Last year around this time BimmerBoost took a look at the B48 2.0 liter inline four-cylinder motor. The B58 is basically the same thing except longer due to offering two more cylinders.
The B48 replaced the N20 2.0 four-cylinder engine. The B58 will replace the N55 3.0 liter inline-6 throughout the lineup. The B58 differs from the N55 in a few ways the most important which is the new bore and stroke of 82mm x 94.6 respectively and the higher compression ratio. Here is a quick overview of the N55 and B58 replacing it:
Bore x Stroke: 84mm x 89.6mm
Compression Ratio: 10.2:1
Torque: 332 lb-ft
Redline: 7000 rpm
Bore x Stroke: 82mm x 94.6mm
Compression Ratio: 11.1:1
Torque: 330 lb-ft
Redline: 7000 rpm
The difference in architecture between the B48 and N55 is greater than the N54 and N55. It has a new cylinder head design to lower carbon buildup, new intercooler, new thermostat, lighter Valvetronic, and new pumps.
While the N55 did go to a single turbo instead of the twins the N54 had it maintained the same basic geometry sharing the same bore and stroke as well as the same compression ratio. The N55 could be essentially boiled down to an N54 with one turbo and Valvetronic added in.
The B58 changes the architecture a bit going to an even more undersquare design. Less bore, more stroke. They also raised the compression ratio. 11.1:1 is definitely getting up there for a modern turbo motor even one that is direct injected. The B58 is slightly larger although this is a minor point. BMW seems to want more stroke and higher compression for low end performance and spool. The N55 is not a high revver by any means but the B58 is geared even more toward low end torque and spool. The higher compression will help with fuel efficiency as well.
The B-Series is going to make nice gains in the aftermarket as all the BMW factory turbo inline-6 motors do. BMS (Burger Tuning) is already at work at this. Let's take a look at their stock and Alpha Stage I tuned figures:
The stock motor through the automatic transmission puts down 331 horsepower to the wheels and 339 lb-ft of torque to the wheels. As is the BMW trend now with the turbo powerplants their crank output ratings tend to be closer to wheel output ratings. 331 to the wheels exceeds the 320 crank rating meaning this motor is closer to 400 horsepower than 320.
This is not all that different from the N54 and N55 era although the N54 started in the 275 wheel horsepower range with a crank rating of 302. BMW actually is moving more towards being conservative in their ratings.
Comparing the B58 F30 340i automatic to the N55 F30 335i automatic at the wheels shows a definite factory advantage for the B58:
298 wheel horsepower and 306 lb-ft of torque for the stock N55 automatic. It is underrated as well just not quite to the same degree as the B58 F30 340i.
Comparing the tuned numbers is tricky. For one, the above graph shows a flash tune. The B58 tuned numbers are from a BMS JB4 piggyback. You will see B58 tuning evolve just like the N55 and N54. Piggybacks will dominate early on and flash tuning will be more prevalent over time. Eventually, both approaches will likely be used as history shows us.
For now, the B58 is at 379 wheel horsepower and 378 lb-ft of torque on the early BMS JB4 Alpha Stage I setup. The horsepower numbers stands out in particular. BMS believes the B58 turbocharger is larger and therefore it holds its torque better toward redline which results in the better horsepower curve toward redline. It comfortably exceeds a flash tuned N55 which drops off much harder toward redline. However, the N55 shows much more peak torque down low. It is almost a 100 lb-ft of torque advantage.
The N55 should not have that large of a torque advantage down low but this is early B58 tuning once again. Look at the BMS N55 JB4 figures:
Also much larger torque figures down low than the B58 and again a sharper torque drop off toward redline. Based on the figures on paper one would almost expect this to be reversed.
With further testing beyond this Alpha stage tune from BMS we expect the B58 to show well over 400 lb-ft of torque at the wheels on pump gas just like the N55 except with a better top end. That should mean in theory more average horsepower throughout the entire curve.
Here are some thoughts from BMS regarding their B58 F30 340i testing:
We will see what happens but the B58 responds well to tuning and will no doubt hit numbers that open eyes just like the N54 and N55 before it.