What max RPM are you all you track guys shifting at???

[Hack]

When I went to the Track Attack the instructors told us to hold it on the limiter on 2 of the straights, if I recall correctly. After I heard that I was a lot less concerned about potentially hitting the limiter every once in a while in my personal car. At the Track Attack I just kept the throttle pinned on the end of those straights and let the engine computer take care of holding rpm at the limit. The car had no problems and my assumption is that was being done by most of the people who drove it at the Track Attack.

On track when I owned a GT350 I shifted by sound rather than use the lights. Probably right around 8,000 rpm or so. Every once in a while I hit the limiter before shifting, but not very often.

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[Jason304]

Shift light set at 8000. Haven’t yet had mine on a track, but I usually shift between 7800 and 8000 when traffic and road conditions allow.

 

[friedmud]

I see a couple of people mentioning things about peak power, etc. I just thought I would toss in here that physics-wise it is almost always “best” for acceleration (and will be in all Mustangs) if you shift at the redline (not taking into account whatever is happening to the track).

Engineering Explained has a good explanation of why here:



(Note: I am not a track guy - just a local engineer trying to shed light on the physics. I completely understand that on a track there are many reasons to shift earlier, etc.)

 

[AdamIsAdam]

That video is great and I was about to post it also before I saw your post. Now I have something to share which will go hand-in-hand with that. Someone plotted the torque values by gear and RPM. I was trying to figure out if our particular cars will do better shifting at redline. (And again, this does not take into account other reasons for staying in a gear, like braking points, turns, etc. This is really more of a drag racer's view of shift points.) So I followed Jason's formulas in excel.

When I did the math, it looks like Jason's video holds true and shifting at redline will be putting down the most power all the time.

 

[friedmud]

That video is great and I was about to post it also before I saw your post. Now I have something to share which will go hand-in-hand with that. Someone plotted the torque values by gear and RPM. I was trying to figure out if our particular cars will do better shifting at redline. (And again, this does not take into account other reasons for staying in a gear, like braking points, turns, etc. This is really more of a drag racer's view of shift points.) So I followed Jason's formulas in excel.

When I did the math, it looks like Jason's video holds true and shifting at redline will be putting down the most power all the time.

Nice - thanks for doing the calculations!

Yeah - you would have to have a VERY “peaky” torque curve before your loss in torque at higher RPMs is more than the loss from the change in gear ratios. With the Coyote it’s hard to imagine that it would ever happen…

 

[AdamIsAdam]

Nice - thanks for doing the calculations!

Yeah - you would have to have a VERY “peaky” torque curve before your loss in torque at higher RPMs is more than the loss from the change in gear ratios. With the Coyote it’s hard to imagine that it would ever happen…

Or a very flat torque curve. My old Pontiac made close to 600 ft lbs, and most of it from about 1800 RPMs. Then the top end falls fast after 5000. In fact, Pontiac drag racers often tested at the strip and found quarter mile times and MPH do not change when shifting at 5,000 vs 5,500 (and fall off dramatically above that).

It's all about that torque.

I should try this math with my Cobra. It's got a 347 stroker with one of the flattest torque curves ever but the HP curve just keeps going until the MSD box cuts off the fun.

 

[friedmud]

Or a very flat torque curve. My old Pontiac made close to 600 ft lbs, and most of it from about 1800 RPMs. Then the top end falls fast after 5000. In fact, Pontiac drag racers often tested at the strip and found quarter mile times and MPH do not change when shifting at 5,000 vs 5,500 (and fall off dramatically above that).

It's all about that torque.

I should try this math with my Cobra. It's got a 347 stroker with one of the flattest torque curves ever but the HP curve just keeps going until the MSD box cuts off the fun.

A flat torque curve would mean it would _definitely_ be best to shift at redline.

The only way that shifting earlier ever makes sense is if you lose a larger amount of torque as the RPMs go up than you would lose in changing gear ratios.

So: if you had a perfectly flat torque curve it could literally _never_ be better to shift early.

You would have to have a torque curve that peaked in the middle and fell off by a huge amount by the end for shifting early to make sense.

 

[Hack]

Or a very flat torque curve. My old Pontiac made close to 600 ft lbs, and most of it from about 1800 RPMs. Then the top end falls fast after 5000. In fact, Pontiac drag racers often tested at the strip and found quarter mile times and MPH do not change when shifting at 5,000 vs 5,500 (and fall off dramatically above that).

It's all about that torque.

I should try this math with my Cobra. It's got a 347 stroker with one of the flattest torque curves ever but the HP curve just keeps going until the MSD box cuts off the fun.

No, it's only with a peaky torque curve (or at least it has to fall rapidly, the torque curve can be flat at low RPM - that doesn't matter). Especially with an older car that only has 3 or 4 forward speeds. The torque drop at the rear wheels from shifting is a lot. For example on a Muncie 4 speed transmission, first gear is 2.56:1 and second is 1.91:1. So shifting from first to second loses you 25% of your torque at the rear wheels.

Only when engine torque falls off a cliff does it become worth shifting.

 

[AdamIsAdam]

Got it. Yes, the torque curve on the big Pontiacs falls off a cliff at around 5000 RPM.

 

[AdamIsAdam]

I plotted my Cobra (2,200lb car) with 347 CID (stroker 302), T5 Levy Racing tranny. Remember, these are chassis dyno numbers, not crank HP/Torque numbers.

 

[tom_sprecher]

I have not had the Mustang on a track, but when I raced my shift points were completely dependent on the track layout.

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