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SuprChrged '00Si
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I've heard that tuners like to have a square bore & stroke (i.e. '06 Si is 86 X 86.) Why is this a favorable condition?
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Dren
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probably has to do with piston side wall pressures and things like that. engine geometry has a lot to do with forces on various parts. that would be my guess. that is a lot of the reason why long stroke engines can't rev very high, the forces on the cylinder walls are quite high.
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bboule
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A square engine is going to be fairly "balanced" in that it should end up making similar torque & horsepower.
The reason a long stroke engine can't rev as well as a short stroke engine isn't the cylinder wall pressures it has to do with the acceleration of the piston & the forces on the connecting rod, crankpins, and crankshaft.
An engine with a 10cm stroke will have double the piston velocity of an engine with a 5cm stroke at the same engine speed.
At any given level of reliability & durability expected, there is a known piston speed which can be maintained with a given type of piston/rod/crank construction. As time goes on the maximum piston speed goes up as materials and design get better.
I am not sure why a tuner would prefer a square engine over an oversquare engine if they are just after a fast car. The RSX-S is square (86x86) for example, and the current S2000 is actually undersquare (87x90?), but take a look at the Ferrari F430... it is 92x81, take a look at sportbikes with their high rev capabilities.. they are extremely oversquare (e.x. CBR600RR - 67x42mm - revs to 15k or beyond) Take a look at an F1 car - if you could actually get an F1 team to tell you their bore & stroke it is going to be even more oversquare then the motorcycle.
The square engine should be more balanced for the street but make it oversquare for outright performance.
Note there are other issues to being able to rev high besides piston speed... such as valve issues.
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gavinjung
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oversquare designs have the ability to rev like nuts (lower piston speeds and lower reciprocating losses), but perhaps more importantly the space to have giant valves. more air = more power. if i had to guess tho, oversquare designs would have issues with piston float (if that's the right term). basically, by making the cylinders wider, the pistons have to grow wider, but they'd also have to be longer in order to stay precisely aligned within the cylinder walls. too short and there'd likely have a tendency for play and float a bit. and then, if the designers were at all concerned about fuel economy, i'm guessing oversquare wouldn't be ideal for complete burn.
gj
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fcd
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According to my engineering text under square engines have greater thermal efficiency (less combustion chamber area, more thermal efficiency). Of course they then have higher piston speeds and more piston travel meaning more friction. Over square engines have the opposite problem, less stroke, less friction, but more chamber area, so less thermal efficiency. Seems like over time a happy medium has been found at square.
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JeffX
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bboule wrote:
A square engine is going to be fairly "balanced" in that it should end up making similar torque & horsepower.
The reason a long stroke engine can't rev as well as a short stroke engine isn't the cylinder wall pressures it has to do with the acceleration of the piston & the forces on the connecting rod, crankpins, and crankshaft.
An engine with a 10cm stroke will have double the piston velocity of an engine with a 5cm stroke at the same engine speed.
At any given level of reliability & durability expected, there is a known piston speed which can be maintained with a given type of piston/rod/crank construction. As time goes on the maximum piston speed goes up as materials and design get better.
I am not sure why a tuner would prefer a square engine over an oversquare engine if they are just after a fast car. The RSX-S is square (86x86) for example, and the current S2000 is actually undersquare (87x90?), but take a look at the Ferrari F430... it is 92x81, take a look at sportbikes with their high rev capabilities.. they are extremely oversquare (e.x. CBR600RR - 67x42mm - revs to 15k or beyond) Take a look at an F1 car - if you could actually get an F1 team to tell you their bore & stroke it is going to be even more oversquare then the motorcycle.
The square engine should be more balanced for the street but make it oversquare for outright performance.
Note there are other issues to being able to rev high besides piston speed... such as valve issues.
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think about the crankshaft on a long stroke motor - the throws are much longer (and thus the crank swings wider off the center of the cylinder bore), so the connecting rods swing out at much wider angles than oversquare motors. This introduces increased side loads on the cylinder walls and though you have more leverage on the crank (torque), there are higher frictional losses because of the increased sideload on the rings AND the fact that it has to travel that much further to complete a revolution, so you run into a point of diminishing returns. Also, since the piston has to move so far to complete a full revolution, you approach the limit of where combustion energy can spin the crank fast enough to make top end power.
Now an oversquare motor can pack many more explosions (revs) per minute because the piston stroke is shorter, but then it loses the leverage on the crankshaft, so generally torque production is reduced. But at the same time, frictional losses are reduced, so power can be equivalent or superior to an undersquare motor, albeit at higher RPMS (no problem really if you have gearing to deal with it) As others have mentioned, square motors obviously ride the fence. In the real world of road cars, it seems that valvetrains are generally insufficient to rev high enough to take full advantage of an oversquare design. Undersquare designs are popular in some applications because with their tendency to produce peak output at the lower end of the RPM range, they tend to feel more "relaxed" while operating (family cars, trucks, suvs, etc...).
Oh, with oversquare (short stroke motors) also you have a limit in terms of the speed of flamefront propagation. When bores start getting really large with comparably small strokes, the combustion event takes too too long to complete (not enough time during the power stroke) so you don't extract the same energy (power) from each explosion of mixture and in fact combustion could still be taking place after the piston has reached BDC. This can obviously be mitigated (F1, superbikes) but the compromises must be too extreme for typical road cars, because to my knowledge, oversquare motors aren't all that common.
Last edited by JeffX on 12-11-2005 08:04
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Conky
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Your explanation of the physics of the question is spot on. Concise, too.
The answer, if any, could be looked at in terms of how various types of racing engines behaved in earlier days, before modern metallurgy and modeling, manufacturing quality, etc., since modern engines are pretty much racing engines. Think Northstar V8 with a wicked ECU, B16* Undersquare is OK -- desireable, if your metallurgy is sound -- in small blown engines. The late 30's to early 50's Alfa 158: DOHC, 1.5L, I8, single stage roots-type blower is the best example. If you care, here:
http://www.conceptcarz.com/vehicle/z10267/Alfa%20Romeo_Gran%20Premio%20159/default.aspx
This engine went from 200 hp @ 7K in 1938 ('scuse me. Good metallurgy.) to 420 hp in 1951 with a twin stage blower -- Type 159. Increased air pressure more than compensated for mechanical costs while providing enough torque for its purpose.
Oversquare is great with a bigger engine because the torque cost isn't as bad. Unless tiny, they can't rev as nearly as quickly, but the raw power and torque bring it big time. The 4.5L V12 Ferrari/Cunningham/Hall/Reventlow -- GT40 (NASCAR, now) engines certainly show the advantage of a big bore. Pressurizing them only makes for more power because of the swept volume.
A normally aspirated 2L I4 almost needs to be square to do, say, a 25-50% increase in power and still be able to get it off the line without destroying the clutch every year and you have to be willing to get the car to tiptoe around on the streets as well, because you have so little torque available. That being said, who cares. Either of us could have, with any luck, won the 1954 12 Hours of Sebring, listening to our iPods with the A/C on when we became uncomfortable, with a stock K20a3 -- Civic or RSX. With the '06 Si and it's sweet version of the K20 & LSD, maybe even the 1952 Italian Gran Prix -- iPod and A/C on.
Last edited by JeffX on 12-11-2005 08:07
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JeffX
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Conky wrote:
Your explanation of the physics of the question is spot on. Concise, too.
The answer, if any, could be looked at in terms of how various types of racing engines behaved in earlier days, before modern metallurgy and modeling, manufacturing quality, etc., since modern engines are pretty much racing engines. Think Northstar V8 with a wicked ECU, B16* Undersquare is OK -- desireable, if your metallurgy is sound -- in small blown engines. The late 30's to early 50's Alfa 158: DOHC, 1.5L, I8, single stage roots-type blower is the best example. If you care, here:
http://www.conceptcarz.com/vehicle/z10267/Alfa%20Romeo_Gran%20Premio%20159/default.aspx
This engine went from 200 hp @ 7K in 1938 ('scuse me. Good metallurgy.) to 420 hp in 1951 with a twin stage blower -- Type 159. Increased air pressure more than compensated for mechanical costs while providing enough torque for its purpose.
Oversquare is great with a bigger engine because the torque cost isn't as bad. Unless tiny, they can't rev as nearly as quickly, but the raw power and torque bring it big time. The 4.5L V12 Ferrari/Cunningham/Hall/Reventlow -- GT40 (NASCAR, now) engines certainly show the advantage of a big bore. Pressurizing them only makes for more power because of the swept volume.
A normally aspirated 2L I4 almost needs to be square to do, say, a 25-50% increase in power and still be able to get it off the line without destroying the clutch every year and you have to be willing to get the car to tiptoe around on the streets as well, because you have so little torque available. That being said, who cares. Either of us could have, with any luck, won the 1954 12 Hours of Sebring, listening to our iPods with the A/C on when we became uncomfortable, with a stock K20a3 -- Civic or RSX. With the '06 Si and it's sweet version of the K20 & LSD, maybe even the 1952 Italian Gran Prix -- iPod and A/C on.
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I just checked that link you provided and I think they got the bore and stroke figures wrong. 100x130 mm is pretty large, and in an 8-cylinder configuration results in a displacement of 8168cc, not 1.5L.
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