Go Back   986 Forum - for Porsche Boxster & Cayman Owners > Porsche Boxster & Cayman Forums > Boxster General Discussions

Reply
 
LinkBack Thread Tools Display Modes
Old 06-14-2006, 07:35 PM   #1
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Shift Points for Maximum Acceleration

I've seen a few posts on the board asking at what RPM to shift for maximum acceleration. The answer is, for any given vehicle speed, the engine could be making a certain amount of torque to the wheels in each gear. The goal is to use the gear that's making the maximum torque at that speed. It's not just a matter of the engine torque but a combination of engine torque gear ratio and vehicle speed.

To illustrate, and for you curious folks, I've attached a spreadsheet that does all the calculations for you. You need to input tire information and engine torque.

As you can see, at any given speed the wheels are getting the most torque from the engine in the lower gear up to redline up until 4th gear, where there is a slight advantage to shifting into 5th a little before redline at about 114 MPH. 5th to 6th similarly has a very minor shift point benefit before redline.

download and delete the .txt from the file extension, it's a .zip file.


Last edited by blue2000s; 06-28-2006 at 12:14 PM.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 04:17 AM   #2
Registered User
 
Join Date: Feb 2006
Location: Toronto Ontario
Posts: 291
Except for the fact that this doesn't acctually work in real life.

Try this at the track with Any car, no matter what it is. If you shift where your torque is at the maximum instead of redline you will always run a slower 1/4 mile time. This is a pretty old myth and one that I fell into myself when I first started drag racing.

I have drag raced about 8 different cars in the last 15 years pretty hard core, right into the 10's in a few of them. They have all been to the dyno, and I have tried shifting them all at peak torque like this graph illustrates. If you shift at peak torque you simply go slower every time.

Your always better to shift at your highest RPM point. As the RPM falls back down, it doesn't matter that it's over the Torque peak when you shift, it's that you land in a higher RPM point when you did the shift into the next gear. The higher you land in the RPM band after your shift the faster the engine will accelerate. If you shift at 5500rpm and fall to 3500rpm you car will be slower than shifting at 6500rpm and landing at 4500rpm because the engine will rev easier and faster to 6500rpm again from 4500rpm faster than 3500rpm to 5500rpm regardless of what your torque numbers are on a graph.

The best illustration of this is with a motorcycle. My CBR did 11.4 @ 126mph. It revs to 15,000rpm but peak HP is 13,000rpm, peak torque was 9000rpm or so. It was significantly slower if I shifted at anything less than absolute redline. The difference was a few 1/10ths and would knock me back to high 11's each time I messed with the shift points. Even in first gear, short shifting the bike made it slower. I did it however because I had to worry about poppin the wheel up hittin second without the clutch at 50mph.

Cool graph, but doens't pan out in real life. Don't take my word for it however, if you want proof hit up some hardcore drag racing forums and ask them this there. Their response will be much the same as what I just told you.


Last edited by 986Jim; 06-15-2006 at 04:21 AM.
986Jim is offline   Reply With Quote
Old 06-15-2006, 07:05 AM   #3
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Quote:
Originally Posted by 986Jim
Except for the fact that this doesn't acctually work in real life.

Try this at the track with Any car, no matter what it is. If you shift where your torque is at the maximum instead of redline you will always run a slower 1/4 mile time. This is a pretty old myth and one that I fell into myself when I first started drag racing.

I have drag raced about 8 different cars in the last 15 years pretty hard core, right into the 10's in a few of them. They have all been to the dyno, and I have tried shifting them all at peak torque like this graph illustrates. If you shift at peak torque you simply go slower every time.

Your always better to shift at your highest RPM point. As the RPM falls back down, it doesn't matter that it's over the Torque peak when you shift, it's that you land in a higher RPM point when you did the shift into the next gear. The higher you land in the RPM band after your shift the faster the engine will accelerate. If you shift at 5500rpm and fall to 3500rpm you car will be slower than shifting at 6500rpm and landing at 4500rpm because the engine will rev easier and faster to 6500rpm again from 4500rpm faster than 3500rpm to 5500rpm regardless of what your torque numbers are on a graph.

The best illustration of this is with a motorcycle. My CBR did 11.4 @ 126mph. It revs to 15,000rpm but peak HP is 13,000rpm, peak torque was 9000rpm or so. It was significantly slower if I shifted at anything less than absolute redline. The difference was a few 1/10ths and would knock me back to high 11's each time I messed with the shift points. Even in first gear, short shifting the bike made it slower. I did it however because I had to worry about poppin the wheel up hittin second without the clutch at 50mph.

Cool graph, but doens't pan out in real life. Don't take my word for it however, if you want proof hit up some hardcore drag racing forums and ask them this there. Their response will be much the same as what I just told you.


I don't think that you completely understand what the graph is saying. For any given vehicle speed, the torque to the wheels is shown. You can see from the graph that the best place to shift pretty much IS redline. This information is accurate if it's read correctly.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 07:44 AM   #4
Registered User
 
Join Date: Sep 2004
Location: Atlanta
Posts: 1,820
redline is usually best. forget torque; you want to shift just after peak POWER, thus allowing revs to fall at or near the peak POWER range.

torque is nothing without motion (work). you can apply torque and never actually move the car (zero work). power describes the rate at which work can be performed (i.e. how quickly can you move a mass of n pounds a distance of x feet). when your car is at peak power, it is performing work at its fastest capable rate.

this is why jim986's real life experience shows him that best acceleration runs occur when you ignore the torque peak and use the power that the almighty RPM helps us generate.
__________________
insite
'99 Boxster
3.4L Conversion

http://i156.photobucket.com/albums/t...1/KMTGPR-1.jpg
insite is offline   Reply With Quote
Old 06-15-2006, 09:16 AM   #5
Registered User
 
Join Date: Jan 2005
Posts: 335
Gnerally speaking any advantage you think you'd gain from short-shifting is negated by the fact that the gear you're going into is further down the powerband.
BuffaloBoxster is offline   Reply With Quote
Old 06-15-2006, 09:17 AM   #6
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Quote:
Originally Posted by insite
redline is usually best. forget torque; you want to shift just after peak POWER, thus allowing revs to fall at or near the peak POWER range.

torque is nothing without motion (work). you can apply torque and never actually move the car (zero work). power describes the rate at which work can be performed (i.e. how quickly can you move a mass of n pounds a distance of x feet). when your car is at peak power, it is performing work at its fastest capable rate.

this is why jim986's real life experience shows him that best acceleration runs occur when you ignore the torque peak and use the power that the almighty RPM helps us generate.
It doesn't sound like you're quite getting it either. Power and toque are directly related, you can't neglect one or the other. The statement you've made is not accurate in that the calculations of acceleration use torque becuase it's units are more directly converted to the units of acceleration. Make no mistake, it's TORQUE that's commonly used in the calculations for acceleration.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 09:21 AM   #7
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Quote:
Originally Posted by BuffaloBoxster
Gnerally speaking any advantage you think you'd gain from short-shifting is negated by the fact that the gear you're going into is further down the powerband.
Actually you can shift into a gear that's higher in the power band but still not have a benefit in acceleration because the torque at the wheels is lower than if you'd stayed in the higher gear.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 09:28 AM   #8
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
There's alot of confusion here.

Depending on the power band, there CAN be a benefit to short-shifting but it's not common in production passenger vehicles and clearly not the case with the Boxster. This is deliberately done by the designers.

I encourage anyone who's really interested in understanding this stuff to look up "Fundamentals of Vehicle Dynamics" by Thomas D. Gillespie.

Last edited by blue2000s; 06-15-2006 at 09:31 AM.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 09:54 AM   #9
Registered User
 
Join Date: Mar 2005
Location: new york
Posts: 17
Correct blue2000s

Excellent data and explanation blue2000s. Its torque at the wheels and only torque at the wheels that accelerates the car. Everything else is just an "old wives tale" repeated over and over, but still not true. If you're getting more torque at the wheels in first gear even beyond engine torque peak, then you stay in first gear until wheel torque is higher in the next gear or you hit redline. Every car (and motorcylce) is different and must be analyzed with this type of chart to determine shift points. I'm a state licensed professional engineer, so I speak with some knowledge of physics and mechanical engineering principles.

Tom
z281974 is offline   Reply With Quote
Old 06-15-2006, 10:14 AM   #10
Registered User
 
Join Date: Mar 2006
Location: Irvine, CA
Posts: 121
I've GOT to invite you guys my next cocktail party!!!!

Cheers!!!!
__________________
Woof Woof!

RONDOG

2005 987S
BLK/BLK
PASM
SPTS CHRONO +
PCM w NAV
19" CARRERA S

http://i112.photobucket.com/albums/n...P4724Large.jpg
Rondog is offline   Reply With Quote
Old 06-15-2006, 10:22 AM   #11
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Quote:
Originally Posted by z281974
Excellent data and explanation blue2000s. Its torque at the wheels and only torque at the wheels that accelerates the car. Everything else is just an "old wives tale" repeated over and over, but still not true. If you're getting more torque at the wheels in first gear even beyond engine torque peak, then you stay in first gear until wheel torque is higher in the next gear or you hit redline. Every car (and motorcylce) is different and must be analyzed with this type of chart to determine shift points. I'm a state licensed professional engineer, so I speak with some knowledge of physics and mechanical engineering principles.

Tom
Thanks Tom, I'm a mechanical engineer as well. Just trying to spread a little knowledge.

Last edited by blue2000s; 06-15-2006 at 10:34 AM.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 10:35 AM   #12
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Quote:
Originally Posted by Rondog
I've GOT to invite you guys my next cocktail party!!!!

Cheers!!!!
Every time I've seen engineers at a cocktail party, they can only entertain each other.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 10:56 AM   #13
Registered User
 
Join Date: Jul 2005
Location: Denver
Posts: 740
Quote:
Originally Posted by blue2000s
Every time I've seen engineers at a cocktail party, they can only entertain each other.
Hmmm, this coming from an Engineer? If by entertaining each other they have discussions like this thread - then YEAH, small surprise there.

Personally, I've got better things to do at a party than bore people with detailed discussions of engineering and physics. For example, you can always bore them with Politics and Religion....
__________________

'06 Cayenne Turbo S, Beige Metallic/Tan

Ex - '99 Arctic Silver, Red Interior, Silver Top
denverpete is offline   Reply With Quote
Old 06-15-2006, 11:03 AM   #14
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Quote:
Originally Posted by denverpete
Hmmm, this coming from an Engineer? If by entertaining each other they have discussions like this thread - then YEAH, small surprise there.

Personally, I've got better things to do at a party than bore people with detailed discussions of engineering and physics. For example, you can always bore them with Politics and Religion....
We talk about what interests us which ends out boring everyone else including spouses and friends. When I see my lady's eyes gloss over (she's an actress with no interest in engineering whatsoever so it happens pretty quickly) I know it's time let her break away.

Usually I can find a couple of other car guys who like to talk about Porsches and Rx-7s.

Now we're totally off topic.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 11:54 AM   #15
Registered User
 
Brad Roberts's Avatar
 
Join Date: Aug 2004
Location: Alta Loma, CA
Posts: 1,334
Great thread!!

I shift when it is safe. LOL

Try this: turn 8 at Willow Springs entering the corner at 120mph almost at redline in 4th gear. In theory I should up shift. No thanks. If I lift for the shift I'm spinning the car at 125mph

I have used this for years in drag racing: shift 500-800 RPM over peak torque to do exactly what has been described above. Hit the sweet spot that keeps the car accelerating.


B
__________________
Engine Builds, Transmission Builds, Engine Conversions, Suspension Installs, Suspension Tuning, Driver Coaching, Data Acquisition, Video, SCCA/PCA/POC/NASA/GRAND AM/ALMS.
We have worked with amateur and professional drivers for over 26 years. In house machinist, In house fabrication. Our cars, our parts, our engines, our transmission's run nationwide at events every weekend. We work side by side with industry names developing parts.
Brad Roberts is offline   Reply With Quote
Old 06-15-2006, 12:00 PM   #16
Registered User
 
Join Date: Sep 2005
Location: Minneapolis/St. Paul, Minnesota, USA
Posts: 3,308
Hi,

I think there's still quite a bit of confusion here.


Horsepower = Torque * RPM / 5252



The number 5252 works only with Torque in ft-lbs. Torque measured in other units such as Newton Meters or kg-m require a different number. Visualize a one pound weight, one foot from the fulcrum on a weightless bar. Rotate that weight for one full revolution against a one pound resistance, it moved a total of 6.2832 feet (π * a two foot circle), and, incidentally, it has done 6.2832 foot pounds of work.

Now, 33,000 foot-pounds of Work/min. equals one horsepower. Divide the 6.2832 foot pounds of Work/revolution of that weight into 33,000 foot pounds, = one foot-pound of Torque at 5252 rpm or 33,000 foot pounds per minute of work, and is the equivalent of one horsepower. If we only move that weight at the rate of 2626 rpm, it's the equivalent of 1/2 horsepower (16,500 foot pounds per minute), and so on.
  • Maximum acceleration at any speed occurs at the HP peak.
    Maximum acceleration in any gear occurs at the torque peak
    Horsepower = Torque * RPM / 5252
    Torque = Horsepower * 5252 / RPM
    Torque = Horsepower at 5252 RPM

What is not being discussed is the Torque Curve, that is, how long an engine can operate at peak Torque throughout the RPM range.

Example: 2 Cars, each producing 300 ft. lbs. of Torque. Car #1 reaches Peak Torque at 4200 RPM and it's curve falls off at 4700 RPM. Car #2 Reaches Peak Torque at 4200 RPM but it's curve doesn't fall off until 5300 RPM. Car #2 will be faster.

Without a look at the curve, the raw numbers can be very deceiving...

Happy Motoring!... Jim'99

Last edited by MNBoxster; 06-15-2006 at 12:21 PM.
MNBoxster is offline   Reply With Quote
Old 06-15-2006, 12:16 PM   #17
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Quote:
Originally Posted by MNBoxster
[list]Maximum acceleration at any speed occurs at the HP peak.
Maximum acceleration in any gear occurs at the torque peak
Wow. Don't confuse torque with power, they're directly related but totally different and can't be used like this.

If you go back to the fundamentals, force=mass*acceleration, torque is a twisting force. It can be directly calculated to force by the wheel/tire radius, therefore it is the property that defines acceleration.

Last edited by blue2000s; 06-15-2006 at 12:22 PM.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 12:23 PM   #18
Porscheectomy
 
Join Date: Mar 2006
Location: Seattle Area
Posts: 3,011
Quote:
Originally Posted by MNBoxster

What is not being discussed is the Torque Curve, that is, how long an engine can operate at peak Torque throughout the RPM range.

Example: 2 Cars, each producing 300 ft. lbs. of Torque. Car #1 reaches Peak Torque at 4200 RPM and it's curve falls off at 4700 RPM. Car #2 Reaches Peak Torque at 4200 RPM but it's curve doesn't fall off until 5300 RPM. Car #2 will be faster.

Without a look at the curve, the raw numbers can be very deceiving...
Agreed, but power doesn't give you this story. The torque curve does.
blue2000s is offline   Reply With Quote
Old 06-15-2006, 12:48 PM   #19
Registered User
 
Join Date: Sep 2005
Location: Minneapolis/St. Paul, Minnesota, USA
Posts: 3,308
Quote:
Originally Posted by blue2000s
Wow. Don't confuse torque with power, they're directly related but totally different and can't be used like this.

If you go back to the fundamentals, force=mass*acceleration, torque is a twisting force. It can be directly calculated to force by the wheel/tire radius, therefore it is the property that defines acceleration.
Hi,

Yes, you are correct in pure physics, but in Autodom, and especially in the context of this thread, the term acceleration is, strictly speaking, misused and does imply a time component and work being done. You can generate 300 ft.-lbs. of Torque at 4000 RPM the instant the Clutch is released (from a dead stop), but the Car isn't yet doing any work.

There is really only Torque. Torque is the only thing that a driver feels, and Horsepower is just sort of an esoteric measurement in that context but adding in a time component.

300 foot-pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would double at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where Horsepower and Torque always come out the same.

In contrast to a Torque curve, Horsepower rises rapidly with RPM, especially when Torque values are also climbing. Horsepower will continue to climb, however, until well past the Torque peak, and will continue to rise as engine speed climbs, until the Torque curve really begins to plummet, faster than engine rpm is rising. However, as I said, horsepower has nothing to do with what a driver feels.

It is better to make torque at high rpm than at low rpm, because you can take advantage of gearing.

An extreme example of this, imagine a waterwheel. A pretty massive wheel (let's say 4 Tons), rotating lazily on a shaft. You determine that the wheel typically generated about 2600 foot-pounds of Torque, and it rotated at about 12 RPM. If you hooked that wheel to the drive wheels of a car, that car would go from zero to twelve RPM in a flash. But, 12 RPM at the drive wheels is around 1 MPH for the average car. In order to go faster, you'd need to gear it up. To get to 60 mph would require gearing the wheel up enough so that it would be effectively making a little over 43 foot-pounds of Torque at the output, which is not only a relatively small amount, it's less than what the average car would need in order to actually get to 60. Applying the conversion formula 12 RPM * 2600/5252 = 6 HP. While it's clearly true that the water wheel can exert a lot of Force, its Power (ability to do work over time) is severely limited.

Happy Motoring!... Jim'99
MNBoxster is offline   Reply With Quote
Old 06-15-2006, 04:42 PM   #20
Registered User
 
Join Date: Mar 2005
Location: Portland, OR
Posts: 18
Waiting with baited breath to find out who wins this argument.

PDX-986S is offline   Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -8. The time now is 01:10 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Content Relevant URLs by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page