I was trying to answer your question in the last post, but maybe I didn't make it clear. In-gear acceleration is all about torque not power or BHP. It's the ability to turn the wheels over a given distance (lb/ft or Nm). An engine with a flat torque curve will have the fastest in-gear acceleration through out that rev range. The torque curve in your link shows it peaking at 1000 rpm & tailng off towards 6000rpm with a loss of 2lb/ft. In theory, this will have fastest in-gear acceleration at 1000rpm. Have you ever read peformance figures that state 30-50mph or 50-70mph times? Thats a true measure of torque. BHP is for 0-60 times. But how many times do you do that in the real world? Edited by: Jon TDI
I'm sorry, but this is what I dont agree with. You are saying that max in gear acceleration is achieved at the peak torque point as well, which is not necessarily true all the time! You can have max in gear acceleration at the peak torque point, I'm not saying this is impossible, but only if there is no other point on the power curve (derived from the torque curve) which is higher than the power figure derived from the peak torque point. I know what torque is, I know what power is, I know the relationship between them, and I dont see how you can have harder acceleration with 200lb/ft torque at 1000rpm, compared to 198lb/ft at 6000rpm. Power is going to be a lot higher at 6000rpm, which is why the car will accelerate faster. You have to think about the size of peak power when comparing a similar reading of torque, at different RPM. POWER = TORQUE X RPM / 5252. When you compare two cars with different torque curves, and they both have the same figure of peak torque (and even the same area under the torque curve) if one peaks at 2500rpm, whilst the other peaks at 5500rpm, the car with the peak at 5500rpm will accelerate faster and harder (assuming the cars are identical weight, etc...) because it will have more area under it's power curve (remember power = torque x rpm / 5252). Can no one on here understand this concept? I know you diesel boys love your low down torque, but ultimately a car with the SAME FIGURE of peak torque that you guys have, albeit further up in the rev range (say 5-6000rpm) will accelerate faster than a diesel/petrol with the SAME peak torque at 2000rpm, with both cars accelerating at the peak torque point. Does anyone know what I'm talking about? Edited by: S1MMA
thats why diesels are better because they develop more torque than an equivallent petrol car, a petrol car with 165bhp would probably only have 165llb ft of torque approxx give or take a bit.and my sad little diesel had nearly 70% more torque hense it was quicker than most similar powered cars out on the road in normal conditions. i think i've confused my self now
You sure about that? What engine have you got? Im just suprised as according to official figures a 150 PD has 236 lb/ft and a 130PD 228lb/ft. This still beats my V6s 199 lb/ft but its nothing like 280 lbs/ft... Edit: and you wont be quicker necessarily because we can hold the revs longer - you need to change. A PD150 diesel and a V6 are pretty even matches overal I am told Edited by: Mikedav
it's chipped, bigger turbo and injectors, boost 1.5 bar, built by Simon Cooper of mk2 tdi fame. it does what it says on the tin, only with smoke, lots of smoke
and why change gear leave it in fifth at seventy and just accelerate all the way to 144mph at the limiter
Sorry about the delay in response, I've been travelling. I'll have to deal with this bit by bit.... Yes. If you mean will the car with the highest torque point always been accelerating faster when it is at that highest torque point. It's always true. If you mean will the car with the highest torque point always been accelerating faster throughout the rev range. No it may well not be. Wrong. The highest torque point is where the maximum force (using the word 'force' here in its real meaning) is being applied to the transmission by the engine. F=MA. This is the maximum possible acceleration regardless of revs, speed, gearing or any other consideration. I think you have them slightly confused actually! Well, there's not much difference in this example, only 2lbft. (It's lbft btw, not lb/ft or even lbft-1!) But F still equals MA. 200 > 198. Now for one of my examples. There are plenty of electric motors that generate maximum torque at zero rpm, thus giving you maximum applied force at zero rpm, ergo maximum acceleration. Work on the maths and you'll see that you get maximum acceleration with zero BHP @ zero RPM! Both BHP and RPM are meaningless with regards to acceleration. Yes. No!! HP does not exist, (it's like centrifugal force, an engineers phantom!) it has no relevance to car performance whatsoever. It is simply a number derived from the combination of torque and rpm. No. I've already shown RPM is irrelevant to acceleration. Torque is the only thing that counts. I could have a revolutionary (no pun intended) engine that only revs to 10rpm. If it generates 1000lbft of torque at 5rpm it will out accelerate a car with 900lbft that is revving at 14000rpm, assuming appropriate gearing and some very strong driveshafts... Another example. Take a jet powered car. We measure jet engine output by 'force' in pounds (lbs). Why? Because there is no rotational vector to take into account. The force generated is the only thing that matters. Car engines produce 'force' too, force over a an applied distance, i.e. lbfts, i.e. Torque! Force is all that counts! F=MA. No, this is wrong. I'll say it one more time, HP/BHP and RPM are irrelevant to acceleration. F=MA. What I think you are getting at is the issue of the torque curve. If a engine peaks at a high torque point, but the rest of the torque curve is significantly below this threshold throughout the rev range then a car with a lower torque point might well accelerate more rapidly if the acceleration is measured as an average throughout the rev range, given appropriate gearing to the road wheels. This is easy to work out, as you've stated, it is the area under the graph on a torque plot, the car with the highest overall torque will have the best overall acceleration. This is the achilles heel of typical diesel turbo engines. Peak torque at low rpms, and high acceleration at low rpms, but torque rapidly drops off as the revs rise, hence the acceleration rapidly tails off too. THe reason, incidently that diesels always have long gear ratios. Summary. 1. At any given RPM, with all else being equal (ie gearing, rolling resistance, aerodynamics etc) the car with the higher torque accelerates the quickest. F=MA. QED. 2. For any rotation engine, maximum acceleration is delivered at the maximum torque point for that engine. F=MA. QED. Cheers, Drew. Edited by: drew
Mostly because 1. They have about twice the compression ratio of a petrol engine on average. 2. The fuel burns slower and more completely, being more efficient at lower RPMS. Cheers, Drew.
