Shift at 3K, 4K or 4.5K RPM for max acceleration?

Given the power/torque graph of the 90HP TDIs where...

• Torque peaks and levels early (2000 to 2500 RPM) then starts slowly dropping
• Torque meets the rising HP curve at 2900 RPM
• Torque continues to drop while HP contines to climb where HP peaks and levels off at 3500 to 4000 RPM. Torque is 130 ft-lb to 110 ft-lb in that same RPM range.

Shifting at 3000RPM I stay of the left of the crossing-point (more torque than HP), shifting at 4000 RPM I stay to the right (more HP than torque).

Taking gearing, HP and torque into account, will max acceleratlion be had shifting at 3000 RPM, 4000, 4500 RPM, or somewhere in-between?

i think shifting at 4k rpm is fine if necesary, i usually shift at 3.
i might have the wrong graph tho. this is the 84kW engine...not sure which one you (or i, for that matter) have

I used this graph http://pics.tdiclub.com/showphoto.php?photo=1591&cat=533

to determine the data used (the blue lines)

Mine is the pre-2004 90HP VE TDI (engine code ALH)

Bob, shift it at 4K for the best performance in a stock car. Modded cars run a bit further up the band.

Bob_Fout said:

I used this graph http://pics.tdiclub.com/showphoto.php?photo=1591&cat=533

to determine the data used (the blue lines)

Mine is the pre-2004 90HP VE TDI (engine code ALH)

Click to expand...

they look pretty much the same. torque tops out at ~2k, horsepower at around 4.

does this mean i should also shift at 4 in my 5.5? after the breakin, of course.
(mk4? sorry, i still have no clue what engine model is in there)

yep, unless the car is chipped then they can run out a lot longer.

Soooo....why does shifting at (4K RPM) and running in the higher HP range (3k to 4k RPM) in each gear yield higher acceleration than the high torque and lower HP range (shift at 3K and run at 2K to 3K) in each gear?

Is it because in that range (3K to 4K) HP is high and torque is decent too vs. the low HP and highest torque of the 2K to 3K range?

Or perhaps I'm missing the real-world relationship between HP and torque?

I dont quite understand what matters about where the hp and torque graphs "cross"? They are on different scales? One is in ft-lbs of torque and the other is HP so when they cross is pretty insignificant.

Torque is the amount of twisting force the motor puts out. HP is the amount of work that the motor is doing at a given instant.

Once you get to the output shaft of the tranny torque doesnt matter any more. HP is all you put to the ground. So you will want to keep the car where maximum hp is for maximum acceleration.

Bob_Fout said:

Soooo....why does shifting at (4K RPM) and running in the higher HP range (3k to 4k RPM) in each gear yield higher acceleration than the high torque and lower HP range (shift at 3K and run at 2K to 3K) in each gear?

Is it because in that range (3K to 4K) HP is high and torque is decent too vs. the low HP and highest torque of the 2K to 3K range?

Or perhaps I'm missing the real-world relationship between HP and torque?

Click to expand...

Power = the rate of change of work done in an instant of time. IOW: how much more work you can do in this instant than the last instant. Torque is work. Lifting a ten pound cinderblock over your head takes about 70 ft-lbs of work. Lifting it in one second takes more power than lifting it over 3 seconds.

Power is what accelerates the car. Torque is how much available work the engine can do at that particular RPM. The maximum steepness of a hill your car can climb is determined by the peak torque available. How fast you can climb that hill is limited by the peak power available.

(Last sentence much simplified by ignoring traction, gearing, etcetera...)

david_594 said:

I dont quite understand what matters about where the hp and torque graphs "cross"? They are on different scales? One is in ft-lbs of torque and the other is HP so when they cross is pretty insignificant.

Torque is the amount of twisting force the motor puts out. HP is the amount of work that the motor is doing at a given instant.

Once you get to the output shaft of the tranny torque doesnt matter any more. HP is all you put to the ground. So you will want to keep the car where maximum hp is for maximum acceleration.

Click to expand...

Yes, different units of measure and scales, but important to my initial question. No matter how the data is displayed, it's still true that in the case of the ALH, to the left of the crossing point, torque is higher for any given RPM, to the right, HP is higher.

nicklockard said:

Power = the rate of change of work done in an instant of time. IOW: how much more work you can do in this instant than the last instant. Torque is work. Lifting a ten pound cinderblock over your head takes about 70 ft-lbs of work. Lifting it in one second takes more power than lifting it over 3 seconds.

Power is what accelerates the car. Torque is how much available work the engine can do at that particular RPM. The maximum steepness of a hill your car can climb is determined by the peak torque available. How fast you can climb that hill is limited by the peak power available.

(Last sentence much simplified by ignoring traction, gearing, etcetera...)

Click to expand...

I'm beginning to understand now... now that you bring Calculus into it.

nicklockard said:

Torque is work. Lifting a ten pound cinderblock over your head takes about 70 ft-lbs of work.

Click to expand...

Torque is force. You can have all the torque in the world but if its not moving then no work gets done. Force applied over distance is work. So thus torque applied over distance is work, torque in itself is not wrk.

david_594 said:

Torque is force. You can have all the torque in the world but if its not moving then no work gets done. Force applied over distance is work. So thus torque applied over distance is work, torque in itself is not wrk.

Click to expand...

Torque = F *cross* L = F X L

|X= the standard notation for the vector cross product operation, not simple multiplication.

Torque = The vector cross product of force over length

Torque is work, not force.

Think about the units: Foot-pounds. Feet is length (L in the above cross product), and pounds is force (not mass, that would be a slug in the english system.)

F = m*a
W = F*d ..........................................replace F with it's equivilant m*a to derive:
W = m*a*d

In reality, you agreed with this, but accidentally slipped up your terms:

david594 said:

Force applied over distance is work.

Click to expand...

100% correct.

In the calculus defenition, power = dW/dt

P = dW/dt = how much more amount of work (torque in this case) you did in this instant of time than the last divided by that interval of time.



Lastly, a comment about torque and power:

Torque (at the crank) gets multiplied by the effective gear ratio. This is why a car with only 155 foot pounds of available (peak) work output can accelerate a 3200 lb car at a reasonable rate. In first gear, what's the effective gear ratio (don't forget the wheel/tire combination)?...multiply that by 155 to get the amount of torque at the rubber-ground interface.

Since power is dependent on torque (which got multiplied by the effective gear ratio) it itself does NOT get further multiplied by the effective gear ratio. Because of transmissive losses, power actually decreases as it gets closer to the rubber-ground interface.

I still stand by the fact that Torque is a force, not a measure of work. When you are talking about axial loads the distance is measered in radians, not feet(or meters). Torque is the force applied around an axis and the distance will be in radians for your work equation.

To find the force on the axis, you will take the cross product of the force against arm connecting you to the axis. My assumpting of torque = force x distance(or length of arm) does hold true when the force is being applied at a 90 degree angle.

http://en.wikipedia.org/wiki/Torque

Taken from there:

Torque has dimensions of force times distance and the SI units of torque are stated as "newton-metres". Even though the order of "newton" and "metre" are mathematically interchangeable, the BIPM (Bureau International des Poids et Mesures) specifies that the order should be N·m not m·N[1].
The joule, the SI unit for energy or work, is also defined as 1 N·m, but this unit is not used for torque. Since energy can be thought of as the result of "force dot distance", energy is always a scalar whereas torque is "force cross distance" and so is a (pseudo) vector-valued quantity. Of course, the dimensional equivalence of these units is not simply a coincidence; a torque of 1 N·m applied through a full revolution will require an energy of exactly 2π joules. Mathematically,
E = τθ where
E is the energy
τ is torque
θ is the angle moved, in radians.
Other non-SI units of torque include "pound-force-feet" or "foot-pounds-force" or "ounce-force-inches or meter-kilograms-force.

See the edits. I tried to explain it better. You are somewhat right in that you say torque = force *cross* length...but you're stuck on the angle thing. You're halfway there. You're just missing that the L term (distance) is the length of the moment arm.

Okay, I tried to clean it up and keep it simple. whew

For torque to accomplish any amount of work, some distance must to gone. The distance will be about the axis of which the torque is applied so

Torque = F X L

Work = Torque * Angular Distance

Work = F X L * Angular Distance

Power = Torque * Angular Speed

Work = Force applied crossed with Length of the arm times the Distance moved as measured in rotations of the shaft about the axis.

Yes. If the car doesn't move (or the crank doesn't turn) no work has been done. (simplified) You are right. I have been misreading what you were originally trying to say. It seemed your words had gotten mixed up. That last post made a lot more sense--your first one had me discombobulated. Other readers, the confusion was mine! Now that I re-read his first post carefully it now makes sense.

Ok... in the end it all makes sense. I was afraid I was going to be having another person saying "im clearly not a physics major"

Back on topic though...
For maximum acceleration you will want to be shifting as to keep the car at maximum levels of horsepower.

Yup. Would you agree with this analysis or do I need more coffee?

Since Torque (F *cross* L)* revolutions is equivilant to work, then Torque*rpm's can stand in for work here,

P = dT/dt *rpm = how much more amount of work you did in this instant of time than the last divided by that interval.


Since (if you don't change gears) L is constant, we can also say that

P = dT/dt *rpm= how much more amount of force (don't forget to formally cross with L to get the power numbers right) you did in this instant of time than the last divided by that time interval.

Force in an engine is a function of cylinder gas pressures (in the simplified sense)...so let's evolve the statement:

P = d(cylinder gas pressures)/dt * rpm= how much more amount of cylinder gas pressure (don't forget to formally cross with L to get the power numbers right...you also need to do conversions from force to gas pressures(complicated)) you did in this instant of time than the last divided by that time interval.

cylinder gas pressures are a function of fuel injection quantitiy and effective compression ratio (itself dependent on the turbo which gets its work from spent exhaust gas pressure from the previous cycles)...so we can evolve the statement further; let's assume a constant turbo boost to make the statement easier:

P = d(injected fuel quantity)/dt *rpm= how much more fuel you burn't (don't forget all the previous conversions (complicated)) in this instant of time than the last divided by that interval.

*These statements should include the theorem of limits...you have to shrink the time interval down to an infinitesimal nothing and you also shrink the d(x) down to an infinitesimal nothing, but I chose to eliminate the wording to keep the sentences cleaner.

You could probably explain it better.


*I need more coffee. I still have mixed units here.

me said:

Since Torque (F *cross* L)* revolutions is equivilant to work, then Torque*rpm's can stand in for work here,

Click to expand...

Should read:

me said:

Since Torque (F *cross* L)* angular velocity is equivilant to work, then Torque*rpm's can stand in for power here,

Click to expand...

corrections from here.

Since Torque (F *cross* L)* angular velocity is equivilant to work, then Torque*rpm's can stand in for power here,

Almost, Your were half right to begin with. The correct statment would be :

Since Torque (F *cross* L)* revolutions is equivilant to work, then Torque*rpm's can stand in for power.

Angular velocity = RPM which is over an interval of time.

Work is not measured over an interval of time, so you would leave it as revolutions. So ....

Torque*revolution = work
Torque* RPM = power

So yes, I agree with his technical jargain (I'm a mech E too)... .but to put everything in simplest terms, you should always have quickest acceleration when shifting at the peak of your POWER band. By this of course I mean horsepower.

To put it simply, torque's what gets your car moving when you have high resistance to movement (i.e.: from a standstill or up a hill). As you accellerate initially (with torque), your resistance to acceleration drops, you have built up momentum and horsepower starts to take over.

Hopefully that makes some sense... this is a hard topic to put simply, but hopefully this makes some sense.

As a motorcycle racer, powerbands usually peak at the upmost limit (like 500 rpm before) redline, usually you program your rev limiter at the upper limit, and as you accelerate you slam the rev limiter and shift gears w/o clutch, as it "automatically" stops the engine for a second.

Anyway.... i'm gonna stop before I confuse myself

ScottieD.

ScottieDucati said:

So yes, I agree with his technical jargain (I'm a mech E too)... .but to put everything in simplest terms, you should always have quickest acceleration when shifting at the peak of your POWER band. By this of course I mean horsepower.

To put it simply, torque's what gets your car moving when you have high resistance to movement (i.e.: from a standstill or up a hill). As you accellerate initially (with torque), your resistance to acceleration drops, you have built up momentum and horsepower starts to take over.

Hopefully that makes some sense... this is a hard topic to put simply, but hopefully this makes some sense.

As a motorcycle racer, powerbands usually peak at the upmost limit (like 500 rpm before) redline, usually you program your rev limiter at the upper limit, and as you accelerate you slam the rev limiter and shift gears w/o clutch, as it "automatically" stops the engine for a second.

Anyway.... i'm gonna stop before I confuse myself

ScottieD.

Click to expand...

Ok, that makes sense. Torque gets you moving. HP, once you are moving, gets you moving faster.

Whatever.. math hater I am.....I have 290ft lbs and it moves the car out NOW. The HP is lower than other diesels similar to mine and I still can outrun them.

jasonTDI said:

Whatever.. math hater I am.....I have 290ft lbs and it moves the car out NOW. The HP is lower than other diesels similar to mine and I still can outrun them.

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You HP is also a lot lower than a lot of gassers and you can outrun them too! People buy HP but drive TORQUE. You've got it all over them in torque and in an RPM range where it's most useful. Whether modded or stock, I love a TDI's meaty power band which makes it so easy and a pleasure to drive.

Ricer driver after getting whipped by a modded TDI in a traffic light brawl: "Hey man...your car is fast. What's it got under the hood?"
TDI driver: "Oh...just a 1.9L turbo-DIESEL. And I get 45-50 MPG with it too."
Ricer: "That's a DIESEL??"
Look on Ricer driver's face: Priceless.

Then tell him he's using the wrong fuel.

n1das said:

You HP is also a lot lower than a lot of gassers and you can outrun them too!

Ricer driver after getting whipped by a modded TDI in a traffic light brawl: "Hey man...your car is fast. What's it got under the hood?"
TDI driver: "Oh...just a 1.9L turbo-DIESEL. And I get 45-50 MPG with it too."
Ricer: "That's a DIESEL??"
Look on Ricer driver's face: Priceless.

Then tell him he's using the wrong fuel.

Click to expand...

And the mods are? List please.