I'm wondering what is the best engines everyone is installing or running? Also the pros and cons? I am building the Zenith CH 650 and have just started. I like the UL350iS or the Lycoming 233 but I see quite a few Corvair engines out there, so what are the pros and cons about them. I do not like the Rotax. I want something simple. So any input will be greatly appreciate.

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..."shooting a pilot...", not a good idea, will certainly improve the show's ratings!  I suggest a show for the ADD spectrum that simply selects a 'drama du jour' from internet forums, tosses in some cleavage, and, voila!

Jackson (somewhat vulnerable to drama, but working on it...)

The best engine? In the case of a Zenith, an engine in the region of 100HP which fits the firewall forward weight range. Beyond that in my view it is a matter of reliability, supportability and simplicity. I prefer knowing exactly what is inside the engine, why and how to maintain that system. Airplanes and engines are not cheap; some are simply less expensive than others. For my purposes the Corvair conversion makes sense with one being built for the 750 STOL and another for a Pietenpol. In both cases they work when the directions are followed which is of course key in any flight endeavor.

Beyond that I'm having too much fun building and learning.


OK, I can state my preferences and engineering opinions -- I am a graduate mechanical engineer and have worked some with engines.  I have been the faculty adviser for the FormulaSAE students at the University of Utah and about 15 years ago, worked on the design and manufacturing of an Opposed Piston diesel.  This particular engine was not designed for aircraft use.  FormulaSAE involves building a formula type race car with a restricted 4-stroke engine that is not larger than 610 cc.  It is common to add electronic fuel injection to a motorcycle engine.

Anyway, I am building a 750 STOL and have a turbo-charged Rotax 914 and believe that this is a good choice for me (8000 ft density altitude at the airport is not uncommon in the summer) but would rather have the new Rotax 915 with electronic fuel injection but it is not yet available.  If you live at lower altitudes, the fuel injected Rotax 912 might be the best choice.  The Rotax engines are lighter weight, have a better fuel burn, and are probably more reliable than any air cooled engine.  They are not the cheapest alternative.

I am flying some older Cessna 172s and am just waiting for one of the Lycoming or Continental engines to quit running in the air.  In my earlier days, I had three different different turbo-charged Corvairs.  They were fun cars to drive and I will admit that I drove them hard but the worse thing was the reliability of the air cooled engine. Anyway, I am not a fan of air cooled engines and if I was designing engines, I would never design an air cooled engine for anything larger than a lawn mower and even then, I would think twice about it.  The problem is that air cooled engines have hot spots and you either need higher octane fuel or lower compression ratios to avoid engine knock so you have the lower efficiency and higher fuel costs.  With the aircraft engines, it is common to use enrichment to cool the engine which is a very expensive method of liquid cooling.  Because air cooled engines have differing temperatures, they also need to have looser tolerances which leads to excessive oil consumption. 

Engine RPM does not matter but engine piston speed does.  The Rotax runs at a max of 5800 RPM and 5500 RPM cruise but the engine piston speed is not that high so gearing can be a good thing if it is done right as it allows the engine to be smaller and lighter and the prop speed to be lower.  My one concern about the Viking (Honda Fit) engine is that the piston speed is relatively high compared with most contemporary automobile engines.  Most are running about 14 or 15 meter/sec while the Honda Fit is about 20 meter/sec and the forces go with the square of piston speed.

The one engine that I saw at last years Oshkosh that was particularly interesting was the Gemini opposed piston diesel. They are not yet available but now that Superior has taken over the design, it looks promising.  They are lighter than any of the other small aircraft engines other than the Rotax engines and have a better fuel burn than any of the gasoline engines and burn JetA.

I am planning on getting my plane flying with the turbo-charged Rotax 914 but I may considering working on modifying a contemporary direct-injection turbo-charged engine such as the GM 1.4 liter engine used in the Chevy Cruze (153 hp at 5600 RPM).

Mr Drake,

I went to sun in fun 1990, and there was a guy there with a Yugo engine in the bed of a pickup giving much the same anti-aircooled message. The guy had a real flair for expressing how terrible they were, and how his liquid cooled Yugo was the wave of the future. how air cooled motors didn't work at all. Reminded me of the film 'Elmer Gantry.'  There was a little crowd following his every word. I watched this from 50 feet back, and looked at the field of 8,000 aircraft on hand, of which 99% had just safely flown there with air-cooled motors, and thought that I was glad to be an American, because in other countries they lock up the mentally ill in cruel settings, but here we are kinder, and let the delusional walk around among us.


I can make a factual case that a factory new Lycoming 320 will run more hours at 75% power output, without halt, than any Rotax. The Austrians make good motors, but you can't claim they are more reliable than Lycomings. I worked at the Embry-Riddle engine repair station, they build all the schools engines, they are authorized to go to 3,000 hr TBO's they have 120 planes that each log 2,200 hrs a year, and they have records to show they get over one million hours between failures, operated by notoriously unkind college students. When you claim a Rotax is more reliable, you are going to have a tough time proving it.


In the last 25 years I have had countless guys with an engineering degree feel that they understood all flight engines, even if they had never worked on them. Many of the claims about loose tolerances, oil consumption, inefficient operation, etc, were great theory, but any A&P with a private pilot rating and two years experience had more practical and realistic observations. Example: I have 25 years of owning and working on air-cooled engines every day, and the majority of them can go between oil changes without using any appreciable amount. Go to Oshkosh and look at a field of 500 air-cooled RV's and tell me how many you say with the belly covered in oil or the soot of it being burned. To be better than that, liquid cooled engines would have to make oil in their crankcases. 


Want to learn something new? Your piston speed theory is not valid. Here is the real factor: The rod to stroke length ratio of the engine is a vital factor your comments are missing. Study a motor with a 3" stroke and a 5" rod, and then change the rod to 6" and watch how the piston lingers at TDC and BDC for many more degrees of crank rotation (this is why diesels have long stroke/rod ratios) Given a set rpm, the inertial loads are much higher with a long rod motor because the piston must accelerate and decelerate much faster. Thus any talk about piston speed without referencing rod ratio is just pleasant simple theory. Maybe I'm not just long haired grease monkey from Florida? Maybe there are things to learn from practical experience?


Your statement that a Superior Gemini's will be lighter than any engine except the Rotax isn't valid. First, a UL is lighter than a Rotax right now, and a Gemini will not be lighter than a UL. This isn't theory, I have physically picked up all three engines, and looked at the numbers, handled the parts, met the designers. You like the theory of two stroke diesels, and I do also, evidenced by the fact I have a Detroit 3-53T transplanted into my pickup, an experience that gave me first hand understanding. Everyone has preferences in engines, and I am no different, but I am careful not to make blanket statements or repeat information that isn't part of my personal, first hand experience


I come across as an arrogant jerk, but I know my craft very well, and I have gotten as far as I have because I have absolutely no allegiance to any theory, I have always just followed the factual data produced by working in the field and testing. If you don't like air-cooled motors, don't use one, but for the rest of your lifetime, many other people will because they actually work much better than people who's experience is limited to textbook theory are yet to understand.


.....and this is an example of why I have a short Christmas card list. We all have the right to remain silent, but some of us don't have the ability. Mr Drake, if you head to Oshkosh, come see me and I will apologize in person, buy you a coffee and talk engines after hours for as long as you like.   -ww,

William,  I will indeed come see you the next time I am at Oshkosh (hopefully this July) and we can talk.  I do have some good things to say about the Corvair engines.  They are simple and easy to work on.  I rebuilt a number of them.  And like I said, I drove them hard and power shifted well above the red line.  They were fun cars for their time and I had one untill the early 80's.

I am not a total Rotax fan but it had more things going for it in my opinion than the other options.  They are not low cost and they are not simple.  I just tried to find the weights of the UL350 (about 173 lb with oil?) but the Rotax information is a bit more difficult (156 lb with exhaust but without radiator or fluids?).  So it needs at least about 8 lbs for oil and probably another 12 lbs for radiator and coolant. So the UL350 may be a few lbs lighter.  Does the exhaust for the UL350 include a muffler?  Anyway, I will state that off-road motorcycles did not go to liquid cooling to add weight and that for a given performance, you can not make an air-cooled engine lighter than a liquid cooled engine.

Piston speed is not my theory and I suspect that it may have been design factor for steam engines but I have no proof of that.  Anyway, it was probably well know at least 100 years ago and yes you can change things by changing the rod length.  You can also play with offsetting the crank from the cylinder axis, etc which will give you faster compression and slower expansion.

Anyway, see you at Oshkosh and we can have a friendly discussion.


I am not sure that I did post my message yet, will try again:

I mentioned that I thought this was a great discussion, a good read!

I have been trying to get the price and availability of the 2775cc piston/cylinder kit. Rachel seemed not to know. Could you tell us?

Also, have any of the builders made their own top and bottom (oil pan) plates?

I would like to close the case with the crankshaft and camshaft and bearings installed. Have not received the shafts yet. Do I need the oil case for this? I would like to put off purchasing this till later.

Thanks for the info, and I hope you have a good tour out west!


I have been trying to get the price and availability of the 2775cc piston/cylinder kit. Rachel seemed not to know. Could you tell us?


We don't want threads to turn into commercial/business activity. I would suggest you contact the vendor directly via private message or their website rather than use the forum for pricing/availability information, etc.



Zenith.aero Forum Moderator


Okay, I will abide.

I was just getting desperate, as my confidence in this enterprise is dropping very quickly.

It's not that I am in a hurry to get this project done, I am just trying to get enough data to plan the financial , technical, and scheduling aspects. When I saw the number of contributions from William I thought aha maybe I can get some movement here!

 I am pretty close to pulling the plug on this, my corvair project, this after putting a fair amount of work into it over the last 5 months (I did attend the Mexico MO #34 session). Well, I guess I cannot comment further on this in order to honor your request, but I think these comments might be informative to the other builders.

I do very much appreciate this site, and I think the contributions from the other builders is quite valuable.

Thanks for you work,


Aye, laddies an lassies, dinna forget--'tis frriction's brrisk, rough rub 'at pr'vides the vital spark! (plagiarized and highly modified from A. R. Martin)

As grown-up boys and girls, we are mostly able to cull the apples pretty well--as my grandmother used to say, "Consider the source."

I'm just as concerned as The Moderator about civility, but too often, in this world obsessed with political correctness, telling the unvarnished truth is considered rude--by the ox that felt gored. And the members of the ox-goring protection league.

Intellectual discourse is not, however, based on playpen antics like "my poppa can lick your poppa--CAN! CAN'T! ad nauseam. Nor is it based on cutting out when a refutation seems to stand. But we can readily recognize the latter, and come to our own conclusions about credibility issues.

I put up with a lot of $#it from my car mechanic because he doesn't want to be bothered with anything but mechanics. He might be (but ain't) the meanest SOB in the valley, but I don't care as long as he fixes my car better than anyone else. I have had butt-kissers for "mechanics" and have consistently gotten my car back more #uck'd up than it was when it went in. "My" A&P is a nice guy, but boy, does he have an eagle-eye! And so on--you get my drift.

A little "flame" doesn't offend me as much as inform me. A lot of "flame" just burns the hangar down.


Opinions differ. Facts don't.

As a former partner in a 601-XLB, we loved the Jab 3300.  Very reliable and inexpensive to maintain, we got 4.5 GPH for the first hour of flight (with STTO and climb) and 4 or less for the second.  No PRSU or mixture to worry about.  Annuals ran about $50-60 for oil, oil filter, fuel filter and spark plugs...all from Autozone.  We religiously followed the Jabiru maintenance schedules for oil changes and plugs and never really had a problem in 800 hours of flight.  Another added benefit is that it sounds like an airplane...not a weed-wacker.  :)

I want to thank everyone for there comments. I agree with a lot of you. I love the thought about building my own engine like the corvair engine or rebuilding a lycoming or what have you. I want to learn every aspect about them. That's why we build our planes to learn and have fun!!!!! You can look at all the different engine sites and read what that've to say. But it's not real world pros and cons about there engine. I'm just looking for your thaughts about your engine. It's like the old fight between chevy and ford lol. So maybe I didn't word my discussion right but I did get a lot of info from y'all and I want to thank all of you.

Good luck, Jeremy. You really started a spirited discussion! Chevy vs Ford analogy is right on! Maybe you should have asked which engine Donald Trump or Hillary Clinton would prefer. LOL


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