# Cars We Drive!

Discussion in 'Fine Living, Home, Design & Auto' started by Bert1568, Jul 18, 2006.

1. ### KookzSenior member

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Was the real, nearly identical 335i vs. 335d not close enough that you want to toss around purely hypothetical situations such as you just wrote?

1/4 mile [email protected] for the diesel, 13.5 @103.8 for the gas.

As for your question re: 400hp, 2000 lb car?

At 60mph, it'll accelerate at ~1.2 ft/s^2. At 100mph, .74 ft/s^2 (ignoring wind and windage losses, of course). (oops, screwed up the units. That should be 1.2 g, and .74 g, not ft/s^2).

Any chance you could do a similar calculation from torque? I didn't think so.

Last edited: Apr 15, 2012

2. ### Arthur PESenior member

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no
M3 has higher HP and lower weight yet is the same speed as a car with more torque, equalize wt and the car with less HP is faster

wrong. I know the answer, you are waaaay off
show your work, it's a single equation, 3 variables

a 400 HP 2000 lb car will take ~ 60 sec to go 60 to 100 mph (88 ft/sec to 150 ft/sec) or a gain of 60 ft/sec at an avg accel of 1 ft/sec-sec. really?
my 400 HP 4000 lb car does it in 6.5 sec, lol

yes I can knowing axle torque and tire radius (we'll assume 1' to make the math easy) what is the torque you want to use?

3. ### Arthur PESenior member

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btw
your 400 HP 2000 lb car would take 140 sec to get 0-60 mph
assume an avg linear accel of (1.24 + 0)/2 ~ 0.62 ft/sec-sec (like most cars in low gears)

88 ft/sec / 0.62 ft/sec sec ~ 140 sec
even if we assume it launches and hits 1.24 ft/sec sec instantaneously still 70 sec
my 400 HP 4000 lb car takes a bit less than 5 sec (4.6 to 4.8)

a 400 HP 2000 lb car would seem a bit faster

Last edited: Apr 15, 2012

4. ### KookzSenior member

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Sigh.

Ok, power = 400hp (and I'm going to take that to be linear over at least some range. Let's say the engine makes 400hp from 5000 to 8000 rpm, which means it makes 420 ft-lbs of torque at 5000 rpm and 262 ft-lbs at 8000 rpm.

Let's say we're in a gear where we're doing 40mph at 5000 rpm. That's 3520 ft/min. Given an average wheel size of 24" (this is small for modern cars, but it's what cars used to run 20 years ago and makes for an easy 1 ft radius), that means that the wheel is rotating at 560rpm.

If our wheel is doing 560rpm and our engine is doing 5000 rpm, our effective gearing is 8.92. So at 40mph we're making (420 ft-lbs x 8.92) 3750 ft-lbs at the rear wheel, on a 1 ft radius wheel, so 3750lbs of force pushing us forward. 3750 lbf = 2000 lbm x a, a = 1.875 g. At 8000rpm, the numbers are (262 x 8.92) 2337 ft-lbs at the rear wheel, 2337 lbf accelerating the car, and 1.17g.

Obviously these are theoretical numbers, but the point being...you actually can calculate the speed of a car from horsepower, and you know absolutely nothing about the speed of a car from simply knowing the torque that it makes.

Anyway, sorry to everyone for getting so far off track. I'm clearly bored out of my mind sitting on a rig off the coast of Equatorial Guinea at the moment.

Back to the show...

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6. ### KookzSenior member

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Yeah, I made the correction above. I was using ft/s^2 when I meant to use g.

Ok how fast does a 2000 lb car with an engine making 400 ft-lb of torque accelerate at say, 40mph?

7. ### Arthur PESenior member

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so you can't do it with only HP (and tire radius)
you need need the speed to calculate rpm to calc torque THEN power (as in P = T x rpm)
lol
my tires are 26" dia so 24" is not real small

you can't calculate speed with HP alone, absurd, you assumed a tire size and road speed

axle torque = 3750
thrust = 3750/1 = 3750
accel = 3750/2000 g or 1.875 g or 60 ft/sec sec
you needed road speed (and wheel speed and gear ratio)
I needed only axle torque/thrust
that is becasue T is a primary variable and determines accel, you need rpm to calculate torque from HP

8. ### KookzSenior member

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I picked those numbers to make a calculation with, but they can easily be changed. The point is the engine was making 400hp, whether it was doing it with 840 ft-lbs of torque at 2500rpm or 105 ft-lbs at 20,000rpm doesn't change the outcome of it. All that was needed was the power, I simply used place holders. You can change the speed as well, and the calculation will still be valid.

I said the car makes 400 ft-lbs of torque. That's the engine output, how do you know the axle torque? (hint: you don't).

9. ### Arthur PESenior member

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you need axle torque or wheel thrust, like measured on a dyno (and which they use to back calculate HP, since it isn't readily measured)
unless you are talking about an old school dynomometer which turns a generator, measure current, voltage and power factor (not easily measured) take the product and divide by 746
I gave you wheel/road HP, 400 HP, give me the axle torque or wheel thrust

Last edited: Apr 15, 2012

10. ### Arthur PESenior member

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it changes everything
with wheel torque/thrust I can calculate accel
with HP alone you can't (you need rpm to calac torque and thrust, secondary varibale, it takes 2 things to get the answer)

again
no speed given
all you know is:
600 wheel HP
3000 lb car
accel?

11. ### Arthur PESenior member

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engine torque = 400
gears = 10:1
wt 4000
x mph

accel = (400 x 10/1) / (4000) = 1 g

engine power = 500
gears 10:1
wt 4000
y mph

y NE x

what is the accel?

hint, I don't need rpm, road speed, etc.
you do
to calculate torque/force/thrust, lol

Last edited: Apr 15, 2012

12. ### KookzSenior member

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Are you under the assumption that I don't know that F=MA and that axle torque x wheel radius = F in that equation? Clearly the argument we've been having was not "if you apply more force will it accelerate faster?" and if you thought that was the case, then I don't even know what to say.

We're talking about cars in this thread. As in, "the new M3 makes 414hp and 296 lb-ft of torque".

13. ### Arthur PESenior member

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the discussion is
which makes a car accel faster, more HP or more torque (for a closed system), torque
can you calculate accel from power alone, no

GTR and e60 M5
480 vs 507 HP
wt ~ the same 4000 lb
0-150
the GTR is 5 sec faster
how so with less power?
could it be it has much more torque due to turbocharging?

let's summarize
to calc accel I need wt, gearing, tire radius, torque
you need wt, gearing, tire radius, power and road speed or rpm
gotcha

2 cars, identical except as follows
one makes 10% more torque over the power band except close to redline, it makes 5% less than the other
so the other has a higher peak HP, but less average torque
which is faster?

14. ### KookzSenior member

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Why do you keep ignoring the two best examples and pull out cars with different aero shapes, different weights, different drivetrains, etc?

VW golf TDI vs. GTI. BMW 335i vs. 335d. For that matter, BMW M3 vs. 335i OR 335d.

You keep talking about axle torque as if cars are advertised by that number. I'm not arguing that the car putting more force to the ground accelerates faster. I'm arguing that knowing the torque at the engine tells you nothing about the force the car is putting to the ground. NOTHING.

15. ### KookzSenior member

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I realize from reading your previous arguments in this thread that you'll drag this on, changing the original argument to suit you, until the other person gives up out of frustration, so I'll just say I'm done posting on the subject.

I haven't owned a car in 5 years, but I've been considering something to take my bike to track days in, and I came across this, apparently the Element is pretty huge inside. It wouldn't be fun to put in or get out, but it can't be that bad.