Having looked at the IO-390 vs IO-360, one can not help but notice that very few STCs have been done, and even fewer OEMs have fitted this engine in place of the IO-360 used in many airframes. 180 or 200 gross HP vs. 210 guaranteed net for the extra weight of dynamic crank counterweights - to me that would be a no brainer.

Why is this engine having such a struggle for acceptance?

  • $\begingroup$ is it cheaper, more reliable, uses less fuel, quieter, and easier to maintain, and most importantly, uses the same mechanical interface and mounting point (i.e. is it a "drop in replacement"?). If not, any of those could be a reason not to use it. $\endgroup$ Nov 27, 2017 at 22:41
  • $\begingroup$ As far as STCs go, per Wikipedia "In January 2009 the base price of the IO-390-EXP version was USD$32,650". So how many of us with planes that have a perfectly good O-360 in them really feel like spending that kind of money? That's more than we paid for the whole plane (admittedly some years ago). $\endgroup$
    – jamesqf
    Nov 28, 2017 at 4:36

1 Answer 1


Short answer: OEMs don't see a big enough return on investment (ROI) to justify the change.

Long answer: Getting an STC is not a simple or cheap process. It costs a lot of money. The recovery off that cost has to be factored into the sales price of the updated aircraft. And there are additional costs associated with supporting a new model of an aircraft -- supply chain, spare parts, warranty, maintenance documentation, flight manuals, etc.

Marketing has to figure how many they can sell at what price. Will it generate new sales in addition to the existing aircraft? Will it cannibalize existing a/c sales? How long will it take to recoup the cost of the change? How much will it add to the bottom line over each of the next 10 years?

Ultimately; what's the ROI? And; is there a better place to allocate our capital for a better ROI?

I'm guessing most OEMs found a better place to spend their development dollars.

  • $\begingroup$ Not sure how your argument applies to OEMs, since they don’t need STCs to swap out engines. Restart Cessna 182s and 206s switched to Lycomings since they are both part of Textron now. Anyone can get an STC to retrofit engines. RAM Aircraft holds the STC for a larger displacement on Lycoming O-320 engines and the replacement engine in my Cherokee makes 160 hp instead of the stock 150 hp. Since there is no STC for replacing a 360 with a 390, apparently it isn”t worth anyones time and effort. $\endgroup$
    – JScarry
    Nov 28, 2017 at 2:22
  • $\begingroup$ @JScarry A change in engine requires a change to the Type Certificate. OEMs can do that through an STC, an Amended Type Certificate (ATC), or through an all new TC. You are correct that anyone can get an STC. The restart Cessnas were getting a new TC anyway, that was the time to change the engine. Your last statement applies to both OEMs and the aftermarket. No one has seen a valid business case to do it. $\endgroup$
    – Gerry
    Nov 28, 2017 at 13:57

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