Acceleration and power vs torque vs "punch in the kidneys"

[SimonMac]

I'm hoping that someone can settle an argument I'm having with a mate. The subject of Power vs Torque for maximum acceleration has been has been covered before here and on this website here, and I understand that power-to-weight ratio has more bearing on outright acceleration times through the gears than torque-to-weight ratio.

 

He claims that his Impreza STI (~280BHP, 270lbft, 1300kg) accelerates much harder than my Superlight-R (190BHP, 140lbft, 530kg) due to having a higher peak torque figure, on the basis that subjectively it *feels* much more accelerative in terms of 'kick in the back' under peak torque when his STI comes on boost. I argue that my superior power-to-weight ratio (even taking into account driver and passenger weights) means that my SLR must have the upper-hand, and test figures for both cars support my argument.

 

Unfortunately the opportunity hasn't yet arisen to settle this once and for all in a drag race. However, having failed O-level Physics ❗ I have no argument to explain away how the Impreza's greater punch in the kidneys does NOT equal greater acceleration. So who is right any why??

 

Edited by - SimonMac on 30 Sep 2005 11:32:07

[caterhamnut]

Perhaps your acceleration is more linear - he gets a perception of 'more' acceleration when his turbo kicks in - this gives the impression (briefly) of that extra shove?

 

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[knowley]

Just take him out in it, he'll soon change his mind

 

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[Grubbster]

That's probably it Angus (with my limited knowledge of physics!) - the point that the boost kicks it does feel like a kick in the back but it doesn't last. On a drag strip the SLR would comfortably be quicker (if it made it off the line cleanly - easier to geet right with the Subaru).

 

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[oilyhands]

Angus,

 

That is exactly it, the turbo cars torque will rise dramtically into a sharp peak and then fall off, with the SLR the engine characteristics give a much more gradual and linear increase. Subjectively the turbo car will accelerate like a number of kicks up the backside rahter than a more gradual push, reckon the SLR will be faster given the power/weight equations until drag factors come into play.

 

 

Oily

[rossybee]

The PTW ratios aren't even close, so I reckon there'll be no contest

 

With the exception of at higher speeds (~100mph & above) where aerodynamics play a greater role...

 

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[Myles]

Yeah - the rate of change of torque comes into the 'kick equation' as do your (transmission and other) losses. Spikes in the curve will be more easily felt.

 

 

 

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[stevefoster]

Is that 530KG car alone? What did you do to it?

 

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[gmdh555]

Never owned an STi but my first scooby (standard UK turbo, 215bhp) always felt more accelerative than my second (UK WRX with PPP, 240ish). The figures said the reverse (despite the weight gain of the later car), and the subjective measurement was purely down to the less linear response of the earlier turbo. Needed a decat on the WRX before it ever felt comparable to the older car.

 

Gareth

 

 

[Peter Carmichael]

More often than not when *impressive* torque is being cited, it is because of presentation rather than fact.

 

When you accelerate in gear, you get the biggest "shove in the back" at the torque peak. Therefore it is tempting to believe that torque is the quantity that determines the acceleration of your car.

 

What the torquophile is ignoring is the engineering certainty that you get more acceleration than you get at the torque peak by changing down a gear and operating above the torque peak at higher power output from the engine.

 

The equation for acceleration is: A = power/speed/mass (if you get your units right).

 

It might feel different in terms of the incremental amount of shove you get for each additional mm of accelerator pedal movement. This sort of subjective influence is deceptive and leads to the wrong conclusion being drawn.

 

Fancy R400+ performance on a pre-SVA roadgoing Seven- Click here now it's reduced to UKP14500 but doesn't have the Quaife sequential box

[SimonMac]

Thanks for the replies chaps - so that's all there is to it, just the difference in linearity of the torque delivery and a deceptive impression of shove when accelerating in gear at peak torque?? I was expecting a bunch of complex Newtonian equations in support of my case - even if I couldn't understand them myself I'd be able to blind my mate with science

 

I will try to convince my mate of the facts, and if necessary it'll have to be settled in an impromptu drag race when we next meet 😳

 

Steve - the 530kg is just a guesstimate of weight without driver or passenger.

[Golf Juliet Tango]

For on the road/track proof of Peter's explanation try the acceleration of a BEC. Not much torque but look at the acceleration.

 

[Tight fart]

Peter could you also please explain why it is that

if women float they must be wiches ?

I think it was w/m x d = burn

 

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[SimonMac]

GJT, does that mean that a BEC has even less feeling of 'shove in the back' ?

[Golf Juliet Tango]

Simon: sorry, I don't know, I have just seen them and seen the figures.

 

[Violet Elizabeth]

No because "shove in the back" isn't just a function of flywheel torque but driven wheel torque.

 

A BEC can produces its torque at higher revs, which means for any given speed we can have shorter gearing = higher mechanical advantage.

 

100 ft.lbs @ 8000rpm will produce the same rear wheel torque as 200ft.lbs @ 4000rpm, with the same wheels AND twice the gear+diff ratio. Same speed, same torque.

 

I think !

 

Interesting topic, have had similar conversations with Audi TT owners who just insist their turboed cars accelerate faster than my natasp Boxster S.

 

They don't of course, it is just a bigger kick in the back rather than a steadyier longer whoosh. But they never believe you !

 

And because I am a true saddo ...

 

Scooby. 280hp*550 = 154000 ft.lbs/sec. 2860lbs.

Cat. 280hp*550 = 104500 ft.lbs/sec. 1166lbs.

 

30mph = 44 ft/sec.

 

Scooby accelerating @ 1.22g

Cat accelerating @ 2.03g

 

 

 

Edited by - Noger on 30 Sep 2005 13:20:50

[LazerBrain]

It'd be quite interesting to put an AP22 or similar accelerometer into each car and look at the g force throughout as a test.

 

cheers,

 

Darren.

[gmdh555]

Audi TT owners who just insist their turboed cars accelerate faster than my natasp Boxster S

I test-drove an S3 (same engine as TT) back-to-back with my first scooby and the "shove in the back" factor was completely absent. Must be the lightest-touch turbo I've driven. But... deceptively quick! Didn't handle for toffee though

 

G

 

 

 

[lofi]

From experience (me infront in my scooby 300bhp, 300 twisty things, mate behind in our 1.8 K-series 140) I can confirm that the caterham stays with the scoob just up to the point the aerodynamics come into play. I drive both quite regularly and the scoob always feels like it accellerates much harder but on the road comparison doesn't bear that out, so I have always put it down to the completely different delivery. In some ways it's part of the joy of a turbo barge, the nothing, nothing, nothing - - here we go. requires a totally different driving style.

 

 

 

LoFi

FAt Car at the Back

[SimonMac]

Thanks for the info. and practical examples chaps, interesting stuff and confirmation of what I'd hoped for

[Peter Carmichael]

I've been thinking a bit more about the subjective aspects of this.

 

Imagine if you will...

 

... an R400 caterham spinning away happily at 6000rpm. 2000rpm to go. You press on the loud pedal and as the pedal moves, the throttle response means that you get as much as you ask for when you ask for it. You are driving the car. This becomes second nature. You drive on the basis of the brain translating the amount of shove you need into slight movements of the foot. It is never more or less than you ask for. It doesn't catch you out.

 

... a Scooby or other car with a big hit of turbo torque or a big engine off cam. You press the loud pedal and nothing much happens immediately. There is a delay. The engine climbs on cam, the turbo spools up. The torque arrives, all of its own. The car is driving you. It feels like a powerful force because you have not metered it out by millimetric degrees by your own muscle motion. It is like getting picked up by a wave.

 

Fancy R400+ performance on a pre-SVA roadgoing Seven- Click here now it's reduced to UKP14500 but doesn't have the Quaife sequential box

[DohNut]

 

Also in the perception of acceleration

The caterham does not squat significantly compared to a saloon which rise at the front and drop at the rear significantly. It will feel like the acceleration is pushing you back in your seat but it is just the seat reclining a little more.

Same with braking - big *event* in a saloon *stands on its nose* where the caterham just gets on with it and you are strapped in tight so dont get thrown around hence the perception is not as great.

 

Nick

[SimonMac]

Peter - my mates perspective is mainly as passenger of the SLR, so if anything you'd think he would find the SLR more accelerative than the Scooby.

 

Just for the record, there is no dispute that the SLR is much more responsive and exciting than his STI

[rossybee]

Nick has a very valid point there too

 

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[Myles]

Same with braking - big *event* in a saloon *stands on its nose* where the caterham just gets on with it and you are strapped in tight so dont get thrown around hence the perception is not as great.

 

That *is* true. I remember when Lotus were developing the first active F1 cars, Mansell had real trouble driving the first versions because the active kit dialled-out the dive under braking. They had to put just enough back in so that the perception was returned but without ruining the aero balance...

 

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