Some practice the shift at red line; others say shift at torque peak. I believe both are correct. 1st & 2nd gear shifts should be at peak torque around 6000 rpms because at that speed there is little wind resistance. Shifting to 3rd & 4th should be at a higher rpm around 6300 or 6400 rpms because you can't afford to allow the rpms to go too low after these shifts in consideration of the quadruppling increase of wind resistance. Too much time would be spent trying to get the engine back up to the peak hp curve from the low rpms.

 

Huh? Torque is absolutely irrelevant once you've launched. You ALWAYS want to be where you're making the most horsepower if you want the most acceleration, whether you're in a hurricane or in a vacuum. F=ma, simple physics.... the F here, at any given speed, is directly proportional to HORSEPOWER. So to maximize a for a given mass (the car), you need to maximize f (hp). I don't understand why everybody out there has a different theory on shift points because their daddy taught them to shift their big block at four grand. Sigh...... I give up....... Somebody please grab a physics book....

Andi

 

Andi, you're absolutely right about shifting at redline to maximize power output. However, peak acceleration occurs at peak torque, not peak hp! Unfortunately, this is often miscontrued as implying that one needs to shift at peak torque for best results. Far from it! Staying in a lower gear till redline allows you to take advantage of GEARING and hence, TORQUE multiplication to the wheels. Even though the LS6's torque output is falling off after 4900 rpm, you're still better off in the lower gear through redline due to torque multiplication. For eg: In 1st gear at 6500 rpm, the LS6 only makes about 323 lb-ft of torque at the flywheel. However, effective torque at the wheels = 323*2.97*3.42 = 3281 lb-ft. Even at the torque peak of 400 lb-ft (@ 4900 rpm), in 2nd gear, you'd only have 400*2.07*3.42 = 2832 lb-ft of effective torque at the wheels. The Z06 will thus accelerate harder in 1st gear at redline than it will at ANY rpm in 2nd! I think that clearly makes the case for holding any gear up to the rev limiter before shifting!

The hp argument works, but torque is THE true fundmental quantity. However, torque arguments without consideration to gearing can be rather misleading, hence the simplicity of the hp case! :)




[Modified by raj, 12:00 AM 10/17/2003]

 

Andi, The best 'general' imfo on optimum shift points, is to look at "driveshaft torque". Driveshaft torque(DT)= flywheel torque X transmition ratio. So if you look at the DT at a given rpm then the DT after the shift you can figuer max theoretical aceleration. I shift @6250-6400 when power shifting depending on what's working with track and air conditions. DR Torque after the shift has everything to do it, when you relate it to DR torque before the shift.
Ric
PS; Rear end ratio is a constant.


[Modified by Power Shifter, 1:11 AM 10/17/2003]


[Modified by Power Shifter, 1:17 AM 10/17/2003]

 

Thank you Raj. You're absolutely right-on, torque after gearing multiplication is what we're finally after.

I'd also like to note that while your tq & gearing multiplication math is correct, it's unnecessary. HP already factors in torque *and* rotational velocity, and as such does not change as it passes through the transmission. That's the beauty of looking at it from the HP side - it skips the gearing math and still gives you the same answer. Why? All the transmission & gearing does is change the proportions of the two components of the horsepower -- torque and RPM. But the HP goes in one side of the transmission and goes out the other as the same amount (barring frictional/inertial losses that are outside the scope of this discussion), and that HP figure is directly proportional to the amount of rear wheel torque you have available to accelerate the car at any given speed.

That's why you can do the shift point analysis using your ABSOLUTELY CORRECT method of tq * gear multiplication (like I did for my Maxima back in the day before I figured this out), or you can just see where HP curves intersect in the before & after gears intersect(like I did for my Supra in this example after I figured this out), and you will see that if you use both methods they will always agree as the math in the former method is redundant as the HP comparison method takes care of everything for you.

Oh, and to Power Shifter.. as I wrote this, I noticed that you replied and said that all that matters is driveshaft torque* (i.e. rear wheel torque, and finally accelerative force). That is correct. And that's what I'm talking about in this thread -- driveshaft torque is directly proportional to HORSEPOWER, not to flywheel torque..... to understand this, look at Raj's excellent example above showing why you're much better off at a higher rpm, higher hp lowre torque point in a lower gear than at peak torque in a higher gear.

*Driveshaft torque: i'm assuming you mean transmission output shaft torque, which would be driveshaft torque in any car with a conventional front-mounted transmission.... and not actaul c5 driveshaft torque that technically really is just flywheel torque..... heh.

Andi

[Modified by Andi, 12:21 AM 10/17/2003]


[Modified by Andi, 12:23 AM 10/17/2003]

 

Andi, Ya what you said;) My way is simple minded,more ME. Wear not all math minded. :seeya Ric

 

:lurk: Good post.

 

Wow!...never knew this post would generate so much interest..

Tuesday should be a very interesting night at the track.

Thanks to everyone for their insights and responses

David In Indy :cheers:

 

I shift right at the rev limiter. Any less looses time...

 

WORD! :cheers: But you still need the gear ratios to figure exactly where the shift will put the engine and thus compare the hp before and after the shift!

BTW, your website is neat! :cool: Didn't realize how peaky those twin turbo outputs are - quite another world from the land of big displacement! ;)