Hi!
>What would I gain if I got a transmission and converter out of a 66-68
Toronado.
>Do they all have the switch-pitch convertors and what benefits do they
give over regular.
>I assume the 3.21 ratio would be better, would this affect my speedo any?..
What you would gain is somewhat dependent on how heavy your GMC is (23 or
26'? Toad? etc) and where you operate it and at what kind of speeds.
I personally feel the factory installed 3.07 axle was selected just because
it was the only ratio GM still had in production, and is less than optimum.
The switch pitch converters were used in all 1966-'67 THM 425 transaxles
(Front wheel drive type like the GMC) - the switch pitch was discontinued
for the 1968 models. This appears to have been a cost cutting measure.
Rolls Royce bought the THM 400 (rear wheel drive version of the GMC
Transmission) for many years and they kept the switch pitch feature for
years after, so thats one vote in favour of the variable stator. I really
like the SP transmissions and have used them in several vehicles.
Obviously most GMC's still have the factory 3.07's and non SP converter and
get around fine so its certainly not a neccessary modification. However, in
probably a majority of cases the addition of either the SP converter and/or
3.21 gears would be a refinement.
For best performance on the road in a relatively heavy and unaerodynamic
vehicle its generally a good idea to select a final drive ratio that puts
the engine on its torque peak or slightly (-400rpm) under it at whatever
represents a normal cruise speed. For a stock GMC engine this is 2400-2800
rpm. Operating in this range will usually deliver the best performance on
the least fuel.
If you multiply the number of revolutions per mile your tires turn (tire
companies can give you this data, often is listed in specs on the web) by
the axle ratio, you'll get the RPM you would see at 60 MPH.
Lets say your tires spin 700 revs/mile, and you have a 3.07 axle, heres
some numbers to play with.
700 x 3.07 = 2149 rpm @ 60 mph.
2149/60 = 35.8 engine rpm per mph road speed
35.8 x 75 = 2686 rpm @ 75 mph
There is also a slight amount of slippage at the torque converter depending
upon load, between 100- 300 rpm. Its largely inconsequential for
calculation purposes as it varies a bit. You can substitute different tire
revs/mile and axle ratio figures to suit your actual situation.
In our example above its clear the original gearing is a little on the tall
side, as the engine is running quite slowly at moderate cruise speeds, but
is about right for 75+ mph cruise. Too fast for me!
GM would have recomended a 3.42 ratio in a 1973 Passenger car towing
something heavy enough that the combined weight was equal to a GMC.
To decide if a different ratio is for you, think a little about what you
find a comfortable highway speed, and the terrain and altitude you usually
use your GMC, as well as the weight of the coach ready for the road.
Do the math and see what numbers you come up with that keep your engine
speed in that 2400-2800 range in the speed ranges you typically drive and
you'll likely be very happy with the performance of your GMC. I think in
nearly every case performance would be better across the booard with at
least the 3.21, and in some cases as far as a 3.90. You'll need to know
your tire rolling revs/mile data and should add 150 rpm to the results for
engine rpm for the most accurate estimate of actual over the road results.
Regarding the switch pitch transmissions, the variable pitch stator offers
extra flexibility without any penalties in other areas. 99% of the time its
like a regular converter, but when its activated, it allows the engine
speed (and hence output) to rise several hundred RPM and also increases the
torque multiplication availible slightly. During this phase of operation,
it will also warm the oil in the transmission slightly faster than a
conventional converter, however this is a minimal concern, as the periods
its activated are usually well under a minute, and the degree of extra heat
is minimal. Because performance is so much better with the SP, the actual
period of extra load may be reduced to a point the heat effect is null.
Also, when deactivated, the converter actually runs much cooler and with
more positive drive due to a lower stall speed than the compromise
converter originally installed. The converter itself is of identical
quality to the OEM ones, but can be rebuilt to incorporate the upgrades
availible in heavy duty units availible aftermarket.
The extra flexibility is especially noticed when accelerating hard at
speeds under 40 mph, and when passing cars, it really does put some extra
zip in heavy vehicles. The first time you have to pass a truck on a two
lane, you'll never go back! They are nice, and despite dire warning os the
transmission shops which never understood them, they are exactly as
reliable as a non SP type. In the event of failure of the controls for the
SP feature, they just revert back to conventional operation. The SP feature
is usually activated about the point the secondaries open. Theres a very
refined control made by Bruce Roe on the Toronado List to operate this
feature with real sophistication also, and Caspro sells a similar unit. The
factory used just a plunger switch on the throttle linkage which works fine
too.
If you're worried about having some firebreathing high revving GMC after
the switch, keep in mind all Air Conditioned 1965-'67 Cadillacs, and
1966-'67 Olds Toronados had 3.21 axles and SP converters. Its not too wild.
Hope this is food for thought~
Brent Covey
Vancouver BC
