This project was a
bit of a mission . The goal was to extract as much power as possible
without resorting to expensive cams and big bore kits .
I do not really like
the long duration 748sps cams and the type of power curve they make. If
i had an unlimited budget to work with i would have installed cams from
a 748R or maybe 996sps intake and exhaust . Result being more mid range
and a nicer power curve (not sure about top end now though) as
this combination has worked well.
The first problem was
cylinder head flow . The combustion chamber on 748 biposto and SPS bikes
need quite an amount of material removed from the combustion
chambers in specific places in order to make the head flow well . The
problem is retaining the compression and needing it as compression also
makes power . A high flow head with no compression would not be worth
doing . What's the point having plenty of charge if we cant compress it and
have a high overall volumetric efficiency. If we were fitting 853cc
pistons (a big bore kit) there would be no issue as the compression
would then be high enough to take some away without hurting power .
After thinking about it i came up with a plan using stock pistons from a 748R
. They have a very high dome and also have deep pockets which
accommodate the sps cams without any further machining .Anyhow to use these
pistons i had to machine the combustion chamber quite a bunch in order to
make them fit . The overall result is a piston with a high dome which
replaced the material i stole from the chamber to make the heads flow
Here are some
pictures of the head work. The 2 outside heads in the first picture are from this engine and
the one in the middle still being from a 748 has been re-worked for an
853 kit .Can you see the differences ?
Asides from the
chamber work the ports have been reshaped , The seats have been re-cut
with 3 different angles and to the valves true diameter ( see lap line on
outer edges of valve in pic 2) This makes the head think it has a larger valve
fitted . The factory's valve job does not quite cut it . The stock
valves have been polished and modified and lastly snug as a glove
fitting colsibro guides were installed and honed to size. Overall making
a reliable and an outstanding head .
The next issues were
with the con rods . The 748sps con rods do not fit inside the 748r
piston . The R piston needs a small end guided con rod to work such as
those from the 996/998 family. I had 2 options either machine the
piston to take the original rods or fit small end guided rods. In
the end some polished 999 items were used . The crank was re-balanced ,
gearbox shimmed, squish set, cylinders honed , ring gap set , heads
fitted and cams degree'd to book specs .
Here are the dyno
results . First up (lower line ) a stock 748sps fitted with an open pipe
and twin cylinder mapping with a powercommander 3. Next up is our
hi-flow head /R piston version also with a 45mm open pipe and again twin
cylinder mapping with the powercomander 3.
The results have led
me to believe with the very well ported heads and the further addition of
an 853cc kit there is hope for 125-126hp thus not being that far behind
the 853R's in the other reports !
Quite a boost indeed
. 12 - 13 hp is pretty good especially to the upper midrange and top end . All the power is in the head !