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Date: Mon, 26 Jul 1999 13:14:23 +0100
From: Liam Boyle <liam_bnopsamnet>
Subject: Re: BHP vs Torque

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 > > -- Liam Boyle

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