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:eek: Did anyone else notice in the S5 study guide, the power output curve shows that the engine develops closer to 380HP vs. the 354HP (@7000 rpm) that is quoted in the table - I wonder how Audi rates the Engine output,- with or without the air-conditioning, generator running.

Do you think we are really getting closer to 380HP? Audi is often conservative with their numbers.
 

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Did anyone else notice in the S5 study guide, the power output curve shows that the engine develops closer to 380HP vs. the 354HP (@7000 rpm) that is quoted in the table
You're reading the wrong scale for horsepower. The scale on the left of the chart is for torque, the right side is for horsepower, and they use the same gridlines. The top of the red line is a little more than halfway between 332 and 369, which would put it right at the stated 354 hp.

Sorry to burst your bubble. ;)
 

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I think it's the same as in the motorcycle world. Power measured at the engine level is the highest. Dyno usually measured power at the rear wheel and therefore takes to account the efficiency (or inneficiency) of the driveline: flywheel, transmission, chain, axles, ... and accerssories like exhaust, filters, cooling, ... + undfer the condition of the test (temp, pressure, ...). The rear-wheel hp is always less than the engine hp. The question is what the car manufactureres (Audi) mean by bhp and engine hp. I think the difference might be:L
- engine: engine at the shaft or flywheel with no accessories
- bhp: rear or front wheel (or maybe like above) with all standard accessories)
So if we want to make the car more powerfull let's drop all unnecessary accessories, perhaps remove also all not needed weight and drive. Is there an A5 package option on the S5 :)?
 

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Not really sure as the scale on the left is in KW which is power. Right is torque. The graph is confusing as the left side scale clearly indicates kW units while the red line is labeled hp.
If the engine power was indeed 380 kW than it would be 516 hp or so.
You're reading the wrong scale for horsepower. The scale on the left of the chart is for torque, the right side is for horsepower, and they use the same gridlines. The top of the red line is a little more than halfway between 332 and 369, which would put it right at the stated 354 hp.

Sorry to burst your bubble. ;)
 

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Wow, this chart is very dissappointing to me actually. I can't believe that the engine makes so little power below 3k, no wonder it is geared so high. Also, for those of you that misread the legend it is:

Left side: HP (KW)
Right Side: FT-LB (Nm)

So, on the left hand side the red line for Power peaks at 380HP / 285 KW
On the right hand side the blue-green line for Torque peaks at 332lb-FT / 450 Nm
 

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Aerodave the scale on the left hand side shows HP and the right is Torque

Therefore the 380hp is clearly shown against the HP scale on the left, right?
Ummm...yeah. You are correct, and I got myself all backwards.

In any case, the graph is screwy and is not to be trusted, because the power and torque curves don't act like they're from the same engine. Horsepower is a direct function of torque and engine speed. You can always relate them as follows, if you use units of lb-ft and hp (different conversion factor for metric):

Code:
(Torque x Engine speed) / 5,252 = Horsepower
But the chart in the SSP doesn't follow that logic. Here's an example: the reported peak torque value is 325 lb-ft at 3500 rpm. The blue line seems to agree with that, too (using the proper axis ;) ). But if you do (325*3500)/5252, you get 217 hp. There's no way you can read that chart to make the red line 217 hp at 3500 rpm. It's almost as bad at the top end. If you use the reported max HP value of 354, and do the formula backwards to solve for torque, you get 266 lb-ft. The blue line is clearly higher than that at 7000 rpm.

Okay, so what if you use the apperent value of the red line (380 hp) and solve for torque at 7000 rpm? Then it gives 285 lb-ft, which is believeable enough. However...when you do the same thing for higher values (7500, 8000 rpm), you find that the curves don't really agree at all. In fact, i took the liberty of turning the blue line, as drawn, into a horsepower curve, and ignoring the red line given in the SSP. Surprisingly, it actually predicts a horsepower rise all the way to 8000 rpm, where it just starts to level off. That is, there is no peak until after the torque curve stops. That's absolutely not right.

So, consider the chart in the self study guide to be nothign more than a notional cartoon. It's not precise enough to read real data from.
 

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Aerodave is correct. The power and torque curves do not correlate, therefore this diagram is fake.

In S.I units (W for power, rotations per second for engine speed, Newtonmeters for torque) you have the following relationship between torque and power:

Power (W) = 2* phi * rps * Torque.

For example, if the torque at 2000 rpm is 500Nm (as for instance in my 3.0 TDI :)), the power output at 2000 rpm is:

Power = 2 * 3.14* 2000/60 * 500 = 104666 W , which is approx equal to 142 hp. (if I remember correctly the correlation between hp and W is that 1 kW = 1.36 hp)..

So for my measly 3.0 TDI, the max power I have to accelerate the car at 2000 rpm is 142 hp. How much have you got in your S5:s at this speed? Well, a proper torque curve would show just that. :D

It would be interesting to see actually! May it be so that the V8 is in fact weaker than the V6 Diesel at low rpm´s ? If so, at what engine speed do you have the break-even point at which the S5 outruns the A5?

Interesting interesting, at least if you are an engine geek like me :rolleyes:
 

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I disagree that the provided power/torque graphs don't make sense. I've seen many dyno results and have not noticed any linear (or any other) relation between the power and torque throughout the entire rpm range. Sometimes both power and torque go up at the same time, sometimes they go down, and sometimes one goes up and the other goes down. Rates of change change all the time. Yes, initailly the relation seems to be semi-linear but some place within the rpm range things go differently. I attach the graphs from my bike (tuned a bit) and the manufacturer's graph of power and torque. I tend to think that the car engines behave similarly.
 

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Considering the weight of the car and the 0-60 times and quartermile times I have seen on the S5 I would think that the motor puts out more then what Audi specified it at.
I think Audi said 0-60 in the 5.1 range but most tests have it at 4.7-4.8. My 425 horse/torque Pontiac GTO weighs about the same but it is just slightly faster at 4.5? When I testdrove the S5 I was very impressed about the power for being rated as it is.

Tonny
 

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martino said:
I've seen many dyno results and have not noticed any linear (or any other) relation between the power and torque throughout the entire rpm range.
I never said that the curves have to go in the same direction. It's quite normal for the power curve to rise linearly over the same region where the torque curve is flat. In fact, it has to happen that way, because power is the product of engine speed and torque. If you have a flat stretch on a torque curve, torque is constant but engine speed is increasing. So, too, will their product increase. The power curve can even continue to rise as torque goes down (if the negative slope of the torque curve is slight enough).

It is an inescapable, undeniable consequence of physics that the torque-vs-rpm curve and the power-vs-rpm curve have a precise, one-to-one relationship. If you give me a torque curve over rpm, I can easily generate the one and only one power curve that corresponds to that. The same is true going the other way. Any differences on a dyno printout are products of measurement error, and nothing else.

You say the graphs can act unexpectedly, with the curves moving in different directions. Sure the curves may not act intuitively to someone who hasn't looked into the math, but a little knowledge of the underlying physics reveals that the two curves move relative to each other in a very prdictable way. In fact, it can be easily shown that the power curve behaves proportionally to the integral of the torque curve. And, vice versa, the torque curve has the behavior of the power curve's derivative with respect to engine speed.
 

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Discussion Starter · #20 ·
A plausible explanation?

I did Physics & Engineering as well:cool:

Could there be a difference between maximum torque and torque which is developed during a maximum HP test?

Does anyone know how manufacturers measure maximum torque - is it the point at which the engine stalls? or drop by some pre-determined speed?

By the way, for fun here are the curves for the RS4 and Q7 - note that the hype about extra power in the RS4 - it is only available after 7000 rpm - not that useable in every day driving...in my opinion.
 

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