Displacement vs. Numer of Cylinders
04-14-2008, 08:51 AM
#32
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Apples and oranges.
When you could get the air in with a turbo (and play with the fuel chemistry) , they had 4s. The 6/12's have very long crankshafts and get amazing harmonics at high rpm. If you have two banks, you can manage the naturally aspirated air better.
When you could get the air in with a turbo (and play with the fuel chemistry) , they had 4s. The 6/12's have very long crankshafts and get amazing harmonics at high rpm. If you have two banks, you can manage the naturally aspirated air better.
04-14-2008, 10:33 AM
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RED MX5, I'll definitely let you know what I think about the book. I love these intense mechanical discussions! Engineering FTW!!! Anyways I posted the same question on k20a.org and someone replied that Ducati did a study like this. I tried to do a Google search but couldnt come up with anything. Im sure engineers for car manufactures test engine design all the time but they wont let us have their results. 
04-14-2008, 12:38 PM
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Do a little research on Honda's oval-piston NR750 superbike. Racing class rules limited engine to 4 cylinders. They figured an 8 cylinder had more power potential. So they built an 8 cylinder engine, then cut out the metal between adjacent cylinders, thus leaving 4 oval-shaped holes to fill with weird pistons.
This is only one example, but I remember reading that the 8 cylinder had more power potential than the 4. Probably because, for a given displacement, more cylinders equals smaller, lighter pistons and shorter stroke. Both of these lead to higher rpm potential.
This is only one example, but I remember reading that the 8 cylinder had more power potential than the 4. Probably because, for a given displacement, more cylinders equals smaller, lighter pistons and shorter stroke. Both of these lead to higher rpm potential.
04-14-2008, 01:05 PM
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04-14-2008, 01:29 PM
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Originally Posted by snakeeater,Apr 14 2008, 03:38 PM
Do a little research on Honda's oval-piston NR750 superbike. Racing class rules limited engine to 4 cylinders. They figured an 8 cylinder had more power potential. So they built an 8 cylinder engine, then cut out the metal between adjacent cylinders, thus leaving 4 oval-shaped holes to fill with weird pistons.
This is only one example, but I remember reading that the 8 cylinder had more power potential than the 4. Probably because, for a given displacement, more cylinders equals smaller, lighter pistons and shorter stroke. Both of these lead to higher rpm potential.
This is only one example, but I remember reading that the 8 cylinder had more power potential than the 4. Probably because, for a given displacement, more cylinders equals smaller, lighter pistons and shorter stroke. Both of these lead to higher rpm potential.
I remember the engine, but didn't follow it's racing. How did the bike do?
04-14-2008, 05:38 PM
#38
Torque is largely a function of displacement. A given displacement will have approximately the same amount of torque whether 4, 6, 8, 10 or 12 cylinders. Maybe a slight advantage to having more cylinders because you can run a higher compression ratio with smaller combustion chambers.
Power is a function of torque and how high you can rev (and how well you can breathe at high revs).
More cylinders with the same displacement will be able to rev higher and breathe better => power advantage. But there is a fuel economy penalty.
INTJ is *almost* on the right track, but somehow concludes that race teams have always reduced cylinder count (?).
In fact, in F1 the "optimum" cylinder count proved to be 10 cylinders. 12's made more power, but 8's got better fuel economy. Way way back in the early 90s, before refuelling and when cylinder count was OPEN (F1 rules really suck these days), Ford V8's competed with Renault V10's and Ferrari and Honda V12s. Each sounded very different, AWESOME sounds! Anyway, the V12's suffered a weight penalty early in the race because they had to carry more fuel. But they made more power and could often run more downforce. In the end, for F1 rules, V10's were found to be the best compromise. They certainly didn't CHOOSE to go to V8's.
Power is a function of torque and how high you can rev (and how well you can breathe at high revs).
More cylinders with the same displacement will be able to rev higher and breathe better => power advantage. But there is a fuel economy penalty.
INTJ is *almost* on the right track, but somehow concludes that race teams have always reduced cylinder count (?).
In fact, in F1 the "optimum" cylinder count proved to be 10 cylinders. 12's made more power, but 8's got better fuel economy. Way way back in the early 90s, before refuelling and when cylinder count was OPEN (F1 rules really suck these days), Ford V8's competed with Renault V10's and Ferrari and Honda V12s. Each sounded very different, AWESOME sounds! Anyway, the V12's suffered a weight penalty early in the race because they had to carry more fuel. But they made more power and could often run more downforce. In the end, for F1 rules, V10's were found to be the best compromise. They certainly didn't CHOOSE to go to V8's.
04-14-2008, 06:13 PM
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Originally Posted by ZDan,Apr 14 2008, 08:38 PM
Torque is largely a function of displacement.
, but torque is also a function of many other factors, including the stroke (all else equal a longer stroke means more torque), rod length, and so on. A longer stroke gives the piston more leverage at the crank, giving the cylinder pressure (BMEP) a longer effective lever to push against. A shorter stroke has shorter crank throws so there is less leverage.Breathing capacity is another issue, related to displacement. Big cylinders can hold more air, but a big piston with tiny valves could pump less air/charge than a smaller piston with much larger valves. The bigger the charge, the greater the BMEP, and the greater the BMEP the greater the torque.
You can also get torque using forced induction on a smaller engine, or by injecting nitrous and extra fuel. Displacement is a major factor, but not the only factor by a long shot, and not necessarily the deciding factor.
As an example, somebody already mentioned comparing a 1000CC twin to a 1000cc I-4 (motorcycle) engine. Same displacement, but the twin, with it's longer stroke and other differences, will almost always produce greater torque, but less power.