Geza

on the A4 board yesterday regarding torque/HP got me thinking about why oversquare* engines (larger bore than stroke) generally make more HP than undersquare engines of the same capacity.

Basically I concluded that oversquare engines (with large bore and short strokes) are naturally able to rev to higher RPMs as reciprocating masses are minimized. In addition, the large bore affords the use of larger valves which aids in high RPM cylinder filling and biases the torque curve towards higher RPMs, thus generating increased HP. On the flip side, those larger valves reduce low RPM cylinder filling and thus, low RPM torque.

The undersquare design (like Audis) generate a lot of torque down low for the opposite reasons. Smaller valves improve low RPM cylinder filling, thus combustion pressure, and that combustion pressure is applied to a longer moment arm (stroke) thus generating increased torque at low RPMs. On the flip side, the smaller valves reduce high RPM cylinder filling, thus high RPM torque.

Well engineered undersquare and oversquare engines of the same capacity will generate the same amount of maximum torque. The issue lies in at what RPM range that torque will be produced, which will translate into how much HP is produced.

* I beleive I have the undersquare and oversquare definitions right - if not, it's the opposite! :^)

Please let me know if I missed anything in the above analysis. Of course, this applies to normally aspirated engines - something few people here are concerned about. :^)

RangeR BoB

undersquare engines indeed have less breathing capacity and lower rev limits (expressed as the same piston travel in distance/time, often feet per minute, the shorter the stroke, the higher the revs for the same piston speed).

Smaller bores relative to the displacement mean more valve shrouding, but a longer dwell area to breathe air. The piston moves farther in its stroke, so there is more volume between the piston head and the cylinder head that is NOT near the exhaust valves at overlap, so less charge goes out the exhaust valves. Thus you get better cylinder filling and higher torque at low revs.

That said, undersquare engines tend to make torque at low revs because the designers make them. It is less efficient to make them high rpm screamers, so they take advantage of what is there and tune them to their strengths rather than going thru inordinate steps to cure their weaknesses.

Oversquare engines are good for raw power. Cosworth established that a 1.3:1 bore/stroke ratio was about ideal for '60s metalurgy to make good power all over, with an emphasis on high rpm power. Is it any wonder then that Ford began to go toward this ratio in both small blocks (4.00x3.00 = 302 CID, 4.36x 3.59 = 429 cid for 5.0 liter and 7.0 liter class racing) in the late 60s.

You can go TOO oversquare. The mid 80s GSXR-750 motor went up radically in bore and down in stroke seeking ultimate power, but they lost cylinder filling and torque down low. A few years later they were back to the original bore/stroke because while the peak power was higher, ridability went down.

However, technology seems to know no bounds, and current F1 3.0l V-10s are churning out 850 HP at 15000 (some say as high as 18000) rpm. Their bore to stroke? Noone will say, but rumors of 3:1 are common. Then again these engines idle at 5,000 rpm and are pushing 1300 lbs of car.

I think you are on track.

Geza

Thanks for your input!

DrHandball

Undersquare engines also produce more torque due to simple mechanical advantage of the longer conn rod.

My $0.02

Geza

I do not beleive that is true. The longer crankshaft throw (stroke) generates the increased torque.

RangeR BoB

Longer rods work to make more power because there is less thrust along the wall of the cylinder so less energy is wasted as heat dragging the piston along the wall. The piston spends more crank degrees moving up and down, and less in the crossover points where a lot of crank rotation yields little vertical movement.

Having an undersquare engine does not mean it has a longer rod than a motor with a shorter stroke. If the motor is using the same block, then the shorter stroke crank will have actually LONGER rods than the stroker motor, assuming piston pin deck height is the same.

Again I reiterate, long stroke undersquare motors make torque because it is easier to get torque out of them than revs, so motor engineers (those clever guys) tune them to take advantage of what they have rather than make themn do what they cannot.

Do or Do Not. There is no Try.

Geza

I agree...DrHandball's post referred to increased torque due to longer conn rods

J. Daniel

If what you meant to say was that;
Undersquare engines also produce more torque due to simple mechanical advantage of the longer STROKE.
This is a common misconception. Yeah, the stroke is longer, so the torque arm is longer, but for a given displacement the piston area decreases in proportion to the increase in stroke. Assuming equal combustion pressure, the force on the piston is proportional to the area. So, an undersquare engine has a longer lever arm but less force pushing on it.
So, as RangeRBoB points out, it really comes down to valve area and engine tuning.

DrHandball

I agree with you. However, whether the mechanical advantage is gained from a longer conn rod or increased crank throw, it is the mechanics of the motion that deliver the torque.

Think of it as using a cheater bar on a wrench to turn a bolt.

My $0.02

unA4dable

I think you may have missed RangeR BoB's point: for a given displacement, there is NO mechanical advantage.

For a given displacement.

As an example, let's say engine A is our standard, and we're going to make a longer-stroke version of it, called engine B. Let's say B's stroke is 10% longer than A's. In order to maintain the same displacement, B's cylinder area will be 10% less. Again, assuming the same combustion pressure (a decent first-order assumption, though in reality it gets more complicated), with 10% less area, there will be 10% less force on the connecting rod during the combustion cycle.

So we've got a longer-stroke engine B with 10% less drive force on the connecting rod than engine A. But to make engine B's stroke 10% longer, we had to increase the crankshaft displacement off-center. By how much? 10%, or course. If the piston originally travelled 10cm and we want to make it 11cm, that implies that the connecting rod connected to the crank at a point 5cm off-center, and that needs to be increased to 5.5cm. 10%.

Now, the torque is found by multiplying the connecting rod force by the off-center distance on the crankshaft (again, it's more complicated than this as we need to integrate the force over the entire combustion stroke and take crank angles into account, but you get the idea here). The end result is that engine B has 10% less force on the connecting rod, being applied at a crank radius (lever arm distance) that is 10% greater, so they've conveniently cancelled out.

That's BoB's point; that the cylinder/crank geometry offers no real mechanical advantage or disadvantage, save some secondary effects, and that it's really a matter of valves and tuning. At least, if that's not BoB's point, it's mine... :-)

-dan