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      08-09-2019, 12:10 AM   #1
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M235i shift points

I find myself shifting at 6600/6700 RPM, am I faster this way?

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      08-09-2019, 10:24 AM   #2
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For maximum acceleration on the M235 N55 motor, the ideal shift points are:

1-2: ~7000rpms
2-3: ~7000rpms
3-4: ~7000rpms
4-5: ~6800rpms

Power peaks around 6000-6200rpms, but doesn't tail off much until 6600rpms. Due to the gear ratios and torque curve, taking the revs beyond 6600rpms is actually ideal based on calculations. If you shift at peak power, you'll be slower. Promise.
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      08-09-2019, 11:16 AM   #3
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Quote:
Originally Posted by ttsidd View Post
I find myself shifting at 6600/6700 RPM, am I faster this way?

M235i xdrive 2016 coupe
I don't have an N55, so I can't help you specifically.

What I did with my B58 was to enter the car's transmission and differential ratios into a gearing calculator. I then looked at where the RPM drop would leave me on Dinan's dyno graph for the stock engine. I came up with 6000 RPM for my shift point. I then confirmed that number with a BMW race shop.
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      08-09-2019, 01:41 PM   #4
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Quote:
Originally Posted by dradernh View Post
I don't have an N55, so I can't help you specifically.

What I did with my B58 was to enter the car's transmission and differential ratios into a gearing calculator. I then looked at where the RPM drop would leave me on Dinan's dyno graph for the stock engine. I came up with 6000 RPM for my shift point. I then confirmed that number with a BMW race shop.
That's way too low for the B58. That motor makes peaks power around 5100rpms and carries it's peak nearly to 7000rpms. Shifting at 6000rpms will undoubtedly result in slower max acceleration. I'm not sure what program you're using, but it can't be right. Look at this dyno plot (run 003 - Stock). Power peaks at ~5000 and holds all the way to 7000rpms. You've got to run the gears out to redline.



It's pretty easy to tell what the ideal shift point is on any late model BMW. Simply put the car Sport+, go full throttle, and let the automatic shift the gears. Those are the ideal shift points.
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      08-09-2019, 03:28 PM   #5
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Quote:
Originally Posted by XutvJet View Post
I'm not sure what program you're using, but it can't be right.
I don't have an automatic, so I can't access that data. When do the autos shift?

I used the untuned engine data from Dinan's B58 V1 dyno graph, as seen here:
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      08-09-2019, 03:41 PM   #6
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Here's a video showing the M240 using launch control in Sport+ It looks like the auto selects the following shift points

1-2 6600
2-3 6600
3-4 7000
4-5 7000
5-6 7000




The M235 appears to have similar shift points

1-2 6500
2-3 6800
3-4 7000
4-5 7000
5-6 7000

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      08-09-2019, 03:55 PM   #7
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Not sure but this feels right, to me for fastest accel.:

1st - 6300-6500rpm short shift to avoid synchro jam (it'll suck it right into second this way)
2nd - 6500-6700
3rd - 6700-6800
4th - 6700-6800
5th - Redline
6th - never redlined yet lol
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      08-09-2019, 03:56 PM   #8
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Quote:
Originally Posted by XutvJet View Post
Here's a video showing the M240 using launch control in Sport+ It looks like the auto selects the following shift points

1-2 6600
2-3 6600
3-4 7000
4-5 7000
5-6 7000
In looking at the flatness of the HP curve in Dinan's graph, I'm wondering what spinning much past 6K is gaining me. That's assuming a drop to 5K after the 4-5 shift is completed and I'm flat again. Any thoughts, when using Dinan's HP line as a guide?

I'll grant you, I'm probably missing something obvious here!
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      08-09-2019, 04:00 PM   #9
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Quote:
Originally Posted by dradernh View Post
In looking at the flatness of the HP curve in Dinan's graph, I'm wondering what spinning much past 6K is gaining me. That's assuming a drop to 5K after the shift is completed. Any thoughts, when using Dinan's HP line as a guide?

I'll grant you, I'm probably missing something obvious here!
I agree, as long as u stay above the rpm for maximum torque ( or near ) then shift halfway between that and the rpm for maximum horsepower.
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      08-09-2019, 04:45 PM   #10
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Quote:
Originally Posted by ScottSinger View Post
I agree, as long as u stay above the rpm for maximum torque ( or near ) then shift halfway between that and the rpm for maximum horsepower.
Right. Using Dinan's results for the stock engine output, there's a negligible decrease in HP when dropping from 6K to 5K during a 4-5 shift, but there's an 18% increase in torque. That seems significant to me. It does look like spinning the engine up to 6500K makes sense for the 3-4 shift, as that would be necessary to be at 5K after the shift.

I was at another race shop yesterday (i.e., not the one with the shift-at-6K recommendation). Over the years, this shop has built quite a number of sports cars for IMSA/Grand Am/World Challenge and done very well at it (e.g., landing sponsors as major as McDonald's for its Daytona Prototypes). I now wish I'd brought this question to them, because I'm certain they'd take one look at the data and say "This RPM is where you want to shift." And the recommendation would have come with a good bit of insight, which with this hobby is something I always feel that I need.

I think I'll try them and see what they say, especially in terms of providing context.
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      08-09-2019, 04:59 PM   #11
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Using the calculator provided below and imputing the torque information in the dyno I provided by 1,000rpm increments starting at 2,500rpms, the gear ratio info in the link below, and 245/35R18 tire (24.8" tall), the ideal shift points are 7,000 rpms in every gear. It's quite possible that the 8AT is shifting at 7,000rpms in every gear, but the tach can't keep up in 1st and 2nd because the gears are so short and wind out so fast. I've seen that in a lot of cars including my M235 6MT when winding out 1st.

http://glennmessersmith.com/shiftpt.html

https://www.bimmerpost.com/goodiesfo...oupe-specs.pdf
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      08-09-2019, 11:04 PM   #12
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Quote:
Originally Posted by dradernh View Post
Right. Using Dinan's results for the stock engine output, there's a negligible decrease in HP when dropping from 6K to 5K during a 4-5 shift, but there's an 18% increase in torque. That seems significant to me. It does look like spinning the engine up to 6500K makes sense for the 3-4 shift, as that would be necessary to be at 5K after the shift.
In shifting from 4th to 5th on the M240i, you also see the gear ratio change from 1.18 to 1.00, so also lose 18% of wheel torque from the gearing which offsets the 18% gain from revs dropping from 6000 to 5000RPM. So 4th to 5th gear will drop the power slightly, but not increase the torque at the wheels.
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      08-10-2019, 02:45 PM   #13
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Quote:
Originally Posted by XutvJet View Post
Using the calculator provided below...

http://glennmessersmith.com/shiftpt.html
...and the OEM baseline torque data from Dinan's V1 dyno chart, Messersmith's calculator yielded the data shown in the first image below. Should I assume that if I'd had a 7K torque data point that the calculator would have recommended a shift point of 7K RPM?

The second image below shows calculated shift points after I guesstimated a 7K RPM torque output value.





Quote:
Originally Posted by aerobod View Post
In shifting from 4th to 5th on the M240i, you also see the gear ratio change from 1.18 to 1.00, so also lose 18% of wheel torque from the gearing which offsets the 18% gain from revs dropping from 6000 to 5000RPM. So 4th to 5th gear will drop the power slightly, but not increase the torque at the wheels.
Thanks for this. I recall coming across this fundamental aspect of the issue some years ago but never internalized it nor have I ever really tried to understand this stuff. Perhaps it's time I gained a better understanding of the subject. I'm afraid I've always asked my tuners to tell me what my shift point should be and then entered that data into the component driving the shift light module. (I don't have a shift light module on this car yet, though I may get one as part of the Aim Solo 2 DL.)
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      08-10-2019, 06:29 PM   #14
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Quote:
Originally Posted by dradernh View Post
Perhaps it's time I gained a better understanding of the subject.
I'll take a stab at what is often made out to be more complicated than it needs to be.

Horsepower and torque are two different ways of presenting the same data, so using one or the other always has to give the same answer. If not, you have made a mistake.

Torque is based on force, which is less abstract than horsepower, which is based on energy. Horsepower is, basically, the rate at which the torque of the engine is converted into kinetic energy.

If you have the complete curve of horsepower vs RPM, you have all you need to choose shift points. Just look at the curve and keep the engine making as much horsepower as you can and you will be increasing kinetic energy at the maximum rate possible.

You can also choose shift points from the torque curve, but you have to adjust the torque at the rear wheels by the gear ratio for each gear. It's a lot more work but it is easier to understand. Unless you're willing to dive into the physics of energy you just have to accept that horsepower gives the same answer.
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      08-11-2019, 06:19 AM   #15
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I must be the first engineer to post in this thread. Here's what you are missing:

Horsepower is torque multiplied by rpm (times some constant to get to the power units of choice). If you shift at max torque or bracketing max torque you are shifting too soon. Two ways to see this:

Using Math, ex:
torque = 300 and is relatively flat
shift at 6000 rpm
c = .0001 to get to giraffe power
300 x 6000 x .0001 = 180 gp

torque = 300 and is relatively flat
shift at 7000 rpm
c = .0001 to get to giraffe power
300 x 7000 x .0001 = 210 gp

Using gut feel:
the same torque in a lower gear puts more power to the road so stay in the lower gear as long as possible.

This agrees with the factory shift points.
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      08-11-2019, 11:52 AM   #16
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Other things that can make this process a bit confusing and complicated is the fact that turbo cars do not make the same power in every gear. Turbos are dependent on load to generate boost. Most cars are dynoed in the 1:1 gear (a higher load gear) which gets us the power numbers we're using for these calcs. However, in gears 1 and 2, less boost is being made because the torque multiplication of those lower gears greatly reduces the load on the engine (i.e., it doesn't have to work as hard), thus less load on the motor means less boost that can be ultimately generated. There's no physical way to generate more boost in those gears short of changing out the turbo.

My stock tune M235 6MT with 93 octane, 60 degree temps, and at a 1,000 elevation makes the following max boost per gear per my Torque app:

1st ~6-7psi
2nd ~7-8psi
3rd ~9-10psi
4th ~10-11psi
5th ~10-11psi

Each psi equates to around 10hp. I'm pretty certain the power curves in the gears 1 and 2 can look a different and more peaky than the flatter power curves found in 3rd, 4th, and 5th where there is sufficient load being generated and the DME is actually bleeding boost to keep boost at target levels (typically in 4th and above gears).

Many turbo cars benefit from slightly short shifting into 2nd (shift at 6,000-6,200rpms; would need to experiment) in an effort to create more load and boost on entry into 2nd gear. Confusing, right?

With that said only real way to find out the ideal shift points is to do lots of runs at the 1/4 mile track and experiment with shift points and paying very close attention to the trap speeds. I'm talking 50+ runs.

The calculator and info I provided should get you real close, especially shifts above 2nd gear.

Lastly, I wouldn't pay attention to that slight dip in power between ~5,200-6,000rpms that both tuned dynos are showing. You need to look at the entire curve and note that power continues to climb/hold steady from 6,000-7,000rpms. That dip in power has little affect on acceleration when the entire powerband is considered.
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      08-11-2019, 01:14 PM   #17
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Quote:
Originally Posted by XutvJet View Post
Other things that can make this process a bit confusing and complicated is the fact that turbo cars do not make the same power in every gear. Turbos are dependent on load to generate boost. Most cars are dynoed in the 1:1 gear (a higher load gear) which gets us the power numbers we're using for these calcs. However, in gears 1 and 2, less boost is being made because the torque multiplication of those lower gears greatly reduces the load on the engine (i.e., it doesn't have to work as hard), thus less load on the motor means less boost that can be ultimately generated. There's no physical way to generate more boost in those gears short of changing out the turbo.
I have no reason to doubt your app regarding the boost levels in different gears. However, your explanation makes no sense to me. The load is the same in every gear - it's the torque of the engine applying force to the road and pushing against the weight of the car. The weight is the same in every gear. In lower gears the force on the road is greater because of gearing is different, but that just means the car accelerates more quickly. It doesn't mean the load is any different.

Part of the difference might be turbo lag. At max throttle you spend so little time in first gear that the turbo might not have time to reach full boost. Maybe boost drops during the shift to second with a manual, then it gets a bit higher in second before you have to shift again since you spend more time in second. And so on. But, to my seat-of-the-pants sense, the turbo spins up quickly enough that this doesn't seem like a full explanation.

It might also be the computer limiting boost in the lower gears, although it would be hard to understand why an app would have that information as I doubt BMW releases it.

Anyone have a better explanation?
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      08-11-2019, 03:24 PM   #18
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Quote:
Originally Posted by Shovelman View Post
I must be the first engineer to post in this thread. Here's what you are missing:

Horsepower is torque multiplied by rpm (times some constant to get to the power units of choice). If you shift at max torque or bracketing max torque you are shifting too soon.

Using Math, ex:
torque = 300 and is relatively flat
shift at 6000 rpm
c = .0001 to get to giraffe power
300 x 6000 x .0001 = 180 gp

torque = 300 and is relatively flat
shift at 7000 rpm
c = .0001 to get to giraffe power
300 x 7000 x .0001 = 210 gp

Using gut feel:
the same torque in a lower gear puts more power to the road so stay in the lower gear as long as possible.
Some engineer always comes along to give the formula for calculating horsepower from torque, as if it explains everything. Unfortunately, it doesn't explain what horsepower actually is. For someone who doesn't understand the physics of energy, the formula provides no insight.

There's nothing wrong with your calculation, expect that it is irrelevant to our cars. Torque is not flat above 5000 rpm, as shown in both the Dynan and Dynojet results, so your calculation of more power at 7000 than 6000 rpm does not match reality.

Finally, your use of torque and power in the same thought contributes to the confusion among people who don't understand the concepts underlying them, which is the source of the common idiocy spouted by journalists that "torque is more important than horsepower". One of the reasons physicists look down on engineers is that often they are so focused on getting results that they don't take the time to fully understand the underlying physics. If you do understand the physics, then you have a professional obligation to use more precise language.
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      08-11-2019, 04:13 PM   #19
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There is no easy answer here as these are complex turbo motors. These motors have an electronic waste gate, don't use a throttle body, and have variable cam timing. BMW can greatly manipulate the shape of the power curve and the way power is delivered. BMW strives to make these motors feel naturally aspirated and significantly modify the boost and timing curves, especially in the lower and midrange rpms. It's also why most tunes pick up significant power in this range.

With respect to load, again, it's a very complex matter because these are turbo motors and not naturally aspirated. The engine simply doesn't have to pump as hard in the lower gears because of torque multiplication at the gearbox thus less boost is generated because the engine doesn't fill the cylinders as much thus can't create the exhaust velocity to spin up the compressor rapidly enough to get to full boost. Think of a multigear bicycle. The bike is the gearbox and you are the engine. Go up a moderate grade at 10mph in the short gears and your legs and lungs don't work as hard and it's relatively easy. Put the bike in a tall gear and go up the same grade and speed and you'll struggle. You'll breath in a lot harder, breath out a lot harder, your legs get stressed (think connecting rods), etc. This heavier exhaling is what would spool up the turbo.

The slight hiccup in upper rpm power most see with tuned B58's (especially piggybacks) is likely the DME trying to make sense out of what it perceives as wonky data from sensors when compared to what it is expecting to see, thus it manipulates timing, fuel ratio, bleed boost with the EWG, etc.

If you want to really understand the full relationship between load and boost, there is plenty of information out there by simply searching "turbo boost, load, gear". As you'll see, it's very complex and highly variable.

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      08-11-2019, 04:20 PM   #20
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Torque is a measure of power.

HP is a measure to do work over time.

Peak torque is meaningless without the HP calculation Torque will always fall after 5252rpms. There's no way around it.

A motor that make 800 ft/lbs but only makes 250hp means the power will come on strong and quick, but the powerband will be very narrow with power focused in the lower rpms (below 4000rpms). This is great for towing because significant torque is available right off idle but isn't great for all out acceleration.

A 500hp/500tq motor wouldn't be nearly as good for towing but will be MUCH faster because the motor can do more work over a larger rpm spread (i.e., power under the curve).

Torque doesn't will races. Power under the curve wins races. Period.

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      08-11-2019, 06:37 PM   #21
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Quote:
Originally Posted by XutvJet View Post
Torque is a measure of power.

HP is a measure to do work over time.

Peak torque is meaningless without the HP calculation Torque will always fall after 5252rpms. There's no way around it.
Torque is dimensionally equivalent to energy (Nm are equivalent to J in the SI system), power is the rate of energy change, i.e. J/s or kW.

There are actually quite a few high revving engines (non in the current BMW line up I know of) that have peak torque above 5252 RPM. In my Caterham peak torque of 150lbft is at 6300RPM and peak power of 210bhp is at 7600RPM.
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      08-11-2019, 06:41 PM   #22
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Quote:
Originally Posted by XutvJet View Post
Torque is a measure of power.

Peak torque is meaningless without the HP calculation Torque will always fall after 5252rpms. There's no way around it.

A motor that make 800 ft/lbs but only makes 250hp means the power will come on strong and quick, but the powerband will be very narrow with power focused in the lower rpms (below 4000rpms). This is great for towing because significant torque is available right off idle but isn't great for all out acceleration.

A 500hp/500tq motor wouldn't be nearly as good for towing but will be MUCH faster because the motor can do more work over a larger rpm spread (i.e., power under the curve).

Torque doesn't will races. Power under the curve wins races. Period.
Torque is not a measure of power. It is a measure of force directed in a way to cause an object to rotate. (More specifically, it is force times the distance between the point at which it is applied and the axis of rotation, and it is only the component of force that is at 90 degrees to the line joining those two points).

Arguing that torque or horsepower is more important is like arguing that heads or tails is more important. They are both sides of the same coin.

If you have the complete torque and horsepower curves and you understand what they mean, you can draw exactly the same conclusions from either.

If the only information you have is the peak horsepower and torque values, you have two indicators of performance which are usually at different RPMs. If you are interested in performance near the RPM that gives peak torque, then peak torque is more useful to you than peak horsepower. And vice versa.

Neither torque nor power is intrinsically "better". The equations that you use to calculate performance with them are fundamentally different despite being derived from the same basic laws. In some circumstances it can be easier to apply one or the other or to collect the data needed to do calculations with one or the other, which are among the reasons that both exist.
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Last edited by albertw; 08-11-2019 at 07:02 PM..
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