A friend of ours is about to install a Rotax 912 ULS in his Zenith CH750 STOL. The firewall forward kit is the standard package from Zenith. We measured the weight of the engine itself, plus all the related components. While I was surprised about how all the small stuff adds up, I believe that it is overall still the lightest engine on the market:

Components Weight, lb
Engine mount 12.2
Exhaust, muffler 6.2
Exhaust, pipes 3.8
Radiator 2.4
Heater hose, throttle cable, etc. 4.8
Mounting hardware, oil cooler and tank, water cooling components, air filter, miscellaneous parts 19
Total parts 48.4
Engine 0.8 gal 5.44
Oil tank 0.8 gal 5.44
Oil radiator, hoses 0.3 quarts (assumption) 0.51
Total oil 11.39
Cylinder heads, water pump, expansion tanks, coolant hose. 1.5 liters / 0.4 gallons as per Rotax installation manual. 3.3
Total coolant 3.3
Engine, dry 131.6
Total fwf, minus cowling 194.69

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Interesting info, Oliver - I appreciate the detail!  You state the FWF weight at 194.69 "minus cowling," but I also assume that is minus a prop?

I've always thought the Rotax was the lightest option for the 750, but real-world comparisons need to include all the systems, accessories, hardware, etc. required to actually operate the engine!  Oliver's weight break-down is by far the most comprehensive (and realistic!) that I've seen ... I was surprised to see the actual operating weight approaching 200 lbs!

My air-cooled Jabiru 3300 has a total (including accessories, exhaust system, etc.) weight of 178 lbs.  However, that does not include the engine mount (no idea what that weighs!).  So, if one takes Oliver's figures and subtracts the mount's 12 lbs, the Rotax would weigh 182.69 lbs vs the Jab's 178.  However, virtually all Jab 3300's in 750's will require an oil cooler, slightly increased oil quantity, and associated plumbing, which I would estimate weighs a few pounds at most, so looks to me that the Rotax vs the Jab are very, very close to the same weight!  I deleted the heavy Bing carb and mechanical fuel pump on my Jab for a Sonex/AeroInjector gravity-fed throttle body carb and lost an additional 7-8 lbs of weight, so that likely makes my much-modified Jab a few pounds lighter than the standard installation.

If one were comparing other engines that happen to be fuel-injected and require return fuel plumbing and/or a header tank, it would only be fair to include the weight of these components, too, because they are required for the engine to actually operate!



Hi John,

You are right, prop, spinner and related installation hardware is also not included.

For accuracy (and simplicity) reasons, we had thrown all the bags with the smaller parts into a bucket after we zeroed the scale. The line item with a total weight of 19 lbs actually consist of three individual measurements of 6.8+8+4.2 lbs. I thought however that it makes no sense to break it down that far since it was always a pretty random mixture of parts.

I used the same scales as when I weighed our AeroMomentum engine, for which I ran a number of sanity checks to confirm the accuracy of the not certified scales. I am therefore confident that the total weight should be accurate to about +/-1 or 2 lbs.

Our friend's Rotax installation will be around 50 lbs lighter than our AeroMomentum AM15, which is currently the lightest auto conversion in this power range. AeroMomentum is working on reducing the weight by 10 lbs, it will then however still be 40 lbs heavier than a Rotax 912.


Good reporting Oliver. If the Aeromomentum AM15 (117 hp) is 40 lbs heavier than the Rotax 912 (100 hp) then it is quite close in weight to the Rotax 914 (115 hp). 

Just noticed that you list 0.8 gallons of oil twice, once for the engine and once for the tank. This is not correct. There is just 0.8 gallons (3 liters) for the engine and the oil tank together. There would be some additional for oil hoses and oil radiator but the 3 liters is what flows from the tank, through the engine and then back to the tank.

Thank you for the correction. From reading the instructions, I (mis-)understood that it would be 0.8 gal. in the tank and 0.8 in the engine and gearbox.

This means the total installed weight, minus cowling, prop and spinner is 189.25 lbs.

Pretty impressive!

Thanks, Oliver, any unbiased info of actual weight really helps clear the fog that seems to surround actual engine weight. Because builders claiming that engine weights are at times not factual and really hard to pin down, I started the below thread awhile back to strictly list completed aircraft weight  from actual weight and balance declarations. Though engine weight is not listed, it's up to the viewer to draw their own conclusions about how a particular engine weight may have impacted the build through the average aircraft weight with that aircraft. 

This list is strictly a factual compilation, and yes, there are variables, and that's why there's a listed average

I'm hoping to have more builders list their weights, particularly with the newer models and engines that are flying, we are lacking somer current models.  I found is a reluctance of a few to list their true weight, so if you can send it to me in a PM it will be anonymous. And I really appreciate those that have taken the time to list theirs, I'm sure it really helps other builders. 



Searching the engine I have found the mostly recommended Rotax 100hp. Also I have found many auto conversions and one of them seems very seductive to be installed on my CH701. This is Viking 130hp. Thirty horses plus comparatively to the 100hp of Rotax is a good point. But I have found very interesting comment on the forum. What you think about the following. Don says:”This is where a lot of home-builders reach a brick wall and end up quitting on the project. Remember cheap is not necessarily the least expensive way in the end. If you go with something other than what Zenith has engineered you will have to fabricate an engine mount, exhaust system, cowling and figure out baffling/cooling systems etc. It will take you a lot longer to fabricate all these systems than it takes to build the airframe. Then when you are finished will you have a reliable powerplant that actually performs as advertised? In my experience there are very few auto engine conversions that perform as advertised. Most end up heavier and put out less horsepower. Rotax 912 is a well proven performer on the 701 and there are some on the used market since they are so popular. Added to that Zenith has a firewall forward package that makes installation a breeze. I have been building and restoring airplanes for over 30 years and have built a RV7 and a Backcountry Supercub from start to finish along with several other homebuilts and restorations. Just finished a Zenith 750STOL that I did about 80% of the work on and just beginning a Supercub rebuild. I have flown auto conversions, two stroke Rotaxs and several 912 and 914 aircraft so I know what is involved. Good luck on your project and I hope you will post down the road showing us you flying it. Don”

UL Power 350is
ULPower 350iS is a 130 HP air cooled airplane engine that weighs in very close to a rotax, maybe lighter as there is no gearbox or water cooling system if you can afford it. No gear box to maintain and pistons move up and down less than 1/2 as many times per minute. No water hose replacements.

PS Thank you Oliver for the real number on a 912 ULS. Could you share the empty weight of the airplane,

Hello Gino,

Talk to someone who is flying a Viking 130 in a 701 and get their perspective. Remember in a 701 even with the lightest, simplest engine you can find there’s not a lot of load carrying capability left over. If you want a few extras like paint and some avionics weight adds up quickly and next thing you know you have built a single pilot airplane with full fuel or a two-place airplane that can’t go anywhere.  As far as 130hp for a 701, that would win you some STOL takeoff competitions for sure and provide gobs of surplus power on high DA days when you’re mountain flying.  Other than that I don’t know what benefit 130hp in a 701 provides. Pitched for STOL operations (5,400 rpm static) I can’t get my 701 to reach 100mph even in a maximum continuous power 5,500 rpm, 1,000 FPM dive with my little 80 hp Rotax, so extra horsepower alone in a 701 won’t equate to higher cruise speed.  I did get a 1,200 FPM rate of climb single pilot and half a fuel load (840 lbs.) on a 54 degree Fahrenheit, 1,000 ft. DA day recently.  With a 150 ft takeoff roll, that’s plenty good STOL performance for me. I don’t need any batteries to keep the engine running, don’t need a header tank, and I have one electric backup fuel pump. Another consideration regarding cost.  I have found Rotax to be expensive, but I feel I get what I pay for in value.  Rotax is very good, almost obsessive, about keeping their owners informed.  You receive the same service bulletins and free on call technical support whether you fly a certified version or not, and the Rotax owner website is a wealth of information and advice, including continuously updated factory service and owners manuals and information videos, and although any business can decide to cease operations with little notice, as the world’s largest recreational vehicle engine manufacturer I never worry about that, and if any one key person decides to leave BRP Rotax, I’ll never even notice it.  Just my $.02

I cruise constantly in a 701 at 85-90 at 4600 rpm 100 mph at 5400 with the 100 hp rotax and really not 100 hp as I don t have the airbox so they say?? If pitched for STOL it will be slower with better climb and less cruise speed? How much speed vs fuel do you want to spend even the Viking 195hp in a super stol won t do much better then 100mph and 2x the fuel burn...701 still the best all around if you don t weigh 250 lbs and want a big passenger because  its more then a little snug?  


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