N55 powerband with s54 top end

[Wolffm3socal]

Something I've always wondered, why is it that BMW is able to achieve peak torque at sub 2k rpm for n55, n20 etc motors, but all aftermarket solutions for n/a motors always spool way later? Every "stage 1" kit I've seen, even if it's with really conservative power figure, spools at 4k rpm. With more displacement than an n55, shouldn't we be able to make an s54 that makes something like 300lb/ft at 2k rpm, and if we limit power to 400hp hopefully preserve the top end? Is the issue simply that any turbo that spools at 2k wouldn't be able to make power at 8k?

[Hallerhan]

im no expert in this, but I can speak from experience with my N54 and N55. The low end torque was incredible, but the turbos "gassed out" right around the 6000 RPM mark. I think the reasoning behind it was they were too small of turbos to push out through the entire RPM band. Again this is completely anecdotal, and it's been a few years since owning either of those engines

[beefaroni]

An engine generates peak torque at 1 operating speed in which the airflow dynamics are optimized. broadening the torque curve above and below that requires some black magic. Ever since BMW started making the M50NV, most of the technological advancements were not to improve peak horsepower, but to broaden the torque curve.

For example, lets look at 2.5L engines.

M50B25 (Non-VANOS) 181 lb-ft @ 4700 rpm
M50B25TU (single VANOS) 184 lb-ft @ 4200 rpm single VANOS helps broaden peak torque curve by 500 rpm
M54B25 (dual VANOS w/ DISA) 181 lb-ft @ 3500 rpm dual VANOS and DISA further broaden torque curve 700 rpm
N52B25 (valvetronic) 184 lb-ft @ 2750 rpm Valvetronic helps broaden the torque curve by 750 rpm as well as at higher rpm for ~20 more HP than previous BMW 2.5 engines

with the N54/N55, the turbos broaden the torque curve substantially

Technology can only go so far while being reliable, emissions compliant, and cost effective for consumer use. So while BMW has been able to make great efficiency improvements with technology.

As stated before, internal combustion engines are really poor at making power over a wide range of speeds due to fluid dynamics. Adding a turbocharger allows a massive advantage since you can strategize an engine design different ways. One turbocharged strategy would be designing the car with very mild cams and prioritize peak torque at low rpm for good cruising fuel economy and emissions while using the turbocharger to supplement airflow in the mid to high rpm range to create a decently balanced turbo engine.

The biggest difference between OE and aftermarket turbocharging solutions really is the R&D budget. It is certainly possible for someone to turbo an engine that was originally NA and have great low end response and top end pull but it would require some good engineering, use of twin scroll/VGT turbos, a change in the length/diameter of the intake and exhaust, cylinder head ports, change in cam timing/cam profile, excellent tuning etc. etc. all things which are possible but usually not feasible for the time and money constraints of the average car guy working by himself.

Aftermarket turbo systems can compliment the low end better when the buyer can restrain himself from selecting too large of a turbo and building based on peak HP gains.I thought I was selecting a small-ish turbo for a previous E30 build, T3/T4 .60/.63 but driving a similar setup with a .50 was such a better experience where I would have guessed it would be too small of a turbo.

[EthanolTurbo]

You could use a GTX30r or GTX35r sized turbo and have more torque at lower RPM with an amazing midrange or top end.

[Arith2]

When I was UTI, when looked at charts of turbos and they explained the efficiency charts.

This article explains the charts