Look at Van's Blather here.

Introduction - Powerplant Choices

RV aircraft are designed to use Lycoming aircraft engines.

The RV-4 and RV-6/6A use 150/160 hp 0-320 or 180 hp 0-360 engines.
The RV-7/7A and RV-8/8A can accommodate O-320, O-360 or angle valve
IO-360 (200 hp) engines. The RV-9/9A is suitable for Lycoming engines
in the 118 hp to 160 hp range. The engine used in the 4-place RV-10 is
the Lycoming IO-540D4A5 rated at 260 hp. Van's recommendation for the 4
place includes any of the parallel valve 540's which are available from
235hp on up to the 260hp version. These engines are the most readily
available, affordable, and reliable of the possible choices. Other
aircraft engines of similar configuration, weight and power might
possibly be used, but only the Lycoming will fit the mounts and cowls
supplied with our kits.

Van's volume allows us to buy appropriate models of new engines at
O.E.M. (Original Equipment Manufacturer) prices direct from Lycoming.
We market these engines to our customers at far less than list price.
This makes them an affordable alternative, even when compared to the
traditional used engine. Van's has similar arrangements with Hartzell
Propeller, Sensenich Propeller and other manufacturers.
Other Engines

We are often asked about using non-aircraft engine conversions. We'd
like to pass along a quote from a colleague in the homebuilt airplane
business:
"the best conversion I know is to take $8000 and convert it into a
good used Lycoming." This may sound a bit narrow-minded, but it
reflects the basic truth: no non-aircraft engine has yet proven to be
as reliable, available, and inexpensive (everything considered) as a
traditional aircraft engine.

It seems that magazines are always printing stories about automobile
engines bought for junkyard prices, mated to inexpensive reduction
drives and flown off into the sunset. It simply doesn't work like
that in the real world. The reliability we have come to expect from
aircraft engines is the result of years of development and refinement
of engines designed specifically for the task. Automobile engines
function well in their intended application: delivering low cruising
power in vehicles with well designed transmissions and power trains.
Using them successfully in an airplane requires continuous high power
outputs and reduction systems coupled to the propeller. This is
completely foreign to their design intent. (You can imagine the car
engine designer banging his head slowly against his desk..."no, no, no.
If I'd known you wanted to do that with it, I would have designed
something different....)

(Why are Lycomings never found in boats, fire pumps, gensets or other
high output and often life-critical applications? They are less
reliable intrinsically than commodity powerplants, and secondarily
ridiculously priced.)

With enough research and development effort, auto engines may be made
to work acceptably or even well in an airplane. We are not opposed, in
principle, to RV builders using alternate engines, but we would hope
that this choice is made on facts, not hopes or dreams. Do you want to
spend your time and effort on engine development or do you want to fly
confidently behind an engine that has already been developed?

(Using that logic why should I spend more money to build your
noncertified, and presumably intrinsically uncertificatable by design,
airframe when less will buy me a PROVEN, certificated aircraft? )


We, too, would like to see "something better" in available powerplants.
We are carefully watching some alternatives. Meanwhile, the proven
Lycomings do the job very well and are the best "available now" option.
Despite the many claims and promises made by promoters, we feel that if
you will look closely at what is actually available, how many are
really flying, and how well they really perform, you will agree with
our conclusions.

(Not "you may" agree, "YOU WILL". Ja wohl Mein Führer! With all due
disrespect, Dick, I don't think you really would like to see any other
powerplant succeed because one, you have a sweetheart deal with
Lycoming, and two, you want your RVs to be alike as production aircraft
to fluff resale and insurability without the bother of type
certification and production. This is called "the tragedy of the
commons" or "why buy the cow if all those heifers will come to you for
you to milk the living daylights out of and they will buy you breakfast
too".)

While we are not opposed to RV builders installing alternate engines,
we simply cannot recommend or encourage the installation of any other
engine - we don't feel it would best serve the interest or safety of
the builder.

(It wouldn't serve OUR interest.)