Updated October 2026
A short block is the bottom half of an engine: the block, crankshaft, connecting rods, pistons, and bearings. A long block is a short block plus the cylinder heads, camshafts, and valvetrain, so it is much closer to a complete engine. Choose a short block if your cylinder heads are in good shape and you want control over the top end of the build, and a long block if the heads are damaged or you want a faster, more predictable install.
When planning a Subaru engine build or replacement, this is one of the first big decisions. Both options have their place, and the right choice depends on your goals, budget, and how much work you want to take on. This guide breaks down the differences in plain terms for WRX and STI owners.
Short Block vs Long Block at a Glance
| Short block | Long block | |
|---|---|---|
| Includes | Block, crankshaft, rods, pistons, bearings | Short block plus cylinder heads, cams, valvetrain, often timing components |
| Cylinder heads | Reuse yours or source separately | Included and matched to the block |
| Upfront cost | Lower | Higher |
| Labor and extra parts | More: head inspection or machine work, gaskets, assembly | Less: closer to a drop-in |
| Best for | Good heads, custom builds, tighter budgets | Damaged heads, faster installs, matched assemblies |
Neither one is a complete running engine. Intake and exhaust manifolds, the fuel system, sensors, and accessories usually carry over from your car or come separately, so check exactly what a given block includes before ordering.
What Is a Short Block?
A short block is the bottom end of the engine. It typically includes the engine block, crankshaft, connecting rods, pistons, and bearings, but not the cylinder heads, cams, valvetrain, or timing components.
Short blocks are a popular choice for Subaru owners who already have good cylinder heads or want more control over their build. Built short blocks with forged internals are also the usual foundation for high-power builds, since the bottom end is where a stock engine typically reaches its limits first. The stock FA20 and FA24 power limits guide explains when upgraded internals become necessary, and the stock STI power limits guide covers what breaks first on the EJ257.
When Does a Short Block Make Sense?
A short block is usually the right option if:
- Your cylinder heads are in good condition or already built
- You want to reuse or upgrade specific top-end components
- You are doing a performance build with custom parts
- You are working with a tighter budget than a full engine allows
What Is a Long Block?
Long blocks are more complete engine assemblies. Each one includes the short block plus cylinder heads, cams, valvetrain, and often timing components, so in many cases it is close to a drop-in solution.
For Subaru owners who want a simpler path, long blocks remove a lot of the guesswork because the top and bottom end are matched and assembled together. They are especially common when a failure has damaged the heads, or when downtime needs to be kept short.
When Does a Long Block Make Sense?
A long block is usually the better choice if:
- Your original engine failed and damaged the heads
- You want a faster, more straightforward install
- You want a matched, professionally assembled setup
- You prefer convenience and predictability over customization
Short Block or Long Block for Your WRX or STI?
IAG Performance offers both for the most common Subaru engines, with clearly defined power levels that make planning easier.
For the 2015 to 2021 WRX, the IAG 600 FA20 DIT short block suits owners whose factory heads are healthy and who want stronger internals, while the IAG 600 FA20 long block adds IAG heads for a fully assembled replacement. On the 2022 and newer WRX, IAG offers the 1000 FA24 closed deck short block for high-power builds.
EJ-powered cars, including the 2002 to 2014 WRX and 2004 to 2021 STI, can use IAG’s EJ25 short blocks, which run from the 550 for street builds to the 750, 950, and 1150 closed deck blocks for higher power. Matching long blocks with IAG heads are available at each level. The IAG short block power guide breaks down what each level is built for, and STI owners can find the EJ257-specific path in the IAG short block guide for the Subaru STI.
Each IAG level also has its own guide: the IAG 550 EJ25 short block guide, the IAG 750 closed deck guide, the IAG 950 closed deck guide, and the IAG 1150 closed deck guide.
Open Deck vs Closed Deck: Why It Matters for a Short Block
Before choosing a short block for serious power, it helps to understand block design. A closed deck block adds support around the top of the cylinders, which helps them hold their shape under high cylinder pressure. The closed deck engine guide for WRX and STI explains what makes it stronger, and the open deck to closed deck guide covers whether a factory open deck block can be converted.
Cost vs Labor: Which Is Cheaper?
At first glance, a short block is cheaper. Labor and additional parts can close that gap quickly, though.
With a short block, you may need head inspection or machine work, new valves or seals, more assembly labor, and extra gaskets and hardware. ARP head studs are one of the most common additions for keeping head gaskets sealed under sustained boost, and the ARP head studs guide for WRX and STI covers which kit to run. A long block typically costs more upfront but often reduces labor time and surprise expenses during the build.
Performance and Reliability Differences
Both short blocks and long blocks can support serious power when properly built. The difference is less about performance potential and more about execution. A well-matched long block offers consistency because all components are designed to work together, while a short block build can perform just as well but depends more on part selection and assembly quality.
Any built engine also needs the right supporting setup. Upgraded engine mounts reduce movement under the extra torque, and the WRX engine build and power goals guide and STI engine build and power goals guide map out the full build path. For a practical example, see the guide on building a reliable 400whp EJ257 STI.
To understand why engines fail in the first place, the most common Subaru STI engine failures guide covers the EJ257, and the EJ207 vs EJ257 guide and EJ257 vs FA24 guide compare the platforms.
Related Build Guides
A new short block or long block is one part of a bigger build. These guides cover the systems that need to keep up with it.
Fuel: the AEM 340LPH fuel pump guide for WRX and STI, the AEM 340LPH 65mm fuel pump kit guide, the AEM 380LPH inline fuel pump guide, the stock WRX and STI fuel system limits guide, and the COBB fuel rails, line kit, and regulator guide for STI.
Cooling and oil control: the Mishimoto aluminum radiator guide for WRX and STI, the Mishimoto radiator guide for 2015+ WRX, the best oil catch cans for WRX and STI, IAG oil catch cans and AOS systems, and the guide on whether an air oil separator is worth installing.
Turbo and boost: the best turbo upgrades for Subaru WRX, the best turbo upgrades for Subaru STI, the supporting mods guide for big turbo WRX builds, the Turbosmart external wastegate guide, and the guide on how much boost is safe on a stock WRX and STI.
Exhaust: the WRX exhaust upgrades guide, the best catback exhaust for Subaru WRX and STI, the downpipe vs catback vs turboback breakdown, the Invidia stainless downpipe guide for WRX, and the Subaru STI exhaust upgrades guide.
Drivetrain and tuning: the ACT HD Street Sprung clutch kit guide for Subaru STI, the Competition Clutch STI Stage 2 guide, clutches from Competition Clutch and ACT, and the WRX bolt-on upgrades guide for tuning and entry-level mods.
Engines and parts: assembled Subaru short blocks and WRX performance parts.
Short Block vs Long Block FAQ
What is the difference between a short block and a long block?
A short block is the engine’s bottom end: the block, crankshaft, rods, pistons, and bearings, while a long block adds the cylinder heads, camshafts, and valvetrain, and often the timing components, so it is closer to a complete engine.
Should I buy a short block or a long block for my WRX?
Buy a short block if your heads are healthy and you want to reuse or upgrade them yourself, and a long block if the heads were damaged in a failure or you want a faster, fully matched install.
Can I reuse my cylinder heads with a short block?
Yes, as long as the heads are in good condition. In many cases they should be inspected or refreshed before installation to ensure proper sealing and long-term reliability.
Is a long block worth the extra cost?
For many builds, yes. A long block reduces labor time, removes compatibility concerns, and is often the better choice when the original engine damaged its heads.
Do short blocks and long blocks support the same power levels?
They can. Power capability depends more on the internal components and build quality than on whether the engine is a short block or long block.
Which option is better for a daily driven Subaru?
Both can work well. A long block is usually preferred for simplicity, while a short block is ideal if you already have good heads and want more customization.
Need Help Choosing the Right Engine Setup?
If you are planning an engine rebuild or upgrade and are unsure whether a short block or long block is the right choice, Wrench Masters Auto can help. The shop works on Subaru WRX and STI builds in St. Petersburg, Florida, so you can reach out with your vehicle details, goals, and power targets. You can also browse the full Subaru engine components category or the IAG short block options for WRX and STI.
