- Mar 3, 2005
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http://www.g-speed.com/pbh/bore-vs-stroke.html
Introduction
We've already discussed the engine theory around rod to stroke ratios, and this time we get even more elemental with questions about the ratio between bore and stroke.
An engine with an equal bore and stroke, say 82 mm and 82 mm, is called "square" for some obscure reason. An engine with a stroke larger than the bore is called "under square," and an engine with a bore bigger than the stroke is called "over square." Their relative values have been the subject of many a night's bench racing since the first Otto motor chuffed and puffed its way into existence.
History and Theory
Early motors were most often wildly under square, with long strokes and relatively small bores. As materials improved and the additional power from high rpm became desirable, more engines sported a larger bore.
What does it all mean? Tradition tells us that an over square engine should make more power and rev higher than an under square configuration. But, comparing different engine families is a lot like comparing apples and oranges. Sometimes the results are what you expect, and sometimes they don't come close. Consider the Super Vee engine example: When VW switched from the air-cooled Super Vees to the water cooled Rabbit-based Super Vees, they went from an over square engine to a slightly under square engine, which was capable of higher revs and made more power more dependably. So much for theory.
Comparing similar engines (for example, a big bore versus a long stroke VW) often doesn't yield much in the way of a difference. Why is that? Darrell Vittone, whose history with VW racing engines has given him plenty of insight into the more practical side of engine theory, said, "It all comes down to breathing. The ultimate limiter of engine performance, if the mechanical can take the power, is the breathing and the integrity of the valve train." On our air-cooled Super Vee example, the valve train left a lot to be desired, probably a major reason for the success of the Rabbit motors.
Breathing is the important thing, then. Over square engines have an advantage here, in theory. In a big bore engine, the edges of the valve are less obstructed by the cylinder wall. This is called "unshrouded" and helps breathing. A big bore can fit larger valves and give them more breathing room, too. The 2002 and 911 engines are good examples of over square motors which benefit from big valves.
The downsides of a big bore are flame travel and burn time, a reduction in strength, and some emissions related questions. A bigger bore is harder to ignite evenly. That's why the 911 engine with dual plus make more power than with a single-ignition setup. Physically, the bigger span of the open bore contributes less to the strength of a motor, too. Finally, a longer stroke motor seems to offer more favorable emissions conditions than a big bore motor.
Why are big bores so popular? One good reason is that it's much easier to increase the size of an engine with the bore. A stroke increase of 6.4 mm, from 86.4 to 92.8, gains only 127 cc (7 percent). But, only a 3 mm increase in bore, from 79.5 to 82.5, gives a 141 cc increase and just about the same 7 percent increase in displacement. on replaceable bore motors, like the air-cooled VW or 911, it's certainly cheaper to replace the bore and pistons than it is buy a special crank, so price is important, too.
A Matter of Evolution
Metallurgy and other technologies have improved race engines for higher rpm levels in search of more power. More often than not, that means going to shorter strokes, both to reduce the reciprocating mass (the weight the engine has to spin around) and allow wider bores for better breathing. The result is radically over square engines.
Let's look at a past and present English racing engine to show how engine design trends have changed. The current 3.5 liter Ford Formula 1 V8 has a 4.0 inch bore (about 101 mm) and a short -- ridiculously short stroke of about 2.12 inch (53 mm!!). This gives the engine the maximum allowed 3.5 liter displacement. This is an over square engine. The Ford beauty breathes through four-valve-per-cylinder heads with a pair of 40 mm intake valves for each cylinder, and it spins to incredibly high rpm -- purported to be in the 13,000 rpm range or even higher.
Compare this to the classic Jaguar 3.8 liter twin cam as fitted in the XK series Jaquars. Don't think that this wasn't a racing engine -- it was the real thing in its day, with a long string of victories that included Le Mans. The Jaguar motor has only six cylinders and slightly larger displacement than the modern Ford Formula One V8. The bore is 3.45 inch (87 mm) and the stroke is 4.17 inch (about 106 mm) or just about the opposite of the F1 V8. The Jag has an incredibly torquey stump puller of a motor, but one with a redline in the 5500 rpm range. Because the rpm is limited by the long stroke and heavy pistons adequate breathing is provided by a single inlet valve in each cylinder. Under square? Radically.
Politics, Too
Our English examples are apropos of another sub-chapter in the saga of the under square/over square squabble. Scratch an English engine from the Fifties or Sixties and you'll find a whopping long stroke underneath. Jaguar, Austin-Healy, Triumph and MG all had strokes that a tractor would be proud of. Ferraris, Porsches, Mercedes and even lowly Fiats had far more over square engines back in those days, and they revved more freely as a result. With a desire to compete in the world market, and to compete in international racing, wouldn't you think that the English would bend in the breeze of modern technology? You probably can't begin to guess the reason why they didn't.
It was pure politics. In the beginning of the motor age, the English were taxed by bore size, unlike the French who were taxed by displacement, or a mathematical "fiscal" horsepower. Some government boffin decided that by limiting bores through taxes, engines would be small. The government was playing the "let's make rules about something we don't understand" game, while the manufactures were just playing the old "dodge the taxes" game. The result was extremely torquey low revving engines, and an automotive industry that became increasing less competitive. By the 1950s the silly law was dropped, but the impact was already made.
The over square/under square question is not as simple as it seems at first glance, or as simple as it is often presented. Engine bore/stroke ratios, like most engine design criteria, are a complex issue. What does it mean to the average engine builder? For most stock and class racing, you're limited to strictly controlled modifications, and theory shouldn't affect your choices there. For street modifications, you probably don't have a wide range of choices and can't radically later the basic relationship of the engine's bore and stroke. If you are sitting down and designing an engine for yourself, you have some interesting thinking to do.
[Well, now you have some ideas about the bore/stroke relationship. That will help when I talk about the Honda "B" block motors (B16A, B17A, B18A, B18B, B18C, B20A...) which mostly share the same bore but with different strokes. Generally longer stroke motors produce more torque, has a lower rev limit, and also revs slower. Short stroke motors are the exact opposite. - Frank]
A table about bore and stroke of different engines
http://www.racewindsor.com/performancepartsplus/ratio_table.html
Some others related sites
http://en.wikipedia.org/wiki/Stroke_ratio
http://www.donsautopages.co.nz/enginespecs.htm
http://www.grc.nasa.gov/WWW/K-12/airplane/stroke.html
http://www.chevelles.com/forums/showthread.php?p=1134445#post1134445
Introduction
We've already discussed the engine theory around rod to stroke ratios, and this time we get even more elemental with questions about the ratio between bore and stroke.
An engine with an equal bore and stroke, say 82 mm and 82 mm, is called "square" for some obscure reason. An engine with a stroke larger than the bore is called "under square," and an engine with a bore bigger than the stroke is called "over square." Their relative values have been the subject of many a night's bench racing since the first Otto motor chuffed and puffed its way into existence.
History and Theory
Early motors were most often wildly under square, with long strokes and relatively small bores. As materials improved and the additional power from high rpm became desirable, more engines sported a larger bore.
What does it all mean? Tradition tells us that an over square engine should make more power and rev higher than an under square configuration. But, comparing different engine families is a lot like comparing apples and oranges. Sometimes the results are what you expect, and sometimes they don't come close. Consider the Super Vee engine example: When VW switched from the air-cooled Super Vees to the water cooled Rabbit-based Super Vees, they went from an over square engine to a slightly under square engine, which was capable of higher revs and made more power more dependably. So much for theory.
Comparing similar engines (for example, a big bore versus a long stroke VW) often doesn't yield much in the way of a difference. Why is that? Darrell Vittone, whose history with VW racing engines has given him plenty of insight into the more practical side of engine theory, said, "It all comes down to breathing. The ultimate limiter of engine performance, if the mechanical can take the power, is the breathing and the integrity of the valve train." On our air-cooled Super Vee example, the valve train left a lot to be desired, probably a major reason for the success of the Rabbit motors.
Breathing is the important thing, then. Over square engines have an advantage here, in theory. In a big bore engine, the edges of the valve are less obstructed by the cylinder wall. This is called "unshrouded" and helps breathing. A big bore can fit larger valves and give them more breathing room, too. The 2002 and 911 engines are good examples of over square motors which benefit from big valves.
The downsides of a big bore are flame travel and burn time, a reduction in strength, and some emissions related questions. A bigger bore is harder to ignite evenly. That's why the 911 engine with dual plus make more power than with a single-ignition setup. Physically, the bigger span of the open bore contributes less to the strength of a motor, too. Finally, a longer stroke motor seems to offer more favorable emissions conditions than a big bore motor.
Why are big bores so popular? One good reason is that it's much easier to increase the size of an engine with the bore. A stroke increase of 6.4 mm, from 86.4 to 92.8, gains only 127 cc (7 percent). But, only a 3 mm increase in bore, from 79.5 to 82.5, gives a 141 cc increase and just about the same 7 percent increase in displacement. on replaceable bore motors, like the air-cooled VW or 911, it's certainly cheaper to replace the bore and pistons than it is buy a special crank, so price is important, too.
A Matter of Evolution
Metallurgy and other technologies have improved race engines for higher rpm levels in search of more power. More often than not, that means going to shorter strokes, both to reduce the reciprocating mass (the weight the engine has to spin around) and allow wider bores for better breathing. The result is radically over square engines.
Let's look at a past and present English racing engine to show how engine design trends have changed. The current 3.5 liter Ford Formula 1 V8 has a 4.0 inch bore (about 101 mm) and a short -- ridiculously short stroke of about 2.12 inch (53 mm!!). This gives the engine the maximum allowed 3.5 liter displacement. This is an over square engine. The Ford beauty breathes through four-valve-per-cylinder heads with a pair of 40 mm intake valves for each cylinder, and it spins to incredibly high rpm -- purported to be in the 13,000 rpm range or even higher.
Compare this to the classic Jaguar 3.8 liter twin cam as fitted in the XK series Jaquars. Don't think that this wasn't a racing engine -- it was the real thing in its day, with a long string of victories that included Le Mans. The Jaguar motor has only six cylinders and slightly larger displacement than the modern Ford Formula One V8. The bore is 3.45 inch (87 mm) and the stroke is 4.17 inch (about 106 mm) or just about the opposite of the F1 V8. The Jag has an incredibly torquey stump puller of a motor, but one with a redline in the 5500 rpm range. Because the rpm is limited by the long stroke and heavy pistons adequate breathing is provided by a single inlet valve in each cylinder. Under square? Radically.
Politics, Too
Our English examples are apropos of another sub-chapter in the saga of the under square/over square squabble. Scratch an English engine from the Fifties or Sixties and you'll find a whopping long stroke underneath. Jaguar, Austin-Healy, Triumph and MG all had strokes that a tractor would be proud of. Ferraris, Porsches, Mercedes and even lowly Fiats had far more over square engines back in those days, and they revved more freely as a result. With a desire to compete in the world market, and to compete in international racing, wouldn't you think that the English would bend in the breeze of modern technology? You probably can't begin to guess the reason why they didn't.
It was pure politics. In the beginning of the motor age, the English were taxed by bore size, unlike the French who were taxed by displacement, or a mathematical "fiscal" horsepower. Some government boffin decided that by limiting bores through taxes, engines would be small. The government was playing the "let's make rules about something we don't understand" game, while the manufactures were just playing the old "dodge the taxes" game. The result was extremely torquey low revving engines, and an automotive industry that became increasing less competitive. By the 1950s the silly law was dropped, but the impact was already made.
The over square/under square question is not as simple as it seems at first glance, or as simple as it is often presented. Engine bore/stroke ratios, like most engine design criteria, are a complex issue. What does it mean to the average engine builder? For most stock and class racing, you're limited to strictly controlled modifications, and theory shouldn't affect your choices there. For street modifications, you probably don't have a wide range of choices and can't radically later the basic relationship of the engine's bore and stroke. If you are sitting down and designing an engine for yourself, you have some interesting thinking to do.
[Well, now you have some ideas about the bore/stroke relationship. That will help when I talk about the Honda "B" block motors (B16A, B17A, B18A, B18B, B18C, B20A...) which mostly share the same bore but with different strokes. Generally longer stroke motors produce more torque, has a lower rev limit, and also revs slower. Short stroke motors are the exact opposite. - Frank]
A table about bore and stroke of different engines
http://www.racewindsor.com/performancepartsplus/ratio_table.html
Some others related sites
http://en.wikipedia.org/wiki/Stroke_ratio
http://www.donsautopages.co.nz/enginespecs.htm
http://www.grc.nasa.gov/WWW/K-12/airplane/stroke.html
http://www.chevelles.com/forums/showthread.php?p=1134445#post1134445
