[ptgltw]

So according to BMW dyno chart It looks like peak HP is 6250 with a flat torque curve from 2300-5500. HP and torque seems to fall pretty sharply after 6750. These are all ish numbers.

Any reason (beside mid corner or corner entry rpm stretch) to shift over 6300-6400 rpm?

My next step is to research real life dyno curves. I've got another 750 miles to go (1500 miles) before I'll feel comfortable putting it on a dyno for testing.

Thoughts?

[ScullyD]

I don't see any point to shift over 6250 on the street, because i think that actually allows you to shift to 3rd before the speed limit. Track might be the same (but i'm not a track guy).

I'm still at 250 miles, and I've been shifting around 5000rpm, but it doesn't feel like i'm missing a ton more torque higher in the rev range.

[Moflow]

Originally Posted by ptgltw

So according to BMW dyno chart It looks like peak HP is 6250 with a flat torque curve from 2300-5500. HP and torque seems to fall pretty sharply after 6750. These are all ish numbers.

Any reason (beside mid corner or corner entry rpm stretch) to shift over 6300-6400 rpm?


Thoughts?

Well theoretically, you'd want to shift higher than that, certainly not before power starts to really drop off. I think it technically depends on what gear you're shifting into and how many RPMs you'll drop with that gear change.

This engine has a pretty broad and flat power peak, generally speaking, you probably want to be shifting toward the top end of that power peak, maybe 7100 or so. That way, when you hit the next gear, you're right in the power band again. If you shift right when the engine hits peak power, 900 RPM lower, you're going to fall further out of the powerband for the next gear once the revs drop.

All of that is only if you're trying to get maximum acceleration out of the car though. If i'm going for it on the street I shift at 7500 RPM because it sounds and feels cool. Not many turbocharged cars rev way up there in the year 2021.

[sdhotwn]

Do the math on the gearing. Torque per instant by mechanical advantage is what is generating your acceleration (when multiplied over time that is where the horsepower comes... the whole torque horsepower argument is circular as they are directly related to one another, just separated by a time quotient). If the ft-lbs delivered to the rear wheels has fallen off enough that the drop in mechanical advantage of the gearing is made up for by the change in pickup point in the torque curve, then shifting make sense.

This is pretty rarely the case.

The math backs out from the simple equation of F=ma, and then the ft-lb's through the drivetrain viewed instantaneously as a statics problem drops off the "ft" part of the equation because of the diameter of the wheel resulting in a force applied in the correction direction at the contact patch acting against the mass of the car to give you the acceleration. That's all that matters at a given slice of time. Hence ft-lbs matters, not horsepower when you want to know about acceleration for a shift point.

[ptgltw]

Originally Posted by sdhotwn

Do the math on the gearing. Torque per instant by mechanical advantage is what is generating your acceleration (when multiplied over time that is where the horsepower comes... the whole torque horsepower argument is circular as they are directly related to one another, just separated by a time quotient). If the ft-lbs delivered to the rear wheels has fallen off enough that the drop in mechanical advantage of the gearing is made up for by the change in pickup point in the torque curve, then shifting make sense.

This is pretty rarely the case.

The math backs out from the simple equation of F=ma, and then the ft-lb's through the drivetrain viewed instantaneously as a statics problem drops off the "ft" part of the equation because of the diameter of the wheel resulting in a force applied in the correction direction at the contact patch acting against the mass of the car to give you the acceleration. That's all that matters at a given slice of time. Hence ft-lbs matters, not horsepower when you want to know about acceleration for a shift point.

Ha thanks for that. I’ll let the race teams figure that out

I think it’s gonna be around 6500 rpm. On upshifts it might drop 1500 max. I haven’t taken to 6500 yet so I don’t know. But this would get me right around 5000 which still gives you a little max torque left in combination with a decent amount of hp.

Again not a scientist. But I think shifting at 7500 might actually make you slower and arguably more wear and tear on your engine. Might not be worth it just for the sound.

All this is tbd as I’m limiting myself to 5000 rpm at the moment

Again all guessing on my part

If I do end up shifting at 6500 it’s good to know I have an additional 1000 rpm to stretch it out thru corners and corner entry.

Or I could just wait until James Clay sorts his CS racing out and just ask him lol

[sdhotwn]

Here's some of the basic math without actual shift points.
Gear Ratios: 1- 4.11, 2 - 2.32, 3 - 1.54, 4 - 1.18, 5 - 1.00, 6 - .85

So a shift from 1st to 2nd... the torque at a given point in the rpm's would have to be 1.77 times greater for it to not to be a loss in acceleration. a 2-3 shift - 1.5X, for a 3-4 shift - 1.3X.

Attached is a graph of the torque dynos from the M3/M4 (assuming the CS uses similar tuning on the S55 to the ZCP and not the M3 CS variant).

Flywheel torque at redline is ~310 ft-lbs. The 3-4 shift has the smallest multiplier I listed putting you at needing about 403 ft-lbs to make even. It looks like you'd exceed this if drop back below 5000 rpm's. However, if you shift earlier, 7,000 rpm's torque is at 350 and you'd have to drop back to a torque above 455 ft-lbs, which doesn't exist (for the ZCP curve). I'm not spending the time to calculate with final drive etc what the actual rpm change is with the shifts either here, but I'm doubtful we are getting 2500+ rpm drop on a 3-4 shift to get you back below 5,000 rpm's anyway, so it further reinforces the situation.

So moral of the story is if you have the ZCP tune type curve in our CS, you shift at redline for maximum acceleration.

[ptgltw]

Originally Posted by sdhotwn

Here's some of the basic math without actual shift points.
Gear Ratios: 1- 4.11, 2 - 2.32, 3 - 1.54, 4 - 1.18, 5 - 1.00, 6 - .85

So a shift from 1st to 2nd... the torque at a given point in the rpm's would have to be 1.77 times greater for it to not to be a loss in acceleration. a 2-3 shift - 1.5X, for a 3-4 shift - 1.3X.

Attached is a graph of the torque dynos from the M3/M4 (assuming the CS uses similar tuning on the S55 to the ZCP and not the M3 CS variant).

Flywheel torque at redline is ~310 ft-lbs. The 3-4 shift has the smallest multiplier I listed putting you at needing about 403 ft-lbs to make even. It looks like you'd exceed this if drop back below 5000 rpm's. However, if you shift earlier, 7,000 rpm's torque is at 350 and you'd have to drop back to a torque above 455 ft-lbs, which doesn't exist (for the ZCP curve). I'm not spending the time to calculate with final drive etc what the actual rpm change is with the shifts either here, but I'm doubtful we are getting 2500+ rpm drop on a 3-4 shift to get you back below 5,000 rpm's anyway, so it further reinforces the situation.

So moral of the story is if you have the ZCP tune type curve in our CS, you shift at redline for maximum acceleration.

Your the man. I guess that’s why it’s there eh! I need to install a shift light now. The rpm cluster is pretty tough to quick glance it without any red

[sdhotwn]

Originally Posted by ptgltw

Your the man. I guess that’s why it’s there eh! I need to install a shift light now. The rpm cluster is pretty tough to quick glance it without any red

I'm going to get the electronic wheel. One of my few mod splurges for the car, and it'll preserve the original steering wheel for the future .

[CarJunkie]

Here is a picture of the effort I made when bored one day after analyzing my dyno. This was a custom Cobb tune, which I don't run anymore, but it demonstrates the concept:

If you have an engine with a relatively flat torque curve all the way to redline, you would always want to shift at red line. A good example is the rotary engine, or some of the small displacement early Honda VTEC engines. You definitely wanted to string it out all the way. I'm guessing the pre-turbo M engines were the same way.

With a turbo engine that has a fat torque curve that is declining toward redline, finding the optimal shift point is a little bit trickier.

You'll notice that for each gear I have wheel torque at engine RPM (bold column) which is essentially the engine torque multiplied by the gear ratio for that gear. The column to the right of the bold numbers is what the RPM will drop to in the next gear when shifting. You essentially want to shift where those torque numbers match up. Notice that the optimal shift point decreases for each gear because the ratios are getting closer together. The color-coded highlights represent the multiplied torque pre-and post shift at optimal points.

I can tell from feel that my custom MHD tune is pretty similar. I make 450ft-lb and only 380 hp at the wheels. Just from the numbers (406ft-lb and 444hp) we know that the shift points will be higher since the torque band is flatter and lasts longer on the S55. I'll copy the spreadsheet and plug in the numbers later and see what it looks like if someone can find me a fairly detailed stock wheel dyno.

[PLF69]

Would love to know this for the N55 M2. (DCT)

[CarJunkie]

OK, I ordered a DCT, so it's moot for me, but it is easy to plug new numbers into a spreadsheet....here you go:
From the ZCP dyno above:

[CarJunkie]

Another, using the purple line in the dyno, which seems to me to be closest torque numbers to the factory claim, and surprisingly suggests redline shifts are the way to use the most torque:

[sdhotwn]

BTW. Awesome spreadsheet, thanks for sharing! It really helps paint a broader picture.

[CarJunkie]

Anyone know BMW's DCT gear ratios?

[CarJunkie]

Found ratios....here is the spreadsheet for DCT using the lower torque dyno above:

[CarJunkie]

DCT with numbers from "ZCP" dyno above: more midrange torque results in some benefit to earlier shifts with the closer ratios of the DCT, but only a few hundred rpms in upper gears. 7th gear is clearly meant for economy and would never be used on track or drag strip.