[Updated on: Wed, 14 April 2004 02:45]

ok...
hmmmmm..

as metallurgists always say "it depends"

there are a couple of things to understand when it comes to this.
i am going to assume you know how piston and crank are connected and related?

startign with the crank. the length of the stroke does more than just increase capacity. when the piston is halfway up the bore, the big end is on the same horizontal plane as the crank centreline, and the distance between them is the maximum moment arm of the conrod onto the crank.
________|Rod_|
ie_______|____|_____crank
________|____|________________
________/____|________________\
________|_*__|______________*_|
________\____|________________/
__________|<----------------------->|
_______________moment arm

if you increase the moment arm of the rod acting on the crank, even with the same force pusing on the rod, you will increase torque, from torque = force x distance
TQ = F x d
so if you increase the stroke of a motor (but somehow keep the force pushing on the rod equal) you will have more torque.


now for the the piston (bore), and a set stroke (ie set crank radius/diameter). for a given setup, ie NA, or a set boost level, and assuming perfect tuning and timing etc, the combustion pressure can be assumed to be fixed.
this means that the force per unit area (ie N/cm^2, or lbs/in^2 (Psi)) will also be fixed, since it is a gas pressure working against a solid surface. F = pressure x area, F = P x A

if you increases the area of this surface, ie increase the bore size, then although the force per unit area stays the same, the area increases, so the actual force of the piston onto the rod increases.

now you put them together, TQ = P x A x d

where TQ = torque at crank,
P = combustion pressure
A = area of piston = Pi x (bore/2)^2
d = crank radius = (stroke/2)

SO.....

Torque = combustion pressure x Pi x (stroke/2) x (bore/2)^2
or
Torque = combustion pressure x (Pi/8) x stroke x bore^2 (A)

as you can see, increasing either bore or stroke will increase torque.

thats about as basic a scientific way to explain it. stroke inscreases carnk moment arm, bore increases force on rod..

it gets difficult when comparing same capacity but different dimensions, BUT from equation (A), you can see that increasing bore has a larger effect (since it is squared), compared to stroke, depends on the tradeoff between bore and stroke.

we can do another substitution,
where capacity = stroke x Pi x (bore/2)^2
ie stroke = capacity x 4 / (Pi x (bore^2))
and bore^2 = capacity x 4 / (Pi x stroke)

so (A) becomes (with either substitution)

Torque = (capacity x combustion pressure) / 2 (A)

and so you can easily see that, regardless of the bore and stroke combination, torque is directly proportional to capacity, so increasing capacity will increase torque.

you will also see that increasing combustion pressure (using boost, CR etc) will also increase torque in direct proportions.

you can use a very basic estimation when working out how much torque you can get from a motor.

if it is 'well developed' but not 'race developed' you can expect a maximum of 100Nm/L per atmosphere of manifold absolute pressure.

this means that for NA, manifold pressure is 1atm and so you get 100Nm/L
for a turbo motor with 1 bar boost over atmospheric, the manifold pressure is 2 bar, and so you should expect 200Nm/L

this gets a little complicated when you think about rod length and timing etc, since most work is done when the piston is near TDC, and maybe the first 60 degrees of crank movement?? by the time the crank has turned 90 degrees, the pressure in the cylinder is way down, and does little work.

increasing rod length, with a given capacity, will increase torque apparently, by increasing the time that the piston is near TDC (so the force on the rod is still high), but the crank is turning, so you get slightly more force at a given crank angle, increasing torque...


phew..
hope this helps... i'm going back to my thesis
Cya, Stewart

[Updated on: Wed, 14 April 2004 03:18]