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Torque vs RPM

  1. Mar 28, 2009 #1
    Now, everyone knows that motorcycle engines work at much higher rpms than car engines.

    They also produce lesser torque than a car, because (and correct if I'm wrong) bikes don't have as much load on them as cars do.

    Can anybody tell me what goes in an engine that decides that when power is being generated in the cylinder, torque increases more than rpm in case of a car engine, whereas in a bike engine torque remains much lower while rpms rise to very high values?

    I'll try to frame this a little better. Power (in kW) = 2(pi)(N)(T) / 60000. N = rpm, T = torque in N-m. Bike engines go upto 14000 rpm. Car engines go upto 8000-9000 rpm. Car engines produce way more torque, while bikes produce lesser torque. How does the engine produce less torque and more rpm in one case and more torque and less rpm in another case?

    Is it a mechanical design consideration? Is it something elementary that I'm thinking wrong?
  2. jcsd
  3. Mar 28, 2009 #2

    Ranger Mike

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    see Mechanical Engineering forum
    Horsepower - Please help - Confused! dated Feb 14 2009
  4. Mar 28, 2009 #3
    Thanks Mike. That was very informative.

    Let me see if I have this right. The force pushing downwards due to combustion, produces torque. That would be - force multiplied by length of crank web on the crankshaft equals torque.

    RPM would be decided then by the time it takes the piston to complete the stroke. Thus, for similar torque produced, rpm would tend to be determined by stroke length and length of con-rod. Is that right?

    So, motorcycles are on low torque since they obviously don't carry as much load as a car does. The high rpm is caused by the design of the engine parts as stated above.

    Am I correct in all this? Or am I still missing something?

    Can anyone tell me more about engine loads? How does one know what full load for an engine is?
  5. Mar 28, 2009 #4

    Ranger Mike

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    in general you are correct on RPM vs. Hp ..but the main reason today's bikes have real hi RPM engines is evolution. Old Indians and Harleys and BSA, Triumphs were brute torque hogs..today's Harley still is the Milwaukee vibrator,one stroking pig!!..lots of low end..the old bikes very way similar to automobiles..then the hi rev aluminum Japanese bikes came in vogue..think about it..a car has 4 wheels weighs 3500 pounds..a mid size motorcycle would top out at maybe 350 pounds...a Harley full dresser with windshield fairings, hot tub..would weigh 500-600 pounds..still 15 % the weight of a car...

    Bore stroke and rod length effect Torque and HP
    an IC (internal Combustion) V8 engine can have the same cubic inch displacement but..totally different torque and HP characteristics
    the big bore, short stroke mill will be a hi end screamer (7000rpm) lacking bottom end torque..the small bore long stroke engine will be a super stump puller that tops out at 4000 rpm.
    Both use the same fuel air mixture , similar valve train , induction and exhaust..though not exactly the same.
    Note one main difference...the 7000 RPM engine will make almost twice the HP for one reason..it is producing almost twice the power during the same time period..i.e. the 4000 rpm engine fires each cylinder 500 times in one minute while the 7000 RPM engine fires each cylinder 875 times per minute...one reason the Formula 1 boys run over 20,000 rpm engines..their rules severely restrict cubic inch displacement..so how do you get competitive...??? more RPMs...p.s. them cars are maintenance nightmares...

    ref: max loads of the engine..I gotta dig in my dyno folder and check but will get back to you when I come back from beer run

    Last edited: Mar 28, 2009
  6. Mar 28, 2009 #5

    Ranger Mike

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    What is engine load...big question - lets break it down
    I had to rebuild a Go Power dynamometer for one of my college course..a dyno is a water brake attached to the crankshaft of an engine. The water brake initiated load of literally, puts the brakes on the crankshaft rotation..it also has a load cell that measures the force applied..the engine is run up to mid range speed and the brake is slowly applied until the engine drags down..under load..you can torture an engine and abuse the dog cra- out of it with this but a good operator will get consistent runs to determine the Wide Open Throttle Maximum load. Things like the Torque curve and Horse Power curve are plotted and now you know the capability for that engine at that time, in that environment at that air temperature with that humidity...

    In my opinion, the Internal Combustion Engine (IC) is a big air pump. It uses a fuel/Air mixture which is converted to mechanical energy through the combustion process. for this discussion I will keep it simple. the IC we discuss is normally aspirated ( not turbocharger or supercharged) and uses gasoline as fuel..

    Air - what we breathe under 14.7 PSI pressure
    Fuel - gasoline, Av. gas if you can get it.note; gas is rated in octane..there are two methods used to rate octane but we won't go into detail here. The lame stuff we have to buy is low octane like 85..it used to be you could get 13 octane pump gas ..had lots of lead that prevented detonation. now illegal...av. gas comes close. The IC compression ration dictates the octane..a 8:1 engine can use 85 octane but if you run 12:1 compression you need av. gas 110 or higher octane. or you detonate...your engine pings, bucks and may even grenade.

    The IC takes in the fuel/air mixture
    Light 1 Light 2 Light 3 Light 4 Light 5 Light 6 Light 7 Light 8 Light 9 Light 10
    Gasoline 17.1 16.0 15.1 14.7 14.7 14.7 14.7 14.0 13.2 12.1
    Alcohol 7.6 7.1 6.7 6.5 6.5 6.5 6.5 6.1 5.8 5.3
    Propane 17.9 16.8 15.9 15.6 15.6 15.6 15.6 15.0 14.0 13.0

    the fuel/air mixture depends on the design. I will discuss carburation as it is simplest to understand. google it for more details
    For this discussion it is a device that mixes air with fuel the mixture is determined by the carb jets used. bigger jets mean more fuel/air mixture..less means higher economy. Carbs are rated in CFM (cubic feet per minute) you can by um up to 1100 CFM
    most small block IC are happy with 650 cfm.

    Airflow requirement = CID / 2 x RPM/1728 x Volumetric Efficiency

    so a total prepared race 350 cube engine at 8000 RPM needs 811 CFM ( race engine is as close to 100 % Vol. Eff. as your are gonna get! ) and this is a requirement at Wide Open Throttle..which is NOT going to be for long as cranking a mill at 8 grand makes the parts guy at the race engine facility real happy.

    So you see, there are many variables that contribute to the amount of Load the IC can handle..
    let me know if you have questions..gotta get another beer
  7. Mar 28, 2009 #6
    The physical load has nothing to do with how much torque a combustion engine will produce.

    One obvious difference in the amount of torque an automotive engine produces over a motorcycle engine is the shear increase in displacement of the automobile’s engine. Many car engines are between 2,000cc ~ 4,000cc’s (2.0 ~ 4.0 liters), and some are even larger. Greater displacement allows greater torque to be produced. Additionally, longer piston stroke increases leverage, further increasing crankshaft torque.

    Examine the specs of Suzuki’s sportbike king, the “Hayabusa” (99~07):

    1,299cc’s of displacement (1.3 liters)
    81mm piston bore, 63mm piston stroke
    155 RWHP @9,750 RPM, 90 ft pounds of torque @7,500 RPM, redlines @11,000 RPM

    Now examine the specs for my 2001 Mitsubishi Eclipse:

    V6 3.0 liter engine (3,000cc’s)
    91.1mm piston bore, 76mm piston stroke
    produces 200 HP @5,500 RPM, 205 ft lbs of torque @4,000 RPM, redlines at 6,200 RPM.

    Comparatively, the engine in my Eclipse produces a little over double the torque of the Hayabusa’s engine, as it has a little over twice the displacement of the Hayabusa’s engine.

    A key factor that limits an engine’s redline RPM is the tension of the valve springs used. If they are provided with greater tension, then redline can be increased, as they are capable of forcing the valves closed quicker thereby eliminating potentially damaging valve float, which could otherwise allow pistons to make contact with the floating valves if the valves aren't returned fast enough under the tension of their valve springs at these higher RPM. The problem with increasing the tension of the valve springs is; they increase wear of the engine, which of course aids in reducing longevity.

    As far as octane goes, you only need to use an octane high enough to prevent your engine from pinging HOWEVER, virtually all modern vehicles employ a “knock-detector” in which case, lower octane fuels can be used quite satisfactorily, as the knock-detector will reduce ignition timing as required to eliminate any harmful ping. For this reason, 87 octane works fine in virtually all modern vehicles.

    P.S. For the past 7 years, I've been running 12.3:1 compression in the performance Hayabusa engine I built. All I ever use is 87 octane and I have no pinging however, I do run two OEM batteries in parallel for the little extra oomph it provides when cranking the engine while hot due to the increased compression. Stock Hayabusa engine compression is 11:1.
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