Which airplane?

wrote:
George Patterson wrote:
: You can get a mogas STC for the 152.

... from Petersen for 91 fuel, no?... isn't the 152 a high-compression
Lycoming O-235 at 110hp? 8.5:1 or 8.7:1 CR IIRC.

The EAA offers an STC for the O-235 and the Cessna 152. There's a note that the
engine "requires modification." Usually the only engine modification required
for an STC is replacement of the fuel pump, but I do not know if that's the case
with the 152.

Retrofitting an auto engine to an aircraft requires very careful packaging to get the
power/weight ratio comparable to an aircraft engine.

Right.

The main problem typically is associated with engine speeds. Auto engines are
usually horsepower rated at high speeds (typically around 5,000 rpm). The
propellors on most aircraft need to turn at no more than about 2,700 rpm. So,
you either need to add a gearbox to reduce shaft speed (which adds weight) or
you limit the engine to 2,700 rpm. Doing the latter means that the engine only
puts out about 60% of its rated horsepower, so you need a bigger engine, which
also adds weight.

In general, builders who need lots of power go with a gearbox. They usually have
larger planes and the weight penalty is relatively small. Running an engine at
high speed with a gearbox means that it's just not going to last as long as it
would when used for normal driving.

If you're going to run the engine at prop speeds, the best thing to do is to
change out the valve train. Go with the equivalent of a 3/4 race cam, replace
the valves with lighter ones, and replace the valve springs with lighter ones.
If carburetted, a main jet change may be in order, but modern car engines are
injected anyway. You may also have to tinker with the computer. All of the
changes reduce the life expectancy of the engine.

You also have to deal with the fact that most auto engines are water-cooled.
Your cooling system will take some expert design and will (guess what) add more
weight. I know of one pretty nice looking aircraft with a Subaru 4-banger in it
that sits on the ground a lot because it overheats easily.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.