A question that we often receive by those restoring antique aircraft is, “Which engine would be the best one for me to put on my airplane?” Unfortunately there is no good, one size fits all answer to that question. However, there are a few factors that will help in making that decision, and this article will look at a few of those things.

First, antique airplanes tend to fall into two categories: 1) those models which almost always use either the original engine or a slight variant of the original, and 2) aircraft that almost never use the original engine. Aircraft in the first category would be theStinson V-77, Beech Stagger wing, and the Cessna 195. It is not that you never see a V-77 with a Jacobs engine or aC-195 with a Russian radial, but it is the rare exception when a non-standard engine is fitted to these models.

Examples in the second category would be the Boeing Stearman, Waco 10s, and all models of Travel Air. While most restorers will choose a W670 Continental or R680 Lycoming for their Stearmans, it is not uncommon to see Jacobs, Pratt & Whitney, or even Wright radial engines on the venerable Kaydet. And though the Waco 10s came with an assortment of engines from Curtiss OX-5s to Hissos, to Wright J5s, J6-5s, and R760s, probably the most common engine seen on fresh Waco 10restorations these days is the W670 Continental.W670 engines are plentiful and the Stearman firewall forward parts can easily be grafted onto the Waco fuselage to make a clean installation.

So what to do when it comes time to make that fateful decision about a powerplant? Several factors might play into that decision. For instance, how supportable is the engine, i.e. am I getting ready to install an engine for which there are no spare parts?Will my proposed engine add to or detract from the value of the completed aircraft?How important is originality to me? Is it OK for the installation to only look original, or is it important to me for it to actually be original? Am I building this airplane to fly around locally and maybe go to Oshkosh once, or will this be a regular cross country flyer?All these things will play into your decision about the appropriate engine for your airplane.

The following chart compares several models of the more common radials available to us today for use on our restorations. “Mechanics friendliness factor” is rated from* (not easy to work on) to ***** (really easy to work on).

CONTINENTAL

Continental W670-6N or–6A 220hp @ 2075rpm - 485 lbs. (dry)
Bore & stroke: 5.125” x 4.625”  
Compression ratio: 5.4 : 1

Strengths:
Most parts are plentiful (especially those that interchange with the W670-9A tank engine)
Good seal & gasket design—can be built to be a dry engine

Weaknesses:
Main bearing failures (solved by installing roller main bearings)

Continental W670-23 240hp @ 2200 rpm - 485 lbs. (dry)
Bore & stroke: same as 220 hp
Compression ratio: 6.1 : 1

Strengths:
Fairly good supply of parts(those that interchange with a220 hp engine)
Constant speed prop capability
Good seal & gasket design

Weaknesses:
Shortage of all parts that are specific to the–23

Mechanic friendliness factor: ***

LYCOMING

Lycoming R680-17 225hp @ 2100 rpm - 512 lbs. (dry)
Bore & stroke: 4.625” x 4.5”
Compression ratio: 5.5 : 1

Strengths:
Very smooth running engine
Still a fairly good supply of parts available

Weaknesses:
Prone to oil leaks—not very positive oil seals
Original exhaust connections in cylinders rust out frequently

Lycoming R680-9 or–13 300hp @2300 rpm - 523 lbs.(dry)
Bore & stroke: same as 225
Compression ratio: 7 : 1

Strengths:
Very smooth running engine
Constant speed prop capability

Weaknesses:
Shortage of some key parts (pistons, intake valves, cams)
Prone to oil leaks—not very positive oil seals
No drawings or support from Lycoming

Mechanic friendliness factor:***

JACOBS

Jacobs R755-9 245hp @2200 rpm - 505 lbs.(dry)
Bore & stroke: 5.250” x 5”
Compression ratio: 5.4 : 1

Strengths:
Core engines and parts are very plentiful
Constant speed prop capability
Good seal & gasket design

Weaknesses:
Seven-ring piston lends itself to stuck rings & compression problems
Exhaust valve problems (corrected by installinghardened steel seats)

Jacobs R755B2 275hp @ 2200 rpm - 505 lbs (dry)
Bore & stroke: same as R755-9
Compression ratio: 6 : 1

Strengths:
Core engines and parts are plentiful
Constant speed prop capability
Good seal & gasket design
Good availability of drawings to PMA new parts (applies to all Jacobs models)

Weaknesses:
Exhaust valve problems (corrected by installing hardened steel seats)

Jacobs R755A2 300hp @ 2200 rpm - 511 lbs.(dry)
Bore & stroke: same as R755-9
Compression ratio: 6 : 1

Strengths:
Constant speed prop capability
Good seal & gasket design
All parts except cases are plentiful

Weaknesses:
Susceptible to main case cracking
Exhaust valve problems (corrected by installing hardened steel seats)

Jacobs L6MB 300hp @ 2200 rpm - 570 lbs.(dry)
Bore & stroke: 5.5” x 5.5”
Compression ratio: 6 : 1

Strengths:
Constant speed prop capability
Good seal & gasket design
Most parts are plentiful

Weaknesses:
Seven-ring piston design lends itself to compression problems & ring burning
Exhaust valve problems (corrected by installing hardened steel seats)
Piston shortage

Mechanic friendliness factor: ****

WRIGHT

Wright R760-8 235hp @ 2000 rpm - 532 lbs.
Bore & stroke: 5” x 5.5”
Compression ratio: 6.1 : 1

Strengths:
Good seal & gasket design

Weaknesses:
Parts are not as plentiful as some other engines in this horsepower range.

Wright R760-E2 350hp @ 2400 rpm - 570 lbs.
Bore & stroke: same as R760-8
Compression ratio: 6.3 : 1

Strengths:
Excellent horsepower to weight ratio (supercharged)
Good seal & gasket design

Weaknesses:
Core engines and parts are very difficult to find and are therefore quite expensive

Mechanic friendliness factor: ***

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