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  Last Update: 19 Jun 2011

- Engine Conversions: Introduction -

Some make sense; Some do not

When you evaluate the wisdom of converting an aircraft from an existing powerplant to a different one, it is essential to consider the important factors which define the suitability of a powerplant for aircraft usage. It is this company's firm opinion that the requirements for a good aircraft engine, in order of importance, are:

  1. Reliability,
  2. Power-per-pound of installed engine weight,
  3. Fuel efficiency,
  4. Form factors,
  5. Cost,
  6. Availability,
  7. Support.

There may be disagreements about the order of (3) through (7), but (1) and (2) are widely accepted, in that order.  In certain situations, item 5 (cost) can become a very important factor. In those cases, we would allow cost to come right after reliability, but never to precede it.

It is also our opinion, based on real-life data, that for engines up to about 325 HP, the best way to satisfy requirements (1), (2), (3), (4), (6) and (7) is to use an appropriate certified engine (Lycoming, Continental, etc.).

Naturally there have been problems with those engines; all devices produced by humans have problems. The costs are high, for a variety of good (and not-so-good) reasons. However, the fact of the matter is that, statistically, the low-to-medium powered “LycoNentals” have demonstrated both excellent reliability and high HP-per-pound of installed weight. New technology makes reasonable BSFC available at cruise power settings. Flat engines fit into most airframes well. (Could it be that they were designed around flat engines.....?)

That being said, there certainly are situations in which an engine conversion can make good sense. The higher-powered "LycoNentals" (325 HP and up) tend to suffer from reliability problems. In order to attain those higher power levels, the engines are changed from normally-aspirated to forced induction (usually turbocharged, but in a few rare instances, supercharged).

The intensity of combustion pressures, caused by the additional heat release which forced induction produces, can cause serious structural difficulties with the rotating and reciprocating components, and more frequently with the basic structure of the engine crankcase itself. If you use a strobe light to watch a normally-aspirated IO-540 or IO-550 on a dyno at a high power setting, you will see an astounding amount of back-and-forth motion of the individual cylinders. That motion is indicative of the high loads and limited structural stability of the engines. The forces which cause that motion also cause the cylinders and crankcases to crack in service. Further, these engines have traditionally had reliability issues with exhaust valves and seats.

When these engines are upgraded to forced induction, the additional structural loads on the cylinders and crankcases simply cause the components to fatigue and crack more quickly. In addition, the heat rejection required for the additional power is always crowding the margins of available cooling capacity, further taxing the already-heavy thermal loads on exhaust valves and seats. BSFC's on the forced-induction engines get much higher (in the 0.70 region) because in order to assist cooling, the engines require very rich mixtures at high power settings. Costs tend toward the extreme. The weight grows, especially with geared versions.

There certainly are applications which demand power well in excess of what can be reasonably extracted from a Lyco-Nental, and which can be reasonably considered for conversion. There are three conversion examples detailed in this section, one of which makes very good sense, one of which was on the cusp of good sense, and one which made no sense at all.

Read on, and then judge for yourself.

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