Slow 6MT Turbo Spool - Actual Torque Curve?

[SuprUsrStan]

So I've got a 6MT M3 that I feel like has an immensely long turbo spool. Lets say I'm in second gear driving normally at 1500 to 2000 rpm and then mash the gas pedal. It feels like it takes FOREVER for the turbos to properly spool up. Basically the entire time from 2000 RPM to 3000 RPM you can hear the whistle on the turbos grow louder as more power builds and its not really until almost 3000 RPM where you actually get full power. That's like a full 1 to 2 seconds of power being built on the turbo.

Many have suggested that peak torque on the S55 comes at 1850 RPM per this torque curve.



However actual dynos of the car suggests peak torque is actually a little closer to right under 3000 rpm. Kies Motorsport Dyno run on a stock motor.



I feel like my experience with turbo spool on the 6MT is much more closely matching the second image and in normal daily driving the spool takes a long time to overcome. So what is the actual torque curve for our cars and why do we keep on talking about peak torque coming on below 2000 RPM?
Thoughts?

[hC1001]

What istep are you running? I noticed a huge change in the stock tune going from a 2016/early 2017 istep to 2020 istep. I think around late 2017/early 2018, BMW neutered the torque down low. There was more torque down low but with the later istep, its as you described, builds up slowly until 3K.

The chart with peak torque at 1850rpm is from an engine dyno. The second chart is a chassis dyno. They are different and shouldnt be compared.

I didnt get a lot of time on the stock tunes because i have bootmod3 stage 1. I updated my istep to 2020 and reflashed bootmod3 but i drove a couple days with the stock tune and really noticed the lack of low end torque.

Also found this thread that proves this theory with dyno runs. You can also find personal experiences with same conclusion. https://f80.bimmerpost.com/forums/sh....php?t=1454386

[SuprUsrStan]

It's maybe as you say. The car is a 2018 M3 ZCP and I'm on F020-17-7-507. The feel I'm getting matches up with your gray line perfectly.

I'm sure another factor is the 6MT and the way I'm driving. It's most noticeable when you're driving normally in 2nd or 3rd gear in low RPMs and decide to hit it hard. In a DCT, the car will normally kick down for you down to 1 or 2 depending on your speed but in a 6MT you wouldn't shift into 1 in most cases, so you're just left waiting for that 2nd or 3rd gear to get up above 3000 RPM. It'd be equivlent to putting the car in manual mode and holding a 2nd gear from 1500 to 3000 and waiting for the turbos to spool up.

I'm just curious why everyone talks about how the turbos are at their max spool and torque at under 2K RPMs when that's just not the case.

[TopJimmy]

I had the same low-RPM-low-Power with my 6MT before tuning...anything under 2K was like pushing a slug. When I got a Stg1 tune installed the power builds earlier. Now with a tune I don't need to shift into first to do a tight sharp turn.

[Chris@VargasTurboTech]

Spool and lag aren't the same thing. Spool is rotordynamically where the wheels start flowing significant air, lag is time-to-torque. What you're noticing is lag, and to some extent is normal although there are tuning (and hardware and/or fueling) tricks to reduce it. People always forget that dyno's do not show lag. They show spool. Sure, all else equal spool and lag are in bed with one another but.... time-to-torque is a real thing and hugely impacts drivability. Obviously picking your gears better would help but that thing you're noticing and not liking much is good 'ole lag.

Chris

[SuprUsrStan]

Originally Posted by Chris@VargasTurboTech

Spool and lag aren't the same thing. Spool is rotordynamically where the wheels start flowing significant air, lag is time-to-torque. What you're noticing is lag, and to some extent is normal although there are tuning (and hardware and/or fueling) tricks to reduce it. People always forget that dyno's do not show lag. They show spool. Sure, all else equal spool and lag are in bed with one another but.... time-to-torque is a real thing and hugely impacts drivability. Obviously picking your gears better would help but that thing you're noticing and not liking much is good 'ole lag.

Chris

As you say, the dyno only shows spool, not lag. I believe what I'm describing is lines up with the exact curve shown on the dyno. It feels more dependent on RPM rather than gear or gas pedal. That's why I thought it'd be turbo spooling based on actual lack of airflow from low RPM rather than lag. At higher RPMs, there's plenty of power and the lag is minimal since there's plenty of air to flow through the turbo.

[Chris@VargasTurboTech]

Take a log, input it into virtual dyno for an idea of what it's putting down and more importantly... when in those gears. Interesting to compare it to a proper WOT in a taller gear.

[TopJimmy]

Hey Chris, I noticed from BM3 logs that even a fast shift drops RPM by 2K...

Is it right that if we have a dyno chart that the best powerband for a 6MT is the 2KRPM wide band that catches the "hump" at the top of the Horsepower curve? (open the pic, green marker)

[Chris@VargasTurboTech]

Usually, but all that matters is what's being put to the ground -which is a function of gearing etc. In a car that the power climbs to redline it's almost always best to shift at redline. Plus remember... on a turbo car when you shift you usually have to build at least some of that boost back, NLS aside -so the dyno chart isn't what you experience on the street. Usually lower gears don't have enough load to command that much boost and make that power (traction limited often times too) -so the dyno chart isn't really what you'll experience on the street unless you do a single gear pull (or doing this on a vehicle without turbos). It's another reason quick spooling low-lag turbos feel so much faster.