Technical Data

Automotive Engines Vs. Certified  Aircraft Engines

The purpose of this article is to set straight some common misconceptions many people have about automotive engines as applied to aircraft propulsion and answer some equally common questions about package performance and weight. This article is based upon on our experience in the field of aviation

Automotive Engines Will Not Take Continuous High Rpm Use

This is the most common misconception put forth by lay, anti-auto power people and is total nonsense. They often go on to say that auto engines were designed for low rpm operation and 15-30 hp as required to cruise a car at 70 mph. This simplistic, flawed reasoning is completely unsupported by facts. When asked to supply facts to support their contention on various forums since 2003, not one person has ever done so.

Examine the facts at hand which completely contradict this notion.

Auto engines today are designed and routinely tested to higher standards than certified aircraft engine requirements. The FAA only requires 100 hours of full throttle, full rpm for certified engines and another 50 hours at 75-100% power, 50 hours of which are required to be at redline oil and cylinder head temperatures. Most auto engine manufacturers today do a minimum validation of 200 hours of WOT at rated hp rpm and some as much as 1200 hours. In addition to this test, they perform cold weather testing to the tune of 1000+ cycles of cold soaking the engine to 0F and immediately taking the engine to WOT and high rpm until coolant reaches 240F. While the engine is still hot, 0F coolant is pumped into the engine until the block achieves 0F and the test is repeated- over 1000 times. Additional tests often include idle testing to 2000 hours with oil temperatures of 260F+ and transmission validation where the engine is cycled from low rpm to shift point rpm at WOT while the transmission is shifted up and down for up to 1600 hours. Not just one engine is put through these tests- dozens are. Wear rates are noted and obviously failures are not acceptable before release of the design.

Specific output is the measure of hp produced per unit displacement of an engine. It is a measure of relative strength and efficiency of an engine design. One 2.5L turbocharged Subaru drag engine has been dynoed at 960hp and completed over 60 passes at this hp level. This equates to 384hp/ liter and all with stock crankshaft and block. This would be 2300hp on a Lycoming! How long would a Lycoming O-360 last at even half this power level? Probably less than 10 seconds. The Subaru has gone over 600 seconds at this power level. This proves the tremendous strength of the Subaru boxer design compared to aircraft engines. A detuned version of the 2.5liter STI engine operating at 200-250 hp and 4500 rpm is obviously nowhere near any critical strength limits.

In Europe, where speed limits are higher (or none), cars are often cruised at very high speeds for a good portion of their lives. This demands high rpm, high hp operation. We just don't see a high percentage of smoking wrecks on the side of the Autobahn or Auto Strada. Engines today must be designed to withstand this kind of use or there would be a lot of warranty claims. Many showroom stock race cars also prove every day how robust OE engines are, spending their lives between 5000 rpm and the rev limiter for many, many hours.


Are auto engines competitive on a hp/ weight basis with popular air cooled aircraft engines? The short answer is maybe. Naturally aspirated auto engines with PSRUs and radiators from firewall forward suppliers have generally not able to achieve the power to weight ratios of a typical Lycoming engine to date. Turbocharged auto engines can be more competitive. Often hindering auto conversions has been the mismatching of PSRU ratios which has not allowed these engines to achieve power peak rpm. This is critically important with naturally aspirated engines.

It should be stressed than auto engine hp can be lower than factory ratings on naturally aspirated versions employing variable induction tract tuning and variable valve timing and lift if these features have been disabled or modified. Careful modification of the induction and camshaft systems should carried out to negate these possible effects and ensure full rated hp potential.

Cooling and Drag

 Air cooled advocates often like to say that liquid cooled engines exhibit higher cooling drag than air cooled installations. This is again another "feeling" they have with no facts to back up the statement. The simplistic reasoning usually offered is that because air cooled engines operate at higher temperatures, Delta T is higher so less mass flow is required. What they fail to consider is the mechanism of heat transfer and the pressure (energy) loss is very different between air cooled cylinders and coolant radiators. A modern aluminum radiator is many times more efficient at heat transfer than aircraft cooling fins per unit area or unit volume. Radiators also can be placed in a very efficient divergent/ convergent duct to maximize heat transfer, minimize core drag and exit the air near free stream velocity for minimal drag with no turning of the cooling air. Air cooled fins by comparison on opposed engines, force the air to turn sharply over 180 degrees past many obstructions to flow, creating turbulence and drag.

It must be said that many experimental aircraft have had relatively inefficient radiator setups and consequent cooling problems. Today, we understand what is required to get good cooling performance with a minimal drag penalty. Designs which continue to place rads in the cowling cheeks with short ducts and poor pressure recovery mixed with inefficient exit paths likely do have higher drag than a well designed air cooled installation. Dedicated ducts mounting proper radiators with oval tubes and proper fin density are required for best performance.

Propeller Speed Reduction Units (PSRUs)

PRSUs are required in most cases to allow the engine to spin up to its power peak rpm while allowing the prop to spin at its most efficient rpm. This is the other part of liquid cooled installations often attacked by air cooled advocates and sometimes rightly so. Many PSRU designs have had less than stellar longevity. This is generally the result of inadequate design and especially testing. Of considerable concern is torsional vibration (TV) problems causing failures of the gears, bearings and shafts of the gearbox and sometimes propeller. Few PSRUs have had proper TV analysis or instrumented testing done on them. Some later designs such as the D2, SA and T4  have Centaflex  coupling devices between the crankshaft and gear set have shown unquestionable  reliability. Lightweight flywheels aggravates  TV problems. There is no reason that PSRUs with proper design and validation should not be as reliable as modern manual transmissions on cars. Gearboxes have been used on hundreds of thousands of aero engines and are very reliable when properly designed. PSRUs do add weight to the installation. Most units weigh between 25 and 50 pounds for engines in the 150-300 hp range.

PSRUs have losses inherent in their gear sets or belts. Lay people have often speculated that these losses are up to 40 hp in the case of a 200 hp class drive. This is absurd as it would represent about 30,000 watts being dissipated as heat. If this was in fact true, the case or belts would melt in just a few minutes. Typical losses for single mesh spur and helical gears is around 2-2.5%. HTD belts run at 3-4%. Twin mesh helical gear sets would then have perhaps a 6% loss as worst case including bearing losses.


Costs vary widely depending on whether the package is DIY or from a commercial vendor. In the case of DIY installations, costs may be 1/6 to 1/2 of what a typical air cooled engine costs. This of course does not take into account the many hours spent designing and building the various components needed nor the testing and modification which invariably follows. Commercial packages generally are about the same cost as an air cooled engine. Where the savings come in is at overhaul time. The core auto engine is a fraction of the cost of the aircraft engine and rebuild parts are much cheaper. In some cases, a complete new engine is under $5000, lowtime used engines are under $1000 as are generally the parts and labor required to do an overhaul. This experience has been born out but many high time gyro operators using auto engine for training where total operating costs for fuel and overhaul reserve is under $25/hr.

Package Reliability

This is the grey area. While the reliability of the popular Subaru engine core in proper repair has been shown to be as good or better than the Lycoming core, inflight reliability of auto engine conversions is dependent on several supporting systems like fuel, PSRU and electrical. Most power losses are attributed to problems with the supporting systems, not the core engine. This is where careful design decisions pay benefits and poor practices often cause engine shutdowns. Until the systems used are proven to be reliable through extensive flight testing, reliability on par with a typical Lycoming is often in question. Shared information on various forums and learning from both good and bad experiences will result in better designs which should improve overall reliability. Auto engines are not for everyone. Some people have had excellent results with them, others dismal. Some owners just want to fly while other like the experimental experience and difference an auto engine provides them. Auto engine conversions open up a second choice for those so inclined.


There are a couple of ways by which car manufacturer's vary the valve timing. The most well-known system is the VTEC which is used on some of the Honda engines. Other systems which some of you might not have heard of are:

 * VarioCam/VarioCam Plus which is used on some of the Porsche engines,

 * MIVEC(Mitsubishi Innovative Valve timing and lift Electronic Control) which is used on the Mitsubishi engines,

 * VVT-i(Variable Valve Timing with Intelligence) and now VVTL-i (Variable Valve Timing and Lift with Intelligence) which is being used on the current Toyota and some Lexus engines,

 * VVL(Variable Valve Lift) which is used on the Nissan engines and also featured in the 350Z is the CVTCS (Continuously Variable Valve Timing System)

 * VANOS(Variable OnckenwellenSteuerung) which is used in the BMW engines and also the Double VANOS system on the new 3 Series and they are many more similar systems used by manufacturers such as Ford, Lamborghini and even Ferrari.

What do all these Vs have in common? Well, in case you don't already know (or haven't yet guessed despite the monster hint in the article's title), the V stands for valves or, more specifically, variable valve timing.

Before you can appreciate how important valve timing is, you have to understand how it relates to engine operation. Remember that an engine is basically a glorified air pump and, as such, the most effective way to increase horsepower and/or efficiency is to increase an engine's ability to process air. There are a number of ways to do this that range from altering the exhaust system to upgrading the fuel system to installing a less-restrictive air filter. Since an engine's valves play a major role in how air gets in and out of the combustion chamber, it makes sense to focus on them when looking to increase horsepower and efficiency.

This is exactly what Honda, Toyota and BMW and quite a number of other manufacturer's have done in recent years. By using advanced systems to alter the opening and closing of engine valves, they have created more powerful and clean burning engines that require less fuel and are relatively small in displacement.

 Before we take a look at each of these variable valve-timing systems, let's rehash how valve timing normally works. Until recently, a manufacturer used one or more camshafts (plus some pushrods, lifters and rocker arms) to open and close an engine's valves. The camshaft/camshafts was turned by a timing chain that connected to the crankshaft. As engine rpm's rose and fell, the crankshaft and camshaft would turn faster or slower to keep valve timing relatively close to what was needed for engine operation. 

Unfortunately, the dynamics of airflow through a combustion chamber change radically between 2,000 rpm and 6,000 rpm. Despite the manufacturer's best efforts, there was just no way to maximize valve timing for high and low rpm with a simple crankshaft-driven valve train. Instead, engineers had to develop a "compromise" system that would allow an engine to start and run when pulling out of the driveway but also allow for strong acceleration and highway cruising at 70+ mph. Obviously, they were successful. However, because of the "compromise" nature of standard valve train systems, few engines were ever in their "sweet zone," which resulted in wasted fuel and reduced performance.

Variable valve timing has changed all that. By coming up with a way to alter valve timing between high and low rpm's, Honda, Toyota and BMW and many more manufacturer's can now tune valve operation for optimum performance and efficiency throughout the entire rev range.

 Honda was the first to offer what it called VTEC in its Acura-badged performance models like the Integra GS-R and NSX (it has since worked its way into the Prelude and even the lowly Civic). VTEC stands for Variable Valve Timing and Lift Electronic Control. It basically uses two sets of camshaft profiles-one for low and mid-range rpm and one for high rpm operation. An electronic switch shifts between the two profiles at a specific rpm to increase peak horsepower and improve torque. As a VTEC driver, you can both hear and feel the change when the VTEC "kicks in" at higher rpm levels to improve performance. While this system does not offer continuously variable valve timing, it can make the most of high rpm operation while still providing solid drivability at lower rpm levels. Honda is already working on a three-step VTEC system that will further improve performance and efficiency across the engine rpm range.

 The camshaft in a pushrod engine is often driven by gears or a short chain. Gear-drives are generally less prone to breakage than belt drives, which are often found in overhead cam engines.

Toyota saw the success Honda was having with VTEC (from both a functional and marketing standpoint) but decided to go a different route. Instead of the on/off system that VTEC employs, Toyota decided it wanted a continuously variable system that would maximize valve timing throughout the rpm range. Dubbed VVTi for Variable Valve Timing with intelligence (Is this a dig at Honda, suggesting their system isn't intelligent?), Toyota uses a hydraulic rather than mechanical system to alter the intake cam's phasing. The main difference from VTEC is that VVTi maintains the same cam profile and alters only when the valves open and close in relation to engine speed. Also, this system works only on the intake valve while VTEC has two settings for the intake and the exhaust valves, which makes for a more dramatic gain in peak power than VVTi can claim.

Several other manufacturers, including Ford, Lamborghini and Porsche have jumped on the cam phasing bandwagon because it is a relatively cheap method of increasing horsepower, torque and efficiency. BMW has also used a cam phasing system, called VANOS (Variable OnckenwellenSteuerung) for several years. Like the other manufacturers, this system only affected the intake cams. But, as of 1999, BMW is offering its Double VANOS system on the new 3 Series. As you might have guessed, Double VANOS manipulates both the intake and exhaust camshafts to provide efficient operation at all rpm's. This helps the new 328i, equipped with a 2.8-liter inline six, develop 193 peak horsepower and 206 pound-feet of torque. More impressive than the peak numbers, however, is the broad range of useable power that goes along with this system.

Several engine manufacturers are experimenting with systems that would allow infinite variability in valve timing. For example, imagine that each valve had a solenoid on it that could open and close the valve using computer control rather than relying on a camshaft. With this type of system, you would get maximum engine performance at every RPM. Something to look forward to in the future!

To close these series of articles on camshafts, you can see that as the benefits of variable valve timing used on cams become more apparent to both consumers and manufacturers, you can expect to see it on just about every vehicle sold in the world. I suspect that in five years, variable valve timing will be like ABS or side-impact beams: only really cheap cars won't have it.

Electronic Fuel Injection  vs. Carburetors

Let me start off by saying that the choice of EFI or carb in experimental aviation is primarily based on preference and possibly cost on some lower HP engines. If you like a carburetor  and it works for you, that's great, you'll probably stick with it. This article is in rebuttal to the misinformation presented by the Carburetors lovers

People with limited experience with EFI in aircraft and automotive worlds and therefore will make many erroneous statements about it. Most of the arguments have no basis in fact or logic and fly in the face of modern developments in both the auto and light aero engine industry today. I'll address each point as it appears in the article:

Power. EFI will often produce slightly more power than a carb. This is because cylinder to cylinder mixture distribution is better than most carbs can offer and secondly, no restrictive venturi is required with EFI. More mass flow through the engine due to less restriction = more power.

Fuel Consumption. Again, due to superior mixture distribution and the precision in which fuel can be metered under all conditions, fuel injection has been shown to reduce fuel consumption markedly. On some Continental O-200 engines, which have very poor induction setups, customers have reported fuel flows were reduced up to 20% over the carb setup for the same TAS.

Carb Heat. Obviously with fuel being injected into the hot port at the back of the valve and without fuel being vaporized in the venturi like in a carb setup, there is no possibility of carb ice and no need for carb heat. Carb heat reduces mass flow and thus power when used as well.

Reliability. The reliability of EFI is well proven in the automotive world (trillions of hours since 1967) and we use the same OE fuel hardware in most cases. It's the single most important technology introduced in that industry which drastically reduced maintenance and improved reliability. I used to fix and adjust carbs for a living long ago and that work pretty much went away entirely when EFI use became widespread. Most EFI cars require no maintenance with the fuel system for the entire life of the vehicle. No production cars have had carbs for over 20 years. EFI is also proven in aircraft. Our systems alone have accumulated over 250,000 flight hours to date.

Reliability according to the users, has been superior to their old carburetors setups and none of them would return to carbs. Fundamentally, there is very little difference between EFI hardware for automotive and experimental aviation. We take the reliable and well proven OEM parts from the auto industry and get the same results in aviation. We've been doing it for over 10 years with installations on 6 different brands of engines fitted to aircraft.

Fuel injection usually makes more power than a carburetor  setup. I've put injection on dozens of carbureted  engines and run Dyno  test  before and after. I've never seen a case where the EFI lost power and seen some significant gains at lower rpms due to superior atomization and equal distribution between cylinders. Cooling from fuel vaporization in a carburetor manifold is better than EFI? Incorrect,the same mass of fuel is being vaporized so charge cooling is identical, just occurs at different places in the engine- port, in the case of MPFI.

Without a carburetor , there is no icing . Since only air flows by the throttle body and there is no venturi, there is no local cooling to condense water vapor to form induction ice with EFI. Impact and airframe icing is possible on any aircraft flown in IFR conditions, obviously you'd want a second induction air source in any aircraft, carb or EFI if you operate there.

“There is always two paths to the hill” - Chinese Proverb

Auto vs. Certified

People who are using or planning to use automotive engines to power their homebuilts often hear lots of negative things about their decision from those on the other side of the fence in the certified engine world. The reliability of the lower power certified engines is well documented. They are pretty reliable pieces with few suffering catastrophic failure. It is equally true that these engines frequently don't make it to TBO without some top end work at least. We read of continuing problems with brand new certified engines suffering from cracked cranks, barrels, heads, camshaft and wrist pin problems with only a couple of hundred hours on them. There is the occasional rod out through the side or an exploding jug. The higher hp six cylinder engines experience cracking heads and cases on a surprisingly regular basis. For engine designs with over 40 years of evolution and the prices being charged, many find this unacceptable in this age of other viable alternatives. The choice of auto engines is mainly driven by economics and to a lesser degree by performance concerns. Something just seems wrong when a certified engine costs more than the whole airframe it is to power.

For people with no experience or background in engine design and building, the certified route is often the best. Auto conversions invariably get a bad name caused by failures related improper assembly, choice of components, improper fuel and spark management and especially modification of critical components without sufficient knowledge. Unless you have a good understanding of engine mechanics and have actual experience building engines, I would not recommend that you practice your skills on an auto conversion to power an aircraft. I am an advocate of reasonable operational limits and hp levels as well. Super high boost pressures and rpm will lower engine life in most cases.

Many lay people often point out that automotive engines are not designed for continuous high output applications and will blow up when installed in an aircraft. This view is a result of complete ignorance in my opinion and is not supported by any credible facts. Modern automotive engines make use of the latest advances in computer design and modeling to optimize the design of everything from port flow, port resonance tuning, combustion chamber characteristics, vibrational node analysis and mechanical stresses. Machining and metallurgy technology is far superior to the old days when the air cooled, flat engines were developed. Technology has indeed progressed on automotive engines in the last 40 years.

Automotive engines are routinely tested during development at full power and maximum rpm for periods of up to 1200 hours on a dynomometer. These engines must be able to withstand whatever stresses a customer might inflict on them such as flat out cruising on the autobahn or endurance racing, without failure. Manufacturer's limits are conservative to guarantee longevity and reliability. The engineering and capital investment that goes into a new engine release dwarfs any similar development by any piston aircraft engine manufacturer. The testing and validation methods FAR exceed those required on piston aircraft engines. In Europe, cars are routinely cruised at speeds (RPMs and load) 50-100% higher than what we see in North America with no ill effects in life span. This is real world, long term hard use.

Just one example of the demonstrated real world reliability on the popular Subaru EJ22 engine was the 1989 record set by 3 Legacy's at an Arizona test track. These cars were run flat out for 17 days straight without failure at an average speed of over 138 mph. Similar records have been set by Saab and Chevrolet. How many people reading this article think that most aircraft engines would survive at 100% takeoff power for 400 hours? Subaru now offers the production 2.5L turbo STI rated at 300 hp, With the popularity of showroom stock endurance racing in the last decade, we get to see just how good the design and engineering is on modern cars. Thousands of Hondas, Toyotas, BMW , Suzuki, Oldsmobiles, Chevrolets, Mitsubishis, VWs etc. are mercilessly flogged to the rev limiter at full throttle for hundreds of hours between rebuilds. A very small fraction of these ever suffer a serious failure. Aircraft use does not put this kind of cyclic stress on an engine, being a constant load, relatively low rpm situation. Most modern car engines outlast the chassis without ever being removed. This performance can be equated into lifespans of between 4000 and 8000 hours. Even operating at 75% of maximum power and rpm limits, it is reasonable to expect a TBO of at least 1000 hours in aircraft use.

From all of the people that we know with about Subaru aircraft engines, I can conclude that mechanically, they are more reliable than certified aircraft engines. In various aircraft uses, the EJ22/25 has accumulated in the tens of thousands of flight hours with no basic mechanical failures attributable to the engine design. Any failures that I have heard of were due to improper modifications made to the engine or supporting systems or unreasonable operating limits. Rotary Airforce has sold over 500 gyros powered by EJ22/25s in the last decade, accumulating an estimated 100,000 flight hours. Pretty well proven.

Many popular domestic 4, 6 and 8 cylinder auto engines are offered in slightly modified marine versions for motivating boats. These engines operate in the same kind of high continuous power and rpm environment as an aircraft engine and are very reliable with TBOs of between 400 and 2000 hours on the popular units, some of which develop in excess of 1.5 hp/cubic inch.

If we look at the latest stock block based NASCAR V8 engines, we can see amazing performance and reliability. These engines produce well over 600 hp at rpm levels in excess of 9000 with excellent reliability in 500 mile races. Taking a version of this engine and derating it down to 300 hp and 4500 rpm would extend its life into the many hundreds of hours as the stresses involved are far, far below any critical limits.

I don't advocate that some person with no engine building experience should go to the junk yard and slap the engine directly into his plane or even rebuild it and do the same thing. This has the potential for serious disappointment and even tragedy. I suggest taking the scientific approach to choice of the engine, complete teardown and inspection by dye penetrant and magnafluxed, modification or replacement of any marginal components such as pistons or rods and careful reassembly of components to proper tolerances. Attention to supporting systems such as lubrication, fuel/spark and cooling are equally important to ensure reliability. Subtle changes to even a seemingly insignificant part or external assembly can sometimes have serious consequences. I go by the theory that the auto engineers are pretty smart people and that something should not be changed from stock unless you can substitute a better and stronger part.

If you have no engine building experience but want to enjoy the advantages of auto power, I would recommend contacting a reputable company which specializes in converting engines. Check out references from their customers and don't be the first one to plunk down your money. If you are on a budget, a clean, low mileage Subaru from a wreck is often more reliable than one that has been improperly rebuilt. Many people have had excellent success with engines like this if operated within reasonable limits. Just be aware of the possible pitfalls.

With the explosion of CNC machining in the last decade, comes the availability of a vast array of super strong, forged and billet components for many popular engines. These are reasonably priced so there is no excuse for using a marginal factory part any more.

A well engineered and assembled automotive conversion will no doubt deliver lower costs, smoother operation and similar fuel flows. A poorly executed one will likely leave you in the trees at the end of the runway. We have many satisfied pilots flying auto engines reliably in their homebuilt worldwide.

While the non-believers and ill-informed sprout nonsense about the inability of auto engines to sustain long periods of high rpm/ high power operation, they seem to have their heads buried in the sand about the real problems with certified air cooled aircraft engines. Seemingly in their perfect world, all make it to TBO without much work along the way. This is nonsense in the real world as many engines require cylinder, piston and valve replacement long before TBO is reached.

As an addition to the thoughts above, I would just like to remind those "certified people" about some of the problems with certified engines in recent years: Lycoming piston pin plug problems prompted the FAA to issue a SAIB with regards to this matter. Soon after, problems with TCM engines surfaced in a series of 7 crankshaft failures on 470, 520 and 550 direct drive engines. The FAA has issued an AD on this matter. As many as 3200 engines were affected, manufactured in 1998. All seven failures occurred on cranks with less than 175 operating hours on them. You can draw your own conclusions from these problems.

Exactly what are you getting for $30,000+ dollars? Here is an engine family with 40+ years of development behind it, still suffering from premature failures, with the major tooling costs long ago paid for. Simply amazing. If someone in the automotive industry produced products like this, the governments of the world and consumer groups would run them out of business assuming that they were even able to survive selling products like this. I am waiting for the day (coming soon) when some newer designs make these dinosaurs extinct.

"Aircraft certified" is a term that many people associate with top quality products and as such think that the price that they pay may be justified. I seriously doubt if the quality is any better or even as good as many off the shelf auto parts. Someone has paid some money to complete the certification tests probably decades ago and a nice paper trail is provided to track defective parts to the source, but "certified" is no guarantee of quality, only a guarantee of high cost.

The Reality of Auto Power

As of 2007, we see increasing numbers of experimental aircraft being fitted with auto conversions as never before. I can only assume that most pilots have good reasons for this switch and that they have not experienced the utopianism that certified advocates purport as reality. Eggenfellner Aircraft have now sold hundreds of Subaru engine packages for aircraft. There are dozens of Wankel powered aircraft flying. There are over 700 production Subaru powered gyrocopters flying. There are many hundreds of other one off auto powered aircraft flying today and there are more companies than ever offering engine packages, parts, reduction drives and propellers to consumers. Operating costs on most of these conversions HAS proven to be a fraction of the cost of a traditional air cooled engines. Overhaul costs are MUCH lower. Engine reliability in most cases has also been good. To be fair, auto engines are not issued ADs when something goes wrong repeatedly however few mechanical issues remain by the time testing and release are accomplished through the OEMs, at least on the engine popularly used in aircraft such as the Subaru, Chevrolet LS, Mazda rotary and Suzuki families. The auto engine manufacturers have generally had far superior design, validation, testing and QC procedures compared to Lycoming and TCM. In fact the big two are now using much of what was originally developed by the auto OEMs 20 years ago!

Challenges remain:

Cooling has been a problem for many installations although with information shared today over the internet, solutions are being worked out and good workable practices are becoming more commonly available. Some quantified experimentation is being done on reducing cooling drag.Weight is almost always higher than with air cooled engines with the same installed hp once reduction drive, radiator and coolant figures are added. This is one of those inevitable engineering compromises- weight vs. cost vs. other advantages. Careful attention to detail and choice of engines can help mitigate this disadvantage to some extent.

Torsional vibration remains a largely untested area in the propeller/ redrive/ engine equation even among leading firewall forward suppliers. This is perhaps the scariest unknown and one of the leading causes of auto conversion failures. Much more data needs to be accumulated for the many different combinations out there.In conclusion, auto engines are a viable alternative to traditional air cooled aircraft engines for many users. The proof is in the several hundreds of thousands of flight hours they have accumulated to date. They are not for everyone to be sure but there appears to be many more satisfied users than dissatisfied ones. I welcome submissions from auto engine users relating your experiences (good or bad). May you continue to enjoy your inexpensive, smooth quiet engines in flight.


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