[Diy_efi] Re: Fairlane

Wed Apr 20 12:21:16 UTC 2005

Torque is the instantaneous twisting (or moment) on a shaft or body.  Rotation of torque through an angle produces work.  Power is the rate of doing work.

50 ft.lbs on a wrench that is not moving does no work but does have stress on it due to the balanced forces on it.  A driveshaft with 50 ft.lbs on it that rotates 1 full revolution does 50*2*pi=314 ft.lbs of work (2*pi radians per revolution).  Imperial measures are poor in that the energy (314 ft.lbs) is indistinguishable from torque.  If that same shaft were spinning at 1000 RPM, you have 9.5 hp.

In SI it makes more sense.  50 ft.lb= 68 N.m of moment.  One revolution gives 68*2PI=462 N.m or Joules of energy (work done).  At 16.7 rev/s (1000 RPM) you get 7130 N.m/s (or J/s. or Watts).  7130 Watts /746 W/Hp gives 9.5 hp.

Brian
============================================================
From: "Michael Richards" <michael at fastmail.ca>
Date: 2005/04/20 Wed AM 12:33:39 EST
To: diy_efi at diy-efi.org
Subject: [Diy_efi] Re: Fairlane

> I'm no expert on the subject but as far as I understand, you "feel"
> torque whereas 400m times are relative to your horsepower and weight. 
> I recently test drove a Golf MkV 2.0 TDi which had 320nm of torque and
> only 103kw of power.  It felt really quick, quite fun to drive, yet it
> takes over 8 seconds to get to 100km/hour.  I guess it's because
> torque is instantaneous power whereas horsepower is torque over time?

This is one of the most misunderstood areas of performance. Something 
you have to realise is that torque is an instantaneous measure of force 
while horsepower is a unit of work - torque applied as the engine turns 
a specific speed.

Think about the concept this way...

You have a dumptruck engine making 400hp. That engine will put out 1500 
ft/lb of torque but only spin to about 3500RPM. Next to that you have a 
turbo honda making 500hp. Only about 350ft/lb of torque and you have to 
spin it to 10,000 RPM. Which engine could move the biggest load of dirt 
up a hill?

With the proper gearing the Honda could. But wait! It doesn't have the 
torque! That's what gears do - they multiply torque. So in actual fact 
the torque figure doesn't mean anything in your go-fast formula. You 
need something with good average horsepower between the operating range 
as you accelerate.

Even though your TDi has lots of torque it's still only making 103kw or 
about 140bhp. Gear it any way you like it's still only able to 
accelerate with the 103kw of energy.

-Michael
_________________________________________________________________
    http://fastmail.ca/ - Fast Secure Web Email for Canadians

_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://lists.diy-efi.org/mailman/listinfo/diy_efi
============================================================