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TDI Power Enhancements Archives on TDI Power Enhancement related items.

View Poll Results: Which car would win in a drag race?
Car A will win because it has twice as much torque. 199 50.90%
Car B will win because the engine spins faster. 73 18.67%
The cars will tie. 119 30.43%
Voters: 391. You may not vote on this poll

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Old February 12th, 2006, 11:29   #61
Fix_Until_Broke
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Quote:
Originally Posted by jnecr
Originally Posted by jackbombay
Do you think Atomic Sushi or any of the other drag racers here wind it up to 4500-5000 RPM when drag racing or do they shift at 2500 RPM to get the revs back down to 1900 as soon as possible as thats where you believe the rate of acceleration is greatest?

Because of gear redution, you can accelerate faster in nearly all of second gear than you can in third gear... Man I wish somebody with a g-meter could just do some basic measurements...
It is true that you can accelerate faster in 2nd than 3rd as you say, but it has nothing to do with gear reduction - it's moastly due to aerodynamic drag (assuming same vehicle). You are delivering the same ammount of power to the wheels, just a larger % of it is going to overcome drag than accelerate the mass of the vehicle.
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Old February 12th, 2006, 11:31   #62
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Quote:
Originally Posted by Fix_Until_Broke
It is true that you can accelerate faster in 2nd than 3rd as you say, but it has nothing to do with gear reduction - it's moastly due to aerodynamic drag (assuming same vehicle). You are delivering the same ammount of power to the wheels, just a larger % of it is going to overcome drag than accelerate the mass of the vehicle.

Hah, that's real funny, you honestly believe that? In which case would you like to answer the question of why we do dynos in third gear rather than 1st or 2nd??
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Old February 12th, 2006, 11:35   #63
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Quote:
Originally Posted by jnecr
http://wahiduddin.net/race/dynotest.htm

about half way down the page you will find the acceleration curves of a vehicle in each gear. Please notice the units on the side of the graph

[/left]
That is not a good example because the TQ peak is very close to the HP peak, but it will work to show where your misunderstanding is.

Quote:
Originally Posted by the link that jnecr posted
For example, in the Spec Racer Ford, keeping the engine turning above 4000 rpm but below 5600 will get maximum acceleration
Now letslook at the HP curve,



and now the TQ curve,



From 4000 to 5600 RPM the torque does nthing but decrease, while if you look at the HP curve the peak HP output is centered between 4000 to 5600 RPM. In short you link proves my point, that max acceleration is acheived when the area under the HP curve maximised not area under the TQ curve.
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Old February 12th, 2006, 11:39   #64
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Originally Posted by jnecr
Hah, that's real funny, you honestly believe that? In which case would you like to answer the question of why we do dynos in third gear rather than 1st or 2nd??
"We" run dynos in 4th because it is a one to one ratio and eliminates trading HP for TQ.

Running a DYNO in 2nd would give you higher TQ numbers at the expense of RPM, so you end up with the same power output.

I'm going to headbutt some nails now.
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Old February 12th, 2006, 11:39   #65
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Quote:
Using the engine torque data, along with the transmission ratios and the tire diameter, it is possible to calculate the rear wheel thrust in each gear.
From:
http://wahiduddin.net/race/dynotest.htm
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Old February 12th, 2006, 11:41   #66
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Quote:
Originally Posted by jackbombay
"We" run dynos in 4th because it is a one to one ratio and eliminates trading HP for TQ.

Running a DYNO in 2nd would give you higher TQ numbers at the expense of RPM, so you end up with the same power output.

I'm going to headbutt some nails now.
No.. you won't "trade" torque for HP, both figures will increase at a ratio that will be the same as the gear reduction ratio. If, of course, you plot the curve in usual format with posting engine RPM rather than wheel RPM.
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Old February 12th, 2006, 11:44   #67
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Quote:
Originally Posted by Fix_Until_Broke
It is true that you can accelerate faster in 2nd than 3rd as you say, but it has nothing to do with gear reduction - it's moastly due to aerodynamic drag (assuming same vehicle). You are delivering the same ammount of power to the wheels, just a larger % of it is going to overcome drag than accelerate the mass of the vehicle.

Quote:
Originally Posted by jnecr
Hah, that's real funny, you honestly believe that? In which case would you like to answer the question of why we do dynos in third gear rather than 1st or 2nd??
It probably depends on the dyno - if it is on rollers, my guess is the limit of traction available between the tires and rollers. If it is on a hub mounted dyno, I don't know the answer. I would think that one would do a dyno run in the gear that is 1:1 with the engine, that would minimize transmission losses due to not driving the counter-shaft. The above are my thoughts - I have not spent any time on a dyno, although I would like to think I am familiar enough with the principles to discuss them intellegently. I'm open to correction.
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Old February 12th, 2006, 11:47   #68
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Let's say that they're both 4-cylinder diesels, running fuel from the same source, the same batch, and the same pump.

Tentatively, I'll say Car B by a few millimeters.
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Old February 12th, 2006, 11:50   #69
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Well the answer is you run a car on a dyno in a gear that is closet to a 1:1 ratio is because if you run it in 1st gear (let's say it's a 4:1 reduction) you will get a torque rating of something like 600lbft of torque...
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Old February 12th, 2006, 11:53   #70
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Quote:
Originally Posted by jnecr
No.. you won't "trade" torque for HP, both figures will increase at a ratio that will be the same as the gear reduction ratio. If, of course, you plot the curve in usual format with posting engine RPM rather than wheel RPM.
You need to re-read this thread, you are not understanding the concepts being discussed.

Gear ratios greater than 1 (1st 2nd and 3rd gears in a VW TDI) DO exchange HP for TORQUE, not HP and TQ output of the engine, but RPM and TQ output of the transmission.

Did you read my explanation of how a 2 HP weedwacker engine canmake 1000 lb/ft of torque? It can because it trades HP for TQ at the expense of RPM that is why the weedwacker can produce 1000 lb/ft at 10.5 RPM, the weedwacker engine cannot possibly run at 10..5 RPM that is why the a gearbox is necessary to "trade" Hp for Tq at the expense of RPM
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Old February 12th, 2006, 11:56   #71
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Quote:
Originally Posted by jnecr
Without a doubt, the car in second gear will accelerate quicker.
Yes, and why is that? Is the gearbox some sort of magical device that creates energy from nothing? No. All it's doing is just converting torque-in to torque-out with a proportional change in the input and output shaft speeds. The common denominator is that the power going into the gearbox is still the same as the power (magnitude) going out of it (well, minus losses, of course).

Quote:
But that is a different scenario than the one of instantaneous acceleration of cars in the same gear travelling at different speeds in a vacuum.
Vacuum or no vacuum , I believe I covered what you are saying in my first post in the thread.:

IF the question were reworded and you set the gear ratios to be the same between the two cars and you stay in the same gear during the acceleration run, then you change everything, and then YES, engine TORQUE alone will determine the acceleration because now you've set the relationship of the rotational speeds of the wheels relative to the engine between the two cars to be constant, and then, the engine with the greater torque wins the race.
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Old February 12th, 2006, 11:59   #72
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Quote:
Originally Posted by jnecr
Well the answer is you run a car on a dyno in a gear that is closet to a 1:1 ratio is because if you run it in 1st gear (let's say it's a 4:1 reduction) you will get a torque rating of something like 600lbft of torque...

BINGO, you are very close to getting this now. Why do you suppose you would end up with 600 lb/ft on a dyno in 1st gear? Because the tranny tradded away a lot of RPM for torque, which has nothing to do with the output of the engine. Sure you had more torque at the wheels but with less RPM at the wheels so you don't have any more power than you would have if you ran the dyno in 4th gear

Are you familiar with the following equation?

HP = TQ X RPM/5252
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Old February 12th, 2006, 12:03   #73
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Quote:
Originally Posted by jackbombay, a question that he REALLY wants JNECR to answer, pretty please :-)
You are drag racing, if you loose the drag race you will loose your job and your house, obviously there is a huge incentive for you to win, at what RPM will you shift your car? Assuming of course you are driving a pre 2003 5 speed VW TDI.
Well?
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Old February 12th, 2006, 12:04   #74
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Quote:
Originally Posted by jnecr
Well the answer is you run a car on a dyno in a gear that is closet to a 1:1 ratio is because if you run it in 1st gear (let's say it's a 4:1 reduction) you will get a torque rating of something like 600lbft of torque...
Yes - and your horsepower will be the same (assuming transmission losses are equal for each gear ratio)

HP = (Torque * RPM)/constant of your choice

The dynomometer measures torque and speed and then calculates horsepower. It does not care what drives it, what gear it's in, etc as long as the mechanical limits of the dyno are not exceed.
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Old February 12th, 2006, 12:26   #75
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Quote:
Originally Posted by Fix_Until_Broke
HP = (Torque * RPM)/constant of your choice
Constant of your choice?!?! Do you think they just plucked a number out of space? The number must be 5252 if you want to convert torque to HP. There is a long equation as to how this number was reached and you can find it on the internet, but I don't really think we need to go into that...


Quote:
Quote:
Originally Posted by jackbombay, a question that he REALLY wants JNECR to answer, pretty please :-)
You are drag racing, if you loose the drag race you will loose your job and your house, obviously there is a huge incentive for you to win, at what RPM will you shift your car? Assuming of course you are driving a pre 2003 5 speed VW TDI.
Well?
Right around 4700RPMs...


So with that mainly unrelated question out of the way..

The original question that we are now discussing is:
Is a TDIs instantaneous acceleration higher at 1900RPM or at 3500RPM?
We can reword this in mulitple ways:
Will a car's instantaneous acceleration be higher at it's HP or torque peak?
Will a car's longitudinal acceleration plot mirror it's torque dyno curve or it's HP dyno curve?


I finally found a good example to show my side of the discussion (not argument, mind you.. ):
So we'll go to number 3 on the wording, "will a car's longitudinal acceleration plot mirror it's torque dyno curve or it's HP dyno curve?"
Well.. there's a very simple conclusion if we examine how a dyno works in the first place.
What does a dyno really measure?
The fact is there is no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula BHP = Torque (ft/lbs) x rpm/5252.
A Dyno measures the force put on the rollers. F=ma (Force equal mass * acceleration)
Force on the rollers is therefore the roller mass multiplied by the acceleration. This force is multiplied by the radius of the roller itself to give torque at the wheels using the following equation:
T = Fr
Where T is torque; F is force; r is radius of application

Therefore we can easily show that acceleration is directly related to torque because, after all, that is how we arrive at the torque curve to begin with.

If it were the other way around (HP related to acceleration) we would directly measure HP on a dyno and have to have an equation similar HP = Tq*RPM/5252.

So have I made my point? If I haven't I will be more than pleased to explain this again in a different example, just gimme time to think of one... But in reality I think this is probably the best one I'll ever come up with.
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