Rotax 912 vs ULPower 350i: Which Engine Best Suits Your Build?

Picking an engine for your experimental or light-sport aircraft is one of the biggest decisions you'll make. Get it right and you'll have years of smooth, reliable flying. Get it wrong and you'll spend a lot of time and money fixing a problem you could have avoided.

The Rotax 912 vs ULPower 350i debate comes up constantly in homebuilder forums, hangar discussions, and EAA chapter meetings across the country. Both engines are popular, capable, and well-supported — but they take very different approaches to power, design, and simplicity. The Rotax leans on decades of proven track record. The UL Power (also written as ULPower) brings modern fuel injection and direct-drive simplicity to the table.

So which one belongs on the front of your aircraft? This guide breaks it all down — specs, weight, fuel options, TBO, cost of ownership, and real-world flying experience — so you can make a clear, confident decision.

Key Takeaways

The Rotax 912 and ULPower 350i are both strong choices for light experimental aircraft, but they suit different builders. The Rotax 912 (specifically the 912ULS at 100 hp) is the safer bet if you want proven reliability, a huge support network, and FAA-friendly documentation. The ULPower 350i offers more raw horsepower (118 hp), cleaner fuel injection from the factory, and direct drive simplicity — but with a shorter track record and a smaller support base in the U.S. Your choice should hinge on how much you value power versus ecosystem maturity.

If you're in the market for an aircraft equipped with either of these engines, Flying411 is a great place to start your search — with listings and resources tailored to the light-sport and experimental community.

A Tale of Two Engines: Very Different DNA

Before diving into head-to-head specs, it helps to understand where each engine comes from. Their backgrounds explain a lot about how they behave in the field.

The Rotax 912: Austria's Aviation Workhorse

The Rotax 912 was first sold in 1989, originally for ultralights and motorgliders. It earned FAA certification in 1995, which opened the door to factory-built light-sport aircraft. Over the years, Rotax steadily pushed the TBO upward — from a modest 600 hours at launch to 2,000 hours today on many variants.

By 2014, Rotax had reportedly produced its 50,000th 912-series engine. That's a massive number, and it means there is an enormous global network of mechanics, parts suppliers, and experienced operators. If something goes wrong with a Rotax engine, you're very unlikely to be the first person to deal with it.

Fun Fact: The Rotax 912 uses a hybrid cooling system — liquid-cooled cylinder heads paired with air-cooled cylinders. This unusual combination helps manage heat at the hottest part of the engine while keeping the overall design lightweight.

The 912ULS is the most popular variant, producing 100 hp at around 5,800 rpm (engine) before the gear reduction brings the propeller speed down to a much more efficient range — typically around 2,400 rpm at the prop. The later 912iS adds fuel injection and FADEC (Full Authority Digital Engine Control), bringing the engine into the modern era at a higher price point.

ULPower: A Belgian Challenger Built for the 21st Century

ULPower was founded in 2006 by a small team in Belgium with roots in automotive racing. Their goal was simple: build an aviation engine using 21st-century technology from the ground up, rather than adapting designs from the 1950s or '60s.

The result is a family of engines that are direct drive, fully air-cooled, and equipped with FADEC-controlled multi-point fuel injection as standard. No carburetor. No gear reduction box. No separate oil system for a gearbox. The ULPower 350i produces 118 hp at 3,300 rpm — and because there's no gearbox, that's also close to what the propeller sees directly.

Good to Know: The "i" in the ULPower model name stands for the injected (FADEC-equipped) variant. The "iS" (350iS) is the higher-compression version rated at 130 hp, requiring at least 93 AKI premium mogas.

Rotax 912 vs ULPower 350i: Head-to-Head Specs

Here's how the two engines stack up in the categories that matter most to builders and pilots.

Horsepower and RPM

The Rotax 912ULS produces 100 hp at around 5,800 engine rpm. Its gear reduction unit (with a ratio of roughly 2.43:1) brings prop speed down to around 2,380 rpm at max continuous power. A slower-turning propeller is generally more efficient, which is one reason the Rotax gets good marks for fuel economy in cruise.

The ULPower 350i makes 118 hp at 3,300 rpm — and since it's direct drive, the prop turns at the same speed as the crankshaft. That's 18 more horsepower than the 912ULS, which translates to noticeably better climb rates and short-field performance. One builder flying a Zenith CH 750 with a 350iS reported getting airborne in about 100 feet at gross weight, climbing at around 800 fpm.

Pro Tip: Horsepower numbers from different manufacturers aren't always measured the same way. Some test with straight pipes; others test with stock exhausts. Real-world propeller thrust is what matters most, and a Rotax 912 with its slower-turning prop can be surprisingly competitive on thrust even against higher-hp direct-drive rivals.

Weight

Both engines come in at surprisingly similar installed weights:

• Rotax 912ULS: approximately 170–175 lbs installed
• ULPower 350i: approximately 172 lbs installed (per manufacturer figures)

The weight difference is small enough that it shouldn't be a deciding factor. However, on an LSA where useful load margins can be tight, even a few pounds can matter. The 350i is also a physically wider engine due to its longer stroke, which can affect cowling design — something worth discussing with your airframe manufacturer.

Cooling

The Rotax 912 uses a clever hybrid approach: liquid-cooled cylinder heads and air-cooled cylinders. This helps keep cylinder head temps (CHT) in check, which is critical for engine longevity. The liquid cooling circuit requires an antifreeze coolant reservoir, overflow tank, hoses, and a radiator — adding some complexity and maintenance items to your firewall-forward setup.

The ULPower 350i is fully air-cooled, which means no coolant system at all. Simpler plumbing. Fewer potential leak points. However, all-air cooling can be more sensitive to ambient temperature and airflow, particularly during ground operations in hot weather.

Keep in Mind: ULPower compensates for the all-air-cooled design with pressure oil lubrication to each cylinder head through an external oil line. This helps cool the valves and guides, which are typically the hottest wear points in an air-cooled engine.

The Induction System Question: Carbs vs. Fuel Injection

This is often the most hotly debated topic between these two engines.

Rotax 912 ULS: Dual Carburetors

The standard 912ULS runs two Bing 64 carburetors — one for each pair of cylinders. These are motorcycle-style carburetors that are generally reliable, but they require periodic synchronization to make sure both sides of the engine are drawing equal air-fuel mixture. An out-of-sync carb setup can cause rough running, uneven power delivery, and unnecessary wear.

Carburetor balancing is a routine part of Rotax maintenance, and most owners learn to do it themselves. It's not difficult, but it is a recurring task. Carburetor ice is also a known risk in certain atmospheric conditions, so carb heat is a required system.

Why It Matters: The dual-carb setup does have one significant advantage: the Rotax 912ULS ignition system is entirely self-powered. Two independent charging coils on the generator stator supply power to each ignition circuit. If the aircraft's electrical system fails completely, the engine keeps running.

The 912iS adds fuel injection and a full FADEC system, which eliminates carb sync and carb ice concerns. However, the 912iS relies more heavily on redundant electrical systems to keep the engine running — which adds complexity and weight.

ULPower 350i: FADEC Fuel Injection

The ULPower 350i comes with multi-point electronic fuel injection controlled by a FADEC engine control unit (ECU). This system automatically adjusts spark timing and fuel delivery based on altitude, temperature, and engine load. There's no manual mixture control and no need to worry about carburetor ice.

The FADEC system also provides an automatic altitude compensation feature — the ECU leans the mixture as you climb, keeping combustion efficient without any pilot input. This can result in better fuel economy and smoother operation at cruise.

However, the ECU and fuel injection hardware do require electrical power to keep the engine running. ULPower uses a dedicated internal alternator to power the engine systems, and the engine can reportedly run for roughly an hour on battery alone if the alternator fails. A dual-battery or dual-ECU setup is recommended for added redundancy, especially for longer cross-country flights.

Heads Up: Unlike the Rotax 912ULS, the ULPower engine's ignition and fuel injection systems share the same electrical bus as the aircraft. An electrical failure scenario requires careful planning. Many builders opt for the dual-battery setup ULPower recommends.

Direct Drive vs. Gear Reduction: What It Means for Your Propeller

This is one of the most important mechanical differences between these engines — and it affects everything from propeller selection to noise to long-term maintenance costs.

Rotax: Gear Reduction (PSRU)

The Rotax 912 spins at up to 5,800 rpm at the engine but uses a Propeller Speed Reduction Unit (PSRU) — essentially a gearbox — to step the prop speed down to around 2,380 rpm. Slower propeller rotation is more aerodynamically efficient because the blade tips don't approach transonic speeds.

The gearbox does add complexity. It uses a separate oil system to prevent 100LL lead deposits from contaminating the main engine oil. It requires its own maintenance intervals, including gear inspections. And at TBO time, the gearbox is typically overhauled along with the engine, adding to the overhaul bill.

On the upside, the Rotax's lower prop speed allows you to run larger-diameter propellers, which are generally quieter and more efficient at low aircraft speeds. This is great for STOL designs like the Zenith CH 701 and CH 750.

ULPower: Direct Drive

The ULPower 350i drives the propeller directly — the crankshaft connects straight to the prop flange with no gearbox in between. This is the same approach used by Lycoming and Continental engines, which keeps the drivetrain extremely simple.

Direct drive means no separate gearbox oil, no PSRU maintenance intervals, and no gear inspection requirements. At TBO time, you're overhauling one system, not two. This simplicity is one of ULPower's biggest selling points for builders who want to minimize long-term maintenance tasks.

The tradeoff is that the prop turns at engine speed — around 2,400–3,300 rpm depending on the pitch setting. This limits propeller diameter, can increase noise compared to slower-turning props, and means propeller selection requires more attention to prevent tip speed issues.

Fun Fact: The torque curve on the ULPower 350i is notably flat — maximum torque arrives as low as 2,400 rpm and stays consistent all the way to 3,300 rpm. This makes the engine feel smooth and responsive at a wide range of power settings.

Fuel Options: Mogas, Avgas, and Real-World Flexibility

Both engines are designed to run primarily on automotive gasoline (mogas), which is typically much cheaper than 100LL avgas. But there are important differences in how each handles different fuel types.

Rotax 912 ULS

The 912ULS has a compression ratio of 10.8:1 and is designed for 91 AKI (regular premium) automotive gasoline. It can also use 100LL avgas when necessary, but extended use of 100LL accelerates lead buildup — particularly in the gearbox and on valve seats. Rotax recommends a gearbox cleaning at around 1,000 hours if 100LL has been used regularly.

Pro Tip: If you plan to fly to destinations where only 100LL is available, keep a small bottle of lead scavenger additive (like Decalin or TCP) in your flight bag. Many Rotax owners use it as a precaution whenever they have to fuel with avgas.

Newer Rotax engines can also run on G100UL, the unleaded replacement for 100LL that is slowly rolling out to U.S. airports. This is an advantage as aviation transitions away from leaded fuel.

ULPower 350i

The ULPower 350i (standard model, lower compression ratio of around 8.4:1) is designed for 91 AKI mogas and can accept 100LL when needed. However, the engine was not originally designed with leaded fuel in mind, and extended use of 100LL has been linked to valve issues in some cases — particularly sticky or leaky valves due to lead deposits.

ULPower has updated the ECU map on newer engines (serial numbers from 223501 onward) to support 87 AKI (regular automotive) fuel, which significantly expands fuel availability, especially in rural areas.

The 350iS has a higher compression ratio (around 8.7:1) and requires at least 93 AKI (premium) mogas.

Time Between Overhaul (TBO): What the Numbers Mean

TBO is a frequently misunderstood concept in experimental aviation. For certified aircraft, TBO is mandatory. For experimental-category homebuilts, TBO is a manufacturer recommendation — you can theoretically fly past it, though most builders and mechanics recommend following it for safety and resale value.

Rotax 912 TBO

The Rotax 912 has a recommended TBO of 2,000 hours (for most current-production engines). This is one of the best figures in the class. For context, it took Rotax years to get there — the original TBO was just 600 hours in 1989, rising to 1,200 hours by 1999, then to 2,000 hours in 2009 for engines meeting certain serial number criteria.

At TBO, an overhaul of the engine and gearbox is recommended. The cost of a Rotax overhaul can be significant, which is a point of frustration for some owners. However, the mature supplier ecosystem means overhaul shops are relatively easy to find.

ULPower 350i TBO

ULPower recently updated its TBO structure. For engines with serial numbers above 250000, the TBO is 2,000 hours (or 12 years, whichever comes first). Older engines carry a 1,500-hour TBO for standard models. Aerobatic variants have shorter TBOs regardless.

The direct drive design simplifies the overhaul significantly — there's no gearbox to rebuild, just the engine itself. This can mean lower overhaul costs compared to a gearbox-equipped Rotax.

Reliability and Field Support

This is where the Rotax 912 arguably holds its biggest advantage.

With tens of thousands of 912 engines flying globally — and well over a million collective flight hours logged — the Rotax is one of the most thoroughly tested small aircraft engines in history. Failure modes are well understood. Parts are stocked at hundreds of distributors. Mechanics certified in Rotax maintenance are relatively easy to find at airports across the U.S.

The ULPower fleet is much smaller by comparison. That's not a knock on the engine's quality — it's simply a function of being a newer, less widespread product. Finding a mechanic familiar with ULPower in rural America can be challenging, and parts lead times from Belgium can occasionally be longer than domestic options.

Good to Know: ULPower engines use several off-the-shelf automotive components — including ignition coils and injectors from the automotive industry. In theory, some replacement parts could be sourced locally in a pinch. However, the ECU and aviation-specific components still need to go through ULPower or an authorized dealer.

Some community forum reports have mentioned issues with piston slap, rich-running conditions, and valve concerns in early ULPower 350 engines. ULPower has reportedly addressed many of these through ECU updates and revised piston specifications. If you're buying a used 350i, checking the serial number and asking about any known service bulletins is highly recommended.

Looking for an aircraft with a Rotax or ULPower engine already installed? Flying411 lists experimental and LSA aircraft for sale with detailed engine information — making it easy to compare your options in one place.

Best Aircraft Fits: Where Each Engine Shines

Both engines are popular in the experimental and LSA world, but certain airframes tend to favor one over the other.

Rotax 912 Best Fits

• Zenith CH 701 / CH 750 STOL — Factory-recommended; excellent match for the gear reduction's slow-turning prop
• Van's RV-12 — Designed specifically around the Rotax 912; factory-supported configuration
• Tecnam P2002 Sierra — Certified LSA platform using the 912 series
• Rans S-7 Courier / S-21 — Common in kit builds; excellent parts availability
• Training aircraft — The proven, self-sustaining ignition (no external power needed for carbureted versions) makes it popular in flight school environments

ULPower 350i Best Fits

• Zenith CH 750 / CH 650 — Factory-supported ULPower installation; builders report excellent climb performance
• High-performance STOL builds — Extra 18 hp makes a noticeable difference on short fields
• Weight-sensitive builds — Similar installed weight to the Rotax but more power per pound
• Builders who dislike carburetor maintenance — The factory fuel injection eliminates carb sync entirely

If you're looking at other engine comparisons for your build, resources like Lycoming vs. Continental vs. Rotax offer a broader look at how these brands compete in the certified and experimental spaces. For builders interested in four-cylinder Lycoming options, this Lycoming O-360 vs IO-360 comparison covers the carbureted vs fuel-injected debate in that engine family.

Cost of Ownership: The Full Picture

Purchase price is just one piece of the equation. Let's look at the broader cost picture.

Initial Purchase

Both engines are available in similar price ranges for new units — though exact pricing varies by distributor and fluctuates over time. Generally speaking:

• The Rotax 912ULS has historically been less expensive than the 350i on initial purchase
• The Rotax 912iS (fuel-injected version) carries a premium — often significantly more than the base ULS
• The ULPower 350i typically sits in a mid-range price point — often comparable to or slightly above the carbureted 912ULS, but less than the 912iS

The 912 also requires additional purchase of items like an air filter box, exhaust system, and carb heat equipment — costs that are sometimes already bundled into the ULPower firewall-forward package.

Why It Matters: ULPower ships each engine with a complete exhaust system, wiring harness, ECU, ignition coil pack, alternator with rectifier and voltage regulator, electric fuel pump, and starter. For some builds, this can reduce the total firewall-forward cost even if the engine price tag itself is similar.

Ongoing Maintenance

• Rotax 912ULS: Carb synchronization required periodically; dual-carb system means carb rebuilds come in pairs; gearbox oil changes at specified intervals; gearbox cleaning recommended if running 100LL regularly
• ULPower 350i: No carb to sync or rebuild; direct drive eliminates gearbox maintenance; ECU and sensors require monitoring; valve seat wear can be an issue with heavy 100LL use

TBO Overhaul Costs

• Rotax overhauls include both the engine and gearbox — costs vary by shop, but can run into the thousands of dollars
• ULPower overhauls cover only the engine with no gearbox — potentially lower overall overhaul cost, though parts sourcing from Belgium may affect pricing

For comparison purposes, see how overhaul cost thinking applies to Continental IO-520 vs IO-550 — a useful reference for understanding how engine overhaul economics scale with displacement.

Key Comparison: 9 Factors to Help You Choose

Here's a summary of the most important factors to weigh when deciding between the Rotax 912 and the ULPower 350i:

1. Horsepower needs: If 100 hp is enough for your airframe and mission, the 912ULS delivers proven performance. If you want 118–130 hp with a simpler drivetrain, the 350i or 350iS wins.
   
    
2. Support network: The Rotax ecosystem is vastly larger in the U.S. If you fly cross-country to unfamiliar airports, finding a Rotax-trained mechanic is much easier.
   
    
3. Induction preference: Hate syncing carburetors? Want to eliminate carburetor ice risk? ULPower's factory fuel injection solves this cleanly — as does upgrading to the 912iS, at a higher cost.
   
    
4. Electrical redundancy comfort: The carbureted 912ULS runs without any external electrical power. The ULPower needs a working electrical system. How you feel about that trade-off matters.
   
    
5. Propeller compatibility: The Rotax's gear reduction allows larger, slower-turning propellers — a real advantage for STOL aircraft. Direct drive limits prop diameter and means more careful propeller selection for noise and tip speed.
   
    
6. TBO and overhaul simplicity: Newer ULPower engines now match the Rotax at 2,000 hours. The direct drive architecture simplifies the overhaul and may reduce cost at TBO time.
   
    
7. Fuel availability on the road: Both engines run on mogas. The Rotax has a longer track record with mixed-fuel scenarios. The ULPower 350i may show valve issues with prolonged heavy 100LL use.
   
    
8. Airframe compatibility: Not all airframes are approved for both engines. Always verify your kit manufacturer's approved engine list before purchasing.
   
    
9. Budget: Compare total firewall-forward cost, not just engine list price. ULPower's bundled components can offset its initial price, while Rotax may require additional accessory purchases.
   
    

Quick Tip: Before committing, try to find a local builder or flight school that flies each engine. Ask about their real-world experience — especially maintenance surprises and parts wait times. No forum post beats a 30-minute hangar conversation.

LSA Rules and FAA Considerations

For U.S. pilots operating under LSA rules, both engines are generally eligible — but there are important distinctions.

The Rotax 912 carries full FAA type certificate approval (for the certified variants) and is specifically listed as an approved engine in many factory-built LSA aircraft. This means repair and maintenance work can be done by FAA-approved mechanics under the standard LSA maintenance framework.

ULPower engines are primarily used in experimental and E-LSA (Experimental Light Sport Aircraft) categories in the U.S. They are not FAA type-certificated as standalone aviation powerplants in the same way the Rotax is. This is an important distinction if you're buying a factory-built LSA rather than building your own kit.

If you're building an experimental aircraft, FAA certification of the engine is not required — the builder is the "manufacturer" and takes responsibility for the airworthiness determination. But if you're buying a factory-built LSA, always verify which engines are approved for that specific model.

For more context on how certified engine choices play out in practice, check out this comparison of the Continental IO-360 vs. Lycoming IO-360 or the Continental O-200 vs O-300 — both of which touch on how certification status affects real-world ownership decisions.

The Verdict: Which Engine Is Right for Your Build?

Here's an honest bottom line.

Choose the Rotax 912 ULS if:

• You want the most proven, widely supported engine in the class
• You're building an airframe specifically designed around the Rotax (like a Van's RV-12 or Tecnam)
• You prioritize electrical independence (carbureted version)
• You plan to fly into unfamiliar airports and want easy mechanic access
• You're a first-time builder and want to minimize unknowns

Choose the ULPower 350i if:

• You want more power (118 hp) from a similar weight package
• You dislike carburetor maintenance and prefer factory fuel injection
• You're building an airframe that supports or recommends ULPower (many Zenith builds)
• You're comfortable doing your own maintenance and have access to ULPower support
• You want a simpler drivetrain with no gearbox to maintain at TBO

Both engines are capable of powering a reliable, enjoyable aircraft. The Rotax is the conservative choice. The ULPower is the choice for builders who want modern technology, more power, and simpler mechanics — and are willing to accept a smaller support network as the trade-off.

For builders who want to dig even deeper into the fuel-injected Rotax side of the equation, this comparison of Continental IO-550 vs TSIO-550 offers useful context on how fuel injection and turbocharging interact in aircraft engine selection.

Ready to find your next aircraft or compare listings by engine type? Head over to Flying411 — your home base for buying, selling, and researching aircraft in the experimental and light-sport market.

Frequently Asked Questions

Is the ULPower 350i FAA certified?

The ULPower 350i is not FAA type-certificated as a standalone aviation powerplant. It is used primarily in experimental-category and E-LSA aircraft in the U.S., where the builder assumes airworthiness responsibility. It is not approved for factory-built type-certificated aircraft.

Can I use regular car gas in both engines?

Yes — both the Rotax 912 ULS and the ULPower 350i are designed to run on automotive mogas. The Rotax 912 ULS uses 91 AKI (premium), while newer ULPower 350i models (post-serial 223501) have been updated to accept 87 AKI (regular) fuel. Both can use 100LL avgas in a pinch, though extended use of leaded avgas is not ideal for either engine.

Which engine is easier to maintain at home?

The ULPower 350i requires less routine adjustment — there are no carburetors to sync and no gearbox to service. However, the ECU and fuel injection system require electrical troubleshooting knowledge, and parts must be sourced through ULPower or authorized dealers. The Rotax 912 ULS has more available mechanics and parts domestically, which can make unexpected maintenance easier to resolve on the road.

Does the Rotax 912 need electrical power to keep running?

The carbureted Rotax 912 ULS does not require external electrical power to run. Its dual ignition system is powered entirely by integrated generator coils on the engine. The fuel-injected Rotax 912iS and the ULPower 350i both rely on electrical systems to power their ignition and fuel injection — though both have backup provisions built in.

Which engine has better resale value in the aircraft market?

Generally speaking, aircraft powered by the Rotax 912 series tend to be easier to resell in the U.S. market due to the wider buyer familiarity with the platform. ULPower-powered aircraft may appeal to a more technically sophisticated buyer base and can command good prices in the experimental community — but the pool of potential buyers is currently smaller than for Rotax-equipped aircraft.