<VV> Question about revs in a PG versus Manual

[Brent Covey]

Most Powerglide cars have 3.55 axles- 3.27 was standard with the 95HP 164
engine and Powerglide. Some earlies have 3.89 with Powerglide.

Cruise RPM is pretty much the same with Powerglide or manual transmissions,
the converters have very slight slip but this varies with load between about
100 rpm and 300 rpm under most conditions. As it takes around 20HP to propel
a Corvair ~60 mph, the slip is very slight on level surfaces and adds a very
small amount to cruise rpm, if a manual trans car runs 2900 rpm at 60 mph, a
Powerglide with the same ratio will run around 3000 or so, to as much as
perhaps 3300 if you really have your foot in it on a hill or into a strong
headwind. At top speed, the slip would eat about 5 mph at the same engine
RPM, however in practice the car is usually limited by aero drag before RPM
is too high and so theres usually no difference in the actual top speed
between the two transmissions, all else being equal. A few months ago
someone posted hard numbers in response to Padgetts request on this topic,
you might see if you can find that thread for more information.

If you want to estimate RPM for a Corvair with Powerglide at a given road
speed, heres the basic calculations-

Original tires roll about 840 revs a mile
60 MPH (~100 km/h) is a mile a minute
hence, 840x(axle ratio)= rpm at 60 mph.

for example, 840 x 3.55 = 2982 rpm at 60 mph in a direct drive manual trans
car. For a Powerglide, you need to add a little 'slip factor' on top of
this.

if you do 2982/60 you get 49.7 rpm, per mph road speed.
this means 100 mph would be 4970 rpm

Multiplying the rpm by the lower gear ratios gives the rpm in whichever gear
once you have the first number

For example, a Powerglide has a 1.82:1 first gear, so 60 MPH in LOW range
produces 5427 rpm- plus some slip

After uoi've come up with the road speed rpm number add 100-300 rpm for a
better estimation of the actual RPM, at 60 mph, add say 100-150 rpm, at top
speed add 300-400 rpm. In our example, a Powerglide with 3.55 would likely
run around 3100 rpm at 60 mph, and 5300-5400 rpm at 100 mph.

At engine speeds below 2000 rpm the converter may go into reduction and slip
far more, the engine can always rev to around 2000 rpm regardless of road
speed you are heavy on the gas.

Hope thats some help,
Brent Covey
Vancouver BC


> I don't have a PG so maybe a stick shift and PG owner can answer
> this. How do the revs of a PG and a manual compare at say 60-70 MPH?
> My Corsa 4 speed has a final drive of 3.55, my guess is that most
> PG's are 3.27? Is the total ratio final drive - gearbox about the
> same between both? Is there a general opinion on which one is more
> reliable?
>
> Thanks,
>
> Guus de Haan
> The Netherlands
> '65 Corsa Turbo-Charged Cvt
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