WTF HP PER CYLINDER
10-14-2007, 10:55 PM
#1
2nd Gear Member
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Join Date: Aug 2005
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Posts: 366
WTF HP PER CYLINDER
i got into an argument with some friends today about the performance of their cars, one guy has a 1992 integra with a GSR and he claims its modified with intake exhaust and chip and making 190 HP, other guy has a 1999 318TI superchager exhaust, he dyned his car and made about 218 hp on the engine. now the guy with the BMW i have beaten him with my jeep grand cherokee with a homebuild intake and a muffler, making around 275 HP. now i know my mustang is faster then my jeep, but these kids are saying that since i have a low HP/ cylinder number that i have a slow *** car.
what the hell does HP to cylinder measure, IMO this has d!ick all to do with performance but i would like to hear what you guys think.
also my jeep will run high 14's like 14.8 or .9 and i beat my buddys 3000 pound superchaged BMW, just wondering what minestang could do, 88 with CAI, headers, catted H pipe, bumber timing to 15 degrees and 3:73 and fresh tires the grab well only 245 though. i did dyno my car and i made about 218 hp 280 tq to the rear wheels so about 250 HP 320 TQ on the engine.
i know my stang is slow especially compared to my old mans CL55 AMG or his SRT 8, but compared to these cars how to you think i could do?
what the hell does HP to cylinder measure, IMO this has d!ick all to do with performance but i would like to hear what you guys think.
also my jeep will run high 14's like 14.8 or .9 and i beat my buddys 3000 pound superchaged BMW, just wondering what minestang could do, 88 with CAI, headers, catted H pipe, bumber timing to 15 degrees and 3:73 and fresh tires the grab well only 245 though. i did dyno my car and i made about 218 hp 280 tq to the rear wheels so about 250 HP 320 TQ on the engine.
i know my stang is slow especially compared to my old mans CL55 AMG or his SRT 8, but compared to these cars how to you think i could do?
10-15-2007, 12:12 AM
#5
6th Gear Member
Join Date: Apr 2007
Location: California
Posts: 10,468
RE: WTF HP PER CYLINDER
HP/cyl is rather arbitrary. If I made 2,000hp per cylinder, but each cylinder was 10 billion cubic inches(yes, obviously a superlative example) that wouldn't be all that impressive. If I made 200hp/cyl but each cyl was 10 cubic inches, that's impressive. hp/cid is a more balanced assesment, but the best is BMEP(brake mean effective pressure)which is a measure of mean cylinder pressure required to make the given torque for an engine of a given cid. You could have 2 engines with the same BMEP, meaning they're both just as efficient in terms of producing pressure in the combustion chamber, but for varying reasons one engine may be 5x as powerful as the other, even though the BMEP is the same for both engines.
BUT....here's the kicker(this part's kinda long and a bit complicated)....let's say you have 2 engines with the same BMEP, similar build in terms of reciprocating weight of each cylinder set(piston rod etc) but one engine has more cylinders, and they both have the SAME cid.....the engine with the larger number of cylinders will end up making more power all the way around. Granted, a larger number of cylinders will result in more internal weight(larger crank, more rods etc) and more friction from extra pistons/rings/bearings....but the more cylinders an engine has the less the torsional excition. Torsional excitation is the vibration generated in the crankshaft caused by the power pulsing(the engine doesn't make continuous power but fires power off from each cylinder in bursts...sudden rapid power and acceleration of the piston, followed by deceleration until the next piston fires). The more cylinders an engine has, the more frequent the power pulsing, and the less violent the torsional excitation is, and ithas an effect on power production. That vibration if left unchecked will actually cause the crank to flex and twist so much that it will snap, and the worse it is the higher the tendancy of the crank to wind up like a rubber band and then try to snap back in the opposite direction of rotation(which is what the harmonic balancer tries to counter, and yes, the crank DOES wind up like a spring, even if only a very minute amount).
So, back to the 2 engines, same size, same BMEP, but the one with more cylinders will end up making more power, because of less vibration...the crank isn't trying as hard to snap back in the opposite direction and negate some of the power produced by the cylinder, more frequent power pulsing means the crank stays loaded for a longer period of time in the direction of rotation, and there's less time for power to continually fall off before the next power pulse winds the crank back up again(like a spring that continually compresses and relaxes). The more the cylinders, the more frequent the power pulse, the more time the crank stays loaded in the direction you want the engine to rotate, the less vibration it generates....less vibration(torsional excitation) the more efficiently the engine operates....remember, the more the crank decelerates before the next power pulse, and the more it tries to snap back in the opposite direction of rotation, the more power will be required to push against it and cause it to rotate the direction you want.
So the next time your buddy with the Integra goes on about more hp per cylinder, tell him that your V8 keeps your crank loaded twice as often and you produce less torsional excition....then see what he says.
http://www.epi-eng.com/ET-PistonExcit.htm If you read through this, pay attention to the graphs....mean torque output is the actual rating of the engine, 400ft-lbs etc, and the graph expresses torque/excitation in a %, 100% is the "rated" torque of the engine, 200% is 2x etc, 50% is half(the graphs areactually mapping the TRUE torque output of an engine across a short expanse of time to show that torque peaks much higher thanrated output, but then drops off to well below rated output until the next cylinder fires)....notice that on certain engines, particularly those with less than 8 cylinders, that the lowest pointoftorque outputin the excitation wave is LESS than 0%.....what that means as that at the approximate center of the crankshaft, such as the balancer and the hub, where the flywheel/flexplate is attached, while the crankpins and pistons etc continue to move in the engine's direction of rotation, the centermost portion of the crank is generating negative torque, or in otherwords, the crankis vibrating so much that the centermost portion of the crankshaft is rotating in the OPPOSITE direction of the rest of the crank and the direction of engine rotation. That means that for a VERY breif moment of time, small fractions of a second, that the flywheel/flexplate of some engines actually stops, rotates backwards for avery short distance and time as the crank vibrates and flexes, and then the next cylinder fires and winds the crank back up, starting the whole process over. And the most amazing part is that our engines actually stay together while doing all this
BUT....here's the kicker(this part's kinda long and a bit complicated)....let's say you have 2 engines with the same BMEP, similar build in terms of reciprocating weight of each cylinder set(piston rod etc) but one engine has more cylinders, and they both have the SAME cid.....the engine with the larger number of cylinders will end up making more power all the way around. Granted, a larger number of cylinders will result in more internal weight(larger crank, more rods etc) and more friction from extra pistons/rings/bearings....but the more cylinders an engine has the less the torsional excition. Torsional excitation is the vibration generated in the crankshaft caused by the power pulsing(the engine doesn't make continuous power but fires power off from each cylinder in bursts...sudden rapid power and acceleration of the piston, followed by deceleration until the next piston fires). The more cylinders an engine has, the more frequent the power pulsing, and the less violent the torsional excitation is, and ithas an effect on power production. That vibration if left unchecked will actually cause the crank to flex and twist so much that it will snap, and the worse it is the higher the tendancy of the crank to wind up like a rubber band and then try to snap back in the opposite direction of rotation(which is what the harmonic balancer tries to counter, and yes, the crank DOES wind up like a spring, even if only a very minute amount).
So, back to the 2 engines, same size, same BMEP, but the one with more cylinders will end up making more power, because of less vibration...the crank isn't trying as hard to snap back in the opposite direction and negate some of the power produced by the cylinder, more frequent power pulsing means the crank stays loaded for a longer period of time in the direction of rotation, and there's less time for power to continually fall off before the next power pulse winds the crank back up again(like a spring that continually compresses and relaxes). The more the cylinders, the more frequent the power pulse, the more time the crank stays loaded in the direction you want the engine to rotate, the less vibration it generates....less vibration(torsional excitation) the more efficiently the engine operates....remember, the more the crank decelerates before the next power pulse, and the more it tries to snap back in the opposite direction of rotation, the more power will be required to push against it and cause it to rotate the direction you want.
So the next time your buddy with the Integra goes on about more hp per cylinder, tell him that your V8 keeps your crank loaded twice as often and you produce less torsional excition....then see what he says.
http://www.epi-eng.com/ET-PistonExcit.htm If you read through this, pay attention to the graphs....mean torque output is the actual rating of the engine, 400ft-lbs etc, and the graph expresses torque/excitation in a %, 100% is the "rated" torque of the engine, 200% is 2x etc, 50% is half(the graphs areactually mapping the TRUE torque output of an engine across a short expanse of time to show that torque peaks much higher thanrated output, but then drops off to well below rated output until the next cylinder fires)....notice that on certain engines, particularly those with less than 8 cylinders, that the lowest pointoftorque outputin the excitation wave is LESS than 0%.....what that means as that at the approximate center of the crankshaft, such as the balancer and the hub, where the flywheel/flexplate is attached, while the crankpins and pistons etc continue to move in the engine's direction of rotation, the centermost portion of the crank is generating negative torque, or in otherwords, the crankis vibrating so much that the centermost portion of the crankshaft is rotating in the OPPOSITE direction of the rest of the crank and the direction of engine rotation. That means that for a VERY breif moment of time, small fractions of a second, that the flywheel/flexplate of some engines actually stops, rotates backwards for avery short distance and time as the crank vibrates and flexes, and then the next cylinder fires and winds the crank back up, starting the whole process over. And the most amazing part is that our engines actually stay together while doing all this
10-15-2007, 12:53 AM
#7
6th Gear Member
Join Date: Apr 2007
Location: California
Posts: 10,468
RE: WTF HP PER CYLINDER
LOL!! At least you read it though, some people see a long post and ignore it because they're impatient. It is amazing when you think about it though, that every time a piston fires the crank twists one way, then the other...and a piston fires, and the crank twists one way, then the other...and we beat on them, and spray nitrous on them, and boost them....and they still manage to stay together. Well, mostly stay together. I read somewhere too that on Top Fuel/Funny Car engines, they have to install the cam with the timing quite a ways off, because when the engine is under power the crankshaft twists and flexesso badly that it's the only way they can get the valve timing to be correct at WOT. Now THAT is some crankshaft flexing
