BigPorscheGuy39

I have a 931, the earlier, lesser water-cooled turbo. And I was looking around my Momo Prototipo steering wheel to see my RPM gauge the other day, when I realized that I was 'looking around it'. It seems the placement of the new wheel is awkward for looking at the RPMs. So I've decided to at least USE the space that's above and to the right in the RPM gauge, and install a shift light. (This'll also involve an electronic box like an MSD or LINKS to switch the light on and off - but let's assume for this example I've got something like that).

<img src="http://boards.rennlist.com/upload/FunkyInterior2931.JPG" alt=" - " />

Now, once that idea went through my head, it occurred to me the question: What's the best time in the RPM curve to shift for a particular gear?

Should the shift point be when the torque is peak? The horsepower? OR can you shift after these peaks?

Typically, I've been shifting when the seat of my pants no longer gets pulled strongly, and at a place where I no longer get turbo lag...and that's obviously kind of lame. It's something around 5200 in 1st and 2nd, and around 5000 in 3rd, 4th.

When do you think it's best to shift in your car? And I understand there are whopping differences between our cars, but I'd like to hear a few experiences and views and then make adjustments for my own situation. Thanks

BoostGuy951

I run 1st and second to 6100, and 3rd to about 5500. If I still feel the need for all out acceleration in 4th, I run that gear out because 5th is much too long for anything but highway crusing.

Chas

Since our cars are so different, I'll only tell you when I shift in terms of tq/hp curve...

When I get the "seat of your pants" pull (max torque), I'll notice where that is in the rpm band (depends on temperature, humidity, what color socks I'm wearing...weird I know), and having my rpm gear ratio per speed graphs (or whatever they are called) memorized, I'll ride the gear till I know that when I shift to the next gear, I'll be well in that high-line torque and hp curves.

Aegean racer

My opinion is that the best point to shift is not only governed by the maximum torque but by the combination of max torque / gear ratio.

In other words, you need to closely look at your torque curve and find out the best torque combination at the point of shifting and right after it. As long as the torque combination is optimum, then you'll get the overall max acceleration. Because you may drive let's say your 2nd gear to redline, shift and then hit the decline part of the curve for the 3d gear. But if you shift a bit earlier and get to the flat part of the curve (max) for 3d gear, then you have max overall acceleration.

I remember reading a nice well-written article about that issue a couple of years back in Sport Compact Car.

Peter

Perry 951

</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">....because 5th is much too long for anything but highway crusing.</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Or a strong pull to a buck sixty five....165!

Typically, you want to shift when the horsepower starts to fall. In my case, it would be at 5900rpm. That lands me at 4300 in the 2-3 shift, right at the torque point. My 3rd gear is super mean.

Running it to 6100 or 6200 is pointless since you are not making anymore power, as shown here.

<img src="http://members.rennlist.com/perry951/Nov11.jpg" alt=" - " />

Songz

i agree part of what "Perry 951" said.

When to shift has got nothing to do with Torque!!!
Its a common mistake.

For optimum accelaration you really have to study your car's setup... most importantly.. "GEAR RATIO".

Generally, if you shift just before redline... you are on the right track!!!!

BigPorscheGuy39

WEE WEE I can see what you're saying. I've been using gear ratio too in my shifting pattern. I do wait a bit to shift so that I know when I hit the next gear I'll have lots of boost available to me.

I think I should dyno the damned thing so that I can get a readout as Perry and the other guys have noted.

And I'm 'trying' to shift before redline! <img border="0" title="" alt="[Wink]" src="wink.gif" />

shaheed

i agree with what perry says, looking at my dyno graph max torque occurs between 3500-4500rpm (hard to tell where exactly, torque is pretty consistent in that rpm range), hp maxes out around 5500rpm (smaller turbo than perry) which is where i shift.

Steve Lavigne

Danno

Uhhhh, it's torque that accelerates your car down the road. HP only determines max-speed. They are actually an integral and derivative function of each other with respect to time. Remember also that the torque you see in the dyno-charts are multiplied and divided by the gearbox. So the torque, and therefore linear thrust at the wheels will be different for each gear at the torque peak of the engine.

To compute instantaneous acceleration at any given point in time, compute the linear thrust at the contact patch on the ground:</font><ol type="1">[*]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">take torque at exact RPM that engine is developing (in lb•ft)</font></li>[*]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">multiply by gear-ratio of gear you're in (bigger lb•ft except for 5th gear)</font></li>[*]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">divide by final-drive R&P ratio (lower lb•ft)</font></li>[*]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">subtract 15% for drivetrain losses</font></li>[*]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">divide by distance between rear hub-center and contact patch in feet, a little less than radius of tire (gives lbs of linear thrust on ground)</font></li>[*]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">plug in lbs of thrust computed in last step into F=ma, a=F/m to compute instantaneous acceleration</font></li>[*]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">using the torque curve, you can integrate the computed acceleration with respect to time using V=aT to get speed</font></li>[*]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">integrate once more with respect to time to get distance covered D=1/2a(T^2)</font></li>[/list=a]<font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Here's a plot of the instantaneous acceleration through various gears :

<img src="http://www.gururacing.net/ImagesMisc/AccelerationCurvesPerGear3.jpg" alt=" - " />

You can see that in the lower gears with a big jump, the next gear up really divides out the torque, so it's better to redline in the lower gears because the acceleration (thrust on ground) will still be higher than what it would be in the next gear up. However, in the higher gears with closer gear spacing, you're better off shifting before redline.