#RevvProjectBRZ SCR Performance: Building the Bottom End

FA20 4U-GSE Bottom End Build: Crawford E85 Piston Design

Article by Brian Hannon. Photography by Kenta Young, Ryan Randels and Corey Davis.

Last week we gave you a behind the scenes look at the magic SLVA was conjuring up with the creation of the wide-body for our BRZ. In this installment, we change our focus to the drivetrain build as we’ve had lots of inquiries into what will motivate this beast.

Building the Bottom End of the FA20 4U-GSE: SCR Performance

Naturally, we turned to SCR Performance to take command of the engine build. They have an extensive background in motorsports applications and a commitment to high-quality builds. SCR also has a productive history with building race-oriented Subaru boxer engines, so they were the perfect partner.

Building the Bottom End of the FA20 4U-GSE: Engine Teardown

In a previous installment, we assessed the overall strengths and weaknesses of the FA20/4U-GSE engine with SCR. We’ll need to keep those valuations in mind as we go for our objective of 500-600 horsepower at the wheels, aided by turbocharging and running E85 (or E98 if needed). This week we’ll apply what we learned about those strengths and weaknesses as we address the performance capabilities and specific build requirements for the bottom end.

These days you often hear of massive builds like 2,000 hp Lamborghinis, 1,000+ hp Chevys and Mustangs, and 1,000+ hp Supras. But you never really hear about 1,000+ hp Subarus. Why is that?

Building the Bottom End of the FA20 4U-GSE: SCR Specification Review

There are many factors that affect the power capabilities of an engine, the first being the strength of the block to contain the power. Cast iron blocks and very well-designed aluminum blocks with proper girdling allow for high-performance applications. Manufacturers are constantly changing and improving their designs. That way they can still run a lightweight block and make serious power. This is typically reserved for high-end manufacturers and serious performance applications.

At higher power levels, you see different stresses. All of your material properties are changing, components deform and stretch, and tolerances change. You can see that proper specifications, experience and technique are so critical in high-horsepower applications

When you look at the power numbers that the naturally aspirated exotics are producing, engine displacement isn’t that big, yet they’re getting double the horsepower versus other manufacturers. What have they done? It comes down to rpm—you can achieve that power level by revving the engine higher. The high-end manufacturers run a lot of exotic materials in their bearings and other components to enable their engines to run at the high rpm without failure; something most other mass-production manufacturers can’t afford to do.

Building the FA20 4U-GSE Bottom End: Boxer Engine Design Limitations

Overall, SCR believes it’s the boxer engine design that is limiting the power output. There are some questions about the strength of the block due to the aluminum, as well as the structural design. Flat-fours are also notoriously difficult engines to use to push the high horsepower numbers. Harmonics also plays a big role at high power as the forces in the engine start to counteract and work against each other as the power increases

Luckily, the FA20 was designed by the manufacturer to be better at revving, so we will try to use that to our advantage. Subaru did a lot of work to engineer components to withstand higher rpms compared to other manufacturers. Take, for example, the design of the solid valve train in lieu of hydraulic lifters to avoid bleed down under higher rpm’s.

But still, the engine is designed around making a couple of hundred horsepower, not one thousand. In order to push the limits of the engine, we needed to build upon the strengths and address the weaknesses of the stock FA20, so we broke it down by component and, most importantly, explain why.

Reinforcing the FA20 / 4U-GSE Connecting Rods

Building the FA20 4U-GSE Bottom End: Crawford I-Beam Connecting Rods

With the relatively high horsepower figures we’re aiming for, there was a concern about bending/twisting the stock rods with higher pressures, which required an upgrade to stronger components. For that we turned to industry stable Crawford and their I-Beam billet connecting rods. Their process and metallurgy used to manufacture make their solution much stronger than stock, along with much more stringent build tolerances, yielding a more perfected and reliable product.

Building the FA20 4U-GSE Bottom End: Crawford I-Beam Connecting Rods

Building the FA20 4U-GSE Bottom End: Crawford I-Beam Connecting Rod Design

The rods share a design nearly identical to stock. The rod caps are split at an angle, much like the OEM components, so you can torque the hardware and install rods while the block halves are together. The length of the rod has increased by a couple of millimeters to increase the rod-ratio, allowing for smoother and more efficient power delivery. This reduces side load and extra forces on the piston when you’re trying to make more power. The change in rod ratio also improves on stock geometry inside the motor, which Subaru already did a good job doing, so it was not something that required too much attention. While the rods might be longer, we actually did not change the stroke of the piston, requiring the pin location to be adjusted accordingly.

Building the FA20 4U-GSE Bottom End: Crawford I-Beam Connecting Rod Design 2

Building the FA20 4U-GSE Bottom End: Crawford I-Beam Connecting Rod Design ARP Hardware

For the rods, we’re also using Crawford spec’d ARP hardware. These stronger bolts don’t have a stretch or yield. You do want a certain amount of bolt-stretch when you assemble, but not when the components are under load. We certainly don’t want the rod to be allowed to stretch because the hardware is soft, so we’re confident in Crawford’s decision to utilize this set of hardware.

Reinforcing the FA20 / 4U-GSE Piston

Building the FA20 4U-GSE Bottom End: Crawford E85 Piston

Now we move on to the most important pieces of the suck-squish-bang-blow process: the piston. We wanted to lower the compression and increase the strength of the pistons so we could achieve our performance objective of 500-600 whp, reliably and attainably. We again turned to Crawford for a set of JE pistons built to their specs with a 10:1 compression ratio and optimized for E85 gasoline. This lowers the compression by 2.5 points, allowing us to run an aggressive amount of boost without having crazy cylinder pressures or requiring race fuel. Lowering compression also makes it more attainable for the average person to have and maintain a turbocharged car. To put it simply, by lowering the compression, you can run a lower octane fuel (rather than say race gas) and still introduce a good amount of boost to achieve good power while not having to worry about destroying the engine with pre-detonation.

FA20 4U-GSE Stock Piston vs Crawford E85 Piston Comparison

The compression was effectively altered by the dish on the piston, going with one much bigger than that the OEM design. With direct-injection, we need to maintain the triangle-shaped recess in the piston to allow for proper fuel mixture of the direct injection system and the Crawford piston design took that into consideration.

FA20 4U-GSE Bottom End Build: Crawford E85 Piston Stronger Allow and Pin Location

The pistons themselves are made out of much stronger alloy and built to a higher specification than their mass-produced cousins. An offset was also designed into the pistons pin location so the wrist pin isn’t dead center. This offset helps with side-loading on the piston and helps keep the piston from cocking sideways and dragging along the boxer’s cylinder walls.

FA20 4U-GSE Bottom End Build: Crawford E85 Piston Rings Turbocharged Application

Moving to the sides of the pistons, we needed to address the piston rings in order to handle the heat and power levels of the turbocharged application. And guess what? We reused OEM factory rings albeit a new set from Subaru. They are actually fairly high-quality from the factory so SCR found no reason to change them. The only change is to the clearances on the ring gaps to deal with more heat and different expansion, because we’re going to see higher heat from the turbocharged application.

FA20 4U-GSE Bottom End Build: Crawford E85 Piston Cylinder Pressures

FA20 4U-GSE Bottom End Build: Crawford E85 Piston Cylinder Pressures 2

As the motor heats up and the cylinders see very high temperatures from the boost, we need to make sure it’s not going to have an issue with the rings binding up or doing anything unexpected.

FA20 4U-GSE Bottom End Build: Crawford E85 Piston Ring Tolerances and Clearances

FA20 4U-GSE Bottom End Build: Crawford E85 Piston Ring Tolerances and Clearances 2

Unfortunately, the specific tolerances won’t be published as this is proprietary information for SCR and this type of information and experience is what you pay for when you go with an industry-leading engine builder.

The FA20 / 4U-GSE Crank

FA20 4U-GSE Bottom End Build: Stock Crank

When it came to the crankshaft, SCR first took stock of the condition of the OEM crank. They checked for any signs of wearnone were foundand made sure all tolerances and bearing journals were the same size across entire crank. Everything was nice and clean, but if it was a higher mileage crank, it might have required that we polish the journals.

FA20 4U-GSE Bottom End Build: Crank Review 2

Since the stock piece has not shown any weaknesses or concerning wear patterns, and seems to be of good quality, SCR found no reason to mess with it for our specific application. Another factor is there is not too much aftermarket support yet for FA20 cranks, so we’ll stand on the shoulders of those before us and push this stock crank to its limits.

The FA20 / 4U-GSE Block

FA20 4U-GSE Bottom End Build: Stock Cylinder Walls and Block

With a boosted application, cylinder wall pressure is sure to be a concern. Fortunately, based on SCR’s assessment, not a whole lot needs to be done to the block. We could sleeve the cylinders with a stronger material, but we don’t feel this is entirely necessary at the power levels we’re aiming for. Since we don’t want thinner, weaker cylinder walls, we’re staying with the OEM bore specs to keep the most material we can.

FA20 4U-GSE Bottom End Build: Stock Cylinder Walls Before Hone
Image shows an example of our FA20 cylinder walls prior to hone.

After the tear down, an inspection of the cylinders showed no damage, so we just ran a hone to clean the walls up while maintaining the OEM bore. Using a torque plate, we torqued it down with hardware, so the block half would be under the same stresses as when it’s fully assembled, then ran a hone through it so that everything was perfectly true and round.

FA20 4U-GSE Bottom End Build: Stock Cylinder Walls After Hone

SCR also checked line hone and everything was great, so they didn’t find the need to machine the block faces. If we were sleeving the cylinders, we would need to machine both the deck face and the face between the two case halves. But that is for a different build on a different day.

Upgrading the FA20 / 4U-GSE Hardware

FA20 4U-GSE Bottom End Build: The Build

We replaced the factory hardware with ARP-spec’d hardware by Dynosty. Since it’s a performance application, it required much stronger bolts. These bolts are not a "torque to yield” bolt so we can get much more consistent torque values across all the "mains.”

Determining proper torque values require assembling the engine following the factory guidelines, ensuring you’re meeting the required clamp load on the block, measuring those values, and then simulating them with the new hardware. This results in much more consistent numbers, because with torque to yield bolts, you turn them 60 degrees and call it good. Well, 60 degrees on one bolt and 60 degrees on the next may not yield the same torque values. Not something that we want to have in a high-performance application.

Maintaining consistent torque across all the mains will help the block keep shape, evenly distribute stress, and keep everything as even as possible so nothing is moving or compromising clearances.

Assessing the OE Bearings of the FA20 / 4U-GSE

FA20 4U-GSE Bottom End Build: Assessing the OE Bearings

As is good practice with any build, SCR replaced all of the bearings. Again, they turned to the OEM-spec bearings, as they found no issues with them for our intended performance objectives. Much like the crank, the aftermarket doesn’t offer too many options yet for these components. There wasn’t much concern with using the OEM bearings, as they are designed for high-rpm usage and intended to withstand extended use.

Machining, Balancing, and Building the FA20 / 4U-GSE

FA20 4U-GSE Bottom End Build: Machining, Balancing and Building the FA20

Minimal machining was required on the bottom end outside of the light hone on the cylinders we already mentioned. The bulk of the machining was concentrated on the cylinder heads, which we’ll delve into in a future article.

FA20 4U-GSE Bottom End Build: Machining, Balancing and Building the FA20 Image 2

FA20 4U-GSE Bottom End Build: Machining, Balancing and Building the FA20 Image 3

Once all of the components were spec’d, sourced and received, the entire bottom end was balanced to make sure all the parts that move are perfectly even so everything runs nice and smooth. SCR balanced everything, from the harmonic dampener bolt, harmonic dampener timing gear, to the crank, flywheel and clutch setup. Everything that gets bolted to the crank and spins is something that should be balanced.

FA20 4U-GSE Bottom End Build: Machining, Balancing and Building the FA20 Image 4 Gasket

FA20 4U-GSE Bottom End Build: Machining, Balancing and Building the FA20 Image 5 Halves Install

FA20 4U-GSE Bottom End Build: Machining, Balancing and Building the FA20 Image 6

Though the rods and pistons are not bolted on, they’re ground down so that their weights are equal to each other so they each have an identical effect on the rotating assembly. The rings, wrist pins and rod bearings are also included in that weight equation. While this is an add-on process for most people, factories rely on build tolerances so there’s not typically a necessity to balance the assembly.

FA20 4U-GSE Bottom End Build: Completed Bottom End

Keep in mind, balancing is done, not for more horsepower, but for smooth operation and not adding extra forces on the bearings. If a single rod and piston combo runs heavier than others, there is a buildup of extra forces that can shorten bearing life and increase drag on the motor. We don’t want to make 500-600 whp for a few runs down the strip; we want this engine to last for a long time!

First the body, now the bottom end. The build is coming together quickly! Next week we will take a look at the top end modifications and fuel delivery system as the build pace picks up. Once again, we have to give a big shout-out to all the partners that are helping this become a reality!

The Subaru BRZ / Scion FR-S Platform Series Partners

Please take a moment to check out our partners, whom without, none of this would be possible:

Heuberger Motors
The world's most enthusiast-friendly Subaru dealership.

SCR Performance
The Performance Expertise Behind the BRZ / FRS Platform Series.
Visit them on the web @

SLVA Automotive Studio
Where Form meets Function.
Visit them on web @

Our Audio and Aesthetics Partner aka the Architects of Fidelity.
Visit them on the web @

AR Design
The Fabrication Masterminds Behind our Forced Induction Systems
Visit them on the web @
Our Sport-Compact Performance Parts Source. . . Never Compromising Quality or Customer Service.
Visit them on the web @
Visit them on Facebook @

Air Lift Performance
The Progressive Behind the Build
Visit them on the web @

Maintaining Integrity & Strength through CUSCU Performance
Visit them on web @
Visit them on Facebook @

Wilwood Engineering
The Stopping Power
Visit them on the web @

Advan Racing
Bringing Together Performance & Aesthetics
Visit them on the web @

Turbo By Garrett
Forced Induction at its Finest
Visit them on Facebook @

Yokohama Tire Company
Power is Nothing without adequate Traction.
Visit them on the web @


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Brian Hannon (BHRP)

Grew up around cars and racing, a passion fueled by my whole family. Participated in numerous track days with cars and motorcycles as well as covered the NASA East Coast Honda Challenge for Grassroots Motorsports. Now that I'm in Colorado I'm enamored with the Pikes Peak International Hill Climb... borderline obsessive...
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