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Power Vs Torque: What Delivers Maximum Acceleration?

Torque

Power Vs Torque: What Delivers Maximum Acceleration?



Some people say you get maximum acceleration at any speed with the engine at peak power, and some people say it’s peak torque from the engine that delivers the maximum possible acceleration.

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As hypotheticals go – I can see they’ve both got merit. But they can’t both be right. So let us unleash some righteous jihad on bullshit using the hi-tech miracle of physics.

First I want to be very clear what I mean by ‘maximum acceleration’. Obviously cars operate through a great many speeds, and the maximum possible acceleration obviously varies with speed. You typically get higher accelerations at lower speeds.

What I’m talking about is taking a snapshot in time, with the car at some arbitrary speed – and then working out how to get the maximum acceleration out of the car at that speed. I’m going to use 75 km/h here (that’s about 45 mph) – but you can use any particular speed – the actual amount of acceleration possible will change, but the revs you need to achieve it will not.

Acceleration depends on only two things: the mass of the car, which is generally pretty constant, and the accelerative force the driveline is capable of applying. That’s Newton’s second law of motion.

And of course you need to subtract any resistances the car is battling against – like rolling resistance of the tyres, and aerodynamic drag, which is hugely speed dependent.

There’s also gravity, which helps if you’re running downhill, and hurts if you’re slogging uphill.

Three things come out of your engine: Power, torque and revs. Power is conceptually complicated, because you can’t see or touch it, but it doesn’t have to be. Power equals torque times revs. It’s that simple – but you have to get the units right.

I’ve done a whole video on this but in brief, you balance everything up by getting the units right. It’s easy. Kilowatts equals Newton-metres times rpm divided by 9549. If you work in imperial units in your neck of the woods, horsepower equals lb-ft times rpm divided by 5252.

I chose the i30 SR as a demonstrator of sorts here for a reason. Because 75 km/h is real interesting in the i30 – first gear is a non-starter, obviously, because the engine would be revving at over 10,000 revs. Good luck with that. But you could easily be in second, third, fourth, fifth, sixth or even seventh at 75 kays an hour.

If you’re in second, the engine’s at 6000rpm making its peak of 150 kilowatts. But third to seventh is interesting. In third, you’re at 4100 revs, and in seventh you’re at 1600 – so the engine’s making its peak torque in all gears above third. 265 Nm. Because the SR makes peak torque from 1500 to 4500rpm.

So – here’s the bottom line: Engine at peak power at 75 kilometres per hour delivers 620 kilograms of tractive effort – thrust, if you like). Engine at peak torque (at 4500rpm) delivers 520 kilos of thrust, and engine still pumping out peak torque, but at the lower limit of 1500rpm this time, delivers only 170 kilos of thrust.

This correlates pretty neatly with the experience of driving the car. Get to 75 in manual mode, upshift to seventh, floor the throttle. It’s just bogging down all over town. Downshift to second, repeat the experiment: big difference.

So you can very clearly see that, provided you can find the right gear, maximum acceleration at any particular speed can only ever occur when the engine makes peak power – because peak power at the engine delivers peak torque at the driving wheels. Every time.

Genius, Innovator, Voice Actor, The Original Imposter, Cat Stroker. I like to step on crunchy leaves.

42 Comments

42 Comments

  1. Fred Pilcher

    August 2, 2017 at 3:43 am

    😀 Thanks John – an intelligent and relatively simple explanation.

  2. MrDeepseadweller

    August 2, 2017 at 4:53 am

    Can you please provide the formula for your level of smugness in relation to the bushiness of your beard? I think it would be logarithmic. Thanks.

  3. WHAT A GRAND INTOXICATING INNOCENCE

    August 2, 2017 at 5:37 am

    Google Maps, WiFi, LotR, FarScape, Mad Max and AutoExpertTV – all good things come from Australia.

  4. Peter Hammond

    August 2, 2017 at 6:35 am

    Great video John. But as you are an owner of a Renault Clio RS as you stated.
    What are you thoughts on Renault here in Australia?? I'm may be looking at buying the current
    model Clio RS. Reliability, Customer service. We all know about VW. Jeep,Ford etc. But I can't remember
    you mentioning Renault at all. After all they now have 5 years unlimited K's warranty..

  5. Elle Jay

    August 2, 2017 at 7:03 am

    Next video…Locations/places on public roads where you can "have a little bit of fun….if you want to…if you can manage the risks"? LOL

  6. P0IS0N

    August 2, 2017 at 10:44 am

    I thought I'd comment on other things, but that hat is amazing!

  7. Bruce Anger

    August 2, 2017 at 11:07 am

    I think I might need to watch it again

  8. Ed R

    August 2, 2017 at 12:03 pm

    What about the i30 N..?

  9. Hugh Jwang

    August 2, 2017 at 10:08 pm

    I have no complaints or issue with the topic JC has discussed here.
    What many people forget though is that "driving" and operating a motor vehicle are two different things.
    Anyone, and in Queensland it has been proven that even a trained monkey can do it, where lever A is pressed and thingy B is moved and round-thing C is turned to get a motor vehicle where it needs to be. That is the basic operation of a vehicle.
    "Driving" is the process of being aware of your surroundings and managing a vehicle to the conditions prevalent at the time. "Driving" on a race track is very different to "driving" on a road (which requires the mandatory & correct use of gears to get the vehicle where it needs to be but also correct use of indicators and a knowledge of how mirrors work combined with a courtesy that stems from owning the vehicle and not the road).

    The science behind HP vs Torque will always be spot on but there will always be division as the experience of people differs. When discussing torque and power, the arguments come from a 30 litre diesel that might peak at 1200 rpm to a 750cc motorcycle that revs to 18,000. In the middle you have Harley Davisons, big block V8s, small block V8s, small turbo engines, etc etc etc.

    The science of the debate may be the final say but there will always be someone to argue the point based on them knowing better (and usually after a couple of drinks with no scientific knowledge at all….)

  10. sid snot

    August 2, 2017 at 10:40 pm

    What this wanabe Einstein/ Clarkson doesnt get is that there is no contest between torque and power. He has created a classic non argument. You need torque to gain access to power. There is a crossover point but max acceleration in any car is achieved by redlining it in every gear. QED mate.

  11. Jon Mal

    August 2, 2017 at 11:17 pm

    great video John, I hope that is not permanent Ink, lol

  12. Shawana Washington

    August 3, 2017 at 12:12 am

    Worthless video. Guy's on an ego trip.

  13. Duane Dempster

    August 3, 2017 at 12:47 am

    Hope that wasn't a permanent pen on the duco… Or is that i30 now an 'educational' tool 🙂

  14. Ian Cavanough

    August 3, 2017 at 1:08 am

    My head hurts. I need a stupid looking hat.

  15. Dead Frt West

    August 3, 2017 at 1:56 am

    You had me at "hat sideways" and "yo"!

  16. Kelvin O

    August 3, 2017 at 4:45 am

    Imma gonna have to watch that a few times John – Awesome edukashun though! Thanks

  17. Kelvin O

    August 3, 2017 at 4:47 am

    John you crippled bastard – you are writing with the wrong hand – that's your problem right there! And wearing your watch on the left hand to fool us that into thinking you were normal? This looks like an article for BS News Network 🙂

  18. Andrew Wilson

    August 3, 2017 at 7:53 am

    Brilliant.

  19. trevorpom

    August 3, 2017 at 9:21 am

    What, no gay shirt this time around John? Have you gone straight?

  20. Endureromex

    August 3, 2017 at 12:57 pm

    A fresh air of science your explanations are, a humble effort to know a phenomenon through The Method. This when –paradoxically– despite all the free knowledge at our disposal, most people indulge in all the bullshit found on the internet.

  21. Jose Martin

    August 3, 2017 at 2:02 pm

    F…..k me…. at 12:07 I thought I was looking at Derin Hinch

  22. fhhsvnggbh

    August 3, 2017 at 2:09 pm

    this guy is a pompous arrogant prick, P,s< Not all aussies are idiots. I apologise on behalf of the rest of my country. If you think that an i30 has go its time to put bowls club hats on the parcel shelf.

  23. Ivan Zhelyazkov

    August 3, 2017 at 8:28 pm

    Bahahaha. John! I love you man! You made my day again! The punt about the constant vehicle mass was nuclear-powered! You absolutely have to screen for university. Clearly the way you present the boring science helps to learn. Just wanted to say, that we appreciate your work here in New Zealand.

  24. CNC_channel

    August 3, 2017 at 9:36 pm

    Finally explanation for idiots. People who think peak torque is where max acceleration happens shouldnt pass elementary school physics. People keep saying my engine has this much torque… Guess what.. Only torque data gives me absolutly nothing about performance data as long as I dont know at what rpm. On the other hand, peak power tells me almost all I need to know about engine performance. Who cares about engine torque when you know power output, transmision will carry out its job, and translate engine power to maximum wheel torque.
    As I commented before, on high end sports cars and racing cars, high rpms are actually preffered over high torque. Engine components have to be much stronger and heavier to whitstand high forces, while on the other hand having small torque and high rpm will result in much lighter engine for same power output. Ofcourse parts have to be balanced, and they will wear out fast due to high rpm, there are also lubrication issues etc.. But high rpm engines will have much better power/weight ratio than high torque engines. They will also deliver higher response due to low mass components having less inertia.
    Relevant data is power at rpms you shift. So if you use your petrol engine from 5k rpm to 7k rpm between gears, and someone drives diesel and shifts between 2500 and 4500 rpm, you just look at the power graph of both engines between those rpm, and you will know which engine will pull harder.
    Diesel will have higher torque, but will need lower gear ratio, while petrol car will have lower torque, and higher gear ratio due to higher rpms. End result is same torque at the wheels, if both engines produce same power.

  25. Bryan Joseph

    August 4, 2017 at 12:35 am

    My life is complete. I thought the video was going to be short, but I checked 20 seconds into it that it was 12 mins. Being an engineer and car guy, I felt compiled to watch it. Actually I was shamed not to watch it and was happy that I have kept my BS to myself in the past. My Thermodynamics…Dynamics professors would be proud of me.

  26. mouse.4d

    August 4, 2017 at 7:25 am

    Beautiful! I really love the videos you make!

    But John, one thing I don't really understand from this one, you say that "Three things come out of your engine: Power, torque and revs." but since Power=Torque x Revs, doesn't this mean that engine produces only two things–torque and revs ?

  27. ProCactus

    August 4, 2017 at 8:05 am

    I love these juicy videos, full of meat with no dick jut a top cock. Most people should probably watch this 3 or 4 times. I know this is scripted but it actually sounded like you know what your talking about, Not that a doubt you for the most part.

    Could you do a vid on something about the time it takes LED lights to turn on vs the incandescent bulbs. I think its crazy most people have no idea about that.

  28. Dunno DoYou

    August 4, 2017 at 8:08 am

    After seeing you out there I need to hear, The Dualing Banjos song. Im going to think of you on the verandah.

  29. ctp Mark

    August 4, 2017 at 3:43 pm

    Another excellent Video..
    Well Done..
    Keep up the great work.. Cheers!

  30. Duck Man

    August 4, 2017 at 3:55 pm

    Dear John u r the greatest, we need u in UK!!!!

  31. Fck Uyo

    August 5, 2017 at 1:12 pm

    TL,DR: Drive an auto and stomp it.

  32. MRT

    August 7, 2017 at 2:54 am

    Excellent stuff John! Pretty much sums up what I have always thought about this topic. Seat of the pants feel is torque, but……..horsepower is what makes something truly quick. This is why a Tesla can beat a Hellcat off the line (high torque 0 RPM) but lose in a race to top speed because of the better horsepower of the Hellcat….and of course AWD vs RWD (launching those Challengers looks like a bitch).

  33. japilander

    August 10, 2017 at 2:43 pm

    John… of course that acceleration would be higher in a lower gear measuring at same speed. Comparing torque and power should be at different speeds on a same gear (speed A at max torque vs speed B at max power)… and the mesure should be acceleration it self (forward G force).
    The issue in racing is when to do the shifting, as if its better overall to shift before red-line (where acceleration isn't decreasing yet), or waiting till red-line because lower gear gets higher acceleration in all revs range than next gear anyway. Please comment.

  34. Martin S

    August 10, 2017 at 6:55 pm

    If two cars of equal mass are accelerating, the one with higher power will always accelerate faster, torque here literally doesn't matter it could be any number

  35. goddesseddog

    August 13, 2017 at 5:43 pm

    Power delivers acceleration but not peak power. Peak torque also doesn't tell much just as peak power. Best to have a flat torque curve and very high revving engine with high peak power and a transmission with as many gears placed as closely as possible as could be efficiently implemented.

  36. Son of Man

    August 17, 2017 at 4:08 pm

    I didn't hear any butt fucking jokes this time … you're going soft on me!

  37. WolfoxBenny

    August 19, 2017 at 7:05 am

    +AutoExpertTV Ok, You answered the question I did in the other video. So CVT's are slow because of their built, that could be the only answer. They could give more acceleration, but are "dummed" down because they structure can't deliver all that torque to the wheels. mmmmm…..

  38. Erik Moore

    September 4, 2017 at 5:59 am

    Can't believe you have to explain this to anyone 😲 Have you ever upshifted to pass in the hiway? Like holy shit I can't believe I'm saying this out loud. If you had more acceleration at peak torque you would never downshift…. You would just stay at that rpm while passing. So for example a CVT would never ever ever rev passed peak torque. I feel stupid explaining this. You did a fine job John

  39. SirtubalotTX

    September 19, 2017 at 6:35 am

    Put quite succinctly and all of the mental gymnastics aside, acceleration is produced quite simply by ~torque at the wheels~.

    Yes boys and girls, it is really that simple. The more torque (or force however you want to measure it) you have at the wheels at any given speed, the quicker you are going to accelerate.

    Now, many of you are now scratching your heads and wondering why things get so complicated when you bring "Horsepower" into the picture. Well, that is really not complicated either. Why? because Horsepower is simply a FUNCTION of torque and represents the AMOUNT OF WORK BEING DONE. The way I like to explain is like this: It takes way more energy/power to produce 500 foot/lbs of torque at 5000 rpm than it does to produce the same amount of torque at 2000 rpm. If we both study and do the math, we get this: (And hopefully an epiphany) lol.

    Hp=Torque (lbft) X Speed (rpm) / 5252

    Sooooo….

    500 ft/lb @ 2000 rpm would be 500 x 2000 / 5252 which equals 190.4 horsepower.
    500 ft/lb @ 5000 rpm would be 500 x 5000 / 5252 which would come out to be 476 horsepower.

    You see, 500 ft/lb at 5000 rpm is doing a LOT more work than the same amount of torque at 2000 rpm.

    Make sense?

    Another way to look at it is imaging yourself on a bike and realizing that it is much easier to apply torque to the pedal crank at lower speeds than it is at higher speeds. The 'stronger' you are, the faster you can go on the bike because you can maintain torque at higher speeds. Understand? lol.

    One of the more enjoyable maths I like to do and explain.

    In summary? TORQUE causes acceleration,..Horespower simply tells you how much work that torque is doing. lol.

    Good day.

    Edit to add: The reason that the constant '5252' exists is an interesting one and I encourage all to Google it as I don't have time to explain that here.

  40. M

    September 21, 2017 at 9:43 pm

    Just started watching these a few ago, but thinkin' about acceleration while the rpm's, are at the peak, asap, vs….w/e…..there's 0 reason for a reply……

  41. ChrisHallett83

    September 27, 2017 at 6:50 am

    To let you skip 12 minutes of blather, torque gives acceleration, horsepower gives top speed. For fastest acceleration, you want to keep the engine in the best part of its power curve (sweeping between the peak torque and peak power RPM points) while you're changing gears.

  42. ROB NAVAEI

    October 8, 2017 at 8:32 pm

    Great video, thank you. I just like to add that there is a thing called the curse of the British. They invented the terrible system of units, called the imperial system, and now they talk and walk the metric system.

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