After months of work, the Subaru STI engine rebuild is finally complete — and it’s time for the critical first startup. The goal for this phase was simple: install the rebuilt motor, prime the oil system, fire it up and begin the break-in process. Using a custom oil-pressurizing tool from NV Auto, the team filled the fresh engine with conventional oil to protect the internal components during its first heat cycle.
Startup went smoothly. The engine came to life with no leaks or strange noises, just some oil burn-off from the headers — exactly what you hope for during a first fire-up. The team followed a proven break-in strategy: keep RPMs between 2,000 and 3,000, avoid steady cruising, and focus on throttle modulation and engine braking to properly seat the piston rings.
While the engine sounded strong and responsive, a check engine light surfaced during the road test. A quick scan revealed a “Starter Relay Circuit Low” error. Although the STI ran fine, the team planned further troubleshooting to ensure the issue wasn’t masking a larger problem.
Beyond the motor, attention turned to refreshing the car’s worn-out brakes. New OE rotors and pads were installed, and broken wheel studs were replaced to ensure safety before any real driving began. The finishing touches included fixing a missing AC belt — a job made more complicated by Subaru’s move to a stretch-fit design requiring special tools.
Despite some minor setbacks, the project marked a major milestone. With the motor broken in and running strong, the Subaru is nearly ready for its next phase: performance modifications.
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Summary/Transcript:
[Opening Scene]
Forget the engine we just built — this is it right here.
We’re going to put about 30 PSI into it.
Whoa, whoa, whoa. Did we even tell everybody what’s in there?
[Music + Applause]
That’s right — we had the crazy idea of putting an air-cooled Porsche engine into our Subaru.
Forget the engine rebuild — this is a $40,000 964 3.8L twin-spark engine.
It’s a monster, potentially making over 300 horsepower.
Honestly, we mostly did it for clickbait.
In reality, this is our buddy Dan’s engine for his 964. It was delivered here for him to pick up, and we thought — why not have a little fun imagining it in the Subaru?
Maybe someday, if we win the lottery.
We bought this cheap Subaru STI thinking we could just throw in a motor and have a VA STI chassis for relatively little money.
Instead, we went down the rabbit hole of a complete engine rebuild.
Today’s goal:
- Install the rebuilt engine
- Prime it with oil
- Do the all-important first startup
- Break it in on the street.
It’s been about a month since we pulled the motor out — hopefully we remember how to put it back in.
[Engine Prep & Installation]
Step one: get the engine off the stand and onto the shop crane to install the flywheel and clutch.
This is also the maiden voyage for our new Maximum 2-ton low-profile shop crane, which fits nicely under lowered cars.
The spec Stage 1 clutch kit is installed — an organic full-face clutch with Kevlar and carbon that can handle up to 402 ft-lbs of torque.
The aluminum lightweight flywheel we installed should give better throttle response.
Spec has changed the clutch cover color from royal blue to a lighter metallic blue, giving it drift car vibes.
We lined up the yellow balance marks so everything stays balanced.
Installing the engine into the bay was fairly easy.
The real challenge was mating the transmission to the clutch and flywheel.
A few tips from NV Auto helped:
- Zip-tie the clutch fork away.
- Loosen the turbo slightly for a bit of play.
After struggling a bit, we used bolts to pull the engine onto the transmission input shaft.
Pro tip: put the car in fifth gear and turn the crank — if the wheels move, the shaft is properly engaged.
Once aligned, we installed the slave cylinder, bled the clutch, and confirmed it was working before bolting everything down.
[Buttoning Up the Car]
Reassembling everything took longer than expected — we forgot where many parts went.
Hoses were installed incorrectly multiple times.
Rust on the lower rad supports was sprayed with Fluid Film Black to stop corrosion and make it look better.
We upgraded the radiator to a Coyo Hyper V-Core, 36mm thick, perfect for handling turbo heat.
The radiator dropped in easily with the stock fans.
We filled the cooling system using an Airlift system, vacuuming the air out first.
We found a small leak at a coolant line, fixed it, and continued filling.
[First Start Procedure]
Our friend from NV Auto brought a custom oil pressurizing tool to pre-fill the engine’s oil system.
We used conventional oil for the initial startup.
The oil was pushed through the system, pre-lubricating everything.
Installed a Pro Series XD heavy-duty battery.
Primed the fuel pump a few times, checked for leaks — none found.
Startup plan:
- Run the engine until fans come on.
- Keep RPM between 2,000 and 3,000.
- Vary RPM to promote proper ring seal.
The motor started and sounded great, no leaks, just a little oil burn-off from headers.
[Initial Issues and Brakes Refreshed]
With the engine running, we moved on to fixing some brake issues:
- Three broken studs on the wheel hub.
- Extremely rusty rotors and worn pads.
Used a Bolt Buster heat induction tool to free stuck caliper bolts.
Installed new OEM wheel studs using a special pulling tool.
Refreshed the brakes with OE+ Pro Series rotors and pads.
[Road Test & Break-In]
We took the car out for a break-in drive, using a downshift-and-coast method to properly seat the rings.
The STI felt strong, smooth, and peppy — but a check engine light appeared.
We planned to visit NV Auto to scan the codes and continue troubleshooting.
[AC Belt Issue & Diagnosis]
One missing step during the engine install: the AC belt.
Modern Subaru models no longer use a tensioner for the AC belt; they require a special tool to stretch the belt on.
NV Auto helped install the belt using the official Subaru belt installation kit.
We scanned the check engine light and found a “Starter Relay Circuit Low” code.
Possible causes: a faulty relay or interference from an aftermarket remote starter.
Further diagnostics were needed.
[Wrap-Up]
The engine runs strong, but the check engine light remains.
Stay tuned for the next episode where we diagnose the issue and start modifying the STI for a full build.
