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​Finding the Balance Between Performance and Reliability: The IAG 550 Short Block versus the STI Type RA Short Block

If you’re like a lot of Subaru enthusiasts, you’ve likely spent a fair amount of time (and money) trying to eke out every last bit of horsepower by modifying your OEM Subaru engine. Finding that balance between power and reliability can be an artform, and there is always that possibility that you’ll end up pushing your engine past its breaking point. These things happen, and when they do, it’s definitely time to reconsider what your new set up should look like.


We’re often asked if the STI Type RA Blocks can hold as much power as an IAG 550 Short Block, and while there are reports that the RA blocks can handle up to 450whp, the real question is whether it can reliably handle the additional power. In this article, we’ll take a comparative look at the IAG 550 Short Block versus the STI Type RA block to help you find the optimal balance between performance and reliability in your Subaru build.

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Photo of IAG 550 Short Block piston, STI piston and Type RA piston.

Photo of Subaru Type RA short block.


The STI Type RA block made its debut in 2018 in what was supposed to be a limited edition run, but became a regular offering in all the STIs from 2019 on. While the stock RA only made 5 more hp than its STI predecessor, it was hopeful that the 2.5 liter turbo boxer engine with 310whp would be capable of accommodating additional power without the risk of a catastrophic engine failure.

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As the piston ringlands have long been a common point of failure for many a Subaru engine, Subaru attempted to address this issue by equipping the RA with a fortified piston that could stand up to extra load on the motor. Unfortunately, the pistons in the OEM Subaru STI EJ257 and the RA blocks continued to be produced using a hypereutectic cast design. This casting method is susceptible to irregularities in both the casting process and the materials used, leading to potential weaknesses in the pistons. While an additional 10 grams of aluminum was added to strengthen the wrist pin area of the RA pistons, the new design still didn’t address the failure-prone ringlands or the casting imperfections.

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Photo of RA piston with cracked ringland.

Photo of a damaged rod bearing from a Subaru.


From an economic standpoint, the RA may initially seem like a pretty good deal. Not only do they cost less than the IAG 550, but there are plenty of power-boosting options available. The thing to keep in mind, however, is that the RA blocks only come equipped with the basic OEM components- from bearings, to the crankshaft, to rods, the only upgraded internal components are the structurally modified pistons. This means you’d be getting an engine that is still susceptible to failure due to cracked or broken ringlands or a spun factory rod bearing. This brings us to another crucial point to consider, which is the fact that Subaru does not warranty their engine blocks, even if they’re brand new and direct from the factory. (Unless the block is installed in the model of car originally equipped with the same short block.) This doesn’t mean that the Type RA doesn’t have its place, but without spending to upgrade the internal components, there’s sure to be costly issues if you’re using it aggressively (which you most likely are).

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So how do you achieve that perfect balance between power and reliability? We’ve devoted years of research and development to come up with our solution. The IAG 550 Short Block is a sturdy upgrade that is capable of handling higher cylinder pressures created by increased boost levels. By incorporating the use of high quality, high performance internal components, we’ve basically built a better block that could stand up to extra load on the motor, while still maintaining its longevity and reliability.

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Photo of IAG 550 Short Block.

Photo of IAG spec JE Pistons for the IAG 550 SHort Block.


Issues like cracked or broken ringlands have been solved with the use of IAG Spec JE 99.5mm Forged Pistons. We teamed up with JE Pistons to create a custom piston strong enough to handle our 550bhp short block. Forged from a blend of aluminum with a silicone additive for flex, these pistons can stand up to engine impact and are able to support increased cylinder pressures that come with more boost. Additionally, the forging process allowed for more control over the purity of the 2618 T6 Alloy aluminum, making the pistons stronger, and ultimately preventing damage to the ringlands.

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Not only did we handle the problematic ringlands, but we also upgraded the bearings to eliminate the chance of a spun bearing. The 550 component upgrades don’t stop there- each IAG 550 Short Block starts with brand new EJ25 Subaru 2.5L, 99.5mm stock bore engine case halves, providing a solid foundation for its upgraded components. The case halves are carefully measured and assembled using a new 2017+ Subaru 2.5L nitride treated STI crankshaft, which is matched to ACL or King race main bearings. The IAG 550 Short Block also features IAG Spec H-Beam Connecting Rods with ARP2000 rod bolts, along with ACL or King race rod bearings. The case halves are sealed with new OEM main case bolts and sealant. Finally, a set of IAG Spec JE 99.5mm Forged Pistons are carefully selected, measured, and installed to finish off the short block.

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Photo of IAG Spec H-Beam Connecting Rods

Photo of IAG technician working on a Subaru.


Again, while a new RA block from Subaru doesn’t come with a warranty for most aftermarket applications, the IAG 550 Short Block comes with a 12-month or 12,000 mile workmanship warranty. You’ll also have the option to extend the warranty to 24 months or 24,000 miles when the block is purchased, installed and tuned at one of our Certified Engine Installation facilities around the country. You really can’t go wrong with the 550, it arrives professionally assembled and ready to install. It’s a quick upgrade and a straightforward solution to a handful of common problems posed by the factory block and components.

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In conclusion, while the STI Type RA block may initially seem like a cost-effective option, its susceptibility to common issues like cracked or broken ringlands and spun bearings, coupled with the absence of a warranty from Subaru, highlight the potential risks of modification. On the other hand, the IAG 550 Short Block offers a comprehensive solution that addresses these issues with high-quality components, as well as a solid warranty. The 550 is the clear winner for high-powered applications, providing a solid foundation that can reliably handle boost and power. Of course, if you’re looking to upgrade or have questions about your Subaru build, we’re always happy to help. Feel free to give us a call at 1(410)840-3555 or send us an email at

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