October 15, 2019

Engine Choices

Here’s what I have to choose from:

EngineStock/ ExperimentalHPTurbo/NA1EFI2Price
Rotax 914ULSStock engine for Sling 41153 / 100TurboNo$31,5395
Rotax 915iSStock engine for Sling TSi1413 / 135TurboYes$38,6675
Edge Performance EP912STiExperimental - modified version of 912ULS154 4TurboYes$34,500
Stol Creek 912ULS (Modified)Experimental - modified with Big Bore kit and Edge Performance EFI120NAYes$26,500

Notes:

  1. NA = Naturally Aspirated
  2. EFI = Electronic Fuel Injection
  3. 5 min max continuous power (for takeoff)
  4. 154HP is the advertised power, but I would probably ask for derated to ~135 HP for compatibility with a Whirlwind propeller
  5. Price when purchased through TAF, as of July 2019

The options I’m considering span a considerable price range (>$12,000), with several pros and cons. Note that I’m sticking with Rotax engines, even though I know there are other options (UL Power, Viking Engines both come to mind) – but I’m pretty sold on Rotax for reliability and longevity reasons, and commonality with the Sling fleet.

The default option – 914ULS – of course will be the easiest for me to install, since that’s the original powerplant that TAF intended for this airframe. But I’ve been considering other options, mainly because I’m not thrilled with having to deal with carburetors, choke, etc., and one Rotax mechanic I’ve spoken to said the 914 can be a finicky engine.

The EP912STi looks very appealing. You get more power than even the 915iS, EFI, a modern ECU to control ignition timing & turbo, and it’s between 1-3kg lighter than the 914 engine. I spoke with Edge Performance, and it sounds like they make a lot of modifications to a base 912ULS engine to ensure the engine will safely handle the power:

  • Welded crankshaft
  • Upgraded camshaft (for altered timing)
  • Replace Rotax proprietary ignition system with a new ECU
  • New forged pistons
  • modified gearbox
  • Replace carburetors with fuel injectors
  • New intake manifold
  • Add Garrett turbocharger

On the negative side, it’s a very experimental engine, so TBO is kinda unknown, and Rotax warranty will be void. It sounds like Edge Performance will offer some warranty for a year (maybe), so that helps. Rotax only offers a year warranty, so it’s not a big loss. I’d also have to find space for an intercooler, and open up a hole in the cowling for an air scoop. They claim to have sold over 200 of these motors already, so it will probably be well supported for many years.

The last option (modified 912ULS) is looking pretty attractive too. It starts with a normal 912ULS, then I would have Stol Creek Aviation modify it with a Big Bore kit and an Edge Performance EFI. It provides slightly more takeoff power than the 914 and roughly 20HP more continuous, loses the turbo and carburetors, all while being about 20 lbs lighter than the 914. On the other hand, being naturally-aspirated, it will produce less power at altitude, which may affect cruise speed and ceiling.

The 915iS is a nice new option, but a lot of people I’ve talked to think it will go through growing pains just like the 912iS did. It’s also heavier and larger than the 914ULS, so weight/CG issues may be a factor. Like the EP912STi, I would have a find a place to put the intercooler.. but id probably try to buy a FFW kit from TAF for the TSi so that I’d have everything I need. The 915 is the most expensive option by about $4000. TBO is currently at 1200 hours, though I’m sure that will be increased with time. For me, it’s hard to want to choose this motor over the EP912STi right now, but I will continue to consider it. TAF doesn’t really recommend this motor on a Sling 4, but I’m not too sure why – they tested it on a Sling 4 before redesigning the wings, and it seemed to work ok.

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