Date: Wed, 28 Jul 1999 21:23:08 GMT
From: (Michael (Steadyı²³) Stangeland)
Subject: Re: BHP vs Torque

Thank you, I appreciate how accurate you were. May I pursue the topic further though. The goal is to apply a Force on the road at a given Velocity. Power = Force x Velocity Now suppose you have two identical cars travelling on a road. The first car is in 3rd gear and the second in 4th. The speed of the two cars and their transmissions are such that car 1 is operating at it's peak HP and car 2 at it's peak torque. Wouldn't car 1 have more acceleration? (Power = Force x Velocity) (I've assumed HP peaks at a higher RPM than Torque) Torque x RPM = Power = Force x Velocity Although Torque is the source of the equation, it seems to me it's power that is the significant factor. Pardon my sarcasm, but if somebody wants a high torque engine; I'll get them a worm gear. Seriously though, I know that there are losses in the gears. Could it be that a "high torque" engine is simple on that has a wide RPM range of good power (typically at low RPM). ie Consider a big truck and a sports car. The big truck has good power from 1000 - 2000 rpm so it will double in speed before the power fall off hits. In the sports car your power is at 5000-6000rpm, giving only a 20% increase in speed. What do you think? Mike On Mon, 26 Jul 1999 13:14:23 +0100, Liam Boyle <liam_bnopsamnet> wrote: >If I try to be accurate, > >Torque is the turning force produced by the expanding combustion gas >acting on the piston, driving it down the cylinder bore, thus forcing >the crankshaft to turn. It is measured in lbft or Nm. This is the force >that provides acceleration. > >Brake horsepower the power apparent at the flywheel when a braking load >is applied, this load is normal a large electric generator of exactly >calibrated efficiency and losses, it produces xx power which measured >and the input power from the engine is the known from the efficiency and >losses in the generator system. > >It is important to remember that an engine produces torque and that >brake horsepower is simply the product of torque multiplied by engine >speed and couple of constants. > >this may not be easy to read this way but I'll write the equation - > >Torque[Nm] x Speed[rps] x 2 x Pi = Power[W] > >reversing the equation gives > >Power[W] / 2 x Pi / Speed[rps] = Torque[Nm] > >nb. 1ftlb=1.356Nm 1kW=1.341bhp > > > >So if my 9000 Aero has 342Nmnopsamrpm that means - > >342 x 30 x 2 x 3.141 = 64465W or 64.465kW which is 86.4bhp > >but it is also 225bhpnopsamrpm that means - > >225 / 1.341=167.8kW > >167800 / 2 x 3.141 / 91.68 = 291.3Nm > > >so that's 342Nmnopsamrpm=84.6bhp & 291Nmnopsamrpm=225bhp, here you can the >first sign of the truth about torque, power and acceleration. If an >engine has a downhill or falling torque curve, that is more torque at >lower rpm than high rpm it will give better acceleration at those rpm. >The uphill or rising torque curve engine will give better acceleration >at higher rpm. > >If any you are wondering why that is, remember you're basic physics at >school. Newton said ...force applied to mass produces acceleration, >simply the bigger the force the more acceleration. > >Saab turbo's are engineered to have a high flat torque between just >above idle rpm to upper midrange rpm, something like 1900-4500 rpm, then >the torque reduces towards the engines maximum speed, 5500-6000rpm. This >makes them feel more like a lumbering big V8 then a sporty revvy 4. > >That sporty 4 feeling of a BMW 318iS coupe (for example) is caused by >the engines rising torque curve and the short gearing, so it is doing >6000rpm in top gear to make 120mph. At 6000rpm in top my Aero would be >going at 176mph, but it will not get to that speed because the engine >has a falling torque curve and there is not enough torque to achieve >that speed on a flat road. > >So if you normally drive between 1900-4500rpm you will experience the >acceleration of a bigger feeling engine, if the 342Nm was developed at >6000rpm that would be 288bhp, which is a lot. The 318iS has 140bhp and >you really have to rev it hard it get it well, it makes for hard work >for the driver too. My Aero will overtake faster in 5th than almost >everything else on the road here in Britain. > >I hope this helps you to understand why it is torque and not power which >is important, but also why the torque curve is important too. > > >In article <7nfuqu$uh6$>, Paul Aspinall <Paulnopsama >> writes >>I have heard a lot about the Torque in SAAB Engines, and I know they are >>really powerful units, but does anyone know the difference between Torque >>and BHP??? >> >>How can you tell an engine with a lot or Torque, and one with a lot of BHP?? >> >>Cheers >> >>Paul >> >>'92 9000 CS Carlsson >>'89 9000 Carlsson >> >>

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