As I wrote earlier, top speed is lower in OD due to rpm being knocked down
on the HP curve.
Jarrid has correctly restated this torque vs HP vs RPM problem.
The OD is a device which changes the usual direct drive (1:1 ratio) of the
engine and trans high gear into an "overdriving" ratio where the engine
turns at say 1000rpm but instead of the trans output shaft also being at
1000rpm it is overdriver to say 1200 rpm.
Most cars have a 15 to 18% overdriving ratio which means that at any road
speed, your engine will be turning 15-18% slower than for the same road
speed without OD. Actually it is more complex than what I just wrote
because the mfr. puts in a rear axle gear ratio for non OD cars which
represent a good compromise for accel. and fuel economy and engine rpm.
Usually, this std. gear ratio also allows the car to reach it's top
potential speed or very close to it because this ratio allows the engine to
rev up to the rpm at which max HP is developed.
When putting in an OD, the mfr. installs a rear axle ratio which allows
higher rpm in 1,2,3,4 gears (than the std car would have) but then when the
OD is engaged the final gear ratio is actually a lower ratio than the std
car is running.
Examples: Non OD car has 3.89:1 final gear ratio and turns say 20 mph per
1000 engine rpm.
An OD car has a 4.22:1 final gear ratio and when running in 4th gear,
locked OUT of OD it gets 17mph per 1000 rpm.
This same OD car when IN 4th gear OD has about a 3.39:1 final drive and
will get about 24 mph per 1000 rpm.
The slower engine rpm of course, can reduce wear on the engine unit and
usually (not always) gets you better fuel economy. The lower engine rpm
does also make the car quieter at speed and perhaps the nicer advantage of
the OD is that you have a 4.22 ratio for really spirited mid range accel
through the gears and yet you can go into the 3.39 ratio for cruising if desired.
On the negative side....the added weight of the OD and the potential for
breakage and costly repairs. Parts are becomming hard to obtain and will
not likely get easier in the future.
I used my various SV cars for cross country touring for years and since I
did not have OD on any car I held on to, I used to install slightly taller
tires and obtain a bit of OD "affect". For this, I used custom alloy
wheels which were properly wider than stock steel wheels and these wheels
would give normal support to taller wider tires and preserve cornering qualities.
Dick T.
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