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Question for car experts: does redlining make you accelerate faster?

Earlier I used to believe the longer you stay in low gear the faster you accelerate. 'Smart' automatic transmissions seem to apply this principle - if you stomp really hard on the pedal, it revs up almost till redline before shifting up.

Obviously, readlining increases engine wear (and sounds nasty) but I used to think its the fastest way to accelerate.

However, both torque and horsepower peaks are way before redline. For e.g. in my humble base-model Jetta torque peaks at 2600 rpm and hp peaks at 5600 rpm.

Logically, the quickest way to accelerate is to shift up as soon as you hit torque peak (i.e. 2600 rpm).

Why does this seem inconsistent with what most people usually do when they want to go fast?
 

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when you shift to the next gear you switch to a different ratio gear... this allows the rpms to drop down, however the rpms can only drop so far... the powerband is broader in the lower gears so in 1st, 2nd, and sometimes 3rd its ok to redline because the powerband will reach those points... in 4th and 5th the powerband is set shorter and in the lower rpms aproximately 3000-5000.... i hope this isn't to confusing...
 

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typically, you should shift at your horsepower peak, or 5600RPM for you. Redlining doesn't do you any good, as the power in your car actually decreases once you pass the peak. Also, redlining will have many adverse effects on your motors life and duration of performance, as you stress parts past their intended limits.
 

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Hmmm I guess I should stop raping every gear when I'm tryin to haul ass. Just kiddin, although I do tend to this, is just naturual recation when your trying to go fast.
 

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I would not take it to redline all the time you will shorten your engines life alot.Unless you like having a car that smokes like a dodge.
 

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yea ill def agree with the majority. redlining isn't a good thing to do with conisistancy.. i personally find my car alot faster when i shift when its pullin hard then i go rite into the next gear. i have tried taken it up to around redline area and it feels to slow u down. b/c the car is laging at the redline point. if u shift while ur car is still pulling u will have a better effect.
 

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WOW TF. I started reading your post, and I was like, "damn he is the shit-a shifting scientist". then, I read at the bottom that you didnt write that. my expectations oif you have seriously gone down. hehe-thanks very informational.
 

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<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by silverGTI:
WOW TF. I started reading your post, and I was like, "damn he is the shit-a shifting scientist". then, I read at the bottom that you didnt write that. my expectations oif you have seriously gone down. hehe-thanks very informational.<HR></BLOCKQUOTE>

 

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The classic mistake is to conclude that the fastest way down, let's say, a 1/4 mile drag strip is to keep the engine RPM at the torque peak (or as close as possible). The technique is usually stated as "shift just after the torque peak", or "shift N RPM above the torque peak so you are N RPM below the torque peak in the next gear when you finish the shift".

Unfortunately, *engine* torque does not tell you the full story. What matters is the torque *delivered to the tires*, including the effects of the transmission. We all know a car does not accelerate as hard in second gear at peak torque RPM as it does in first gear. The transmission amplifies or multiplies the torque coming from the engine by a factor equal to the gear ratio. So to determine how much the car is accelerating at a particular instant, you have to know both the torque output of the engine as well as the gear ratio.

To figure out your shift points knowing only torque, generate tables of transmission output torque vs. RPM for each gear. To get transmission output torque, multiply the engine torque by the gear ratio. You are simply comparing gear to gear, so the final drive ratio can be ignored. You may also need to know the relationship between RPM in one gear and RPM in another gear (which is RPM * (gear2ratio/gear1ratio) at any particular vehicle speed.) Then it's easy to see what shift points to choose to maximize your transmission output torque at all times.

Shift at the redline, not at the torque peak!

Walk through an example. You're hammering down the track in 1st gear. Engine RPM is 6000, just past the engine's torque peak. Do you shift? Well, if you do, the engine will be pulled down to 3600 RPM, and 2nd gear will send 246 ft-lb of torque to the wheels (actually, to the differential first, which amplifies the torque by a constant factor and sends it to the wheels). Don't you think it would be better to hold it in first gear? Torque is dropping off, but it's still 389 ft-lb at 7000 RPM, right before the 7200 RPM redline. So, for this powertrain, first gear is *always* the best deal for acceleration, at any speed, except that you can't accelerate past the redline.

The 1-2 shift at 7200 RPM pulls the engine down to 4400 RPM, where 2nd will deliver 265 ft-lb of torque. Not only did you win by maintaining the high torque of 1st all the way to 7200 RPM, you are now better off in second gear.

Same thing goes for the 2-3 shift. 2nd gear output torque at the redline is still greater than 3rd gear output torque at any engine speed, so you wind her out as far as she'll go before you shift to 3rd. Same for the 3-4, same for the 4-5.

But, you ask, isn't your acceleration greatest at the torque peak? Yes, it is! But only within that gear. The next gear down will give you even greater acceleration at the same speed, unless the vehicle speed is too high for that gear.

To use engine torque to understand how your car performs, you MUST include the effects of the transmission.

Maximum Acceleration VS. Power

OK, so what about power? As has been noted by a previous contributor, Power (hp) = Torque (ft-lb) * RPM / 5252. Note that power is also force * velocity, specifically:

Power (hp) = Force (lb) * Velocity (MPH) / 374

That's net horsepower, which is engine power minus losses like transmission and tire friction. The force is the sum of the longitudinal forces at the contact patches of the two driven tires.

Hmmm... P = F * V ...rearrange to get F = P / V ...

that means that you get the maximum force pushing the car if you maximize your *Power* at any given velocity. This gives us another useful rule:

Shift to maximize engine POWER, not engine torque!

The tires don't see quite these numbers due to [friction and aerodynamic] losses, but I'm going to assume that the losses are comparable from gear to gear and that the overall shape of the power curve remains the same.

Exceptions

There are no exceptions; a car running at its (net) power peak can accelerate no harder at that same vehicle speed. There is no better gear to choose, even if another gear would place the engine closer to its torque peak. You'll find that a car running at peak power at a given vehicle speed is delivering the maximum possible torque to the tires (although the engine may not be spinning at its torque peak). This derives immediately from first principles in physics.

However, note the following: - Transmission losses are not shown on engine power curves. The net power curve (power delivered to the ground) may have a different shape or even a different peak RPM as a result. This would result in different shift point. Best results are obtained from a power curve measured by a chassis dynamometer. - The discussion above assumes negligible tire slip. If you exceed the maximum traction available from the tires, then additional power doesn't help. That's why it's sometimes no loss at all to shift early when the tires break loose, and in fact it can be a benefit.

To the Point

Torque and power are (almost) flip sides of the same coin. Increasing the torque of an engine at a particular RPM is the same as increasing the power output at the same RPM.

Power is just as useful and relevant in determining vehicle performance as is torque. In some situations it's more useful, because you may not have to play with gear ratios and a calculator to understand what's going on.

A car accelerates hardest with gearing selected to stay as close as possible to the engine *power* peak, subject to the traction capability of the tires.

Not all cars should be shifted at the redline for maximum performance. But it's true for many cars. You can determine optimal shift points by graphing horsepower vs. velocity or transmission torque vs. RPM. Engine torque alone will not determine shift points.

I found this on the net. There is no way I would type all that shit up myself.


Also, I will never let my BABY redline. Love her way too much for that.


[ February 28, 2003: Message edited by: TF242 ]
 

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holy crap tf, what an informative post. thanks man!
 

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I have found a new transmission god. maybe you can help me out. I have an 81 rab pick up with a 4 speed ive modified it a little cam,header,muffler,msd,ignition,injectors, clutch and some other nic nacs. i was wondering how to find my red line. the motor will wind out to 8000 rpm but i know damn well thats not good for a 133000 mile old engine can you help im stumped.
 

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good research Tf. it was interesting. didnt read it all, i am to lazy but very interesting. yeah, when i get a new car, i will not redline either. i actully think taht is waht caused my burning oil problem. damn brother telling me that an occational redline helps by blowing shit out!
 

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HOLY SCHNIKES !!! Thomas, perhaps a new signature is in order for you:

Dr. Thomas, Ph.D
 

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<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>Originally posted by JimmyC:
HOLY SCHNIKES !!! Thomas, perhaps a new signature is in order for you:

Dr. Thomas, Ph.D
<HR></BLOCKQUOTE>

All I did is relay the information to fellow VWF members. I didn't write this up myslef. (As I mentioned above).

I might take Doctorate in the future but Ph.D? Now you are pushing it. LOL
 
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