--- title: Subaru Engine Failures | a 'built' motor makes more power, but why? description: Subaru Engine Failures | building power is easy, if you know what you are doing, choose your build components and builder wisely. You do want reliable power ??? image: https://www.5250performance.com/hubfs/Engine%20Failures/Snip20170622_6.png --- [![5250 Performance Dyno Shop Colorado](https://www.5250performance.com/hs-fs/hubfs/artwork_5250/Snip20170305_123.png?width=122&height=302&name=Snip20170305_123.png "5250 Performance Dyno Shop Colorado")](https://www.5250performance.com/) - [Home](https://www.5250performance.com) - [About Us](https://www.5250performance.com/about-us) - [Fab](https://www.5250performance.com/fabrication-services) - [Gallery](https://www.5250performance.com/5250-performance-car-gallery) - [Tuning / Upgrades](https://www.5250performance.com/performance-tuning) - [Need a Tuner](https://www.5250performance.com/tuner-page) - [Custom Fabrication](https://www.5250performance.com/fabrication-services) - [Line Card](https://www.5250performance.com/5250-line-card) - [Haltech ECU](https://www.5250performance.com/haltech-ecu) - [Mil Spec Wiring](https://www.5250performance.com/mil-spec-wiring-for-high-performance-cars) - [Repair](https://www.5250performance.com/automotive-repair) - [Current Gallery](https://www.5250performance.com/performance-gallery) - [IAG Subaru Blocks](https://www.5250performance.com/iag-subaru-engine-blocks-colorado) - [Most Popular IAG Blocks](https://www.5250performance.com/our-popular-iag-blocks) - [Dyno](https://www.5250performance.com/dyno-tuning) - [Performance Blog](https://www.5250performance.com/blog) - [Contact Us](https://www.5250performance.com/contact-us) [![Need a Tune?](https://no-cache.hubspot.com/cta/default/532455/e98cee66-8412-4c1b-b0be-676df9108236.png)](https://cta-redirect.hubspot.com/cta/redirect/532455/e98cee66-8412-4c1b-b0be-676df9108236) # 5250 High Performance Dyno Shop Blog ## [Subaru Engine Failures | a 'built' motor makes more power, but why?](https://www.5250performance.com/blog/subaru-engine-failures-a-built-motor-makes-more-power-but-why) \[fa icon="calendar"\] Jun 22, 2017 11:00:00 AM / by [Mike Wiener](https://www.5250performance.com/blog/author/mike-wiener) ![](https://www.5250performance.com/hs-fs/hubfs/Snip20171127_8.png?width=35&name=Snip20171127_8.png) - [Tweet](https://twitter.com/share) ## [![Subaru Engine Failures](https://www.5250performance.com/hs-fs/hubfs/Engine%20Failures/Snip20170622_6.png?width=320&name=Snip20170622_6.png "Subaru Engine Failures")](https://www.5250performance.com/blog/building-subaru-motors-street-or-race-engine-discussion)Subaru Engine Failures | choose performance upgrades wisely Having a "built" motor means you have replaced some or all of the factory components with upgraded aftermarket ones. The term "built" is very loose term and varies from only a few improved components to a well built one with major upgrades, choose wisely. A common thing to do is what is called a piston slap. A new or used stock shortblock has it's pistons removed and replaced with upgraded ones. This is a very economical way to upgrade a motor, but it doesn't always yield the best results. In the case of a used shortblock, installing new pistons without properly preparing / honing the cylinder walls will cause ring seating problems and engine failure. This is probably the cheapest way to get a "built" motor and you will get what you pay for. This is where taking shortcuts will lead you to failure | A proper built motor will have the entire rotating assembly upgraded. This includes crankshaft, rods, and bearings in addition to the pistons. There are hundreds of ways to piece these parts together, but all of them should start with a new or newly machined engine block. The cylinder walls must be perfectly round and have the proper cross-hatching for the rings to seat properly. If the rings don't wear in properly, there will be excessive blow-by and a loss in compression. Improperly seated rings will lead to low power, oil consumption, and failure of the motor. When building your shortblock, the pistons are the single most critical decision. There are many to choose from and some may not be correct for your application. Factory pistons are hyperutectic. This means they are very strong, but very brittle. Similar to a pane of high quality glass, they can take enormous stress loads, but a sudden shock load can cause them to shatter. Detonation, or knock, is the primary killer of pistons. It only takes one fairly mild detonation event to shatter a stock piston. Aftermarket pistons are of the forged design and are made of a softer material than hyperutectic ones and can handle a sudden knock event without breaking. These are not bullet proof though... a very strong detonation or a series of them can still destroy forged pistons. [![Subaru Engine Failures](https://www.5250performance.com/hs-fs/hubfs/Engine%20Failures/Snip20170622_7.png?width=591&name=Snip20170622_7.png "Subaru Engine Failures")](https://www.5250performance.com/blog/5250-subaru-engine-builds-what-are-your-hp-goals)When pushing an engine towards higher power, knock will happen. Forged pistons are what allows a tuner to push to the limit, then back off a little bit when that limit is found. Without the ability to take these light knock events, max power cannot be found. Even on a perfect tune, knock happens. Bad gas or very high operating temperatures can casue a detonation. It's extremely difficult to prevent all knock, so make sure your pistons are up to the challenge, choose your build wisely.   [![Subaru Engine Packages](https://no-cache.hubspot.com/cta/default/532455/334f0bc8-d28f-47d6-aa38-e7b2074e5fd4.png)](https://cta-redirect.hubspot.com/cta/redirect/532455/334f0bc8-d28f-47d6-aa38-e7b2074e5fd4)   All of our in house performance engine builds start with a strong conversation of where you are at, where you want to get to, and how we can work together to build a motor that is quality and performance. Anything less is just a bunch of parts, we don’t work that way. Topics: [Subaru Engine Failures](https://www.5250performance.com/blog/topic/subaru-engine-failures) ![Mike Wiener](https://www.5250performance.com/hs-fs/hubfs/Snip20171127_8.png?width=100&height=100&name=Snip20171127_8.png) #### Written by [Mike Wiener](https://www.5250performance.com/blog/author/mike-wiener) ### Subscribe to Email Updates - Recent - Popular - Categories ### Lists by Topic - [Building Horsepower (9)](https://www.5250performance.com/blog/topic/building-horsepower) - [Dyno Tuning Colorado (7)](https://www.5250performance.com/blog/topic/dyno-tuning-colorado) - [Subaru Firewall Clutch Pedal Repair (7)](https://www.5250performance.com/blog/topic/subaru-firewall-clutch-pedal-repair) - [Superchargers (6)](https://www.5250performance.com/blog/topic/superchargers) - [Turbo Upgrades (5)](https://www.5250performance.com/blog/topic/turbo-upgrades) - [Dyno Tuning (4)](https://www.5250performance.com/blog/topic/dyno-tuning) - [Fabrication Welding (4)](https://www.5250performance.com/blog/topic/fabrication-welding) - 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