Rich (I believe it was Rich but my PC, in homage to the benevolent and all
powerful Gates, won't allow me to open the appropriate archive [says it's
in use] so I can't check. If I am wrong I apologize but at least I am
consistent.) is once again correct when he states that, for hydrodynamic
bearings, friction varies with speed. He states an exponential
relationship and I have no reason to doubt him. However, lubricated
sliding surfaces, by my recolection, are not speed sensitive. They may be
effected by the secondary effect of inertia from the valve train and
pistons but these effects would be at the most limited to a factor of 2
since if the inertia exceeds the strenght of the valve spring you get valve
float.
Even if all of the frictional forces do go up, if they were a limiting
factor then F1 cars wouldn't turn 15,000 rpm in order to make power. The
real thief at high rpm is pumping losses. This is governed by head,
intake, and exhaust design which, on MG's, is not exactly optimal. The
BSFC (brake specific fuel consumption) curve is relatively flat across the
rpm range but it does peak. However, that peak can be tuned to be just
about anywhere in the rev range with careful air/fuel management. For a
given bottom end, a BSFC curve that peaks at a lower rpm will peak higher
assuming that the peak is optimized. In other words, on modern cars, the
overdrive allows tuning for lower rpm BSFC peaks and thus greater fuel
economy.
If MG's run better gas mileage with OD, I would guess that it is not
because of careful design but because of a unique characteristic of or
charismatic SU carburators. Ignoring acceleration, SU's adjust fuel
delivery based on air flow only, not engine load so there is no way to lean
out the mixure at low loads and get the optimum fuel economy. In this
case, a more heavily loaded, slower turning engine would be a better match
between need and "best power" fuel mixture settings that the SU always
delivers. It would be interesting to compare the plugs from people who get
better fuel mileage with OD and those who don't and see if one group is
tuning leaner than the other.
Regards,
Bill Eastman
61 MGA Never had OD, never will. Still cruises at 70 and has a standard
bore at 85k miles.
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