Jay; I think you pretty well have a handle on this OD stuff, between
my mail and Jarrids.
Where you might have gotten a little confused is the "gearbox" ratios
(not the OD ratio).
To perhaps make a final clarification I offer the following.
The 4 speed gearbox is direct drive in top gear (4th) as are
virtually all other makes of manual gearboxes in their top gear.
Direct drive is another way of saying the input rpm from the engine
is the same as the output shaft rpm when in top gear (4th). This
equalized rpm is spoken and written as being 1:1 ratio. What went in
is exactly what came out!
When an OD is added behind the 4 speed gearbox, the 1:1 ratio is
unchanged as the power passes through the "unengaged" OD. In other
words, when the OD is NOT working, the input/output rpms remain at
1:1 ratio.
When the OD is "engaged" the gears of the OD change the input rpm
from the gearbox to a slightly higher rpm by a process called
"overdriving". As an example, let us say the engine is at 3000 rpm
and you are in 4th gear with OD not engaged. The trans front shaft
(in the clutch) is spinning at 3000rpm and due to being in 4th gear,
the trans. output shaft is also turning at 3000rpm (1:1 ratio). This
3000rpm is fed into the unengaged OD unit at passes through the OD
unchanged ! The drive shaft connected to the OD tail shaft will be
spinning at 3000rpm.
Now the driver engages the OD and KEEPS the engine at 3000rpm. The
OD feels the same 3000rpm input from the gearbox, as before but now
due to certain gears being engaged, the OD "overdrives" the internal
shafts to that the output shaft connected to the drive shaft, is now
spinning about 20% faster than the 3000rpm of the engine. Of course
if the driver keeps the engine at the original 3000rpm the car will
increase its mph by the same 20%. If the driver chooses to not
change his mph, he can lower the engine rpm by that same 20% and
maintain the original mph.
So you see, the OD allows the engine to be slowed down about 20% at
highway speeds.
For the reasons mentioned in prior postings, it is NOT a good idea to
use this OD to lower rpm too low and overstrain the engine. OD is
best used at speeds over say 60mph on a mostly flat terrain.
When OD is used under the "intended" conditions of high road speed,
flat land no big hills, the carb throttles can be kept fairly closed
and high manifold vacuum results in extra fuel economy !
Contd. on next message. Dick T.
This archive was generated by hypermail 2b30 : Tue Sep 05 2000 - 09:56:34 CDT