"Give me a lever long enough and a fulcrum on which to place it, and I shall move the world."
Archimedes
Archimedes didn't say he could move the world quickly.
Theoretically, infinite torque can be achieved from any engine, through
infinite gearing. You can lift your car with a jack, but imagine how fast you'd
have to pump the jack to lift the car quickly.
Many people believe that torque determines acceleration,
since torque is angular force and acceleration = force / mass, in an ideal friction-less vacuum, ignoring how
much power is required to create and sustain that force. Torque, alone, doesn't get
anything done. When you apply force over a distance and get something done,
it's called work. Once you divide the amount of work by the time it took to
perform it, it's called power.
A peak torque of 300 ft-lbs
at 3000 RPM (171 hp) is not unreasonable for a Chevy 350. With a 10:1 gear reduction, this
becomes 3,000 ft-lbs at the drive axle. You can hook up bicycle pedals to your
car and gear it down 30:1 and assuming you create 100 ft-lbs at the pedals,
you'll create 3,000 ft-lbs at the axle, but can you pedal at 9,000 RPM? This
modified 1.5 liter Porsche
engine can. Horsepower
is the ultimate goal. Horsepower is determined by Hp= (Tq*RPM)/5252. 1 ft-lb of
torque at 5252 RPM is 1 Hp. So is 5252 ft-lb of torque at 1 RPM, and ½ ft-lb at
10,504 RPM. It's even simpler in SI units: One watt is equal to 1 Newton-meter
per second (Nm/s).
In the simplest terms,
burning fuel faster makes more horsepower. A larger engine burns more fuel, per
revolution. All things being equal,
a larger engine makes more horsepower. The problem, as has been noted, all
things are never equal. Engines with a smaller per
cylinder displacement,
simply rev higher to achieve the same horsepower, if it's built for it. Torque
will be lost, but gearing will correct that. Remember, horsepower is most
important. Force (torque) is an instantaneous measurement, while power is a
calculation based on how long that force is sustained.
Peak horsepower is dependent
on the "top-end" of the engine, or more specifically, how much air
flows through it. In real world engines, the Chevy 4" bore small block is a good demonstration of this. It
came from the factory in displacements of 350 ci (4"x3.48"), 327 ci (4"x3.25"),
302 ci (4"x3"). A common modification is to stroke it to 383 ci
(4.03"x3.75"). The slight bore increase is actually an insignificant
result of rebuilding the engine. With identical camshafts, heads, compression
ratios, intakes and exhausts all four of these displacements will have only
minor differences in peak horsepower, primarily due to friction differences and
different rod/stroke ratios. With identical, performance-oriented top-ends:
The 302 will make slightly
more horsepower at higher RPMs and considerably less torque that peaks at
higher RPMs, and won't be flat or wide. On a racetrack, where you can keep the
RPMs up and in the peak power range, the small increase in horsepower is an
advantage. Road racers accomplish this with close ratio standard transmissions
and changing the rear gear ratios to suit the track. Drag racers use high stall
torque converters that slip at low RPMs. Also, with less input torque for any
given horsepower level, the transmission and driveshaft can utilize smaller,
lighter components to reduce reciprocating weight, which more than compensates
for any increased friction that comes from more gear reduction. The all-out racecar
will perform better, but you really don't want to drive it to work in traffic
everyday. This is why the Chevy 302 was only produced in just enough numbers to
satisfy SCCA Trans-Am rules
for stock production 5 liter engines. Once de-stroking was allowed, they only
produced 350s, in street cars with the 4" small block.
The 383 will make slightly
less horsepower at lower RPMs, but it will make considerably more torque in the
lower RPMs and have a much wider, flatter torque curve. In a car with a stock drive
train, this approach allows you to make more power by increasing flow but
keeping the RPMs near stock. Drastic performance gains can be achieved without
drastic drive train changes or sacrificing drivability on the street. In a car
with a stock automatic transmission you can just mash'n'go! The transmission
will downshift appropriately and you'll accelerate with ease. With a stick
shift, you'll shift the same and won't have to burn out the clutch on the
launch. This is a popular modification for pickup trucks.
The reason trucks tend to
have large, high torque engines is not as simple as most people think. It's
about power band and gearing, not torque. Power is still the important factor
but large engines have wide power bands and require less gear reduction to
produce the massive torque needed to get a heavy vehicle rolling, relative to a
small engine that makes the same power. For the sake of comparison, let's say
we have two engines. They have comparable power bands but one is from 1,000 to
3,000 RPM and the other is from 7,000 to 9,000 RPM. They are both geared so
that the top RPM in first gear is 10 mph. Engine "L" (low revving)
must slip the clutch or torque converter until it's going 3.3 mph and engine
"H" must slip to 7.7 mph. Second gear is set so that the bottom RPM
is 10 mph. L will need to shift to third gear at 30 mph and H will need to
shift at 12.9 mph. L will need to shift to fourth gear at 90 mph and H
will need to shift at 16.5 mph. At this rate, H will top out tenth gear at 96 mph. These are exaggerated to
make a point, of course, but they do reflect the real reason heavy
trucks don't come with small high-revving engines.
Addendum:
I recently got hold of a dyno simulator. I ran the four engines with identical settings, except stroke of course.
CI
HP
FT/LBS
95% peak torque
306 320 @5500
354 @4000 337 @3000 - 351 @4500
27%
332 322 @5000 ≥375
~3750* 358 @2500 - 365 @4500 40%
355 321 @5000
396 @3500 378 @2000 - 391 @4000
40%
383 323 @4500 ≥418
~3250* 411 @2000 - 406 @4000 44%
CI
HP
FT/LBS
95% peak torque
306 625 @8500
475 @5500 462 @5000 - 463 @6500
18%
332 621 @7500
501 @5500 491 @5000 - 477 @6500
20%
355 614 @7500
520 @5500 499 @4500 - 507 @6000
20%
383 ≥602 ~6750* 540
@5000 530 @4500 - 517 @6000 30%
Heads: Wedge/Pocket Porting Edlebrock
Victor Jr.
Valves: 1.94"/1.50" 2.08"/1.6"
Compression: 9.0 12.5
Induction Flow: 750 cfm 1000 cfm
Manifold: Dual Plane Tunnel Ram
Exhaust: Small Tube w/mufflers Large Step-Tube
Cam:
Edlebrock Performer Crane
284-292