[Mgs] MGC rear axle

[Bob Howard]

Paul,
   Yes, it reduces both acceleration and top speed.  
   Carl Cedarstrand, T-series guru in California, put together a text
about ring & pinion exchanges in TD/TF cars, which are geared for
effective stump-pulling but not for 65 mph cruising.  He wrote that the
axle ratio fitted by the factory achieves maximum possible top speed, as
engine speed at which peak power occurs, 5200 rpm, is that rpm required
to drive the car at top road speed.  I have read in other places that
gearing for maximum speed was standard practice for almost everyone until
the 1970s.  Cedarstrand goes on to note that the decrease in 4th gear
hill-climbing ability will be greater than reduction in top speed or
reduction in acceleration. 
   His charts show curves for horsepower available and horsepower
required to shove a TD through the air.  Changing from original 5.125 to
4.30 reduces top speed by about 3 mph, not significant for the use most
of us give the cars.  His chart of effects on acceleration and hill
climbing shows greater changes.  Comparing the 4.3 to the original 5.125,
in 4th speed the acceleration ability is 90 % at 30 and 40 mph, 87% at 50
mph, 82% at 60mph, and 70% at 70mph.   The hill-climbing performance is
affected more. 80% at 30mph, 78% at 40 mph, 75% at 50 mph, 67% at 60 mph,
and 50% at 70 mph.
   In TDs, that first speed becomes useful, one finds that second and
third are more useful over a greater speed range, then one engages fourth
at 30 mph or so for puttering about or at 50+ for higher speed driving.  
   What are comparative advantages of changing gearing vs switching to
15" wheels on MGB?  I do not have any wheels to measure for
circumference, but the formula for working up mph/1000 rpm is     (1000
rev/mile) (60 min/hour)  /    (gear ratio) (tire revolutions per mile)
 Bob

 
 
On Thu, 15 Nov 2007 09:23:08 -0000 "Paul Hunt"
<paul.hunt1 at blueyonder.co.uk> writes:
> Wouldn't that achieve the inverse of the Holy Grail and lose out on 
> both  acceleration *and* top speed?  The ratio in 4th is 3.909:1, OD
4th 
> 3.2:1. 
> Top speed is 105mph, which means 5,800 rpm in 4th or 4,773 in OD 
> 4th.  So  the engine is already at or past its maximum gearing (mine
seems to 
> pull a  high top speed in OD 4th than 4th, but maybe that is just
because I 
> cringe  at the mechanical noises anywhere near the yellow line).
> 
> PaulH.
> 
> ----- Original Message ----- 
> > Any thoughts on fitting the 3.07 in lieu of a five speed or 
> overdrive in a  4
> > cylinder B?