Modern Transmissions

After yesterdays post about upgrading transmissions, I was thinking about modern transmissions.  And by that, I mean ones that have 6+ forward gears.  After all, if a 4 speed automatic is good, then an 8 speed would be twice as good, right?

As much as I love manual transmissions, I think that with more and more gears in them, we will soon hit the limit on the number of gears a person would want to deal with on a manual.  I know the new Corvette has a 7 speed manual, which actually surprised me.  I thought they would pretty much top out at 6 speeds.  Given how many people don’t even want to deal with shifting themselves, as the gear count goes up, fewer people would want to deal with it.

But back to the thought of an eight speed automatic.  At first glance, it would seem why so many?  The answer lies in the need to increase fuel economy along with more power.  As cars get heavier, due to demand for larger cars with more equipment, along with more and more safety features, cars have in general gotten larger and heavier.

Having more gears allows the engine to have a smaller RPM drop with each shift which will help the engine stay where it makes the most power, or gets the best fuel economy depending on how hard one pushes on the pedal.

Since I am a fan of Mopar cars, I looked at the gear ratios for the new 8 speed they are using in the pickups and the Charger/300 with the V6.  First, the ratio for first gear is 4.696, which is considerably deeper than even the 700-R4 with a first gear of 3.059.  That means you can use a taller rear gear and still get the same acceleration.  In fact, to have the same overall first gear ratio (transmission ration x rear gear ratio) as the 700-R4 with 4.10 gears, you would need 2.67 rear gears with the 8 speed.

Combine that with a top gear that is slightly taller, and you can see why the 8 speed looks good.  The overall top gear ratio of the 700-R4 with 4.10 gears is 2.85, and with the 8 speed with the 2.67 gears is 1.78.  That gives you a 33% reduction in engine RPM at a given speed, with the same starting gear ration.

The other advantage is that there is less of an RPM drop with each shift.  When the 700-R4 shifts from 1st to 2nd, assuming it is shifting at 5,000 RPM, the engine speed in second right after the shift is 2656 RPM.  With the 8 speed, when it shifts to 2nd, the RPM is 3332, then 3361 in 3rd, then 3962 in 4th, which is a close match to 2nd in the 700-R4.

So not only do you get a lower starting gear, you also get a closer gear spread.  That is why an automatic with 8 speeds is being made.  It allows the engine to stay where it is most efficient for acceleration, and still give a good cruising speed for highway MPG.  And that is why modern transmissions are the way they are.

That is also why the new Charger and 300C with the V6 and the 8 speed auto are faster 0-60 than the earlier ones with the 5.7 Hemi V8 and the 5 speed automatic.  I think the new transmissions are here to stay.

And as a note, the new 8 speed that Chrysler is using, is actually a ZF unit used by many companies.  The shift logic is manufacturer specific, so each one will feel different though.


Leave a Reply

Your email address will not be published. Required fields are marked *