The IAG 750 sits in a specific part of the EJ25 build spectrum. It is not the entry point into closed-deck territory. It is also not a competition engine. Instead, it is the block for Subaru WRX and Subaru STI owners who have outgrown the 550 tier and are building toward a serious street or track setup in the 600 to 750 horsepower range. The component specification steps up meaningfully, and the closed-deck architecture provides the cylinder wall stability that aggressive boost levels demand.
This post covers what the IAG 750 is built with, who it suits, and what the rest of the build needs to support it.
What the IAG 750 Is Built With
The IAG 750 starts with brand-new Subaru OEM EJ25 case halves. These are converted to a closed-deck configuration and bored and honed to 99.75mm. The closed-deck conversion fills in the open areas around the cylinder bores. This stiffens the walls and reduces the flex that causes head gasket failure and cylinder distortion under high boost. For a closer look at why those failure modes are so common, the post on most common WRX engine failures covers the specific weak points in detail.
Inside the assembly, the 750 uses IAG-spec Tri-Beam forged connecting rods. These are secured with ARP 625+ rod bolts and paired with ACL or King performance bearings throughout. The Tri-Beam rod is a step up from the H-Beam specification used in the 550 tier. It offers increased rigidity under the higher loads and RPM that a 750 horsepower build generates.
The short block uses IAG-spec JE FSR 99.75mm forged pistons manufactured from 2618 alloy. These feature a skirt coating for improved durability under sustained high cylinder pressure. Additionally, a new Subaru nitride-treated STI crankshaft and OEM Subaru case and main bolts complete the rotating assembly. Every 750 short block ships fully assembled using 100% new components. There are no reused factory parts in the build.
Compression Ratios
The IAG 750 supports multiple compression ratio configurations. The right combination depends on the cylinder head and head gasket used. With EJ257 heads and an OEM head gasket the ratio is 8.9:1. With a 0.039″ JE ProSeal head gasket it drops to 8.6:1. EJ255 D-head configurations run 9.5:1 with an OEM gasket and 9.1:1 with the ProSeal. S20 head builds without combustion chamber modification run 9.6:1 OEM and 9.2:1 ProSeal. The fuel strategy and tuning approach for the specific build should guide the final decision.
Who the IAG 750 Is For
The 750 suits WRX and STI owners who are past the point of mild reliability upgrades. These are builders putting together a purposeful high-power street or track car. Typically, the buyer runs a large-frame turbocharger, E85 or a flex fuel setup, and a professional custom tune. They have already addressed fueling and intercooling. The short block is therefore the next logical step in building a foundation that sustains the target power level.
Specifically, the 750 is the right tier for builds where the 550 does not provide sufficient headroom and the 950 is more than the build requires. An owner targeting 600 to 700 horsepower on a built street car with occasional track use is the core buyer. The Tri-Beam rod and JE FSR piston specification give the 750 the internal strength to handle that range under sustained load without stepping into the more expensive component specification of the 950.
For STI owners planning a build at this level, the STI engine build and power goals hub covers how the 750 fits within the broader EJ257 build path. WRX owners building on the EJ platform can find the equivalent context at the WRX engine build and power goals hub.
What the Build Needs to Support the IAG 750
A short block at this tier raises the ceiling on what the engine handles. However, the supporting systems need to match that ceiling. The build needs to be planned as a complete system, not as a block swap into an otherwise stock car.
Turbocharger
The IAG 750 supports turbo setups capable of producing 600 to 750 horsepower. A stock or mildly upgraded turbocharger is not the right pairing at this build level. Large-frame upgrades in the TD06SL2 class or equivalent are commonly matched with the 750 tier. The WRX turbo airflow upgrades hub and the STI turbo airflow upgrades hub cover the options at this power level.
Fuel System
At 600 to 750 horsepower, particularly on E85, the fuel system must match the engine’s demand. Upgraded injectors and a high-capacity fuel pump maintain safe air-fuel ratios under load. The WRX fuel system upgrades hub and the STI fuel system upgrades hub outline what fueling the build requires at this tier.
Head Studs and Head Gaskets
At 750 horsepower, the head sealing system needs to match the cylinder pressure the build generates. ARP head studs are the standard fastener upgrade for any serious EJ build. At this power level, the IAG/ARP 14mm head stud set paired with IAG Fire-Lock head gaskets is the appropriate sealing combination. The post on supporting mods for big turbo WRX builds covers the full list of what a build at this level needs beyond the block itself.
ECU Tuning
A professional custom tune is required after installing the IAG 750. The ECU needs calibration for the compression ratio, the new rotating assembly clearances, the turbo setup, and the fuel system changes. Running a stock or off-the-shelf map on an engine at this level causes damage. The tuner should have specific experience with high-power EJ platform builds.
Oil System
At sustained high power and RPM, oil control becomes critical. An upgraded oil pickup and oil pan baffle are standard additions at this build level. Additionally, an IAG air oil separator reduces oil vapor ingestion into the intake system. This protects both the turbo and the new short block internals over time.
IAG 750 vs the Tiers Above and Below
The 550 uses H-Beam rods with ARP2000 bolts and targets builds up to 550 horsepower. It upgrades the internal components over the stock EJ257 but retains the open deck block architecture. For owners whose power goals sit comfortably below 600 horsepower and who are not yet stepping into closed deck territory, the 550 is the more accessible starting point.
The 950, however, uses Tuff H-beam rods with ARP 625+ bolts, JE FSR pistons with electroless nickel-plated crowns, and H13 wrist pins. It targets builds approaching four-digit horsepower. For builds in the 600 to 750 horsepower range, the 750 provides the right internal specification without the additional cost of components engineered for power levels the build will not reach.
A full breakdown of how the tiers compare is available at the IAG short block hub.
Shop the IAG 750 Closed Deck Short Block
The IAG 750 ships in 1 to 2 weeks and is priced at $5,499.99.
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