Ducati 748hc
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The first
of the current Desmoquattro series was a 748, which appeared in 1986.
It was built on a 750 (748cc) crankcase and the frame was actually made
up of pieces cut from the air-cooled endurance bike frame.
Michael
Feldman already has a large displacement Ducati (M984) with a limited
RPM range. He bought the 748 because he wanted a different version of
the concept.
When it
came time to consider which modifications to make he decided that the
result should retain the same characteristics that it began with; smooth,
high reving power with the displacement at 748ccs.
The way
to accomplish that is through breathing and compression.
The breathing
part is already there, the valve spacing for all of the 4-valve engines
was first layed-out for an 88mm X 61.5mm engine.
In fact,
the 748 could take the same 37mm inlet and 31mm exhaust valve combination
that was used on the 955 Superbike engines.
There are
some practical reasons why we didnt go that large.
1)
That would require sinking the inlet valves quite a bit, which
increases the combustion chamber volume.
2)
It would require installing larger inlet valve seats.
3)
The lift of the stock cams is, really, not enough to properly utilize
a valve of that size.
Ive done
several engines with 35mm inlets and 30mm exhausts, so I have a reasonable
idea of what flow numbers to expect.
The real
problem was, how to get increased compression. Though I havent measured
the 748 SP pistons Ive seen SP2 pistons, as well as 748SP pistons and
they dont offer an appreciable increase in compression ratio. The worst
part about using the SP pistons is that, since we're using Strada cams,
we don't need the valve clearance cut-aways in the pistons.
In addition
to that, the big valve head-work would increase the combustion chamber
volume lowering the CR even more.
There is
a school of thought that holds that the output of an engine is limited
by the ports (I believe that it is fairly well accepted.) Of course, a
smaller displacement engine would have to rev higher to accomplish that,
(and that creates more friction losses.)
If that
is true, I should have expected a bit over 100HP. Even when one of my
usually accurate engine programs predicted 108 HP I wasn't sure.
The reality
was; I didn't dare predict that. Even though the compression is very good,
I couldn't make myself expect much over 100HP.
(At this point, I wont say how we got the compression, but we got it! I can't tell everything.) This was the previous comment on the subject of compression ratio.
Now, I'll divulge that I had longer rods made, which pushed the piston out of the bore by 2.25mm (1/2 the difference stroke between 916 and 748.)
When I started to look at the pistons I discovered that the ring placement of both 916 and 748 pistons was the same relative to the wrist-pin. Because of that I could safely lower the deck area of the 748 pistons to the same distance above the top ring. That allowed the me to push the piston out of the bore by 1/2 the stroke difference. That's 66 minus 61.5 (4.5mm) divided by 2, which is 2.25mm! (748 and 916 rods are the same part, therefore they are the same length.)
Then I
had to machine the piston crown and the valve cut-aways which left us
with a compression ratio around 13:1. With careful mapping of the ignition
timing we had a bike that would run on 92 octane pump gas.
The stock
piston weight is 443 grams while the pistons we used weighed 408 grams
when they were finished.
The Carrillo
rods weighed 410 grams, the stock rods weigh 512 grams.
When the
crank came back from Falicon the counter-weight O.D. was smaller than
when it left, and there was only the smallest balancing hole in it. Apparently
they had turned down the O.D. until it was very close to the correct weight
for proper balance. The result is a very smooth engine.
Now that
we had the compression we wanted, and the airflow we expected, all I had
to do was assemble it.
In the
US the only models we get have the 'Strada' cams. So, that is what we
used.
The question
of cam timing came up (when it was time to time the cams) the solution
was stock cam timing.
That doesnt
mean that I didnt check it, like it does when customers say that. It
means that the cams have been adjusted to the stock timing specifications,
which are well suited to this task.
Until I
can take the time to run further cam timing tests, the stock cam timing
figures will do just fine.
Below is
a graph after the chip development.
