BMW B46/B58 vs B46TU/B58TU

Updated November 17, 2023  

BMW introduced the B-series of modular engines in 2016. But there was a major update done in 2018 and then another for 2020. The B46 4-cylinder is pretty simple with only minor updates. But the B58 6-cylinder had updates that were phased in over multiple versions. B58A being the original. B58C having some major changes (but only used on a few models). And now the B58D with even more changes but now used throughout the model range. The multiple different specifications requires a detailed breakdown of what we've found. We focus mostly on the B58 since that had the most changes.  

TL;DR - Knowing which B58 you have can be key to ordering the right parts or upgrades.  
B58A = "F" chassis BMW models - F22 M240i, F30 340i, F32 440i, G01 X3 M40i (2019), G02 X4 M40i (2019) and "early" models of 540i/740i/840i.  
B58C = "G" chassis BMW models that are not "M Sport" models - 2020+ G30 540i, G12 740i, G15 840i, G05 X5 40i, G06 X6 40i, G07 X7 40i. This is the spec with the unusual internal exhaust manifold and turbo setup. It's also used in the 2019-2020 Supra.  
B58D = "G" chassis BMW "M Sport" models - G42 M240i, G20 M340i, G22 M440i, G29 Z4 M40i, X3 M40i, X4 M40i, and 2020+ 540i/740i/840i. It's also used in the 2021+ Supra. Also known as B58TU. 
B58N = “G” chassis BMW 5/7/X5/X6/X7-series models. Evolution of the B58 that combines the B58D with the 48V Mild Hybrid system found in V8 models. Also known as B58TU2.



Background  

First some background. The B engines are supposed to be modular in design - many of the same components are shared between 4-cylinder and 6-cylinder engines. However, BMW changed the B58 with three iterations in just five years and the only shared parts are small items like sensors and hardware.  

Second, BMW has way too many abbreviations for their engine specs. The best is the full 8-digit engine code. However, internal BMW docs will often reference a shorthand code for some reason. But there are multiple shorthand codes that mean different things. As a new-from-the-ground-up engine family we expect there to be updates along the way, as well as a system for keeping track of the changes. However, these shorthand abbreviations just muddy the waters and create additional confusion. It's not even enough to be an insider, the abbreviations are so nuanced.  

Third, some translation of the full 8-digit engine code.  
 

Click below for an example:

BMW will then offer abbreviations that are not as clearly defined. Here are the ones we're aware of:  
 

You can geek out learn more about BMW engine codes by clicking here.  



Model Year Changes  

The B46 was largely un-changed from 2016-2019. For 2020 it was updated to the B46D spec with a lot of changes (see more detail below).  
The B58A was used in mostly F-chassis which mostly ended production by 2020. 5/6/7 and X5/X6/X7-series models were introduced with the B58A engine until mid-life updates were released with the B58C engine around 2019. The B58D was introduced along with new 2/3/4-series models for 2019. The B58N design was released beginning in 2023 for 5/6/7 and X5/X6/X7-series models and incorporated the 48V hybrid drive in the 8-speed transmission. 

As a car owner you might not have any crossover at all but some owners will need to pay strict attention to model years and notes so they get right parts. We laid out the model changes here with this chart:  

 

 
B46 engine bay differences.

 
B58 engine bay differences.

Specific Changes  

So what changed? Quite a lot over three iterations! The B58C is more than an in-between spec. The biggest change here was the cylinder head that includes an internal exhaust manifold (see below). Interestingly, BMW abandoned this design for the B58D spec. But other changes carried over from C to D, such as the cooling system. These are the differences we know of (there could be others).  

Crankshaft  
The C/D crankshaft is 2.3lbs lighter due to the use of lighter materials and different machining.  

Timing Chain  
The C/D has a 1-piece timing chain instead of a 2-piece. The original design had dual upper and lower chains with additional guides and sprockets. This was  probably due to the layout of the accessories in the engine bay which forced the chain drive to the back of the engine. The new design changes to a more familiar and simpler single chain that eliminates two camshaft sprockets, one intermediate shaft, two guide rails, and one chain tensioner.  
 

 
 

VANOS Cam Gears  
Because of the changes to the timing chain, the cam gears on the VANOS system are also different.  

Serpentine Belt  
The belt has also changed due to the revised cooling system and overall layout of the engine accessories. The belt is now shorter with fewer deflection pulleys. Other parts of the belt drive also changed - crankshaft vibration dampener, water pump, and A/C compressor.  
 

Cooling System  
The C/D engines have an innovative cooling design that separates the cooling supply into two zones: cylinder head and engine block. Not unlike different cooling zones in your house. The cylinder head and turbo receives a constant supply of coolant but cooling to the engine block can be reduced or turned off. During a cold-start, warm-up, or normal driving coolant circulates through the cylinder head and turbo but is cut off from the engine block. Only when needed is the valve to the engine block opened, allowing coolant to circulate through the crankcase. The water pump and thermostat are now part of this Heat Management Module (but available for replacement individually).  
 

Fuel Injectors  
New fuel spray pattern and pressure is specified for each engine variant.  

Engine Computer  
Each version uses a different ECU.  

Cylinder Head-Exhaust Manifold-Turbo (B58C medium output)  
On the B58C medium-output engines the cylinder head casting includes an internal exhaust manifold. The usual coolant passageways are routed around these internal exhaust runners. The coolant lowers the exhaust gas temperature which improves the life of the catalytic converter and the engine uses less fuel. It also helps to increase the coolant temp faster, bringing the engine up to operating temp sooner. This has eliminated the traditional external steel exhaust manifold for a very different design with a single 2>1 layout instead of two banks of 3>1. Instead of six exhaust ports on the outside there are just two. The cylinder head is obviously a different casting, however, most of the valvetrain, VANOS, and Valvetronic is the same across all B46 and B58 variants.  

BMW only used this engine on 2020+ 540i, 740i, 840i, X5, X6, and X7 "40i" models as well as the 2019-2020 Toyota Supra. The "M40i" M Sport models used the B58D engine with a traditional exhaust manifold.  
 

B58C 48 Volt Mild Hybrid System  
For 2021 the B58C in 5/7/8-series was offered with an optional 48 volt electric motor combined into the alternator/battery/belt drive system. This created a "mild hybrid" design which used the alternator to spin the engine via the belt system. The alternator is referred to as a Starter-Generator (SGR48). By driving the crankshaft via the belt, this allowed combustion to be turned off completely during certain low-load situations, such as coasting and decelerating to a stop. This improved fuel economy and reduced emissions. It also added an instant and seamless 8kW power boost in some situations (eBOOST). Although still classified as a B58C there are several big changes to the overall design and most of the water pump and belt drive systems are different than a standard B58C. The 48V battery is installed in the engine bay with the standard 12V located in the trunk.  
 

B58N 48 Volt Hybrid Drive System 
In some 2022 models, BMW released a new version of the 48V Mild Hybrid. Internally, BMW calls it Generation 1.5. The same fundamental design was released as Generation 2.2 for the G60 5-series and G70 7-series. We'll refer to it as Hybrid Drive instead of Mild Hybrid. Instead of a alternator+belt machine, the 48V power comes from the ZF 8HP transmission (8HP60XMH). The 48 volt electric motor is combined into the torque converter of the ZF automatic. The deployment of the system remained the same - certain low-load situations, such as coasting and decelerating to a stop. For G60 and G70 the system is now active in normal forward drive at low speeds as well (parking lot).

Note: in addition to the “Mild Hybrid” system used on “40i” models, BMW also produces a PHEV model known as the 50e. Even though both may fit the broad definition of PHEV, the 50e should be a MEHV - Mostly Electric-Hybrid Vehicle. The 50e models have full battery cells in the floor in addition to a B58N 6-cylinder gas engine. 50e and 40i 48V share very few components, even on the gas engine. 


B46 vs B48  

Why are there two versions of the 4-cylinder engine? Short answer is that BMW wanted a performance-spec for their special models like the X2 M35i and the MINI John Cooper Works. The B46 is the Super Ultra Low Emissions spec (SULEV) and that is the standard engine in the US market. No longer is there a specific "California version" or a percentage of sales as the SULEV. Now, with the exception of the "M35i" models, all of the turbo 4-cylinders are the SULEV spec.  

The B48 M35i models are more than just a boosted B46. It has a reinforced crankshaft with larger bearing surfaces, higher compression pistons, different connecting rods, larger turbo, revised fuel injectors, and better flowing intake and exhaust systems.  
 

Z4 vs Supra  

Disclaimer: we're not Toyota experts and have no access to genuine Toyota parts diagrams. The following info is what we have been able to deduce from comparing the parts, diagrams, and info already made public.  

The G29 Z4 M40i was released for the 2019 model year and always had the B58D engine in the US market. This is the B58O1, Obere, or "Upper", output spec with 382hp and was shared with G20 M340i and G22 M440i.  

When the Supra was released as a 2020 model it used the B58C spec with 335hp (aka the B58M1 medium output). The B58C spec was also shared with 2020+ BMW G30 540i, G12 740i, G01 X3, G02 X4, G05 X5, and G06 X6. It was also used in the European-market Z4 M40i to meet the EU6 emissions regulations.  

There are significant differences between the two B58 specs - see the section above regarding the cylinder head and turbo.  

For the 2021 model year the Supra was granted the B58O1 engine with 382hp. We assume it's all of the same parts as the G29 Z4 M40i (but we can only assume right now).