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The S55 in the F80 M3 and F82 M4 holds its crank hub in place with clamping force, not a key. Stock, that’s fine. Add a tune, bigger turbos or an E85 map, though, and the S55 crank hub becomes the part that decides whether the engine keeps its valve timing. Atlanta’s cool fall air is a popular time to chase more boost. Ask the hub question first.
Quick Summary: Key Takeaways
- Clamped, not keyed: one bolt holds it all.
- A slip moves cam timing: valves can hit pistons.
- Torque is the load: BMW rated the 2015 M3 and M4 at 406 lb-ft, and tuned cars run far more.
- Cold air adds torque: the same map pulls harder in November.
- Do it in one visit: pair the hub with turbos, fueling or a new map.
The S55 Crank Hub Is Held On by Friction, Not a Key
The weak point is the joint, not the crankshaft. BMW’s launch release for the 2015 M3 and M4 describes a forged, torsionally rigid crankshaft with increased torque-carrying capacity.
The trouble sits on its nose. The hub and the timing-chain drive sit on the snout and rely on the squeeze of one central bolt to stay put. Nothing mechanical locks them. As long as that clamp holds, the timing chain stays exactly where BMW set it at the factory, which on a stock car at stock torque is how things usually stay.
→ BMW tuning and ECU remapping
A Slipped Hub Takes the Valve Timing With It
If the hub turns on the crank, the timing sprocket can turn with it. Then the camshafts no longer agree with the crankshaft about where each piston is.
A small slip may only set a timing fault and leave the car running rough, which is the lucky outcome. A bigger one lets valves and pistons try to occupy the same space at the same moment. Now it’s a teardown. The conversation moves to cylinder heads, and a car that came in for a tune turns into a long engine repair with parts on order through the holidays.
Tuned Torque Is What Loads the Joint
Start with the factory number. BMW’s December 2013 launch release rated the new M3 and M4 at 406 lb-ft of torque from 1,850 to 5,500 rpm, figures it marked as preliminary.
Tuned cars go well past that. An F80 M3 in Solo’s shop, on a pair of Stage II Pure turbos, made 533 lb-ft at the wheels on the in-house Mustang dyno. That’s at the wheels. The crank sees more, since the drivetrain absorbs some on the way to the rollers, and there’s no published figure where an S55 hub starts to slip, so anyone quoting one with confidence is guessing.
→ BMW F80 M3 turbo upgrade on the dyno
Cold Atlanta Air Makes More Torque on the Same Map
Cool air is dense, and a turbo engine turns density into torque. A map that felt mild in August pulls harder on a cold November morning on GA-400.
So fall is boost season. It’s also when the hub sees its biggest loads, in the same months many owners pick to add power. A load-bearing dyno session logs boost and air-fuel ratio against rpm, so what’s on paper matches what the car does on a cold day. Watch the launches and hard shifts from the M-DCT dual-clutch gearbox on sticky tires as well, because they spike torque in a way a smooth highway pull never will.
→ dyno tuning on a load-bearing Mustang dyno
Aftermarket Crank Hub Fixes Add a Mechanical Lock
The fix is a mechanical lock. Aftermarket upgrades for the S55 replace or reinforce the stock arrangement so the hub can’t rotate on the crank under load, instead of trusting bolt clamp alone. Designs and installation steps differ by maker, so ask exactly how a given kit stops the hub from turning before you buy it.
Whichever one goes on, the cam and crank timing has to be set correctly afterward, so it’s a job for a shop that already works inside these engines. One customer’s Stage 2 Pure turbo M4 that Solo tuned went from 396 lb-ft at the wheels stock to 513 on E85, and its tune prep list included an upgraded crank hub before the car ever went on the rollers.
→ S55 Stage 2 Pure turbo M4 build
Frequently Asked Questions
Q: Does every S55 crank hub spin?
A: No. Slips are tied to load, so a stock car driven normally is the low-risk case. Risk climbs with torque, launches and aggressive shifts.
Q: Do I need an M4 crank hub upgrade for a stage 1 tune?
A: There’s no published torque figure that makes it mandatory. The stronger case is any build adding turbos or E85, since each one raises the torque the clamp has to hold, and frequent hard launches add a sudden shock load on top of it.
Q: What happens when an S55 crank hub spins?
A: The cam timing stops matching the crank. A small slip runs rough; a large one can let valves hit pistons.
Q: Does an S55 crank hub upgrade add horsepower?
A: No. It adds no power on the dyno. It protects the valve timing so the power a tune adds doesn’t cost you the engine.
Plan the Hub Before the Next Power Step
Start with the parts list. Then add the fuel you plan to run and whether it sees launches or track days, because those answers decide whether the hub goes first or rides along with the next round of parts. Earlier S55 builds, dyno numbers included, are posted on solomotorsports.net, and the Alpharetta shop at 11255 State Bridge Rd, Alpharetta, GA 30022 is a sensible place to start that conversation.