The key de terming factor in the gearing of an engine is the torque
the engine produces.

Push rod V 8 engines are designed to produce high torque, partly
because the push rod design limits the design to 2 valve per cylinder.
This design, while economical, limits the rpm that the engine can
turn. They usually red line at relatively low rpm (5000 to 6000 rpm)

Modern fours, using overhead cams can uses three or four lighter
valves with better modern spring, can turn 7000 or 8000 rmp. But
since the are smaller displacement they produce much lower torque.

The formula for horsepower is:

Horsepower = torque * revs/minute * minute/60 seconds * 2*pi * 1/550
Horsepower = torque * revs/minute * 1/5252

In a very simple way:

Suppose your car requires 100 hp to move along at 90 mph.

A v-8 rated at 300 ft-lb at 2000 rpm will be producing 114. An
engineer will pick the gearing and rear end to produce the right rpm
at the wheels.

A smaller 4 cylinder engine, producing 140 ft-lbs or torque would have
to turn nearly 4300 rpm to produce the power necessary to move the
same car at the same 90 mph.

As for the gears, the turbo engine puts out 28 % more power than the
normal engine, but the full turbo pressure does not kick in until 4500
rpm. the red line is at 6500, so the maximum power band fairly
narrow. If your gears are too wide, you can fall off the power band
when you up shift, so having more gears allow you to keep the engine
turning at high rpm while you accelerate. (its also more fun)



On 20 Apr 2006 06:13:29 -0700, "laocmo" > wrote:

>I've recently acquired a 05 MazdaSpeed Turbo, 6-speed, a retirement
>gift for myself. Love it so far. It's been 40 years since as a single
>guy I drove a '64 Vette 4-speed. My driver for the last 13 years has
>been a 6-cyl '93 T-Bird auto transmission. Now the questions. At 60mph
>my Mazda is turning exactly 3000 rpm in 6th gear. My old T-Bird loafs
>along at 1750. Both have roughly the same hp. My 300 hp V-8 Corvette,
>with a 308 rear end turned about 2750 in 4th if I remember right.
>Anyone know a good source for the relationship between rpm, highway
>speed, horse power, number of gear speeds, etc. How can that old Ford
>engine cruise along at 1750 when it takes the MX-5 3000 rpm? Why do I
>need 6 gears in the Mazda when 4 worked fine in the Corvette? I
>probably knew all these answers 40 years ago, but I need a refresher.