E. J. - 993 Alumni

[quote]Originally posted by Mark D - TT Alumni:
<strong>

Yes, exactly. If you can't gain more than a second with the upshift, then hang at the redline (below the rev limiter) and hold the lower gear.</strong><hr></blockquote>

Mark are you advocating feathering the throttle just below the limiter for 2 seconds? I don't know what to do either, but it sounds slow.

E. J.

SundayDriver

[quote]Originally posted by E. J.:
<strong>

Mark are you advocating feathering the throttle just below the limiter for 2 seconds? I don't know what to do either, but it sounds slow.

E. J.</strong><hr></blockquote>

If the time difference is less than a second, then yes, that is what I am advocating. If it is really slow, then you would be giving up more than a second and should do the shift.

It is an easier call in my SRF as the ideal shift point is ~5600, no rev limiter and it will safely rev past 6000. In those cases, I simply let it rev to 6000 and avoid a shift. With a rev limiter, you are held back, but the advice still holds. If it costs you more than a second, make the upshift and downshift.

E. J. - 993 Alumni

[quote]Originally posted by Mark D - TT Alumni:
<strong> If it costs you more than a second, make the upshift and downshift.</strong><hr></blockquote>

Glad to agree with you now that you weren't so verbose...

tom_993

Don’t forget that hanging at the rev limiter for a couple of seconds is pretty stressful on the engine. It’s the right thing to do if winning is your highest priority, but for those of us who want to get 200,000+ miles out of our engines, it might be better to short shift into the higher gear.

ScottMellor

Yikes! I thought the "Drive it like you stole it" sticker on the back window of my TT was a factory OEM sticker.

ScottMellor

Yup. He seems so normal too............
It's a great car, and Karen and I are still tickled with it. Karen insisted that the sticker come off though.

SundayDriver

[quote]Originally posted by ScottMellor:
<strong>Yikes! I thought the "Drive it like you stole it" sticker on the back window of my TT was a factory OEM sticker.</strong><hr></blockquote>

That actually came as part of the 520HP package. Almost OEM.

SundayDriver

[quote]Originally posted by tom_993:
<strong>Don’t forget that hanging at the rev limiter for a couple of seconds is pretty stressful on the engine. It’s the right thing to do if winning is your highest priority, but for those of us who want to get 200,000+ miles out of our engines, it might be better to short shift into the higher gear.</strong><hr></blockquote>

Why do you think that is stressful on the engine? The redline is a not to exceed value - there is no damage or extra wear for running there vs. a little lower. Part of standard durability testing in engine development is very long periods of full throttle operation very close to redline.

Steve Weiner-Rennsport Systems

Hi Mark:

There is a world of difference in mechanical stresses between WOT at full load and just holding it at that same RPM with little or no load, such as when feathering the throttle.

Just one example is the loading on the rods; it changes from compressive to tensile when the throttle is closing or closed. Engines rarely ever puke under load at high RPM, it happens when one has backed off. The only exception is a lubrication or bearing failure of some kind.

Unless certain components are upgraded, its really prudent to minimize that high RPM "hang-time",....

Good gearing is everything, as you know.

tom_993

Things start breaking just above redline. Things don’t break at 3000 RPM. The closer you are to the point where things break, the more stress you are inducing.

SundayDriver

[quote]Originally posted by Steve Weiner-Rennsport Systems:
<strong>Hi Mark:

There is a world of difference in mechanical stresses between WOT at full load and just holding it at that same RPM with little or no load, such as when feathering the throttle.

Just one example is the loading on the rods; it changes from compressive to tensile when the throttle is closing or closed. Engines rarely ever puke under load at high RPM, it happens when one has backed off. The only exception is a lubrication or bearing failure of some kind.

Unless certain components are upgraded, its really prudent to minimize that high RPM "hang-time",....

Good gearing is everything, as you know.</strong><hr></blockquote>

Thanks. I had not thought about the rod loading - good point. Not an issue in my race car, I just keep my foot in it and let it rev past the shift point.

PeanutinCA

So does that mean the increased mph from changing gears does not negate the time taken to make that shift? On Rev limiter means on limited mph, making the shift might mean an extra 5-10 mph into the corner? The downshift shouldn't be included in the time calculation as you are on the brakes into the corner anyway. One might argue the loss of the held gear under braking versus clutch in and downshifting perhaps? What's the time calculation for extra mph minus shift time?

Then again, you could all just shift a little quicker!!! <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" /> Just another perspective. Now, could someone tell me how to swap into my Momo race wheel?

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=3&t=004247" target="_blank">Changing Steering wheel</a>

Cheers,
Dylan.

David '96 993

Another thought about shifting vs. rev limiter and optimum shift points: I have had instructors point out the obvious...less shifting is easier on the transmission. Soooo, using as much of the powerband will be easier on transmission (especially as we have a fair amount of torque), but depending on if you are on your limiter it would be harder on the engine. I guess it is about managing the entire drivetrain... <img src="confused.gif" border="0">

Chris W. - '96 Targa

For those who like mathematics, there's a pretty good discussion (a whole chapter, actually) of shift points in "The Auto Math Handbook", by John Lawlor (ISBN 1-55788-020-4). It's a great book, and I highly recommend it.

From the book, in short, the right point to upshift is the point at which the loss of torque on the driveshaft is the smallest. From the book, to calculate it, you would need a chart showing engine torque at different RPMs, and the following formulas:

RPM After Shift = (gear ratio shift into)/(gear ratio shift from) x (rpm before shift)

Driveshaft torque = flywheel torque x transmission ratio

1 Nm = 0.7375622 lb-ft

Here's an example of the above calculations, using a shift from 2nd (2.05:1) to 3rd (1.41:1) at 6000 RPM in my Targa:

RPM After Shift = 1.41/2.05 x 6000 = 4127 RPM

Looking at the torque curves published in my owner's manual, at 6000 RPM my engine is developing ~325 Nm (~240 lb-ft) of torque at the flywheel. At 4127 RPM, I'm at ~330 Nm (~243 lb-ft).

Using these values, you can calculate the driveshaft torque before/after the shift:

Driveshaft torque before shift = 325 x 2.05 = 666 Nm (491 lb-ft)

Driveshaft torque after shift = 330 x 1.41 = 465 Nm (343 lb-ft)

So, the loss in torque at the driveshaft from that shift would be 201 Nm (148 lb-ft), or about 30%.

Repeating the calculations for a shift point of 6500, I get the following results:

RPM after shift = 4471
Flywheel torque at 6500 RPM = 300 Nm (221 lb-ft)
Flywheel torque at 4471 RPM = 340 Nm (251 lb-ft)
Driveshaft torque before shift = 615 Nm (454 lb-ft)
Driveshaft torque after shift = 479 Nm (353 lb-ft)

Driveshaft torque loss = 136 Nm (100 lb-ft), or about 22%

So, in the above example, 6500 is a better shift point from 2nd to 3rd.

To find the optimal shift point, repeat the calculations, solving for the lowest driveshaft torque loss. Repeat for each gear.

The book also mentions a drag racer's rule of thumb: the best shift point is at an engine speed 10% beyond the horsepower peak. By the stats in my owner's manual, that would be at ~6930 RPM (which, of course, is past the redline).

Happy calculating! <img src="graemlins/xyxwave.gif" border="0" alt="[bigbye]" /> <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

-Chris

PeanutinCA

O.k. now I'm just trying to figure out where I'm going to mount my calculator to punch those numbers in whilst I'm grabbing 4th to 5th at 100 plus!! <img src="graemlins/roflmao.gif" border="0" alt="[hiha]" /> Do you think there is something in the regulations about mounting calculators?

Just kidding. Those are some technical points and calculations. I can see how they are pertinent down the front straight grabbing each gear. But not sure how they related to the original question of grabbing that next gear for a couple seconds or leaving it in the current gear and boucing off the redline (o.k. feathering at the redline)?

Still interested if anyone has a theory on the extra mph/time gained in changing Vs. time saved not changing?

Cheers. <img src="graemlins/drink.gif" border="0" alt="[cherrsagai]" />