valves, like a DOHC. The intake valves are opened from directly overhead through a solid lifter. (with shim adjustment) The exhaust valves are opened with tiny little roller rockers, one for each valve. The exhaust valves are smaller and lighter than the intake valves, and they don't open as far... so the added reciprocating weight from the rocker arms is less of a factor, and high RPMS are still possible. There is also less stress on the cam drive compared to a DOHC. Just one (shorter) chain is required to turn the camshaft, and a larger sprocket is used for more torque multiplication and less stress on the chain. The chain also doesn't have to bend through a complex path, so a simpler cam chain tensioner (with less potential for failure) can be employed. Valve adjustments are slightly simpler than a DOHC, because the exhaust valves have threaded adjusters at the ends of the rockers, which eliminates half of the tedious shimming work.

Honda introduced this design for 4-stroke motocrossers and off-road bikes. The reduction in bulk and mass is especially important to compete with flyweight 2-strokes. It made sense to employ this design on the big V4 for the VFR1200F. V4s are bulky engines and are tricky to package compared to the typical inline 4 on most sportbikes and the Unicam design helps make it a bit more compact. (smaller overall than the DOHC VFR800 motor) Very little performance is sacrificed compared to a DOHC and maintenance is a bit simpler.

No comments:

Post a Comment