Rolls Royce Turbo Jet Engine for Blue Birds

[Jeff Miller]

On the way up, it's about power/weight ratio (comparing different
year 'Birds). My '77 FC31 weighed in around 23,000lbs with all tanks
full. With several engine upgrades, and 4.63:1 gears, it would run
well on the flats at 75mph or better. Slight inclines would slow it
down, but doing some simple calculations showed that it added only
minutes to each day's drive to slow a few MPH on the climb.

Coming down the hills was different. In a diesel rig, you CAN NOT use
the brakes to control speed. If you're having to use the brakes to
keep your speed down, you need to slow down and downshift. This makes
the retarder a huge advantage plus the safety of being able to slow
down without using the brakes as the grade changes. This is why Mr.
Cummins invented his Jacobs Brake in the (?) '30s? I believe it was a
runaway when he was driving into Wolf Creek Pass or something like
that, it's been a year since I've heard the story.

On a NA engine you lose 3% of your horsepower for every 1,000' above
sea-level. At 10,000' altitude, you have likely lost about 63hp, or
have around 147hp remaining. Since much of the horsepower is used for
frictional losses in the drivetrain, engine accessories, rolling
resistance, aerodynamic drag, ... this probably leaves enough power
to run around 50mph on the flats and nothing for climbing until you
gear down, and down. With our '77 again as an example I was able to
climb over 10,000', but with much black smoke which had to be
regulated by lifting the throttle so bicycles could see to pass ;~)
The turbo burns much cleaner at altitude and maintains most of its
rated power up there.

Our '77 was 210hp originally, and 23,000lbs. Our '84 was 250hp, and
33,000lbs. The '82-up coaches are heavy and need the turbo. Still,
look at the numbers, the '77 was 109.5lbs per horsepower, the '84 is
a much lower-performing 132lbs per horsepower. This gave my '77 a
performance advantage at low altitudes over the much heavier '80s
models even with the Na engine. At 10,000' however, the '77 would
have to pull 156.5lbs with each horsepower, so the advantage of the
turbo is back.

Our '88 FC was more powerful still, with the 300hp and around
33,000lbs, its power/weight ratio is around 110lbs/hp so almost
identical to the '77 (at sea level). The overdrive transmission and
high-altitude performance of the turbo (and retarder) make it a
rocket in comparison though.

The calculations are a little more complex than all of this of
course, but the basic idea is there and FCs pretty much have the same
aero-drag regardless of the year. I don't know if Ernie has retained
the hp & climbing calculation from the old forum, if not I can re-
submit it.

Turbos are good, very good. The low-boost on the 3208 road engines is
pretty much not going to hurt a well-maintained engine, not like it
does in the 380-435hp marine versions. The engine simply doesn't wear
out very often, it is more likely to suffer a cooling system or other
failure that damages the engine. Retarders are also very useful, and
a safety feature also. I've seen what happens when an FC driver over
drives his brakes with no retarder on a grade, it's hard on the coach
even when he's lucky (this one was).

- Jeff Miller
in Holland, MI


--- In WanderlodgeForum@yahoogroups.com, "davidkerryedwards"
wrote:
>
> Just crunched the numbers. Comparing a 210hp NA with a 250hp T, if
> there is a 30% loss on the NA, and none on the T, there is a 70%
> horsepower advantage with the T at altitude.
>
>
>
> --- In WanderlodgeForum@yahoogroups.com, "davidkerryedwards"
> wrote:
> >
> > The main advantage I see in a turbo is maintaining sea-level
> > horsepower at altitude. Here in Colorado an NA engine looses
about
> > 30% of its horsepower going over the passes, strictly as a result
of
> > the thin air. A turbo overcomes most of that loss. At lower
> > elevations, the advantage is much less significant in my opinion.
> >
> > Kerry
> > Denver
> >
>