Could I ask the forum collective about these two engines and in particular the maintenance issues of one over the other.
I’ve always heard (from the Lycoming and Continental world) that turbo engines have more issues and more maintenance. I’ve heard the cylinders in turbo engines rarely make TBO.
Is this also true for Rotax 914 engines?
As someone with little engineering skills who’s going to have to do some basic maintenance on such an engine (and who would like to keep running costs as low as reasonably possible) would a 912 be significantly preferable over a 914? Or does the turbo really add little in terms of maintenance costs / skills required?
For the avoidance of doubt, the extra performance at altitude isn’t of huge interest to me. I wouldn’t be planning to operate about 10K feet very much.
Internet wisdom, fwiw, is that the 912 80hp is the least “delicate” of the bunch. Mine is ~900hrs and 24 years old and is fine.
Sorry I meant the 912s (100hp) v the 914. Too late to edit my post. Perhaps Peter could change the title and body to add an s top 912 to make it 912s v 914
Speaking for Lycomings and Continentals: The cylinders can make it to TBO, however TBO of turbocharged engines is typically lower than their normal aspirated counterparts (for example 1800hrs instead of 2000). Good maintenance makes all the difference, fe: it doesn’t hurt to have the Turbo cleaned of carbon buildup at some point in between. Monitor the engine, check, rotate and clean spark plugs when necessary. frequent oil changes. etc.
I think if you don’t need the turbo, go for a normal aspirated engine. Less moving parts → less things can break. But good maintenance is always key, it might seem like it costs a little more but in reality you’re saving yourself money and troubles in the long run.
The cylinders can make it to TBO
I am sorry but could not avoid laughing at that one. Now seriously, do you really know turbo examples that routinely fly Up in the teens whose cylinders do make it to TBO?
The 912 UL/A/F etc is the original 1200 cc, low compression, 80 HP engine. It is considered the most reliable, or longest lasting engine.
The 912 ULS/S is the high compression 1400cc, 100 HP engine. It’s probably the best selling engine, but it requires min 98 octane to achieve the power without risk of detonation (the only Rotax engine that do).
The 914 is the turbocharged 1200 cc 912 UL/A/F engine. 115 HP. Not that many around.
My experience is with 912 UL and ULS (I own an ULS). I would say they are the same. Both have Bing carburetors that needs fiddling and maintenance 
Perhaps where the S/ULS will go for 3000 h, the UL will go for 6000 h? It’s not a practical consideration for most people. I have only tried the 914 twice. Not 100% sure, but the 914 requires CS prop, or at least variable speed. I would think you have a hard time controlling the power and MAP without it. It also has Bing carburetors, AND turbo. That seems like lots and lots of fiddling to me Perhaps a 912 S/ULS with CS prob is a sweet spot?
In short, where the 912 S/ULS is a dead simple engine, the 914 is not IMO. For the WT9 Dynamic we use as a glider tug, the engine was replaced/moded some years ago. We went for big bore and FI from Edgeperformance. Big bore instead of turbo for simplicity (they both have 120 HP), and FI mostly to remove the Bing headaches. No need for constant speed on that aircraft as the only consideration is climb performance.
dublinpilot wrote:
For the avoidance of doubt, the extra performance at altitude isn’t of huge interest to me. I wouldn’t be planning to operate about 10K feet very much.
I think the Internet wisdom for that one is that if you don’t need extra power, then go for 912ULS.
Even if you needed to fly in the teens (below FL200), 912ULS should be fine – allegedly UAV operators fly those routinely in the ~ FL250 range, so good enough for you.
Extra factor to consider is which particular S/Ns of 914 are you talking about? New one has got better TBO and (allegedly) are better in terms of maintenance.
And if you are talking about a new engine, many insist on 912iS – it has got an injection and is significantly more economical, plus no carb ice.
What is your plan, aircraft-wise?
A 100hp Rotax in an ultralight (up to 600kg) is ok. In one of the 750kg aircraft (B23, Aquila, Katana, etc.), it‘s bit of a dog, both for short/soft field takeoffs and climbs above 6000 feet (remember these engines don‘t have a proper engine leaning feature). Of course, it also depends on the prop.
Thanks for the info everyone.
To add some context, I’m looking to buy an MCR4S (750KG), not building one. So I don’t have a choice of engine but rather a choice of aircraft with an engine already in it. Some of them have been built with a 912S (100HP) and some with a 914.
My preference would be the 912S because it seems more economical, and less complicated and I’ve little need for the high altitude capability. (Various reasons but FL100 will be more than enough for me).
But there are some aircraft that may be of interest but which are built with a 914.
So I’m wondering how much extra complication / risk / expense (in terms of maintenance) would buying an aircraft with a 914 be over a 912S?
Is it just a minor maintenance complication or a significant one?
Great summary @LeSving
I have zero knowledge on the type.
However, it being a four seater I am guessing it is performance-limited for those short rebel-type runways you like to fly out of. The turbo will help you performance wise on hot summer days in exchange for the added complexity, I am sure there is a reason why some of the MCRs have a 914 other than high-altitude performance.
