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# Re: 4wd Mechanics

```Glen,

>  Horsepower = torque x RPM. What this means in the
>  real world is that an engine might develop say X
>  torque. If said engine produces X torque over a
>  certain RPM range then HP is highest at the top
>  end of said RPM range,

OK so far

>BUT, acceleration is no
>  better at the top of the range Vs the bottom, in
>  spite of the fact that the HP number is larger at
>  the top of said RPM range.

Not too sure about this.

>So, the same rate
>  of acceleration is acheived at any point in the
>  RPM range where torque remains the same.
>  Since HP = torque x RPM, sacrifices in torque
>  to achieve a higher peak HP number for marketing
>  purposes (engineering for specmanship) will not
>  guarantee faster acceleration and higher performance,
>  but certainly make for good ad copy for the uninformed.

Just as engine HP = engine torque x engine RPM, the torque applied to the
rear wheels, or axle torque = axle HP / axle RPM.  The axle torque
determines the force applied through the tires to accelerate the car.  The
axle RPM is subject to the transmission ratio and final drive ratio.  So,
if you have regeared the car to take advantage of the higher RPM horspower,
it will accelerate faster.  This is why race cars with very high RPM
engines and very high power go like h**l (see previous track).

>  High torque at usable RPM is what you want for good
>  performance.

Very true in a road car.

>Don't get carried away by the big
>  HP numbers hype.  An engine with pitiful torque but
>  capable of VERY HIGH RPM could produce an
>  impressive HP number but be a total slug when
>  trying to accelerate the mass of a real car.Torque
>  moves the car, not HP. Final drive ratios, transmission
>  ratios, the number of gears and tire size all conspire to
>  greatly complicate the overall system.
>

I definitely agree that modifying a car to lose torque but add high RPM
power is a net loss in a normal road car.

Richard Funnell,
San Jose, California
'83 urQ

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