BMW M50 / M52 / M54 / S50 / S52 / S54 ZF 8HP45 Swap – Using N20 and N55 Gearboxes

Probably the best transmission swap for BMW M5x and S5x engines.

The ZF 8HP45 is a compact, fast-shifting and remarkably strong 8-speed automatic transmission. Despite its official 450 Nm torque rating, the transmission is significantly overengineered, with real-world tuned BMW applications showing that a healthy stock 8HP45 can survive well over 600 Nm and, depending on use and calibration, even 700+ Nm. Combined with excellent shift quality, a wide gear ratio spread and good availability, this makes the 8HP45 an excellent transmission choice for both street cars and performance BMW builds.

This article covers the engine-to-gearbox connection – starter mounting and flexplate/torque converter. The rest of the swap depends on the car and isn't covered here.

The key is choosing the correct version of the transmission.

BMW 8HP45 transmissions originally fitted behind N20, N26 and N55 engines use the N20/N55 bellhousing pattern. This pattern is very close to the older BMW M4x/M5x/S5x engine family, allowing the gearbox to bolt to engines such as the M42, M44, M50, M52, M54, S50, S52 and S54 with only minor mechanical modifications and without a conventional engine-to-gearbox adapter plate.

The main differences are the starter mounting and the torque converter/flexplate combination.

N55 8HP45 – the easiest combination

The N55 version of the 8HP45 uses a flat-face six-bolt torque converter.

With an N55 torque converter, an N54/N55-style 8-bolt flexplate can be used on compatible older BMW 8-bolt crankshafts. Together with our starter adapter, this creates a very simple mechanical 8HP45 conversion for M50, M52, M54 and related engines.

If you already have an N55 8HP45 complete with its torque converter, this is generally the simplest configuration - you will only need the starter adapter.

N20 / N26 8HP45 – the affordable and widely available option

N20 and N26 8HP45 transmissions use the same N20/N55 bellhousing pattern, but have a different C-shaped torque converter mounting face.

These four-cylinder 8HP45 transmissions are generally easier to find and often considerably cheaper than the N55 version. The downside is that the N20/N26 torque converter does not work with the usual N54/N55 flexplate arrangement.

This is exactly why we developed our modified 8HP45 flexplate.

It allows the original N20/N26 8HP45 torque converter to be retained when installing the transmission behind an older BMW 8-bolt crankshaft engine.

This means there is no need to search for a more expensive N55 gearbox or replace a perfectly usable N20 torque converter just to complete the conversion.

Two ways to build the swap

8HP45 Starter Adapter

Our starter adapter allows an older BMW M4x/M5x/S5x-style starter motor to be installed with the N20/N26/N55-type 8HP45 transmission. The adapter is designed to fit the starter already on the engine – any starter that comes with the respective engine from the factory fits, so there is no need to source a different one.

It can be used with several flexplate and torque converter combinations.

For example, an N55 8HP45 can be combined with its N55 torque converter and a compatible N54/N55-style 8-bolt flexplate.

The adapter is therefore also useful if you already have your preferred flexplate and converter combination and only need to solve the starter mounting.

8HP45 Starter Adapter + Modified Flexplate Kit

Our complete kit is aimed particularly at the much more commonly available N20/N26 8HP45.

It includes the starter adapter together with our modified flexplate, allowing the N20/N26 torque converter to be retained.

This turns the inexpensive and widely available four-cylinder 8HP45 into a practical transmission option for older BMW engine swaps.

Which engines can it be used with?

The conversion is particularly suitable for BMW engines from the M4x/M5x/S5x family with the compatible bellhousing geometry and 8-bolt crankshaft flange, including:

M42 • M44 • M50 • M52 • M54 • S50 • S52 • S54

This covers many popular BMW E30, E34, E36, E39, E46 and Z3 engine and chassis combinations.

Other BMW engines may share parts of the same bellhousing pattern, but crankshaft flange, engine installation angle, starter position and flexplate compatibility should always be verified before installation.

S50, S52 and S54

The S-series engines use the same setup as the M50, M52 and M54. Our starter adapter fits, and with an N20/N26 gearbox the modified flexplate kit works the same way.

Why use an N20/N26 8HP45?

The 8HP45 provides eight closely spaced gears, a wide overall ratio spread and fast shifting in a compact package.

More importantly for older BMW projects, the N20/N26/N55 version requires surprisingly little mechanical adaptation.

With the correct starter adapter and flexplate/torque converter combination, an M50, M52 or M54 can be converted to an 8HP45 without the large adapter plates normally associated with modern transmission swaps.

And because N20/N26 8HP45 transmissions are much more commonly available than N55 units, they can make the conversion considerably more affordable.

N55 8HP45: use the N55 converter with a compatible 8-bolt N54/N55-style flexplate and our starter adapter.

N20/N26 8HP45: retain the N20/N26 converter and use our starter adapter + modified flexplate kit.

Always verify the exact transmission, torque converter, flexplate, output flange and vehicle configuration before ordering.

FAQ

Does the 8HP45 swap work with S50, S52 and S54 engines?

Yes. The same starter adapter and flexplate options apply as on the M50, M52 and M54.

Do I need a different starter?

No. The adapter fits the starter that came with the engine from the factory.

How is the gearbox controlled?

The 8HP needs a controller, and the choice is yours. We recommend MaxxECU.

How much torque can the 8HP45 handle?

ZF rates it at 450 Nm. In practice a healthy stock 8HP45 has handled well over 600 Nm in tuned BMWs, and 700+ Nm depending on use and calibration.

N55 or N20 8HP45 – which one should I get?

With an N55 gearbox and its torque converter you only need the starter adapter. With an N20/N26 gearbox you need the starter adapter + modified flexplate kit.