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400 HP turbocharged engine

If you think that a 400-450 HP engine is a useful / fun / crazy discussion topic, welcome here. This thread is about a Continental TSIOL 550 derivative. It may or may not ever happen, but it can keep our neurones busy and that alone is a good reason for it to exist :)

Here is where we start

The TSIOL 550 is not a new design. It is derived from the TSIO 520 / 550 crankcase/crankshaft assemblies and re-uses many TCM subsystems. This is good because TCM did many things right and the “bottom end” has a reputation for being very strong and trouble free.
It is bad because the conversion to liquid cooling failed to deliver a robust and dependable product.

And so I propose that we start by listing and commenting the causes of the failure. Here my input, and please add your observations:

1) The liquid cooling system was designed for minimum weight, at the expense fo robustness. As a result, the engine was excessively vulnerable to maintenance induced damage and chafing.

2) Engine instrumentation was lacking/misleading and the Extra 400 POH did not adequately cover the prevention/detection and handling of coolant issues. This in turn caused catastrophic engine failures and even accidents.

3) Maintenance skills in the field were lacking

4) Metallurgy issues in the manufacturing of cylinders. More specifically the aluminum cast cylinder head and cooling jacket.

5) The market positioning of the Extra 400 (a “business aircraft”) made it intolerant to the low dispatch rate that results from the above. Hangar queens develop more issues…

More input on this matter?

Last Edited by Flyingfish at 30 Apr 11:05
LSGG, LFEY, Switzerland

Chapter 2 the untold potential of liquid-cooled Lycosaurs

Engine management of turbocharged Lycosaurs revolves around the dreaded “red fin” – the area of operation where detonation may happen as a result of a combination of unfavorable factors.

The remedy is as brutal as the threat: stay clear from the area of maximum engine efficiency and power output.
Too rich or deep in the lean of peak domain where efficiency is good but power is limited.

A water cooled cylinder operates at much lower CHTs: 270 F is my “personal red line” as opposed to 400 F in air cooled turbo Lycosaurs.
How much efficiency/range/payload/power are we throwing away by simply adopting the detonation safety margin of air cooled counterparts?

LSGG, LFEY, Switzerland

Chapter 3 Exhausting exhaust issues

A stainless steal exhaust may enjoy a decent life when operating in turbocharged air cooled engines because it naturally sheds heat to the strong airflow down from the cylinders’s cooling fins.

Not so if the cylinders are liquid cooled.

The exhaust becomes extremely hot, causing major issues to the components around it and the stainless steel tubing fails very quickly.

Add to this the tight cowling of the Extra 400, the spaghetti of SCEET ducts that blocks air flow in many places and you get the idea.

And to make things worse, the exhaust heat shield, a long, riveted assembly of stainless steel sheets, is a maintenance nightmare:
It is very difficult to remove and re-install, develops fatigue cracks and ends up being a nasty generator of well hidden chafing points.

My conclusion is that the exhaust system needs a complete redesign for successful operation in the context of liquid cooling.

LSGG, LFEY, Switzerland

More input on this matter?

Yeah. Buy a Jetprop

One KA90 owner who got a dual engine stoppage in a Baron at FL200 over the Irish Sea, restarting at 2000ft, sold it immediately and bought a KA90. He said to me: the only thing better than a PT6 is two of them

You could probably run a TBM for the cost of your Extra 400.

Shoreham EGKA, United Kingdom

The debate is about how to make a 400-450 HP engine with the specific advantages of a turbocharged piston engine vs the only available turbine of comparable power: the Rolls Royce 250.

This turbine was originally meant for helicopters. It is much lighter, reliable, and pleasant sounding than any piston engine, and unlike most turbines it is reasonably suitable for low altitude ops. But it also has drawbacks – judge for yourself:
- Purchase price around 600 KUSD new and presently no used inventory on the market.
- Unable to handle icing due to flame-out
- Loses power fast with altitude, practical ceiling is FL200
- Specific fuel consumption is high, especially at low alt, negating some of the mass advantage

I believe that a modern turbocharged piston engine could be a good alternative to a turbine in this power range.

What do you say?

LSGG, LFEY, Switzerland

I would say that a reasonably clever engineer could fix all drawback of the RR250 and, and his colleague who’s commercially responsable of the business stuff, could agree that being the only one on the market, there would be a takeover by lowering the price of this engine.

Last Edited by greg_mp at 04 May 06:52
LFMD, France

This has been tried over and over again and a modern piston engines in this power range seem simply too heavy with all their cooling etc. Plenty available in automobiles like the one in my car but those are huge chunks of metal! Do not reinvent the wheel, the proper engine for such an appplication is a turbine. Maybe a pre owned or timed out PT6A-21 from a King Air.

turbotech is doing 130bhp for 80kg package. Downsizing is difficult, but upsizing should work. we can even put 2 or 3 of them.
Edit: 160bhp with strong french accent

Note that we are arriving at the performance of O320 lyco, a bit worse actually, but keeps performance with altitude.

Last Edited by greg_mp at 04 May 09:08
LFMD, France

greg_mp wrote:

turbotech is doing 130bhp for 80kg package. Downsizing is difficult, but upsizing should work. we can even put 2 or 3 of them.
Edit: 160bhp with strong french accent

You mean like TURBO-Seminole? :)


During the heyday of aviation, Aero Commander sold their 685 piston model alongside the Turbo Commander turbines, for those who preferred pistons. Basically the same airframe as the Turbo Commander 681, but with fire—breathing pistons instead. The geared GTSIO-520-K put out a whopping 435hp each and subsequently, didn’t last very long. They have a 1200hr TBO and frequently need top end work halfway through. Not only that, to overhaul them is very costly, so you’d actually save money on a per hour basis running turbines.

But it’s possible.

The 685 is otherwise a sweet ride with some drawbacks. It’s very quiet, very roomy, but not great runway performance or climb. And the pressurization system run by hydraulics is more or less unobtanium.

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