Lycoming O 235 vs Rotax 912: A Comparison Guide

Choosing between the Lycoming O-235 and Rotax 912 can feel like picking between two great friends. Both engines have powered thousands of aircraft across America. Both have loyal fans who swear by their choice. But these two engines are very different in how they work and what they cost to own.

The Rotax 912 came along in 1989 with a fresh take on small aircraft engine design. It brought lighter weight and better fuel economy to pilots. By 2014, Rotax had produced its 50,000th 912-series engine, proving the design's popularity worldwide. The Lycoming O-235 first flew in 1942 and has been a workhorse ever since. It powers the famous Cessna 152 trainer that taught countless pilots to fly.

One engine uses a gearbox and liquid cooling. The other sticks to air cooling and direct drive. One sips regular pump gas. The other needs aviation fuel. One costs less to buy but needs special maintenance every five years. The other costs more up front but asks for simpler care.

Most pilots face this choice when building a kit plane or buying a light aircraft. The wrong pick can mean spending thousands more than needed. It can also mean dealing with maintenance headaches or fuel bills that add up fast.

This post breaks down what makes each engine tick and help you figure out which one fits your flying life.

Key Takeaways

The Lycoming O-235 works best if you want simple maintenance and don't mind higher fuel costs. The Rotax 912 saves money on fuel and weighs much less, but needs more complex care including rubber hose replacement every five years. Your choice depends on whether you value simplicity or efficiency more.

Why the Engine Matters More Than You Think

Your engine choice shapes every flight you take. It decides how much weight you can carry. It controls how much you spend at the fuel pump. It determines who can work on your plane and how often you visit the mechanic.

Think about fuel costs first. A plane burning 7 gallons per hour at $6.50 per gallon costs $45.50 for each hour in the air. Drop that to 4 gallons per hour at $4.50 per gallon, and you pay just $18 per hour. Flying 100 hours per year means the difference between $4,550 and $1,800 in fuel bills. That extra $2,750 adds up fast over the years you own the plane.

Weight makes a huge difference too. Light Sport Aircraft have a strict 1,320-pound weight limit. Save 80 pounds on the engine and you can carry an extra passenger, more fuel, or camping gear for a weekend trip. Some planes simply cannot use heavier engines and still meet weight rules.

Maintenance access matters just as much as the work itself. The Lycoming O-235 has been around since World War II. Almost any general aviation mechanic knows how to fix it. Parts arrive quickly from dozens of suppliers across the country.

The Rotax engine needs specialized training. Not every shop has a mechanic who knows Rotax systems. You might drive three hours to find qualified help. Some owners learn to do their own maintenance to solve this problem.

Engine reliability affects insurance rates and resale value. Buyers pay more for planes with well-maintained engines that have good reputations. They ask tough questions about overhaul history and maintenance records. An engine known for going past its rated life without problems becomes a selling point.

The sound and feel of the engine changes your flying experience. Some pilots love the deep rumble of a big Lycoming turning slowly. Others prefer the smooth whir of a high-speed Rotax. This subjective part cannot be measured in numbers, but it affects how much you enjoy flying.

Your engine also determines fuel availability. Many small airports sell 100LL aviation gas but not avgas or regular mogas. If your engine can burn pump gas from the station down the road, you gain flexibility and save money. If you need 100LL only, you must plan fuel stops around airports that stock it.

A Look At The Lycoming O-235 and Rotax 912

The Lycoming O-235 comes from a different era of engine design. It uses four cylinders arranged in a flat layout. Each cylinder measures 4.375 inches across and the piston moves 3.875 inches up and down. This gives 233 cubic inches of total displacement. The engine bolts directly to the propeller with no gearbox between them.

Air flows over cooling fins on each cylinder to remove heat. Two magnetos provide spark to the plugs. A carburetor mixes fuel and air in the right amounts. Everything works through mechanical parts you can see and touch. The design has changed very little since the 1940s because it works so well.

Most O-235 models make 100 to 118 horsepower. The engine turns between 2,400 and 2,800 RPM depending on the specific version. Power comes from burning fuel at a 6.75:1 to 8.5:1 compression ratio. The whole engine weighs about 246 pounds without oil or accessories.

Carburetor systems on the O-235 heat the intake to prevent ice. This happens when moisture in the air freezes inside the carburetor throat. The heating system pulls hot air from around the exhaust pipes. Pilots control this with a lever in the cockpit.

The 912 ULS takes a completely different approach. It spins at 5,800 RPM inside but uses a reduction gearbox to slow the propeller down to 2,400 RPM. This lets the small engine make good power from just 82.5 cubic inches. The gearbox ratio is 2.43:1, meaning the engine turns 2.43 times for each propeller rotation.

Rotax designed a hybrid cooling system. The cylinder heads have water jackets connected to a radiator and water pump. The cylinder barrels use air cooling with fins. This combination keeps temperatures stable in all weather conditions. An overflow bottle holds extra coolant that expands when hot.

Electronic ignition replaced old-style magnetos. Two separate systems fire the spark plugs. Each one works on its own even if the other fails. The electronic system adjusts spark timing automatically as conditions change. This helps the engine run smoothly and efficiently.

Dual Bing carburetors feed fuel to the four cylinders. Each carburetor handles two cylinders. The 912 ULS can also come with fuel injection as the 912 iS model. This adds computer control for even better fuel economy and easier starting.

The Rotax weighs just 132 pounds dry. Add the gearbox, cooling system, and other parts and you still stay well under 200 pounds installed. This light weight opens up possibilities for aircraft that cannot carry heavier engines.

Both engines use horizontally opposed cylinder layouts. This means two cylinders sit on each side facing each other. This design balances vibration naturally and fits into slim cowlings. The low profile helps with visibility over the nose.

Biggest Question: Traditional Power or Modern Efficiency?

Your flying habits determine which engine makes sense. Do you fly 150 hours per year or just 30? Do you keep the plane at a busy airport or in your garage? Can you buy mogas nearby or must you use 100LL? These questions matter more than just comparing specifications.

Fuel burn differences add up fast with frequent flying. The O-235 typically uses 6.5 gallons per hour at cruise power. The 912 burns about 4.5 gallons per hour. Flying 100 hours per year means 650 gallons versus 450 gallons. At current prices, you save roughly $1,500 to $2,000 per year on fuel with the Rotax.

But the Rotax demands a $3,000 rubber hose replacement every five years. Over 20 years of ownership, that adds $12,000 to maintenance costs. The Lycoming avoids this expense entirely. It just needs regular oil changes and valve adjustments every 100 hours.

Mechanics familiar with the O-200 and O-235 work at almost every airport in America. These engines share many parts and design features. A mechanic who knows one can fix the other. Rotax specialists exist but you might drive past five airports to reach one.

Weight sensitivity changes everything for Light Sport Aircraft builders. An 80-pound weight difference between engines means carrying full fuel plus camping gear for the weekend. Or it means meeting weight limits without cutting back on instruments and radios. Some airframe designs simply cannot use the heavier Lycoming and still fly legally.

The TBO number tells you when manufacturers recommend rebuilding the engine. Lycoming sets 2,400 hours for most O-235 models using genuine parts. Rotax rates the 912 at 2,000 hours. Both engines often run longer with good care. But certified aircraft must follow TBO rules or get special approval to extend.

Private owners flying experimental aircraft can run engines "on condition." This means watching oil analysis, compression tests, and metal particles. Good results let you fly past TBO safely. Many O-235 engines reach 3,000 hours this way. Rotax 912 engines have documented service past 4,000 hours in careful hands.

Overhaul costs hit your budget hard. Plan to spend $24,000 to $30,000 for an O-235 field overhaul. Rotax overhauls run $12,000 to $15,000. But buying a new Rotax costs only $19,000 versus $66,000 for a new Lycoming. The math often favors replacing the Rotax instead of overhauling it.

Cold weather starting differs between the two. Lycomings need good priming technique and patience. Some days require several tries before the engine catches. The electronic fuel injection on the 912 iS starts reliably in any weather. Just turn the key and it fires up. The carbureted 912 ULS falls somewhere in between.

High-altitude performance tips toward the Rotax. The gearbox lets it maintain power where the O-235 struggles. At 8,000 feet on a hot day, the Rotax still makes decent power. The Lycoming loses about 3% per thousand feet and feels sluggish.

Lycoming O-235 vs Rotax 912

Let's dive deep into what sets these two engines apart. Each one solves problems differently. Each one costs different amounts to own and operate. Understanding these differences helps you pick the right power for your plane.

Engine Design Philosophy

The Lycoming O-235 follows a design approach from the 1940s. Engineers built it to last using proven technology. Four large cylinders sit opposite each other in a flat layout. The crankshaft connects directly to the propeller shaft with no extra parts between them.

This direct drive system keeps things simple. The propeller bolts right onto the engine. Fewer parts mean fewer things that can break. The whole package weighs about 246 pounds without fluids or accessories.

Air flows over metal fins on each cylinder to remove heat. The faster you fly, the more cooling you get. Cowl flaps let you control airflow in different weather. Open them for extra cooling on hot days. Close them to warm up the engine in winter.

The Rotax 912 came from snowmobile and motorcycle racing. Rotax engineers knew how to make small engines produce big power. They built a compact four-cylinder engine that spins very fast. The secret sits in the gearbox bolted to the front.

Inside the engine, pistons zip up and down at 5,800 RPM. That speed would destroy a propeller in seconds. The reduction gearbox slows things down to a smooth 2,400 RPM at the prop. This lets the small engine make excellent power from just 82.5 cubic inches.

Rotax mixed cooling systems to get the best of both worlds. Water jackets surround the cylinder heads to control temperature precisely. Cooling fins on the cylinder barrels use air cooling. A radiator and electric fan handle the liquid cooling part.

Power Output and Performance

Raw horsepower numbers tell only part of the story. The O-235 makes 100 to 118 HP depending on which version you buy. Most pilots fly behind the 115 HP model. Peak power comes at 2,800 RPM.

The Rotax engine comes in two main power levels. The 912 UL makes 80 HP for ultralight and light sport use. The 912 ULS bumps that to 100 HP with a larger displacement and higher compression. Both versions spin to 5,800 RPM before the gearbox cuts that to propeller speed.

Power delivery feels different between the two. The Lycoming pulls smoothly from idle to redline. You feel steady acceleration without sudden jumps. The Rotax delivers power with a different character. The electronic ignition helps it run smoothly across the entire power range.

Altitude affects these engines differently. The O-235 loses about 3% of its power for every thousand feet you climb. The Rotax handles altitude better thanks to its high RPM design. The gearbox lets the engine spin faster to make up for thin air.

Fuel Systems and Economy

The O-235 uses a Marvel-Schebler carburetor mounted under the engine. Warm oil from the sump heats the carburetor body. This helps prevent ice from forming inside the carburetor throat. Float bowls inside the carburetor store fuel at just the right level. A mixture control lets you lean the mixture for best economy.

The 912 ULS mounts two Bing carburetors up top between the cylinders. Each carburetor feeds two cylinders. Fuel injection comes standard on the 912 iS model. Two electronic control units manage everything. Sensors measure air pressure, temperature, and throttle position. The computer calculates exactly how much fuel each cylinder needs.

This electronic fuel injection system eliminates carburetor ice worries. It also improves fuel economy by 20% to 30% over the carbureted version. Cold starts become easy. Just turn the key and the engine fires right up.

Fuel burn differences really show up on long trips. The O-235 typically uses 6.5 gallons per hour at 65% cruise power. The Rotax sips fuel compared to the Lycoming. Expect 4.5 gallons per hour from the 912 ULS at cruise settings. The fuel-injected 912 iS does even better at 3.5 to 4.0 gallons per hour.

Fuel type makes a huge difference in operating costs. The O-235 burns only 100LL avgas. This blue-tinted aviation gasoline costs $6 to $8 per gallon at most airports. The Rotax runs happily on 91 octane mogas from any gas station. Premium unleaded costs $4 to $5 per gallon. This fuel flexibility saves real money over thousands of flying hours.

Ignition Systems Compared

The O-235 uses dual magnetos for spark. Each magneto is a self-contained electrical generator. It makes electricity from a spinning magnet inside. Two separate magnetos mean the engine keeps running even if one fails. Magnetos need inspection every 500 hours.

Electronic ignition on the Rotax works completely differently. Sensors detect the crankshaft position. Electronic control units calculate when to fire each spark plug. Solid-state electronics trigger the spark with perfect timing. No mechanical parts wear out or need adjustment.

Two completely independent ignition systems provide backup safety. Each one has its own sensors and control unit. If one system fails completely, the other keeps the engine running. Spark timing adjusts automatically on the Rotax. The computer makes thousands of tiny adjustments per minute.

Lycoming Engine Overhaul Cost and Time to Expect

Planning for lycoming engine overhaul costs helps you budget for aircraft engine ownership. The Lycoming O-235 needs a major rebuild at its TBO of 2,400 hours. Many engines reach TBO and keep running fine. But certified aircraft must follow TBO rules.

Breaking Down Overhaul Expenses

Field overhauls from local shops typically cost $24,000 to $30,000. This price covers labor and most common parts. Shops quote these prices assuming your core engine has serviceable cylinders and crankshaft. If they find cracks or excessive wear, costs jump quickly.

The crankshaft inspection alone costs $800 to $1,200. Crankshafts with deep scratches need grinding. First grind costs $2,000 to $3,000. A new crankshaft costs $7,000 to $9,000.

Cylinders make up the biggest variable cost. Good cylinders with passing compression can be reused. Expect $800 to $1,200 per cylinder for this work. New cylinders run $2,500 to $3,500 each. Four new cylinders add $10,000 to $14,000 to your overhaul bill.

Factory overhauls cost more but come with benefits. Lycoming charges $38,000 to $50,000 for a factory overhauled exchange. The exchange includes a warranty and zero-time logbook. Brand new engines from Lycoming start at $66,000.

Time Frame for Overhaul Work

Plan for your plane to be down for eight to twelve weeks. Busy shops might take longer during peak season. Spring and summer fill up the shop calendars.

Week one typically involves teardown and inspection. Weeks two through four cover parts ordering and machining work. Weeks five through eight focus on reassembly. Final weeks include accessory installation and test running.

Most shops want 50% payment when you drop off the engine. Another 25% comes due at the midpoint. Final payment of 25% happens when you pick up the completed engine.

When to Overhaul an Aircraft Engine: Hours, Signs, and Safety Tips

Smart pilots watch for warning signs long before TBO arrives when to overhaul an aircraft engine. The aircraft engine talks to you through gauges, sounds, and how it runs. Learning to read these signals prevents surprise failures.

Hour-Based Guidelines

Most general aviation engines reach TBO between 1,800 and 2,400 hours. The similar O-200 Continental engine rates 1,800 to 2,000 hours depending on the model. Calendar time matters just as much as hours. Lycoming recommends overhaul at 12 years even with low time.

Private owners can extend time past TBO if they fly experimentally registered aircraft. This "on condition" approach requires careful monitoring. Oil analysis every 25 hours shows metal content. Compression tests measure cylinder health. Good compression readings stay above 70/80 on all cylinders.

Warning Signs That Cannot Wait

Metal particles in the oil filter scream for immediate attention. Drain the oil and cut open the filter. Large chunks or lots of fine metal mean something is breaking apart inside. Ground the plane immediately.

Unusual sounds point to specific problems. Clicking noises often mean valve adjustment issues. Knocking sounds suggest bearing problems. Any new sound that persists needs investigation before the next flight.

Oil consumption tells you about ring and cylinder health. Sudden increases in oil use mean rings are worn or cylinder walls are damaged. Oil pressure changes indicate bearing wear. Temperature increases beyond normal range signal cooling or mixture problems.

Making the Overhaul Decision

Compare overhaul costs to aircraft value before deciding. A $70,000 airplane justifies a $28,000 overhaul. A $30,000 airplane might not. Consider your flying plans too. Will you keep this plane for five more years? Then overhaul makes sense.

Get multiple quotes from different overhaul shops. Prices vary by thousands of dollars. Ask exactly what the quote covers. Get it in writing with a not-to-exceed price. Plan the timing to minimize downtime. Winter months often work best.

Quick Side-by-Side Summary

Power and Performance

The Lycoming O-235 delivers 100 to 118 horsepower at 2,600 to 2,800 RPM. Most common versions make 115 HP at 2,800 RPM. The engine pulls steadily from idle to redline without sudden changes. Power feels smooth and predictable.

The Rotax 912 produces 80 HP (UL model) or 100 HP (ULS/S models). The engine spins to 5,800 RPM internally but the propeller only sees 2,400 RPM through the gearbox. Some pilots notice a buzzing feel at high power settings.

Weight Comparison

• Lycoming O-235: 246-252 pounds dry, about 280-300 pounds installed
• Rotax 912: 132 pounds dry, about 170-200 pounds installed
• Weight savings: 80-100 pounds favoring Rotax

Fuel Economy Numbers

Lycoming O-235 burns:

• Full power: 10.7 gallons per hour
• Cruise (75%): 7.3 gallons per hour
• Economy (65%): 5.8 gallons per hour

Rotax 912 burns:

• Full power: 5.8 gallons per hour
• Cruise: 4.5 gallons per hour
• Economy: 3.5 gallons per hour

Cooling Systems

Lycoming uses pure air cooling. Metal fins on the cylinders transfer heat to passing air. Baffles direct airflow for even cooling. Simple and reliable but less precise.

Rotax combines liquid-cooled heads with air-cooled cylinders. A radiator, water pump, and hoses circulate coolant. More complex but maintains exact temperatures.

Maintenance Schedules

Lycoming O-235 requires:

• Oil changes every 50 hours
• Valve adjustments every 100 hours
• Annual inspections
• TBO at 2,000-2,400 hours or 12 years

Rotax 912 requires:

• Oil changes every 50-100 hours
• 5-year rubber hose replacement
• Gearbox inspection at 600 and 1,000 hours
• Carburetor overhaul every 200 hours (recommended)
• TBO at 2,000 hours or 15 years

Cost Breakdown

Purchase prices:

• Lycoming O-235 overhauled: $24,000-$30,000
• Lycoming O-235 new: $66,000
• Rotax 912 overhauled: $12,000-$15,000
• Rotax 912 new: $16,000-$19,000

Operating costs (100 hours/year):

• Lycoming fuel: $4,225 (650 gallons × $6.50)
• Rotax fuel: $2,025 (450 gallons × $4.50)
• Annual savings: $2,200

How to Choose the Right Engine for Your Flying Style

Match the engine to your actual flying patterns. A plane that sits in a hangar 11 months per year needs different priorities than one flown weekly. Be honest about how much you really fly before deciding.

Choose the Lycoming O-235 if you:

• Fly less than 50 hours per year
• Keep your plane at an airport without mogas
• Want the simplest possible maintenance
• Work with mechanics unfamiliar with Rotax
• Build an aircraft with plenty of weight margin
• Prefer traditional engine sounds and feel
• Plan to do valve adjustments yourself
• Live far from Rotax-trained shops

Choose the Rotax 912 if you:

• Fly 100 or more hours per year
• Have easy access to 91 octane mogas
• Build a Light Sport Aircraft near weight limits
• Work with a Rotax specialist mechanic
• Want the lowest fuel costs possible
• Enjoy modern engine technology
• Fly from high-altitude airports often
• Can handle the 5-year rubber maintenance

Budget considerations matter too. Calculate total ownership costs over 10 years:

Lycoming O-235 (100 hours/year):

• One overhaul: $27,000
• Fuel (6,500 gallons): $42,250
• Routine maintenance: $8,000
• Total: $77,250

Rotax 912 (100 hours/year):

• One overhaul: $13,500
• Two 5-year services: $7,000
• Fuel (4,500 gallons): $20,250
• Routine maintenance: $9,000
• Total: $49,750

The Rotax saves about $27,500 over 10 years if you fly 100 hours annually and have mogas access. Fly less and the savings shrink. Fly more and the savings grow.

Resale value follows different rules. Buyers pay premium prices for low-time engines with good logs. A Lycoming with 400 hours since overhaul sells easily. A Rotax at the same point also attracts buyers but may take longer to sell.

Some builders pick engines based on their specific airframe. Zenith designs work great with either engine. But the Zenith 750 needs the lighter Rotax to meet Light Sport weight limits. The same plane with a Lycoming ends up too heavy for the LSA category.

Think about your long-term plans. Will you keep this plane for 20 years or sell it in five? Do you plan to take long cross-country trips or just fly around the local area? Will you move to a different airport with different fuel options?

Insurance companies sometimes charge different rates based on engine choice. Ask your broker for quotes with both engines. The difference might surprise you.

Conclusion

The Lycoming O-235 and Rotax 912 both serve pilots well in different situations. The Lycoming brings proven reliability and simple maintenance. The Rotax delivers modern efficiency and light weight. Your flying style and budget determine which engine makes sense.

Heavy flyers who burn lots of fuel save thousands per year with the Rotax. Weekend pilots who fly 30 hours annually might prefer the Lycoming's simpler upkeep. Light Sport builders often need the Rotax to meet weight limits. Experimental builders with heavier airframes can choose either one.

Both engines will carry you safely through the sky for many years. Both have passionate supporters who love their choice. Make your pick based on facts that match your real flying life. Then enjoy the satisfaction of flying behind an engine that fits your needs perfectly.

Want more expert guidance on aircraft ownership and maintenance? Visit Flying 411 for detailed articles, buying guides, and technical advice that helps you make smart aviation decisions. We cover everything from engine comparisons to pre-purchase inspections.

Frequently Asked Questions

Can I convert my Cessna 152 from Lycoming O-235 to Rotax 912?

Some European countries have approved this conversion under EASA rules, but it remains unavailable in the United States. The modification requires extensive changes to the engine mount, cowling, cooling system, and aircraft documentation.

Do Rotax 912 engines really last longer than their TBO?

Many Rotax 912 engines exceed 2,000 hours when properly maintained and flown regularly. Non-commercial operators can continue running them "on condition" with careful monitoring. Some engines reach 4,000 hours with clean oil analysis and good compression.

Which engine handles ethanol fuel better?

The Rotax 912 tolerates up to 10% ethanol in mogas without modification. The Lycoming O-235 requires fuel system components rated for ethanol use and careful monitoring. Most operators avoid ethanol blends when possible to prevent corrosion issues.

What causes the 5-year rubber replacement on Rotax engines?

Rubber hoses degrade from heat, vibration, and chemical exposure over time. They can crack internally before showing external damage. Rotax requires replacement to prevent coolant, fuel, or oil leaks that could cause engine failure or fire.

Can I do my own maintenance on either engine?

Experimental aircraft owners can maintain either engine themselves with proper training. Light Sport Aircraft must use certificated mechanics for most work. The Lycoming requires basic tools and skills. The Rotax needs specialized knowledge and some special tools.