This article is an attempt to answer the many questions that arise over the choice of engines used in the RV series of aircraft. (RV-4, RV-6, and RV-6A)
Originally, the aircraft was designed around the most plentiful and least expensive aircraft engine available to the homebuilder, the O-320 Lycoming.
This choice has worked very well in fulfilling the requirements of the RV's in horsepower and reliability.
It is not, however, the only possible choice. Here at Van's we have in recent years installed an O-360 in one of our prototypes and found no problem in it's installation or operation with this aircraft. The engine choices, O-320 @ 150 or 160 HP and O-360 @ 180 HP, are the PARALLEL VALVE versions of Lycoming's engine line. They are available in carbureted or fuel injected versions, and either fixed pitch or constant speed configurations.
There are other 4 cylinder Lycoming engines that, at first glance, would seem to work. Questions are often asked about the IO-360 200 hp engine. Van's does NOT recommend this engine, for a couple of reasons. First, it is more power than the airframe was designed to take and will power the airplane very close to red line in cruise. Just because you can make an airplane go faster does not mean that is safe to do so. Second, the ANGLE VALVE design of this engine means that it is wider, and will not fit the RV cowls without completely rebuilding the front of the airplane. Please understand the difference between the IO-360 180 hp parallel valve engine and the IO-360 200 hp angle head engine: same displacement, different engine.
Not every O-320 or 0-360 will fit. Lycoming has been generous over the years, essentially custom building engine versions for specific airframes. Carburetors, fuel injectors, induction tubes, governors and other items have all been mounted in different places to satisfy the needs of aircraft designers.
Engine configurations that cause problems on RVs include HORIZONTAL DRAFT CARBURETORS or INJECTOR BODIES, that are mounted on the rear of the oil sump. The 0-360-A4K is one example of a problem. These engines use horizontal draft carburetors mounted at the very rear of the sump, facing the firewall, and will not work on the RV's. Another problem area is the "H" engine. Lycoming produced an entirely different O-320 for the 76-77 Cessna 172. The fuel pump was mounted over the front left cylinder, the accessory case at the rear is integral and a different shape. As supplied, the RV mount will not fit with this engine, but modifying the mount is not difficult, and several RVs have been flown with this engine. In general if the induction system faces forward or is mounted toward the front of the oil sump then interference with the mount itself is not very likely.
Lycomings also come with three different engine mounting systems. The most common and modern is the DYNAFOCAL TYPE I which has a bolt attach angle of 30 degrees to a line that would parallel the crankshaft. The second most common is the CONICAL which has bolts which directly parallel the crankshaft. The third and least common is the DYNAFOCAL TYPE II which has a bolt angle of only 14 degrees to the crankshaft. Van's can supply mounts to accomodate any one of these engines. Another major consideration would also be that the engine have a mechanical fuel pump on the accessory housing. Many high wing aircraft do not use engine driven fuel pumps. If one of these engines is to be used on an RV then either a machining job is in order or a new accessory case must be located.
Another caution is that in the RV-4 if a constant speed propellor is to be used, then the engine's propellor governor location should be on the accessory case as opposed to those engines with governors located near the propellor flange. The front mounted governors can be used but only with a cowl modification to accomodate them.
A nearly complete list of these engines is included in the engine section (chapter 11) of the RV construction manual.
Performance differences with the differing horsepowers are not as great as many assume. Other things being equal, the 180 will give another 200'/min in climb and about 6-7 mph in cruise. Of course, HP comes from GASOLINE, not the engine, so if you insist in putting in a bigger engine and keeping the throttle in to enjoy that extra performance, it will cost you at the pump. Performance with the smaller engines is better than just about anything else in the sky, so ask yourself "is it worth it"? Keeping these caveats in mind, most O-320 and O-360 engines will work in RVs.
So far, no alternative engine has come forth to rival the Lycoming for power-to-weight, reliability, or (surprisingly enough) cost. Summary: most O-320/O-360 150/160/180 hp Lycoming engines will work in the RV designs, but there are enough variations to warrant serious research. A call to us here at Van's is a good idea if you have any doubt.
Comments and feedback welcome, please send to... hovan@flash.net