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Discussion Starter · #1 ·
Ok assuming:

Any engine that produces 300 ft-lbf of torque at 3000 rpm has 171 hp at that speed. Regardless of it's stroke, bore, displacement, supercharger, etc. Also, any engine that makes 300 ft-lbf of torque at 5000 rpm will produce 286 hp.

This is due to the simple relationship between power and torque:

power = (torque * rpm) / 5250

where power is in horsepower, and torque is in ft-lbf. Also note - at 5250 rpm, hp is equal to torque numerically.

I know that my standard GT TDi Pd 150 produces max 150 at the flywheel and max torque of 245 ft Ib. Now the question is if we can work out at what RPM the engine produces its max BHP does that mean that its best to shift up gear rather than reving to make use of all the 245 ft Ib of torque again.

From the calculation above: power = (torque x rpm) / 5250

therefore 150 (Max BHP flywheel) = (245 x X) / 5250

I get 3200 RPM value for X ??? Does this mean theoretically that max power is at 3200RPM ??

Discuss.
 

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I have a basic understanding of this, but wouldn't that only hold true if the power band was a straight line?

As we all know the 1.9 TDI's aren't known for their smooth/progressive delivery [;)]
 

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Nope, as torque curves are rarely flat and engines rarely produce peak torque and peak power at the same time.

Peak power on a 150pd is around 4krpm from memory, so you've got approx 197lbft at peak power.

On the basis that each subsequent gear is 25-30% higher than the previous one, the engine would have to be making 25-30% more torque in the next gear at the change up point, so 250lbft ish.

If you change up at 4k it will drop your revs to about 3K at which point you're going to be making 210-220lbft.

In simple terms you will make fastest progress by holding each gear to the power peak or just Beyond it.

But there is a very narrow band around 2-2.5k where if you change up a gear you will momentarily go faster.

If you want to test it, get a stop watch and time each 10mph increment from 1000rpm to max revs for each gear, and then compare each gear.
 

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Discussion Starter · #4 ·
But the torque curve is relatively flat so if you are in the power band you should be making 245 ft Ib of torque right through to peak power of 150BHP? If you rev past peak power, has the torque all but gone also to carry you forward? It would seem so as reving the *** of a diesel car just doesn't work!
 

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Tim

The torque curve is anything but flat. As I've said you're making 245lbft at about 2000rpm, and just under 200lbft at 4krpm.

If you change up a gear the torque at the wheels will be reduced by 25-30% because of the change of gearing, so you'd have to be making 23-30% more torque at 25-30% less revs for it to work.

If you check my sig, I have a 130PD as well, and it is fastest against the stopwatch changing at about 4k rpm. The torque does drop off rapidly after the power peak.

My 18T's making around 245lb ft at 7k rpm, but 320lb ft at 4k rpm. It is much, much quicker if I rev it to 7K.

Its an old argument, but a car will accelerate at its fastest if it is kept around peak power by it gearing. Most engines make very close to peak power over around 1000rpm, so the gearing is designed that it will drop approx 1000rpm when you upchange just past peak power.
 

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Good thread! [:D]

Just one question John. You say:

engines rarely produce peak torque and peak power at the same time

I would have thought max torque and max power are at the same RPM, by definition? Seeing as it's a simple linear relationship of power = torque x RPM x some_constant

Excellent description about hanging on until after peak power, though, due to the next gear up having (obviously!) a larger ratio.
 

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Quote: posted by AndrewB on 23/12/2004 12:32:01

Good thread! [:D]

Just one question John. You say:

engines rarely produce peak torque and peak power at the same time

I would have thought max torque and max power are at the same RPM, by definition? Seeing as it's a simple linear relationship of power = torque x RPM x some_constant

Excellent description about hanging on until after peak power, though, due to the next gear up having (obviously!) a larger ratio.

Andrew as I said they don't

Peak torque for the diesels is at around 2000rpm, but peak power is at 4000rpm.

Torque*revs=power, so the same torque at half the revs= half the power.

A 150pd makes 245lbft at 2K rpm which is 93hp, my18T makes 245lbft at 7k rpm which is 325hp. Same torque, big difference in power.
 

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Discussion Starter · #8 ·
Maybe you need to tread my OE post again! It ios not as simple as Torque = RPM x Power mate. You've forgoten to divide by 5250! Look it up in a physics book but here it is again anyway!

"This is due to the simple relationship between power and torque:

power = (torque x rpm) / 5250

where power is in horsepower, and torque is in ft-lbf. Also note - at 5250 rpm, hp is equal to torque numerically."

In any car, whether diesel or petrol, turbo, naturally aspirated, or supercharged , at 5250 RPM the power and torque are the same. This is fundamental physics/mathematics!
 

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Tim

I think you'll find that power does in fact= torque x rotational speed. It is the rate of doing work.

The 5252 factor is just down to the fact that we keep quoting torque in lbft, and power in hp, which are stupid imperial units that need stupid conversion factors.

Just for ref 245*2000/5252=? (93hp)

245*7000/5252=? (you guessed it 325hp)
 

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I read this thread with great interest! and have alos stumbled across some info which is what both you guys have been saying take a peek!

:http://www.allpar.com/eek/hp-vs-torque.html
 

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Quote: posted by tim5150 on 23/12/2004 12:48:37

Maybe you need to tread my OE post again! It ios not as simple as Torque = RPM x Power mate. You've forgoten to divide by 5250! Look it up in a physics book but here it is again anyway!

"This is due to the simple relationship between power and torque:

power = (torque x rpm) / 5250

Yup, that's what I said! [:D] Power = Torque x RPM x some_constant

where in this case, some_constant is 1/5250. That's exactly what I said! Power is a function of torque and RPM, a linear relationship, multiplied by a scalar which is (for lbft to bhp conversion's sake) 1/5250. If changing from lbft to kW (as per European) the multiplication factor is summat else instead of 1/5250.

But either way, power in an engine is DIRECTLY proportional to torque x RPM.

I think you're forgetting that dividing the whole lot by 5250 is just the same as multiplying by 1/5250 [;)]
 

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Quote: posted by tim5150 on 23/12/2004 14:01:39

Are you claiming that your car has 325BHP then at 7000RPM???

um, are you aware of what IHI means mate? [:I]
 

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Quote: posted by Wilko on 23/12/2004 12:37:56

Quote: posted by AndrewB on 23/12/2004 12:32:01

I would have thought max torque and max power are at the same RPM, by definition?

Andrew as I said they don't

Peak torque for the diesels is at around 2000rpm, but peak power is at 4000rpm.

Ah! Sorry, I'm being dense. I understand what you mean now John.

Surely a good example would be if a given engine produced 200lbft at 2000RPM, but only 180lbft at 4000RPM... The latter example is clearly more power, because it's double the RPM. Despite being slightly lower torque. So its peak torque does not coincide with peak power.

I'm fully aware that power is a direct function of RPM and torque, though - no argument on that front. That's what the diesel folks often misunderstand. A petrol car with half the torque of a diesel does not mean half the power - because if it's producing half the torque at twice the RPM, then the output power is the same. Yes?
 

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Discussion Starter · #18 ·
Ok, got you! Hence the IHI badge.......Is this IHI conversion just for petrol cars or can diesels get it to?
 

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Sorry Tim but there's not a lot more you can do to increase the power of your diesel without going to Nos.

The amount of power an engine makes is down to how much oxygen you can get into the cylinders. Chipped diesels already run 1.5 bar of boost, which is about what a heavily boosted petrol motor runs as well. Theoretically the amount of oxygen (air) you can get into an engine goes up directly with boost, so at 1.5 bar of boost you'll make 2.5 times the torque that you'd make with no boost.

Unfortunately as air flow goes up, you get bigger pumping (pressure) losses, by trying to force more air through smaller holes (valves, intake tract etc). You also get more back pressure from the turbo as it's having to spin faster to move the air/generate pressure.

1.5 bar boost for an engine is a realistic limit before the you start to gain very little airflow for the extra pressure. There's only so much air you can get through a small hole into the top of a cylinder. As velocity goes up, the pressure losses go up by the square of velocity. There are some nutters who boost 2 bar or more though.

The problem with diesel is that it doesn't actually burn very well, and burns too slowly to provide complete combustion during the time available at high rpm. Hence why few diesels rev over 4.5K.

Most tuners just run the car richer (higher fuel to air ratio) an only up the boost slightly on Tdi's. 150tdi's run about 1.3 bar as standard, but don't run excessively rich, as this is where smoke etc comes from. With a diesel there is excess air available, so tuners add extra fuel to use up the extra air and make more power.

The VW tdi's could probably gain some extra power by reducing intake and exhaust losses, as they have an old 2 valve per cylinder head. A 16 valve ported head may improve it.

Nos will work well on diesels though. You're adding 2 oxygen molecules for every nitrogen molecule, where as using air you're adding 1 oxygen molecule for every 3 nitrogen molecules.

Petrol engines of the same capacity at the same boost are always going to make more power than a diesel, as they can rev so much higher, making similar torque but at much higher RPM, so consequently more power.

On the power/torque thing, I much prefer SI units, as 1Nm at 1 radian per second= 1W.

So 1 Nm at 1 rev per second (60rpm) = 1 x 2 Pi = 6.282W.

745.7W =1Hp

1.356Nm= 1lb ft

A KW and Nm dyno graph would cross at 9551rpm
 
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