IAG 950 EJ25 Closed Deck Short Block: What It Supports and Who Should Build It

The IAG 950 is the most popular closed deck short block for serious WRX and STI builds. Not because it is the most powerful IAG option, but because it fits the majority of high-performance street and track builds. This guide covers what the 950 supports, what builds it suits, and whether it is the right foundation for your Subaru WRX or Subaru STI.

What the IAG 950 Rating Actually Means

IAG rates their short blocks in BHP at the crank. The 950 BHP rating means the block is engineered to support that level reliably. At the wheels, after drivetrain losses, most fully built 950 applications produce 700 to 800 WHP.

For street and track builds targeting 400 to 600 WHP, the 950 gives significant headroom above the power goal. That headroom is what makes the block reliable. Running close to the ceiling puts the engine at risk. Our post on how much power the IAG 950 can handle covers the real-world numbers. And what closed deck construction means for the EJ25 explains why the architecture matters at these power levels.

What the IAG 950 Closed Deck Short Block Includes

The IAG 950 short block is a complete rotating assembly. IAG machines the open deck EJ25 block to a closed deck design by bridging the cylinder bores at the top of the block. Forged pistons, forged connecting rods, and a forged crankshaft complete the assembly.

It fits the WRX, STI, Legacy GT, and Forester XT on the EJ25 platform. The short block does not include cylinder heads, valvetrain, or accessories. Those come from the existing engine or separate sourcing depending on the build. The IAG 950 EJ25 Closed Deck Short Block is available now.

What Builds the IAG 950 Is Designed For

The 950 suits serious performance builds that are not at the extreme end of the power range. Three build profiles fit it best.

Street and track builds targeting 400 to 600 WHP on E85 with a big turbo. High-boost EJ257 STI builds that have pushed past what the open deck block handles reliably. WRX builds on a large frame turbo where the stock block has become the risk point. Running the 950 at 450 WHP means the block has reliability headroom built in from day one. The STI engine build guide and WRX engine build guide cover how to plan a build around the right block spec.

IAG 950 Short Block vs IAG 950 Long Block

The short block is the rotating assembly and closed deck block without heads or valvetrain. It suits builds where existing heads are being reused or replaced with separately sourced units.

The IAG 950 Closed Deck Long Block adds IAG 950 spec cylinder heads. It suits builds where a complete matched assembly is the goal. Our short block vs long block guide covers how to decide between the two based on the build plan.

The 2.35L Destroked Option

The IAG 950 2.35L Destroked Long Block is a competition-specific option. A shorter stroke crankshaft reduces displacement from 2.5L to 2.35L. This raises the rev ceiling and suits motorsport class rules where displacement is regulated. For street and standard track builds, the full 2.5L short block is the right choice. The destroked version suits specific competition applications only.

IAG 950 vs IAG 1150: Which One Do You Need

The 950 covers the majority of serious WRX and STI builds. The 1150 suits extreme applications — dedicated drag builds and competition builds targeting 700 WHP and above at the wheels. For owners building toward 400 to 600 WHP on the street or track, the 950 is the correct block.

If the power goal falls in that range and the build is not a dedicated competition application, choose the 950. Browse IAG short block options for WRX and STI to compare both spec levels. The IAG 950 vs 1150 power comparison covers the specific numbers behind each option.

Supporting Mods the IAG 950 Build Requires

A short block upgrade does not work in isolation. Every system around it needs to match what the build produces.

The turbo must match the power goal. Browse WRX turbo upgrades to compare options for the EJ25 platform. Fueling must supply the demand. The WRX fuel system upgrades hub covers injectors and fuel pump requirements at serious power levels. A tune written for the specific combination is not optional on a built engine. Our post on what EFI tuning does for your WRX explains why the tune unlocks the build. Solid engine mounts for WRX and STI keep the drivetrain stable under the torque a built engine produces.

Build on the Right Foundation

The IAG 950 is the right short block for the majority of serious WRX and STI builds. It delivers closed deck structural integrity, forged internals, and power headroom above what most builds actually target.

The IAG 950 EJ25 Closed Deck Short Block is available now. Browse all IAG short block options to compare the full lineup and confirm the right spec level for the build.