Best measure Torque or BHP

[obi_waynne]

What is the best measure of a cars power and performance, the Torque or BHP figures?

We tend to compare based on BHP if we are honest, but this is often only a peak figure reached at a tiny part of the upper rev range and really doesn't tell the whole story.

 

[HDi fun]

Torque is a force, it's torque which makes a car accelerate. Both figures are relevant.

 

[old-git]

Yes, they are, but they serve different purposes. BHP determines top speed and lb/ft determines how quickly you get there.

 

[SLEEPER]

or bhp is how fast you hit the wall torque is how far you move it

 

[HDi fun]

Not quite, I think vehicle mass would determine how far it moves. Then again, lots of torque would let you carry on pushing at said wall.

Nice analogy

 

[old-git]

Agreed, once car makes contact with the wall, velocity plus mass will determine distance, not lb/ft. But I had to think about it for a little while

 

[HDi fun]

Double the speed and the inertia increases 4x. Perfect square law

 

[MADRod]

You cant separate the two they are bound together.
Horsepower = Torque x rpm/5252 or Torque = Horsepower x 5252/rpm
Any Power/Torque graph you see will cross at 5252rpm if it don't its a dud as maths don't lie.
The Topspeed of a car is determined by the Peak Power however if the Torque dies too early it may take a long time to get there. What you need is a big flat Torque curve so that when you change up at peak power your revs drop back onto that Torque & off you go again in each gear.

 

[800jer]

Both Torque and BHP run hand in hand. One is just as important as the other when it comes to cars.

 

[obi_waynne]

I wish manufacturers produced power graphs add this really shows where the power is.

 

[HDi fun]

You cant separate the two they are bound together.
Horsepower = Torque x rpm/5252 or Torque = Horsepower x 5252/rpm
Any Power/Torque graph you see will cross at 5252rpm if it don't its a dud as maths don't lie.
The Topspeed of a car is determined by the Peak Power however if the Torque dies too early it may take a long time to get there. What you need is a big flat Torque curve so that when you change up at peak power your revs drop back onto that Torque & off you go again in each gear.

Rod, that's nonsense. Torque is a force and BHP is power, ie. rate of work - units of work done per unit of time. The number 5252 is irrelevant, it's not revs. It's a ratio, ie it has no units!!!

Change any unit of measure (eg. measure torque in Nm; power in kW; angular velocity in radians per second) and 5252 disappears.

The arithmetic is correct, but the maths is wrong. Work and angular velocity both includes units of time and thus cancel each other.

 

[old-git]

Sits back with popcorn and prepares to learn

 

[thexav]

I look at the acceleration times 30-50 and 30-70 when comparing cars but its hard to get these figures without trawling the internet.

Most advertise a BHP figure but without a redline and torque curve it is pretty meaningless IMO.

I too am getting my popcorn ready OG!

 

[MADRod]

OG not worthy of a reply guys a nut. To argue with someone who has done this all his life been a Senior designer at BAe systems & done more calcs than this rather silly Mr. founder is not worthy of my time. SIR YOU ARE AN IDIOT !!!!!!

 

[HDi fun]

OG not worthy of a reply guys a nut. To argue with someone who has done this all his life been a Senior designer at BAe systems & done more calcs than this rather silly Mr. founder is not worthy of my time. SIR YOU ARE AN IDIOT !!!!!!

Suit yourself, Rod.

Just because something is said enough times does not render it accurate.

There's no need to be offensive either.

The formula you give is correct and can be used to calculate Brake Horse Power from engine speed measured in Revs Per Minute and Torque measured in foot-pounds. This works for any engine speed.

I am merely point out that 5252 is just 5252. It is a ratio. It's not 5252 Revs Per Minute.

 

[old-git]

I thought that 5252 was the RPM where the BHP and lb/ft curves crossed?

 

[HDi fun]

Yes but 5252rpm is not actually significant. It is a unitless ratio, as are all ratios. Measure torque in Nm and 5252 will disappear from the equation. (incidentally it's lbft not lb/ft).

We cannot possibly say torque and power are the same when they are different units of measure. Even if the numbers are identical.

It's like comparing gallons and acres. Different units for different pruposes.

In response to the original question though both BHP and torque are relevant measurements.

 

[old-git]

(incidentally it's lbft not lb/ft).

A little research resulted in me being even more confused. Even learned gentlemen and scientists disagree on which is correct. I have seen lb/ft, lbft, lb-ft, lbf. I guess they all convey what the writer means.

Lb/ft seems to = Torque

Ft/lb = Work

 

[HDi fun]

The / infers lbs per foot or feet per pound depending upon order.

lbft, ftlb, lb-ft and ft-lb are all one and the same. All will interchange quite safely. lbf likewise is fine, although possibly might be mistaken for pounds force which is a linear force.

Torque is an angular force (although through an infinitely short arc it can be considered to be linear) but is still acting about a centre of rotation.

 

[old-git]

When used in lb/ft the / doesn't infer lbs per foot any more than it does in 07/10/2015

However, I will use lbft in future when talking torque in order to avoid any confusion

 

[HDi fun]

I agree, in the context of the discussion it won't matter at all. Same as Nm is frequently written as N/m.

 

[thexav]

If in the "context of the discussion it won't matter at all" then why are we discussing it

 

[HDi fun]

Why not? This all stemmed from Dr Rod's comments to me.

 

[SLEEPER]

Yes 5252 is a value.......
But what Madrod said was the formula means the two lines have to cross at 5252 revs and this is correct.
And if two terms are in an equation then they are related

 

[HDi fun]

Really?

Provided that we use BHP and lbft as units. Plot power in kW and 5252 disappears.

It is simply an equation to calculate BHP from torque and angular velocity.

Torque and power are related but the crossing of lines at 5252 is not relevant.

 

[obi_waynne]

I'm thinking I should snip and move creating a new thread about "5252 and the Torque Curve" as we're going off topic and there does seem to be plenty of debate left here. Any objections guys?

I'm also happy to leave this discussion here as it is in it's present form. (At least until things start getting thrown!)

 

[HDi fun]

Why not?

 

[obi_waynne]

The thread for 5252 is here https://www.torquecars.com/forums/threads/5252-and-the-torque-curve.36200/

If I've missed out any posts from this thread into the new topic please let me know. I did this quickly and I've already spotted one missing which I've moved across now.

Let's keep this one for the original topic
What is the best measure of power Torque or BHP

 

[SLEEPER]

why would we plot something in kW in a thread about bhp ?

MR and I have said that the lines cross at 5252rpm which on a bhp graph is a fact and that was all that was said to which as far as I could see your reply was. ........ nonsense

Since the topic was bhp or torque the discussions are about just that bhp graphs not ones with kW involved,

So our comments are the lines crossing are correct and not nonsense,( and only relate to bhp graphs which as said before was the topic.

If you want to talk about a graph with kW in start a kW thread

 

[SLEEPER]

Wayne the comments were relevant how else do you answere the question

 

[obi_waynne]

Wayne the comments were relevant how else do you answer the question

The question was What is the best measure of power Torque or BHP - we seem to be debating where and why the points cross on the dyno graph, so this is not relevant to the original question (at least to my mind but I'm only one person). That is why I've started a fresh thread about the 5252 figure to hopefully keep this one focussed on the original question.

I guess that is the thing with forums, topics drift around and debate happens. I don't mind really, but that little thing in my head is trying to create order and consistency. I'll never survive out there in the big world will I?

 

[obi_waynne]

It's more important to look at the power curve IMO, and the point where you get into the peak power zone that matters.

I like fat wide torque/power bands.