[spool twice]

Does our load targeting N54 work similar to a boost targeting system to compensate for altitude?

[MilehighM3]

Above 4k feet in elevation, the car adds 1 psi of boost for every 1k feet. Up to 8k feet. The car isn't targeting a certain boost level, it's just trying to maintain a similar response and feel of being at lower elevations.

[Carl Morris]

It may do that stock, but with the Cobb there seems to be no way to make it target more than about 16psi at ~5500ft above sea level. They seem to actually be derating the boost at higher elevation to avoid overworking the turbos. I tried to discuss it with Cobb and didn't get anywhere.

[old account 2]

Nice writeup Shiv!

The S4 is nice in that it now allows us to control boost to a degree. When tuning first started for that car, we were the only company who figured out how to fully close the bypass valve. As such, we were making as much boost on the stock pulley as others were making with an upgraded pulley.

The bypass valve on the TVS1300 essentially acts similar to a wastegate in a turbocharged setup. And with instant response, the superchargers are certainly a worthy competitor to turbchargers.

[JamesM3M5]

Originally Posted by Thomas@GIAC

The bypass valve on the TVS1300 essentially acts similar to a wastegate in a turbocharged setup. And with instant response, the superchargers are certainly a worthy competitor to turbchargers.

True, but can they build the same boost at low RPMs as a turbo? Coming from the individual throttle S14, S50Euro, and S38 world, turbo lag was a new thing for me. Fortunately the tiny little coffee cake roll sized turbos we have are rather fast at our pressures. Does the S4 blower make significant boost at, say, 3000-3500RPM? Or is it better to just start higher up, maybe 5000-6000? Just curious...

[old account 2]

Originally Posted by JamesM3M5

True, but can they build the same boost at low RPMs as a turbo? Coming from the individual throttle S14, S50Euro, and S38 world, turbo lag was a new thing for me. Fortunately the tiny little coffee cake roll sized turbos we have are rather fast at our pressures. Does the S4 blower make significant boost at, say, 3000-3500RPM? Or is it better to just start higher up, maybe 5000-6000? Just curious...

In my experience, yes it will make boost faster than a turbocharger. While they do not make huge torque like the newer turbocharged cars, they will make it earlier (80% of peak torque available by 2800 RPM).

[raceyBMW]

Originally Posted by Carl Morris

It may do that stock, but with the Cobb there seems to be no way to make it target more than about 16psi at ~5500ft above sea level. They seem to actually be derating the boost at higher elevation to avoid overworking the turbos. I tried to discuss it with Cobb and didn't get anywhere.

Carl, on stage 2+FMIC, I hit about 18psi and taper pretty quickly to 12psi in 3rd gear. I know from my logs that I hit nowhere near the load targets though.

[pits200]

Originally Posted by Thomas@GIAC

In my experience, yes it will make boost faster than a turbocharger. While they do not make huge torque like the newer turbocharged cars, they will make it earlier (80% of peak torque available by 2800 RPM).

Aren't our cars 100% of peak torque by 1800 rpms?

[Carl Morris]

Originally Posted by raceyBMW

Carl, on stage 2+FMIC, I hit about 18psi and taper pretty quickly to 12psi in 3rd gear. I know from my logs that I hit nowhere near the load targets though.

I hit 17.5 on my ATR maps, but 1.5 of that is overboost. The target doesn't go above 16. If your boost target is going over about 16.5 I'd be curious to see the log.

[Beemw335]

The supercharger makes boost according to engine rpm, so no it will not make as much power in the lower rpm as a turbo. However there is no "lag" with a blower, however on our cars there is basically no lag anyways.

[raceyBMW]

Originally Posted by Carl Morris

I hit 17.5 on my ATR maps, but 1.5 of that is overboost. The target doesn't go above 16. If your boost target is going over about 16.5 I'd be curious to see the log.

Here ya go! Taken at Bandimere during some runs. This was with about 104octane in the tank.

They are actual PDF files of the graphs...I can make them JPG if you want, or let me know if you want the datalogs.

BTW, is it normal for the timing corrections at shift?

[Carl Morris]

Thanks. So what I'm seeing there is absolute boost requested peaking about about 12.5+14.7psi = 27.2psia. Odds are that your ambient pressure at the time was probably around 12psia, but could have been as low as 11.7psia or so. So probably the highest your requested boost could have been is 27.2-11.7 = 15.5psig. The rest was overboost.

Unfortunately timing corrections are normal at shifts (at least for my 6MT), but can be somewhat tuned around in the timing tables.

[raceyBMW]

Thanks, IC how that reads now, interesting that requested is so low.

[Carl Morris]

I believe that Cobb is reducing the target boost as ambient pressure drops, if the people running 18psi at sea level actually have a boost target that high.

[raceyBMW]

BTW, these were all on v3.01, stage2+FMIC Aggressive...I haven't logged on v4.01/2 yet.

[CoralRed335XI]

Awesome write-up!