Auto vs. Certified

People who are using or plan 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, 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 correspond 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 magnaflux, 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 homebuilts worldwide.

Probably the best source for information is the 2 book set edited by Mick Myal entitled "Alternative Engines". These books have a vast amount of useful information on engine conversions, cooling, propellers etc. from real world experience as well as articles on theory by engineers. Well worth the money. Available from Aircraft Technical Book Company. Those interested in the field should also subscribe to Contact! magazine This is THE source for up to date technical information on automotive engined aircraft.

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.

Update 08/30/99

I just finished reading some incredible details about Continental (TCM) engines that our readers might find interesting. After having customers complain for years about problems with their TCM engines, Beechcraft (Raytheon) has compelled TCM to produce "special improved" engines for installation in their airframes. Customer complaints included excessive vibration, camshaft and cylinder corrosion, low compression and high oil consumption. Let's remember that these are brand new, certified aircraft engines worth in excess of $30,000 each.

TCM's standard and "improved" tolerances for balancing parts are as follows: Crankshafts, counterweights and gears- 21 and 12 grams respectively, connecting rods- 14 and 2, pistons 14 and 2. Combustion chambers are now balanced within 3ccs vs. 8 on standard engines. Japanese automotive engines routinely come from the factory with all of these tolerances under 2 grams and race prepared engines usually have this reduced to under 0.5 grams as weight imbalances have long been understood to reduce life and increase vibration. Chambers are usually die cast or fully machined to less than 0.5 ccs in volume difference between chambers. TCM has implemented many other changes to make the performance and reliability satisfactory which is absolutely routine in the automotive world. I find TCM's standard specs appalling.

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. As Dennis Miller says "of course, this is only my opinion. I could be wrong".

Update 09/02/99 The Saga Continues

I Just finished reading more scary things about TCM engines in the excellent "The Aviation Consumer" magazine, July '99. Just to plug this publication, they accept no commercial advertisements so they have no vested interest in any brand, only to tell the true story. Some manufacturers don't like them as a result. They have excellent product reviews on all things for general aviation.

The latest fiascos involve "spotty" quality control on valvetrain parts, mostly on valve seats and guides. Apparently in their testing, TAC's A&P found 60% of the heads examined were outside manufacturers specs and many of these were OEM new or remans! Problems included non-concentric valve seats, improper seat widths and excess guide to stem clearance. Any automotive shop machinist understands the importance of a properly performed valve job for engine life and performance. Again, the question begs to be asked, what are you getting for all of this money?

TAC recommends getting ALL heads checked out by a good A&P before installation to be sure everything is within spec. I concur.

The second problem on TCM engines is some aftermarket and OEM rod bearings shedding layers. Another worrying situation for certified engine owners.

06/26/00 More Bad News for TCM Owners

That latest problem for Continental engine owners is a series of crankshaft failures on 360, 470, 520 and 550 series engines. 11 failures have occurred in the last 10 months and about 1100 engines made in the last 2 years are affected. One failure happened with only 15 hours on the crank. These failures have been traced to a metallurgical flaw. TCM has issued MSB 00-5 to have cranks checked and the FAA is expected to make this mandatory with an AD soon. TCM has now implemented metallurgical testing of ALL new cranks. Makes you wonder what they were doing before this.

12/05/00 Lycoming Woes

A class action lawsuit has been filed against Lycoming concerning an alleged 10% inflight failure rate on its TIO-540-AE2A engines powering Piper Mirages. This engine has a TBO of 2000 hours however, a survey of 92 owners found that only 4.3 percent made it to 1500 hours. The average was 727 hours. 41% needed a top everhaul before 1000 hours and many needed topping at 200-300 hours. Why Lycoming sets the TBO at an unrealistic 2000 hours is puzzling when probably not one engine ever reached that without topping. If it was set at a more realistic 750 hours, owners would expect a more realistic operating cost.

04/04/02 More Lycoming Woes

Another AD has been issued on Lycoming TIO-540 and LTIO-540 engines concerning those with crankshafts made between March 1 and Dec. 31, 1999. This AD was prompted by 14 crank failures to date, apparently caused by a metallurgical defect.

11/18/02 Yet More

More failures with crankshaft bolts and crankshafts on IO-540 series engines has prompted Lycoming to initiate more quality control procedures and they are now paying to fix over 1000 affected engines.

09/10/03 Oil Consumption

One more irritating thing about certified engines is their high oil consumption, frequent changes and Jurassic break in procedures. Contrast this with my 2002 BMW 330ci. It comes from the factory with synthetic oil. First oil change is at 24,000 km. I added my first liter at 12,000 km. Average speed was 60 km/hr. That's 200 hours and should be 400 hours between changes. It still looks quite clean. At 15 hours on an O-360, the oil looks like mud and at 40 hours it's changed. No progress here for 45 years.


I was amused to read something on Lycoming's website regarding operating the TIO-540 engine. In their introduction, they stated that " An automobile engine probably never reaches its full rated power in its entire lifetime. The aircraft engine does it every day on every takeoff roll. NASCAR racecars operate at high power for extended periods of time, up to 500 miles, after which they are disassembled and rebuilt. The aircraft engine is expected to run at high power for up to 2000 hours without a hiccup."

Wow, whoever wrote this must be wearing rose colored glasses! First of all, unless you are you are a very timid driver, EVERY auto engine sees full rated power every day you floor the gas pedal and crack off a redline shift in your car. Auto engines do not have limitations on full power duration as aircraft engines do. Lycoming's own literature limits ground runnning at full power as well as limits full power in flight to 5 minutes or even less on certain turbocharged engines. Let's remember that at altitude, atmo engines do not deliver rated takeoff power anymore. NASCAR engines are rebuilt often because they produce 4 TIMES the specific output of most aircraft engines. For a NASCAR engine to produce 175 hp like an aircraft engine might, it would only run at about 3500 rpm. I have no doubt it would last 2000 hours running like this. We only have to read the problems above suffered by aircraft engines to see that the final statement is wishful thinking on many aircraft engines. Again, when we see aircraft Lycoming and Continental engines pushed even into the 500-600hp range racing at Reno in the Sport Class, the lifespan of these engines are measured in minutes, not hours.


Interstate wins $97M judgement against Lycoming prompted by multiple crankshaft failures 3 years earlier. Lycoming attempted to blame crank forging company Interstate for the failures, despite evidence that Lycoming called for increased vanadium added to the materials mix to aid post forging processes- aginst the advice of Interstate.

02/28/06 Not Again?

How long will the Lycoming crankshaft saga continue? They have now issued a mandatory service bulletin which mandates the replacement of thousands of 4 cylinder cranks before 2009. Not a pleasant prospect for owners of those engines.

08/13/07 Have They Learned their Lessons?

It does seem now that TCM and Lycoming have learned that if you change processes without proper validation and you let QC slip, you are going to have failures, deaths and pissed off customers. Both companies have implemented sweeping changes in QC and have apparently started to address historically weak areas in their designs. Only time will tell if these practices will be effective.

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.

Insurance and resale values remain other problems in some countries, especially the US. The former may eventually be a lesser problem as more auto engines fly. The latter is more of a perception which may or may not change with time.

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.

Update 12/18/07

It seems I was wrong again. Lycoming has not learned their lessons about QC. More problems recently with oil holes not being drilled, incorrect cylinders being sent out, cross threaded cylinder stud holes. If this is certified, it is pretty scary. Just how do mistakes like this get made still after all these years and all this experience?