<VV> 1959 Impala - Grade Retard - NO CORVAIR

[Brent Covey]

Hey Seth,

> That was needed to compensate for the Drum brakes still prevalent on all
> American big cars at the time. Downshifting - (or GR in this case) - was
needed
> to preserve the brakes on downhill highway stretches.

This brings us full circle on the government Corvairs-

Powerglide's demise was a direct outcome of the US Federal Govt purchasing
agency's (General Services Administration, I think was responsible back
then) demands that cars have certain safety features if they were to be
considered for Fleet Purchases for the government. One of these requirements
was any automatic transmission car MUST have a range for engine braking
lower than 'direct' and in Powerglides case, this meant removing the low
inhibitor that prevents low from engaging at very high speed that would
cause engine damage, until the car reaches a speed its unlikely to hurt the
engine (usually around 50mph). Some of you with 1966-up Powerglides may have
discovered this feature, locked in Low range a Powerglide will not upshift,
and low is availible at any road speed, even if it results in 9,000 rpm or
whatever.

These GSA purchasing requirements were why the '66's got outside mirrors,
backup lights, padded dashes and a bunch of  what had been '65 options like
that, as standard at the very last minute before '66 production. They
amounted to an early safety standard, in effect, before the FMVSS a couple
years later on.

Earlier Powerglides will not engage Low and in some vehicles Reverse range
if the car is rolling fast enough. Three speed automatic is the cure, the
intermediate ratio gives engine braking at any road speed, and low gear is
usually still inhibited (THM 400 wont shift to first over about 4800 rpm,
but seconds always availible). There are no real advantages of a three speed
automatic generally from a driver;s perspective particularly, performance
and economy are pretty unaffected in most cars. Powerglide effectively
amounts to a three speed minus the three speed's low gear, its sort of like
driving around in second and third gear from a modern 3 speed with low
locked out.

Any of you who had cars with the old 1956-up 4 speed Hydramatic may recall
even locked in Low, by 85 mph the car would have gone to max rpm in each
gear, shifted to the next, 1-2-3 and finally to 4th. Taking your foot off
the gas it would downshift thru the gears again back to first at high rpm.

Brent Covey
Vancouver BC