Orphaned Engine

Most Experts Aren’t. That's something the late Smokey Yunick said
back when I was a seaman deuce. Every month my mail delivers one or
two messages saying it's still true. The messages usually come from
some superbly experienced fellow who has literally spent his life
working on cars or trucks. He is the Local Guru when it comes to
engines for homebuilts and he's taking the time to let me know that
the automotive engineers I like to cite in my articles aren't quite as
bright as I seem to think they are, offering an experience-based
example to prove his point.

Unfortunately, the offered example invariably deals with cars or
trucks, things in which the fellow has a life-time of experience,
whereas automotive in the sense used here, does not, although it could
include them. To an engineer, automotive means something that can move
under its own power. Like an oil tanker, the Space Shuttle, or a gold
dredger.

My usual reaction is to hit the delete key. I get more mail than I
want, most from people with real problem, some of whom I can help. But
it's always sad to hear smart people say dumb things. And on the
whole, these are smart people, even though a life-time of experience
hasn't tipped him off that we're taking about two different meanings
for automotive.

We all start out pretty dumb. As we age we gather information and gain
experience and, assuming a fair share of native wit, we end up a bit
smarter than when we began. Mebbe all this guy needs is a nudge in the
right direction. So you say hello and the odds are the fellow is
having the same problems as everyone else except he was a bit too
proud to say so.

With this type of Expert you'll often discover his life-time of
experience has been with just one type of engine or perhaps one type
of car and he has been trying to transfer that experience to a Corvair
or a Volkswagen and isn't having much luck. I mean, who ever heard of
a head torqued to only eighteen foot-pounds! That has to be wrong...
right?

If the fellow hasn't figured out the meaning of automotive there's a
good chance he won't have any idea in the blue-eyed world about Class
of Service but a good understanding here is the real key to a
successful conversion so you give it a shot.

A car or light truck uses a variable speed, high-rpm, low-torque
engine whose nominal output approximates 25% of its peak output.
Nominal output is defined as the amount of power the engine was
designed to deliver for approximately 98% of its service life. The
only time it’s expected to produce more… that wayward 2%… is when
accelerating or climbing a hill. Once on the flats -- once you've
reached a Stable State of cruise -- the figures are a good match. For
hilly regions vehicle manufacturers offer different ratios for the
rear-ends. Economy takes a hit but over-all, the figures match up.
Respect an engine's Class of Service and you'll be rewarded with 2,000
to 5,000 hours between overhauls.

You can always demand more output from either type of engine but
doing so will reduce it's service life. With a converted VW, for
example, your Mean Time Before Failure will typically fall from about
2,000 hours in vehicular service to about 200 hours when powering a
plane.

By comparison, an aircraft engine is a single-speed, low-rpm, high-
torque engine whose nominal output approximates 75% of its peak
output. Peak output may be defined further as maximum sustainable
output, and as Peak-sub I, meaning an instantaneous value or dyno
blip, something you might use to impress the newbies.

Since our goal is to produce thrust throuigh the rotation of a
propeller, our primary interest is in the amount of torque that
appears in the crank, and in the propeller's efficiency at a given
rpm. The measurement of thrust is quite simple and articles describing
different types of homebuilt thrust stands have appeared in the
literature and on the internet. You will note that horsepower, which
serves no useful function at this stage, has not been mentioned.

It usually takes an exchange of half a dozen messages or thereabouts
to arrive at this point, if in fact we arrive at all. In the
overwhelming majority of cases the Local Expert simply vanishes. Which
is doubly unfortunate because the best is yet to come.

When we convert an auto engine for use in an airplane we are trying to
convert it from one Class of Operation to another to make it more
suitable, usually in the area of mechanical reliability. By
comparison, the typical flying Volkswagen starts out as a marginally
suitable auto engine that is then made even less suitable for aircraft
use by turning it into a hot-rod enigine. Why? Usually because the
person doing the conversion has little understanding of an aircraft
powerplant. Indeed, most such experts are merely the local Guru grown
old, selling dune buggy engines to the kiddies. And after all, it
does fly the plane, right?

So why even bother.

Well.... because we should. A properly built engine is more
efficient. It produces the required torque at a lower rpm and wear
increases exponentially with rpm. That means a properly built engine
uses less fuel to deliver the same thrust and last longer, too.

But a properly built engine is also a lot less expensive to build and
nowdays that's becoming a critical factor.

See that chart down there? The one title BORE VS STROKE? (It's
embedded in the article in the blog; you guys on r.a.h. will have to
go dig it out and print yourself a copy.) The chart shows the bore &
stroke combinations for most common conversions and for everything
using 88mm jugs or larger, or a 78 mm or longer crank, is going to
have to machine the crankcase & heads to match. What they'll end up
with is a dune-buggy combination -- a high-rpm engine that produces
most of its torque up high. Itty-bitty toothpick of a prop. Not very
efficient at all. Lots of machining to do. Lots of tricky bits to go
wrong during assembly... which is why some folks don't even offer the
thing assembled.

But it's all a bit of a joke because no matter HOW BIG the engine,
it's MAXIMUM SUSTAINABLE OUTPUT is going to be between 35hp and 45hp.

Yeah, I know -- everybody is selling 80hp and up. Which is a dyno
blip, not a steady output. Lotsa cubes is going to get you out of the
weeds quicker but once you get the puppy cleaned up you're flying
behind your basic 40hp engine, depending on the local atmosphere.

The limitation has to do with the heads, not the displacement. The
cylinder heads only provide enough fin-area to manage the waste-heat
from about 40hp. Unless its nice and cold or you are nice & high.
But the dune crowd only knows how to build big-bore strokers.

Now go take anohter look at that chart. Limit your jugs to the stock
85.5mm.s and your cranshaft to a 78mm. At those sizes there's NO
MACHINING REQUIRED. Your displacement is 1791cc, your maximum
SUSTAINABLE output is about 45hp and your peak torque is going to come
in at about 2800rpm.

Did I mention that no machining is required?

You've altered your cam timing but you're running a stock cam or a
Schneider 'chugger,' the one used in the orchard-blower engine.
You're running SINGLE PORT HEADS... because you're now an airplane
engine, not a hot-rod. Your Volumetric Efficiency is pushing 70% and
you're about a $1000 dollars ahead of the game because you haven't had
to buy all that machining and you're using a higher percentage of
stock, off the shelf parts. You're also running a longer, more
efficient prop -- hopefully one you've carved yourself.

The thing starts on the first flip because it has an efficient
ignition system, one that automatically adjusts itself to the load and
a 20A. electrical system. But no starter, please. As it is, it
weighs about twenty pounds less than any engine offered by anyone
else.

But of course, it's not a dune-buggy engine. And the Instant Experts
will stand in line to damn it with faint praise for that fact alone
even while it flys circles around them and is still going strong when
they're doing their second valve job of the year.

In my opinion, this is the perfect engine for an aerodynamically clean
single-seater, like Bruce Kings little beauty. Had fate dealt me a
different hand, that's what it would be going into. It would also be
a good match for a KR-1, the early Jodel, Druine, the Teenie Two and
similar designs.

Kill the parent, you got orphans. And that applies to engines, too.

-R.S.Hoover