# Car forward power

Discussion in 'Community Discussion' started by madamimadam, Aug 1, 2006.

Joined:
Jan 3, 2002
#1
Hi, I know this doesn't really suit any of the rooms but I thought I'd throw the question out there.

I need to know the power my 1.8L Lancer has in 3rd and 4th gear between the speeds of 60 and 85KM/h

Ultimately I want to use verified/creditable stats to show the impact that a hill of x incline has on the car.

I've called Mitsubishi and they don't have this sort of info available.

I am able to find the incline of hills in my area so I could do tests between the legal speeds of 50 - 60KM/h but this would not be creditable, not correctly show the same situation from 60 - 85KM/h and would have an error range that would be exceptionally difficult to measure.

2. ### saabmp3 macrumors 6502a

Joined:
Jul 22, 2002
Location:
Tacoma, WA
#2
Ok, well what you want to find is horsepower & torque curve vs. RPM's. The horsepower curve isn't going to change, no matter what gear you're in. So in 3rd and 4th, it'll be the same, just dependant on the RPM you are currently at.

However, the torque DOES go up, depending on gear. For this you need to find the gear ratio of 3rd and 4th (4th is probably 1:1 unless you have a 6 speed). If 3rd is a .70:1, then your torque goes up by that ratio at any specific RPM.

Does that help?

Ben

Joined:
Jan 3, 2002
#3
Not really sorry

I understand that torque is a rotational measure and I don't know how to work out the forward power based on the wheel rotational power.

4. ### groovebuster macrumors 65816

Joined:
Jan 22, 2002
Location:
3rd rock from the sun...
#4
Sorry, but the torque produced by the engine is not depending on the gear. Just as the hp curve it only depends on the rpm of the engine and is related to the hp produced. That's why you usually see the two curves in one diagram.

Then there is the transmission with it's gear ratios. So the torque measured at the wheel depends indeed on the gear you are in.

So what saabmp3 said wasn't completely wrong... it was just... 'incomplete'!

Maybe this Wikipedia article helps you:

gear ratio

... and this one ...

torque

groovebuster

5. ### After G macrumors 68000

Joined:
Aug 27, 2003
Location:
California
#5
From wikipedia:

Code:
```power = work / time
work = force * distance
force = torque / radius of wheel
distance = radius of wheel * time * angular speed
angular speed = 2 pi * rpm```

You end up getting:

Code:
```power = (torque / wheel radius) * (wheel radius * time * angular speed) / time
= torque * angular speed
= torque * 2 pi * rpm
```
The power will be the same even if you use torque to calculate it because the power equation removes the dependence on the dimension of the wheel.

Edit, darn, beaten. Good job groovebuster. I thought it might be more useful written out though in more familiar terms.

If you want RPM, you can read the Wiki page on torque to convert it. I just wrote the formula out to explain how torque applies to power.

6. ### groovebuster macrumors 65816

Joined:
Jan 22, 2002
Location:
3rd rock from the sun...
#6
Careful... the torque in the equation by 'After G' is the torque at the wheel and not the torque produced by the engine...! This torque still needs to be calculated!

groovebuster

7. ### After G macrumors 68000

Joined:
Aug 27, 2003
Location:
California
#7
Maybe I'm oversimplifying a bit, but ...

1) He could find the engine torque from the torque - rpm - HP curve, right?
2) Wouldn't the torque at the wheel be what he gets after multiplying engine torque through by the gear ratio, which he could get from the torque - HP curve?

Just using other posters' advice as a stepping stone.
It's easier to find a curve than to calculate the shaft torque by hand.

Joined:
Jan 3, 2002
#8
This is some pretty great stuff guys

Only thing is, where do I get the curves?

9. ### garybUK Guest

Joined:
Jun 3, 2002
#9
Stick it on a rolling road and get the thing calculated for you

10. ### After G macrumors 68000

Joined:
Aug 27, 2003
Location:
California
#10
I found a way (after some googling) to calculate your torque and HP

Torque = HP * 5252 / RPM (from wiki)

HP = weight / (ET / 5.825)^3
(where ET is the time it takes you to do a quarter mile) or

HP = weight * (speed / 234)^3
(where speed is the speed at which you are going at the end of said quarter mile)

You can get RPM from looking at the RPM gauge when driving.

I can admit when I made a mistake, I guess calculating torque is easier Hope you can figure out what you need to with the info we've provided.

When i had to find this sort of information for a Yamaha FZR-600, I had a librarian at university help me out. That was easier than being accurate by calculating how much power a certain fuel intake gave.

Usually, you make your own torque-RPM curve with a dynamometer, though you could use a stopwatch and run your car a few times to make your own rough curve. There's also a device that mounts on your dashboard ... forget what it's called though. You might also find one in a repair manual for your car. That's if you don't like doing the math.

Joined:
Jan 3, 2002
#11
So I've done up a dummy calculation based on a different model lancer (2.4L instead of 1.8L)

(torque * ratio) * 2pi * 3500
= (220 * 1.03) * 2pi * 3500
= 226.6 * 6.28 * 3500
= 4,980,668

How do I use this to calculate acceleration?

This particular car weighs 1255Kg

EDIT AFTER READING LAST POST:

So that I can see how to get to the end point let's assume that the torque is engine torque and I'll do the other calculations later so:

220 * 2pi * 3500
= 4,835,600

EDIT 2:

I've found that the 1.8L Auto has the following specs

Max Power (PS/RPM) - 122/5500
Max Torque (KGM/RPM) - 16.5/4000
Kerb Weight (Kg) - 1125

Joined:
Jan 3, 2002
#12
So, I've worked out that force applied to the car by gravity = 5240Nm

I can't get an exact figure so I'm going to work on the idea that torque at engine is 210Nm @ 3500 RPM

Thing is, I still can't work out how I know how much acceleration the car can have at maximum torque

13. ### aquajet macrumors 68020

Joined:
Feb 12, 2005
Location:
VA
#13
Why not use a dynamometer? That'll give you some exact figures to work with.

Joined:
Jan 3, 2002
#14
Yeah, I do have one of those in my back yard

Oh well, I'm about to leave for court so I just hope the info I have will be good enough

15. ### aquajet macrumors 68020

Joined:
Feb 12, 2005
Location:
VA
#15
This is where a little resourcefulness comes in. Ask around at your local garages. You're likely to find somebody that can help you out. It might cost you a small bit, but it'll give you the information you need.

Joined:
Jan 3, 2002
#16
Thing is, I don't see why I should pay anything

I've been wronged by the police but I should pay a solicitor and a mechanic to prove this???

17. ### Sun Baked macrumors G5

Joined:
May 19, 2002
#17
The force of gravity on the vehicle will provide a counter to the torque, based on the incline of the hill -- got to do you sine and cosines here to get the force horizontal to the road slowing the vehicle down.

As long as the torque of the vehicle isn't too much, at some point a car load of people will slow the vehicle down when you start going up the hill.

Edit: lack of torque, the incline, and number of asses in seats will all do wonders to slow the vehicle acceleration down on the hill.