Hi folks, just ran across some intriguing information about the WW II
Allison V-12 airplane engine on the "Unlimited Excitement" hydroplane
website(check it out, extremely good) which reminded me that these are
legal for SCTA racing in some classes. Over 70,000 of them were made
between '31 and '48 so there must be a few around somewhere; not the
sort of thing you put out for trash collection.
The Allison and it's contemporary Rolls-Royce/Packard Merlin apparently
were copies of an earlier Curtiss V-12, and were very similar. Biggest
problem for our purposes is the weight around 1600 lbs, as much as
Jack's whole Nebulous Theorum.
The last batch of 750 fully-developed Allisons produced 2,250 hp @ 3,200
rpm at "War Emergency Rating" (ten minutes max duration, I think, and
then this meant an engine swap and teardown) with 35 psi manifold
pressure from the gear-driven centrifugal blower, super-high octane
leaded avgas, plus a spray of 50/50 water-methanol "anti-detonation
injection".
The Allison had a 5.5" bore and 6" stroke and at 3200 rpm, a piston
speed of 3200 fpm. The rpm was allowed to rise to 3500 in a dive.
An experimental version with turbo-compounding(exhaust discharging into
a turbine geared to the crankshaft before release to the atmosphere) was
tested at 2980 hp, but the temperature in the turbine receiving the
exhaust gas could not be kept down to a safe level so the experiment was
not pursued for production.
Some hydroplane teams claimed to get 4000 hp from these engines, but one
would wonder how, nitromethane perhaps? They'd have to convince me this
was more than rhetoric intended to scare their competitors, since
Allison already developed the unit to its limit of mechanical
reliability.
Just a few technical tidbits of this 1710 cube monster, that show it was
really just a big brother to the engines we run at speed trials. The
Allison cylinder head had four valves in a pent-roof form, with 45
degrees between the intake and exhaust valve stems, SOHC with roller
rocker arms, and two plugs, one on each side under and between the
valves. A tidy design. Two Bendix-Scintilla mags fired all 24 plugs
through two separate distributors rotating at half crank speed, with
fixed timing of 28 degrees for the intake side plugs, and 34 for the
exhaust side(don't ask me why the difference in spark lead). To start
this monster, which was hard to rotate, they didn't try to crank it with
the full-advance mag sparks to fight(besides, the mag spark was not much
at very low cranking speed), instead, a coil ignition setup fired sparks
for the intake plugs only, about 30 degrees retarded. I gather the
pilot would flip on the mags when the thing came to life.
Valve timing was mild Isky specs, 48-62 intake and 76-26 exhaust, and
lift at all valves .533" with .015"/.020" valve lash intake/exhaust.
In the latest issue of the journal of the Aircraft Engine Historical
Society, P.A. Miller did some calculations of the cylinder pressure a
little after top center in one of these engines running full power at 35
psi manifold pressure, which he gives as 2,250 psi. The engines ran wet
cylinder liners, with .188" thick walls, and he computes that the liners
would 'bulge' outwards .0072" at full power, which with high hours of
use sometimes led to breakage up near their joint with the head, as one
of the fatigue failure modes of this type of engine(as did the similar
Merlin).
Just one more little item, the specified gap for the spark plugs of
these monsters was .012/.015". Too bad they didn't have any MSD in
those days! I'll stop with this now, don't know how many characters the
List allows for dissertations! Bill
/// unsubscribe/change address requests to majordomo@autox.team.net or try
/// http://www.team.net/mailman/listinfo
/// Archives at http://www.team.net/archive/land-speed
/// what is needed. It isn't that difficult, folks.
|