BMW S55 tuning: stages and the crank hub.

The S55 is the engine in the F80 M3, the F82 and F83 M4 and the F87 M2 Competition. Three litres, six cylinders, two turbos, and a reputation as one of the most tunable M engines BMW has built. There is one thing you need to know before you start adding torque, and it is further down this guide. First, what the engine does from the factory.

What kind of engine is the S55

The S55 arrived in 2014 and replaced the naturally aspirated V8 from the previous M3. Where that V8 had to be revved, the S55 delivers its torque from low in the rev range. That makes it a very different car in daily use, and it also makes it more responsive to software: there is a lot to gain from a tune alone.

Worth noting: all three deliver 550 Nm. The difference is in the power up top, not in the torque. On the M2 Competition peak power runs between 5250 and 7000 rpm and peak torque between 2350 and 5200 rpm.

Stage 1: software only

You flash the original engine computer through the OBD port with the bootmod3 licence for the S55. Nothing to remove, nothing to ship, and going back to stock is possible at any time.

Bootmod3 quotes these gains for the S55:

• Stage 1 on RON95: up to 20 percent more power and 22 percent more torque
• Stage 1 on RON98: up to 22 percent more power and 23 percent more torque

That is a solid step for software alone, and the difference between RON95 and RON98 is smaller here than on the newer S58. So the S55 is less fussy on RON95, but RON98 remains the better choice once you start tuning.

Stage 2 and Stage 2H

Stage 2 assumes a freer intake and exhaust with matching software:

• Stage 2 on RON95: up to 23 percent more power and 26 percent more torque
• Stage 2 on RON98: up to 24 percent more power and 26 percent more torque
• Stage 2 on E30 or race gas: up to 26 percent more power and 26 percent more torque

Above that sits Stage 2H, where H stands for upgrade hardware. That is where the numbers climb sharply:

• Stage 2H on RON95: up to 38 percent more power and 30 percent more torque
• Stage 2H on RON98: up to 42 percent more power and 30 percent more torque
• Stage 2H on E30 or race gas: up to 45 percent more power and 33 percent more torque

Do the maths for an M3 with the Competition Package: 45 percent on 450 hp puts you in the region of 650 hp. That is the point where the rest of this guide matters more than the numbers.

On the intake side we have the Eventuri carbon intake system for the F8X and the system for the F87 for the M2 Competition. If you only want to replace the filters, there is a panel filter set. There is also a carbon chargepipe for the S55, and that is not a cosmetic part: the original plastic chargepipe is a known weak point at raised boost.

We do not sell downpipes. You do need those for a full Stage 2, so give us a call if you want to go that way.

The crank hub: read this before you tune

This is where the S55 differs from the newer S58, and not in a good way.

On the S55 the crank hub is not mechanically fixed to the crankshaft. There is no key and no pin through it. BMW holds it in place with a single central bolt and a sintered washer with high friction. From a factory standpoint that is an efficient solution, because such a joint is easy to automate and makes setting the timing simpler. The same construction is used on the N54 and the N55.

In stock form that friction joint holds up fine. But as soon as torque rises, and especially when it comes on fast as it does with two turbos, the hub can rotate slightly relative to the crankshaft. Then the valve timing is no longer correct. If it goes far enough, valves and pistons can meet, and then you are looking at a new engine.

The fix on the market is a one-piece hub that is keyed or pinned mechanically to the crankshaft, so it no longer has to rely on friction. We do not sell those in the shop, but we do fit them. If you are heading towards Stage 2H or ethanol, call us before you order so we can put things in the right order.

To keep it in proportion: an S55 with Stage 1 on RON98 alone is not in the danger zone. It is a concern that grows as you ask for more torque, not something that happens to every tuned M3.

What else to watch out for

The engine computer may be locked

If your S55 has a software level of March 2019 or later, the DME is locked and needs to be opened first. We do that with the DME bench unlock in our workshop. Cars with older software can be flashed straight away.

The chargepipe

The original chargepipe is plastic. At raised boost that is a known concern, and it is a relatively cheap part to get ahead of.

Ethanol takes hardware

Reaching the E30 figures above means fitting a flex fuel sensor so the software knows the ethanol content. That is what the FlexFuel Kit High Flow is for, in the F series S55 or N55 version. That kit is flow optimised, which matters once you load the original fuel system to its limit.

DCT gearbox

If your car has DCT, the GTS and CS transmission flash is included with the bootmod3 licence. That saves a separate purchase and it makes a noticeable difference to how it shifts.

Maintenance

• Oil. With track days or a lot of full-throttle driving we prefer to change it earlier than the service schedule says.
• Spark plugs. More boost asks more of the plug. From Stage 2 upwards, shorter intervals than standard.
• Let it warm up. Wait for oil temperature before full throttle, not coolant temperature. Those two are far apart.
• Let it cool down. After a hard drive, take it easy for a bit before you shut it off.
• Read along. Datalogging in the bootmod3 app shows you whether something is shifting before you feel it in the car. On the S55 that is not a luxury.

What you need to get started

Besides the licence, a bootmod3 wifi adapter or an ENET OBD cable, and with software from March 2019 onwards a DME unlock first.

What we do with it

We work with bootmod3 daily in our workshop in Elburg. You can order the licence from us and flash it yourself, or have us do the whole job. On the S55 we recommend the latter more often than on other engines, precisely because of the order in which you want to tackle things.

More background: what Stage 1 tuning actually is and how tuning relates to your warranty, insurance and roadworthiness test.