WOT auto shifts @ 6,000 RPM

[studio460]

Question for you all . . .

For those of you who DO NOT yet have Dennis' chip (which I know fixes all the shift point problems):

When flooring your Marauder, do your auto up-shifts to 2nd and 3rd consistently occur at 6,000 RPM? My 100% stock Marauder auto shifts, whenever I'm at or near WOT, at 6,000 RPM. Is this normal? Thanks for your help.

[cyclone03]

Thats what mine does.

[SergntMac]

Yep, that's the program. Even chipped, 6K. My MM maxes out the power at 5750RPM, why go higher for nothing?

[TripleTransAm]

Originally posted by SergntMac
Yep, that's the program. Even chipped, 6K. My MM maxes out the power at 5750RPM, why go higher for nothing?

It all depends on how much power is waiting for you in the next gear after the upshift. Depending on the nature of the engine's torque curve, some cars need to fully wind out in each gear for max performance, some benefit from earlier shifts. Based on the curves I've seen, I believe the MM needs all the RPM until redline to deliver the best numbers.


Warning: anyone not interested in the physics behind this, stop reading NOW!
=================

I don't know if you guys remember those simulated power graphs I posted when trying to model the MM's output and top speed, a few months back. If you do, you'll see that even as the torque is dropping after going beyond the RPM where it peaks, the torque it's producing in that gear multiplied by that gear ratio will probably be greater than the torque it would be producing at the lower RPM after the upshift multiplied by that new gear ratio.


Using arbitrary numbers here:

Let's say engine produces 300 lb-ft at 5500 RPM, and let's say that's where the HP peak is (ie. torque is dropping off faster than RPM is increasing, since HP = torque x RPM / 5252). Let's say it also produces 150 lb-ft at 3700 RPM.

Let's say our example car has a 3.00:1 first gear and a 2.00:1 second gear.

So, let's say we're charging hard in first gear and we're producing 900 lb-ft of torque at the output of the transmission (300 lb-ft x 3.00 <- gear ratio). The power starts dropping off, so a decision is made to upshift here.

At that moment, after the upshift, the engine is turning 3700 RPM. This is from the calculation 5500 / 3.00 x 2.00 . At this RPM, the engine is now producing 150 lb-ft instead of 300 lb-ft, and the output of the transmission is now seeing 300 lb-ft total (150 x 2.00) instead of 900 lb-ft total.

In general, the result is slower acceleration after this upshift, since there's less torque being delivered to the wheels, and torque is the direct input for acceleration (just like Force = Mass x linear acceleration, Torque = Moment_of_Inertia x rotational_acceleration).

If you take a lower-RPM motor like my 400, maybe it makes it's peak 300 lb-ft at 3700 and by 5500 it's only making 150 lb-ft. In that case, in first gear you'd see 450 lb-ft (150 x 3) at 5500 RPM and after an upshift, you'd see 600 lb-ft at 3700 RPM... then it's definitely clear the upshift should have come WAY sooner, and performance was lost.

[TAF]

Originally posted by TripleTransAm

Warning: anyone not interested in the physics behind this, stop reading NOW!
=================

I LOVE the "disclaimer" 3TA...I DID read on...and learned, thanks!

[TripleTransAm]

Originally posted by cyclone03
Thats what mine does.

And yes, mine does that too. I'm sure it's for the best, as well, given the nature of the 4.6l powerband.

My WS6 also needs to wind out to its 6200 RPM redline for best performance (manual tranny). My GTA (automatic) upshifts at 5200 RPM but by then it's fairly breathless... it would probably prefer an upshift at 4800 RPM or so, since the lower RPM is so much meatier and there's no point keeping it up there. My '78 gets better highway passing acceleration by simply keeping it in high gear and using the 2000-3500 RPM range... if it downshifts to 2nd gear, it makes a lot more noise but I don't feel it's any faster (maybe even slower?)

[cyclone03]

3TA,
You HAVE to come to the meet in Dallas.I'm sure we can talk for hours!
The thing about shift points is the RPM AFTER the shift at the bottom of the next gear.A perfect shift would land the RPM right at the torque peak this would give the best acceleration to the HP peak and the next shift.
The truth is in the above I lied.The 1-2 shift comes BEFORE 6000rpm about 5800,then the 2-3 at 6000.The RPM after the 2-3 is less than 3500 if I remember correctly.

[TripleTransAm]

Originally posted by cyclone03
The 1-2 shift comes BEFORE 6000rpm about 5800,then the 2-3 at 6000.The RPM after the 2-3 is less than 3500 if I remember correctly.

I'll have to find an opportunity to wind it out through first gear to see where mine actually shifts, but I was under the impression I saw 6000 RPM upshifts from 1st to 2nd. It's been a while, though, so either my memory if off or the car behaves differently after break-in.

Besides, factory tachs can be so inaccurate... even among cars of the same model year!

I'd love to join you guys in Dallas, but it's a long-a$$ haul, to be sure. I couldn't even make the New England slumber party and tuning session due to work and family commitments.

If there's one thing I love, it's talking cars, as if no one figured it out already!

[looking97233]

Mine is stock. 1-2 and 2-3 WOT upshifts occur at 6k rpm.

[SergntMac]

Originally posted by TripleTransAm
If there's one thing I love, it's talking cars, as if no one figured it out already!

I have...

[martyo]

Originally posted by SergntMac
I have...

Oh? And what was your first clue??

[SergntMac]

Ummm...No. I'm not going there thank you.

[martyo]

Originally posted by SergntMac
Ummm...No. I'm not going there thank you.

Damn, and I was so hoping you would. Obviously, I am bored.......

[Fast4Door]

Well, mine only has a couple hundred miles on it, but I couldn't resist one WOT accel...

Anyway, I swear the 1-2 clicked off at 6250 RPM, and it felt like the right time to me.

I guess I'll have to schedule some more WOT operation and see if everything's alright...

[LincMercLover]

Yah, mines a hair past 6000 too.

3TA,

Dayum....