944 Turbo Engine Performance Modifications

Introduction

There is no single best way to modify a 944 Turbo engine. Each car is a special case, and modifications should be tailored to individual goals and available resources. The most important decision to make at the outset is how much performance is desired, because as you approach the physical limits of the car's capabilities, performance modifications become exponentially more expensive.

As a general principle: modifications should be planned as a coherent system. Many of the individual modifications discussed here interact with each other, and a modification made in isolation may not deliver its potential gains until complementary parts of the system are addressed.

Computer Chips

Aftermarket chips are among the most cost-effective ways to increase power on a stock 944 Turbo. A set of chips installed in the DME and KLR units can yield significant horsepower gains.

How chips achieve this: the stock 944 Turbo runs a maximum boost of approximately 11 psi, with overboost protection set above that. Aftermarket chips raise or eliminate the overboost setpoint and increase maximum allowable boost (typically to around 14–15 psi). In addition to the boost setpoint increase, many chip sets modify the fuel and ignition maps to match the engine's demands at the higher boost levels. Some setups include a device to retard pressure buildup from the turbocharger discharge to the wastegate — typically an orifice in the sensing line or a bleeder valve.

Advantages: relatively inexpensive for the power gained; installation is straightforward and within the ability of most home mechanics.

Disadvantages: chips must be matched to the system they are running. If additional modifications are made later (such as a different turbocharger), the chips may no longer be optimal and may need to be remapped or replaced. If significant further modifications are anticipated, a fuel management or stand-alone engine management system is a better long-term investment than chips.

Air Flow Measurement

Accurate air flow measurement is fundamental to any engine management system.

Stock air flow meter: a flap-type ("barn door") meter where a gate swings open proportional to air flow. Includes a temperature sensor for density compensation.

Mass Air Flow (MAF) sensors: use a heated wire element whose resistance changes with air flow. Typically more accurate than the flap-type meter and eliminate a significant air flow restriction.

Manifold Absolute Pressure (MAP) sensors: measure intake manifold pressure, then density-compensate with an inlet air temperature sensor signal. Primarily used with stand-alone engine management systems.

Both MAF and MAP sensors can be used with remapped chips (for the stock DME), with a piggyback fuel management system, or with a stand-alone engine management system.

Fuel Management Systems

Piggyback fuel management systems tap into the existing engine management and manipulate the pulse signals going to the fuel injectors. This allows tuning for modifications without buying new chips for each change.

These systems typically include a programmable control module (adjustable by dial or laptop) and are monitored with an air/fuel ratio meter during tuning. They may work with the stock air flow meter, or may require a MAF or MAP sensor.

Advantages: tunable for a wide range of modifications; generally straightforward to install.

Limitations: limited control over ignition timing, which is critical in high-performance applications. Separate fuel and ignition control systems can work against each other if not set up carefully.

Stand-Alone Engine Management Systems

Stand-alone systems completely replace the factory engine management. They offer full control over both fuel and ignition maps in a single unit, which can be reprogrammed as future modifications are made.

Installation is significantly more involved than a piggyback system — it may require replacing the engine wiring harness and most factory sensors. Most stand-alone systems require a crankshaft trigger wheel and sensor mounted at the front of the engine; some can use the factory crank trigger signal.

Advantages: complete fuel and ignition control; typically includes direct-fire ignition via coil packs (eliminating the distributor); many systems support data logging, knock control, boost control, and sequential fuel injection with a cam trigger.

Disadvantages: high cost; demanding installation and setup; significant time required to learn the tuning software.

Turbochargers

Turbocharger selection is most effective when the turbocharger is matched to the engine's actual air flow capabilities. A turbocharger that is too large for the engine's flow rate will not operate at peak efficiency and will produce excessive turbo lag without delivering a proportional power increase.

For a completely stock 2.5L engine, the air flow capability is relatively fixed, making turbocharger matching more straightforward. When displacement, cylinder head, or intercooler are changed, turbocharger selection becomes more complex. When making these combined changes, provide the turbocharger supplier with all specifications — including head flow data from a bench test if available — and select a supplier with experience matching turbos to specific engine combinations.

If using the stock engine management, confirm that the turbocharger supplier can provide chips mapped specifically for the new setup. With a fuel management or stand-alone system, this is not necessary because the system can be retuned.

Adjustable Fuel Pressure Regulators

An adjustable fuel pressure regulator (FPR) is only useful when the stock injectors cannot flow enough fuel at stock differential pressure for the desired power level. At boost levels up to approximately 15 psi with a stock turbocharger and aftermarket chips, an adjustable FPR provides no benefit.

When used, the adjustable FPR must be paired with chips (or a fuel management system) that are mapped specifically for the new base fuel pressure. Installing an adjustable FPR alone — without matching fuel maps — will not improve performance.

Fuel Injectors

Larger injectors can only be used effectively in conjunction with chips or a fuel management system mapped for the increased injector flow. Installing larger injectors without matching the fuel management can damage the DME injector drivers.

Injector impedance: the stock DME uses peak-and-hold (low impedance) injector drivers. The 944 Turbo uses injectors with a resistance of 4.5 ohms (spec: 3.5–5.5 ohms). Most aftermarket low-impedance injectors are in the 2–3 ohm range. To use these with the stock DME drivers, ballast resistors must be added in series to match the equivalent impedance. Aftermarket suppliers typically offer complete kits with injectors and matched ballast resistors for the 944 Turbo application.

Increasing Engine Displacement

Displacement can be increased by increasing bore, stroke, or both. None of these options are inexpensive.

Stroker Kits (Increased Stroke)

The most common approach is to install a 3.0L 944 S2 crankshaft into a 2.5L block, increasing displacement to approximately 2.758L (commonly called the "2.8L stroker"). This requires custom pistons to accommodate the increased stroke and maintain proper valve clearance at TDC. Pistons must be either specially coated for compatibility with the stock silicon-impregnated alloy cylinder walls, or the cylinders must be bored and sleeved with cast iron to allow use of standard uncoated custom pistons.

If the factory connecting rods are retained, the main bearing saddle sides must be machined to clear the taller rod bolt shoulders at the increased stroke. Aftermarket rods with lower-profile bolt shoulders eliminate this machining requirement and are lighter and stronger than the factory rods.

Displacement combinations:

Stock 2.5L (100 mm bore × 78.9 mm stroke): actual displacement 2.479L
3.0L S2 crank in 2.5L block (100 mm bore × 87.8 mm stroke): 2.758L (the "2.8L")
3.0L S2 block bored to 104 mm with 2.5L crank: 2.680L (the "2.7L")
3.0L S2 (104 mm bore × 87.8 mm stroke): 2.983L

Overbore (Increased Bore, Stock Stroke)

Boring the 2.5L block to increase displacement is generally not recommended for high-performance applications — the 2.5L cylinder walls are not thick and the cylinders are free-standing (unsupported at the top), making heavily bored cylinders prone to movement under high load, which causes head gasket failures. The 3.0L S2 block is a much better candidate for overboring, as its cylinder walls are thicker and the cylinders are tied together at the top with webbing.

Using the 3.0L Block

For displacements above 3.0L, the 3.0L S2 or 968 block is the appropriate starting point. The 944 Turbo cylinder head will not mate to the 3.0L block directly due to differences in cooling passages at the front of the head. Options include:

Modify the 944 Turbo head: weld a boss over the existing cooling passage and machine a new passage to match the 3.0L block.
Use the 1989 2.7L normally aspirated head: this head uses the same block as the S2 engine and mates directly; requires swapping to a high-temperature exhaust valve.
Use the 3.0L 16V head: highest flow potential but requires custom fabrication of intake and exhaust manifolds and custom high-temperature exhaust valves.

Turbocharged 3.0L NA Engine

Turbocharged conversions of a normally aspirated 3.0L engine require high-temperature exhaust valves and a compression ratio reduction to below 8.5:1. This requires custom pistons; no off-the-shelf piston is compatible with the stock alloy bore at the appropriate compression ratio. Options are custom-coated pistons for the stock bore, or bore-and-sleeve with standard custom pistons.

Wastegates and Boost Controllers

The factory wastegate spring weakens over time, reducing the gate's closing force and allowing boost pressure to fall below the intended maximum. Options for correction:

Shim the diaphragm: install shims between the diaphragm and valve body to preload the spring. A temporary fix only.
New OEM wastegate: effective but costly.
Aftermarket replacement wastegate: most aftermarket wastegates are not direct bolt-in replacements and may require custom flange fabrication and welding. One exception is a conversion that retains the factory wastegate valve body with a different diaphragm for single or dual-port operation.

Boost controllers come in two types: manual (mounted in the engine bay or cockpit) and electronic (solenoid in the engine bay with a cockpit control unit). Manual boost controllers are simpler to install and set up. Electronic controllers offer potentially more precise and repeatable boost pressure control. Either type allows boost pressure to be set above the chip-set maximum if desired.

Oiling System

The 944 is known for issues with the number 2 rod bearing under hard cornering and high-load conditions. Multiple factors likely contribute, and several modifications are used to address the problem:

Run oil level slightly high: maintaining oil at or just above the maximum dipstick mark reduces the risk of the pickup tube becoming uncovered. Especially recommended for track use.
Cross-drill or perp-drill the crankshaft rod journal: drilling an additional oil passage through the rod journal (straight through = cross-drill; at 90° to the existing passage = perp-drill) improves oil supply to the rod bearing.
Rebaffled oil pan: adding a hinged flap to the pan prevents oil from surging away from the pickup tube in cornering. Also includes a modification to lower the effective pickup point by banding the screen.
Knife-edge the crankshaft: reduces oil foaming and windage losses.
Drill main bearing saddles: allows air to flow freely between saddles, reducing pressure waves from piston movement.
Accusump: a pressurized oil canister that supplies oil to bearings if pressure drops. Results with this system are mixed.

Crankshaft and Flywheel

Reducing rotating mass allows the engine to rev more quickly.

Crankshaft lightening: removing material from the crankshaft reduces rotating mass. Removing too much weight causes the engine to stall when dropping back to idle — staying within approximately 4.5 kg (10 lbs) of reduction is a safe limit. Knife-edging (as noted above) also reduces rotating mass.

Flywheel lightening: the factory flywheel can be machined to reduce weight, or an aluminum replacement flywheel can be used. A lighter flywheel improves throttle response and rev speed but reduces the engine's rotational inertia at low RPM, which can affect driveability in traffic.

Cat Bypass Pipes

Note: Catalytic converter bypass pipes are not legal for street-registered vehicles and must only be used on track-only cars. Verify local regulations before installing.

Removing the catalytic converter reduces backpressure on the turbocharger discharge. This lowers turbo spool-up time and reduces turbo lag, with a noticeable effect on low-to-mid-range power delivery.

Summary

Engine performance modifications for the 944 Turbo span a wide range of complexity and cost. The modifications discussed here interact with each other — in particular, any modification that changes the engine's air flow or boost level should be accompanied by a matched fuel management solution (chips, piggyback system, or stand-alone system). Plan modifications as a system rather than in isolation, and work with suppliers who have specific experience with the 944 Turbo platform.