Date: Thu, 17 Jan 2002 18:34:49 EST
Reply-To: NotaJeep@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Steven Denis <NotaJeep@AOL.COM>
Subject: Yer torquin me....<grin>
Content-Type: text/plain; charset="US-ASCII"
In a message dated 1/17/02 5:46:56 PM Eastern Standard Time, mdlind@rica.net
writes:
<< Having the same torque at the flywheel at any given road speed
would not translate into having the same torque at the wheels with
two differently geared transmissions. >>
OK I LIED..
Here's the deal..it takes "X" hp to drive the car on a level road with no
head wind..Yes?
Let's say 50 mph (breakneck in a diesel westy) takes 5 HP...IF the engine can
develop that 5 hp though a combination of available torque and rotational
speed then the van maintains speed...Now it may take ALL the torque that the
engine can produce at the "50 mph" rpm..fine..don't plan on passing..you,
friend, have hit terminal velocity under those conditons...fine..IF you are
PAST the rpm of max torque then you can gear UP and accelerate! it's known as
"torque back" (spent many an uphill mile with my eyes glued to the tach on a
220 Cummins so I knew when to shift UP to make the pass)
So if you are BELOW the rpm of max torque you need to speed up the engine
(drop a gear..or six) and get to a level of torque that is greater then the
requirement for level flight...Are we together here?..FINE..this is REALLY
REALLY noticeable on a petrol engine..the torque curve looks like a
hill..climbs and drops off steeply..the DIESEL torque curve is a slight
bowing of a horizontal line.
FINE you say..but what about my "breaker bar"?..the higher geared tranny will
cause me slower acceleration!..
Well..yes..sort of..the torque is flat, the frictional and inertial forces
are linear but the AIR DRAG goes up exponentially on that box of
yours...Sooooo..
with a torque "curve" that isn't and the low drag at low speeds and the crazy
high drag at high speeds the two cars will just shift in different places
(remember torque back??) and get to 50 mph at about he same time..really!
from 0-5 mph the "diesel" pulls ahead (3 inches) then shifts..the "gas" is
still winding up from a slightly slower start (yes, less total available
torque to the wheels) but as the diesel shifts, the gas is getting "on the
cam" and it pulls away..and this differential effect stays WITH the cars,
back and forth but..it's made almost un noticeable due to the rapidly
increasing air drag...basically they are BOTH done at about 70 mph..one is
turning 5000 rpm and is on the back side of the torque curve and the other
one is turning oh, what..4200? and it really really close to the max torque
but it's got a smaller "breaker bar" via the final drive and can't lever the
car though the air any faster....
NOW I'm done....
steve...
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