Bore scoring repair, proven upgrades and how to unlock maximum NA power

If your Porsche 997.1 Carrera S (2005–2008) is facing an engine rebuild — most commonly due to bore scoring — this is the ideal time to consider a performance upgrade alongside the repair.

One of the most proven “while you’re in there” upgrades is the 3.9-litre engine conversion, achieved by fitting 100mm pistons and a compatible cylinder liner solution. Rather than simply fixing the problem, a 3.9 rebuild delivers stronger midrange torque, sharper throttle response and meaningful power gains when paired with the right supporting modifications.

This guide explains what the 3.9 conversion is, the available build routes, which components should always be replaced during a rebuild, and the performance upgrades required to achieve maximum naturally aspirated power.

What is a 3.9L conversion on the Porsche M97.01 engine?

A 3.9 conversion increases engine displacement by enlarging the bore size to 100mm while retaining the factory stroke. This takes the M97.01 engine from 3.8 litres to approximately 3,900cc.

When carried out during a rebuild, the conversion offers excellent value, as the engine is already stripped and machined.

Key benefits include:

• Stronger midrange torque

• Improved throttle response

• Increased horsepower potential

• Excellent drivability gains on the road

Why upgrade during a bore scoring rebuild?

Bore scoring is a known issue on early water-cooled flat-six engines. Common symptoms include excessive oil consumption, knocking or ticking noises from one cylinder bank, smoke on startup and metallic debris in the oil.

A proper rebuild replaces the damaged cylinder surfaces and pistons — making it the perfect opportunity to increase capacity and performance at the same time.

Step 1: Choosing your cylinder liner route

Before selecting pistons, you need to decide which cylinder surface your build will use.

Option A: Ductile / cast iron liners (most common)

This is the most widely used solution for bore scoring repairs. The original cylinder surfaces are machined out and replaced with ductile iron liners, which are then bored and honed to final piston clearance.

Ductile liners are reliable, cost-effective and supported by most engine builders.

Ductile iron cylinder liner set (set of 6):
https://www.design911.com/porsche/997-carrera-s-2005/engine-rebuild-cylinder-liner/

Browse all liner options:
https://www.design911.com/porsche/997-mki–911–2005–08/engine-rebuild-cylinder-liners/

Option B: Nikasil pistons with specialist cylinder service (premium route)

Nikasil cylinder finishes are popular for long-term and performance-focused builds due to excellent wear resistance, strong ring seal and stability under heat.

With this route, Design911 supplies the correct piston kit and the engine builder uses a specialist partner to apply and finish the Nikasil cylinder surface.

Step 2: Choosing pistons for your 3.9 build

Mahle Motorsport 100mm pistons (ductile / iron liner builds)

Designed specifically for use with iron or ductile liners, these pistons are a popular choice for strong road builds.

https://www.design911.com/p/engine-piston-set-for-cast-iron-cylinder-100mm-bore-repair-liners-porsche-997-38l/

Mahle Motorsport 100mm pistons (Nikasil builds)

These pistons are matched to Nikasil cylinder finishes and are ideal for premium rebuilds.

https://www.design911.com/p/engine-piston-set-for-nikasil-liners-porsche-997-3-8l-bore-100mm/

Wössner forged pistons (100mm options available)

Wössner forged piston kits are a proven alternative for customers who want forged internals and precise bore sizing.

https://www.design911.com/porsche/997-mki–911–2005–08/w-ssner-pistons/

OE-style replacement pistons (standard rebuilds)

For customers rebuilding to factory capacity without a conversion.

https://www.design911.com/p/engine-pistons-porsche-997-38l-c2s—c4s_2/

Step 3: Supporting components you should always replace

Connecting rods (recommended for 3.9 builds)

A 3.9 conversion increases torque output, so upgraded connecting rods are strongly recommended — especially for spirited road or track use.

https://www.design911.com/p/connecting-con-rods-set-of-6-porsche-996—997_2/

Bearings (always replace)

All bearing shells should be replaced during a rebuild.

Main bearing sets:
https://www.design911.com/porsche/997-mki–911–2005–08/main-bearing-sets/

Big end and thrust bearings:
https://www.design911.com/porsche/997-carrera-s-2005/porsche-engine-shell-bearings-big-end-con-rod-thrust/

Important: Bearing sizes must be selected after crankshaft measurement (standard or undersize).

Complete engine gasket set

A full rebuild should always use a complete gasket set including head gaskets.

https://www.design911.com/p/engine-gasket-set-porsche-997-s-38l-261kw—355hp-m9701/

IMS upgrade (manual cars)

If the engine is already out, this is a sensible preventative upgrade.

https://www.design911.co.uk/porsche/997-carrera-2005/porsche-ims-bearing-replacement-and-upgrade-kits/

Timing chains, guides and tensioners

Recommended while the engine is apart to avoid future labour costs.

https://www.design911.com/porsche/997-mki–911–2005–08/porsche-timing-chains-and-tensioners/

How much power does a 3.9 conversion make?

A standard Porsche 997.1 Carrera S produces around 355hp from the factory.

A properly built 3.9 conversion can deliver:

• 380–415hp

• 410–450Nm of torque

• Gains of 25–60hp, depending on supporting modifications

The biggest improvement is in midrange torque and overall drivability.

Performance upgrades to achieve maximum BHP

Intake upgrades (plenum and throttle body)

Upgraded plenums improve airflow and throttle response.

https://www.design911.com/porsche/997-carrera-s-2005/porsche-performance-plenums-and-intake-upgrades/

IPD competition plenum + 82mm throttle body:
https://www.design911.com/p/aluminum-intake-plenum–competition–82mm-and-throttle-body-porsche-997-carrera-ipd/

Headers / exhaust manifolds

Performance headers reduce restriction and improve exhaust scavenging.

https://www.design911.com/porsche/997-carrera-s-2005/manifold—headers/

200-cell sport catalytic converters

Sport cats reduce back pressure and support higher power output.

https://www.design911.com/p/exhaust-catalytic-converters-200-cell-porsche-997-1-carrera/

Browse all cats and bypass options:
https://www.design911.com/porsche/997-mki–911–2005–08/porsche-catalytic-converters-and-bypass-pipes/

Complete exhaust packages

Matched exhaust upgrades offer a simple route to improved flow and sound.

https://www.design911.com/p/sports-exhaust-package-1–and18-to-27-bhp–porsche-997-c2—c4/

ECU remap (essential)

A 3.9 conversion changes airflow and fuelling requirements. A custom ECU remap is essential to unlock power safely and improve throttle response.

Final thoughts

If your 997.1 Carrera S requires a rebuild, a 3.9-litre conversion is one of the most rewarding upgrades you can make. It fixes bore scoring properly, improves reliability and transforms how the car drives.

With the right pistons, liners, supporting components and airflow upgrades, the result is a naturally aspirated flat-six that feels stronger everywhere — not just on paper.