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Race Engine Technology Magazine

As you read this page, please keep in mind that one of EPI, Inc.'s primary specialties is Aircraft Engine Conversions.

Also keep in mind that two of our primary product lines are Propeller Reduction Gearboxes and Liquid-Cooled V8 Aircraft Engines.

Yes, we like doing engine conversions.

     And yes, we like to sell engine and gearbox systems.

BUT

When you apply the cold, hard engineering realities, it turns out that most of the time, replacing a Lycoming or Continental air-cooled aircraft engine with a liquid-cooled piston engine is a bad idea.

That being said, there are a few cases where it is an excellent choice. If you are convinced that you have decided on a sufficiently-reliable powerplant to replace the existing one, here are some thoughts to help decide whether to go further.

1. When you do a true, accurate, apples-to-apples comparison of the total, firewall-forward weight of a complete, flying Lycoming or Continental powerplant against that of a complete, flyable liquid-cooled powerplant of the same or greater power (real, measured SAE Horsepower, not "BlantonPower"), the liquid-cooled installation will almost always outweigh the Lyco-Nental by a considerable amount. There is a table at the end of this page showing details supporting this statement.
2. NEXT, the CG of a V8-Gearbox powerplant will typically be quite a bit farther forward than the CG of the air-cooled powerplant.
3. The combination of the greater weight and greater overhung moment means that the engine mount structure of a V8-gearbox powerplant will typically be heavier than that for the air-cooled engine you are replacing, because it has to support (a) a larger weight (b) whose CG is further from the firewall. (There is a short discussion of mount structures later on this page, and a complete page dedicated to engine mount design.)
4. The combination of greater weight and greater overhung moment applies proportionately larger tensile, compressive and shear forces to the support structure behind the firewall to which the engine mount attaches. MOST airframes (fuselage / nacelle structures) are not strong enough to support these larger forces. In fact, in most conversions we have done or assisted with, significant reinforcement of the airframe was required.
5. The greater weight of the powerplant and the structural additions to the airframe to support it will increase the empty weight of the aircraft, which will either (a) reduce the payload (if you understand dynamic airframe loading) or, if you DON'T understand dynamic airframe loading and you arbitrarily decide to "DECREE an increase of the gross weight", it will significantly reduce the ability of your aircraft to survive in-flight maneuvering loads.
6. The combination of greater powerplant weight and further-forward powerplant CG location will move the empty CG of the aircraft further forward. You will have to compensate by moving the wing forward or by moving some other masses rearward to re-establish the proper relationship between the aircraft CG and the wing center of lift.
7. Unless you move the wing forward (not usually a simple task, nor one without other potential pitfalls) the adjustments needed to bring the aircraft empty CG back into range, combined with the more-forward CG of the powerplant, will increase the Mass Moment Of Inertia (mmoi) of the aircraft about the pitch and yaw axes (AND, in the case of a twin, around the roll axis as well).   That change alone can have a highly undesirable effect on the flying qualities of a previously pleasant-flying aircraft.
   - - It reduces the stabilizing influences of the rudder and horizontal stabilizer on the aircraft.
   - - It makes the aircraft less responsive to control inputs, (more "sluggish").
   - - It will reduce the resonant frequencies of the aircraft about the pitch and yaw axes (and roll in the case of a twin).
   
   That change in resonant frequencies can introduce some nasty dynamic behavior in flight, such as pitch or yaw instability.

The Bottom Line is: JUST HOW COMPETENT AN AIRCRAFT DESIGNER ARE YOU?

A Bad Example

One example of a really bad conversion idea is the replacement of the IO-540 recommended for Van's RV-10 aircraft with an LS-1-based aluminum V8 engine. This glaring example of "denial-of-reality-engineering" was actually written up in one of the goofy experimental aviation magazines as being the greatest idea since sliced bread. This idea exhibits ALL of the problems discussed above.

A Very Bad Example

The bad ideas get even WORSE.........Here is an example of some of the inquiries we get regarding conversions:

Dear EPI
I am looking for an engine to fit my RV7A, we have an LS1 out of an (sic) 2001 Firebird with about 65k miles on it, how much will it cost to have your reduction unit intalled and have the LS1 engine rebuild (sic) to your specs, I would also need the engine mount and electrical wiring done with a new computer, also a veriable (sic) pitch prop governor.
thanks

xxx xxxxxxxxx

And here is our standard response to such inquiries:

Dear Mr. xxxxxxxxx:

Thank you for your inquiry about our products.

Regarding your proposed engine project, it is a strict company policy that we will neither sell any equipment nor do any work at all on engine conversions which obviously violate many of the known laws of physics and engineering.

PLEASE read this page on the EPI website:

www.epi-eng.com/aircraft_engine_conversions/additional_weight_considerations.htm

It might also be helpful if you read carefully the entire section on the EPI website regarding engine conversions:

www.epi-eng.com/aircraft_engine_conversions/conversions_contents.htm

Sincerely,

Jack Kane, EPI, Inc.

And Here is a Horrendously-Bad Example.

Several years ago, we worked on an engine conversion project on a popular piston twin, in which the management's goal was to STC the replacement of the aged and overtaxed Continental GTSIO-520 engines with a pair of certified, liquid-cooled Orenda-V8 engines, which had substantially greater takeoff and cruise power.

The Experimental-R&D prototype aircraft was impressive to fly. The takeoff, cruise and engine-out performance of the prototype with 1200 HP on tap were stunning, especially if one was accustomed to the performance of the unmodified aircraft. However, the installed powerplant weight was a major engineering flaw which doomed the project.

The total, firewall-forward weight of the original GTSIO-520 ( engine, gearbox, prop, turbo, heat-exchangers, mount, cowling, baffles, ductwork and plumbing ) was about 930 pounds. The comparable firewall-forward weight of the V8 installation (absolutely apples-to-apples comparison) was slightly over 1260 pounds, which added about 660 lb. to the empty weight of the aircraft.

That increase in the aircraft empty weight had a number of undesirable side-effects, including:

1. The wing-structures to which the V8 engine mounts attached were unable to safely support the substantially higher torsional and bending loads imposed by the V8 engines;
2. The large increase in nacelle overhung moment reduced the resonant frequencies of the wings in torsional and flapping modes;
      Oh, and, BTW,


3. The resulting aircraft was a functionally-useless "full-fuel, zero-person" aircraft.

The project manager DECREED that this huge weight increase was not a problem, because he insisted he could easily obtain, as part of the STC, a gross-weight increase based on the claimed "similarity" of this model of the aircraft to a "similar" turbine-powered model which has a certified gross weight approximately 1000 lb. greater, and which has substantially lighter and more powerful (TPE-331) engines.

An unfortunate snag to that decree was the fact that the allegedly "similar" aircraft was very DISSIMILAR in the wing structure (to which the engine mounts attach). And there were other serious snags, including the ones described in (a) and (b) above.

This was a prime example of a "don't confuse me with facts - my mind is already made up" project.

COLD HARD NUMBERS

Admittedly, the conversion described above was an extreme example. But look at the following table, which shows representative comparisons of the installation weights of several Lycoming engines against a lightweight (for a Liquid-Cooled-V8), normally-aspirated, 500-HP V8 engine.

NOTE 1: The weights listed in the table are generally from measured components being used on actual aircraft. The Lycoming weights are direct from Lycoming documentation, verified from hands-on experience. AND, this is not an exhaustive list--- It shows only the major items.

NOTE 2: The engine weight given in the following table comes from actually weighing an EPI Gen-1 Aircraft Engine. For a conversion based on a GM Gen-4 engine ( LS-3 / LS-7 / LS-9 ) on an apples-to-apples comparison, about 20 to 25 pounds can be deducted from the table engine weight. 

NOTE 3: The EPI Gen-1 V8 in the chart below DOES NOT include any turbocharging considerations. To estimate a turbocharged V8, add 60-90 pounds to account for the weight of (a) the turbocharger and wastegate, (b) turbo support structure, (c) intercooler, (d) inlet plumbing and miscellaneous other stuff.

NOTE 4: The table weight listed for the propeller systems in the table are for typical steel-hub, aluminum-blade Hartzell propellers used in those respective systems. If your project will use a composite prop (such as an MT or composite Hartzell), then reduce your sum by the difference between listed and actual prop weights.

These numbers can be made to vary a bit by altering the specific configuration. Composite propellers can be substituted for metal ones.  Cowling, ductwork, and engine mount numbers can vary;  Things can be omitted; If your IO-360 uses a set of 4-into-one headers with an optimized collector, the exhaust weight will be more than listed; etc, etc, etc. But the table is a good representation of reality, taken from actual flying aircraft.

Note that the only engine in the table which makes sense to replace with a "lightweight" V8 is the Lycoming TIGO-541 (the engines on a Piper Navajo). There are a few others which make sense too, including Pratt & Whitney radials (R-985 and R-1340) and most installations using the GTSIO-520 / 550. But we have had people come to us wanting to replace all the other engines listed in the table with some form of liquid-cooled V8.

You might have noticed that the engine mount numbers in the table above show a considerably greater weight for the V8 installation than for any of the Lycoming installations.

WHY IS THAT?? Because Lycoming engines are cantilevered off the back face, which allows the mount to be quite short and stiff, therefore light. Pictured below is a 19-pound mount for a 300 HP Lycoming IO-540 engine, and it includes the nosegear support structure as well.

IO-540 Mount for a Lancair-4

The picture here (below) shows the engine mount (still in the fixture) for one of our cropduster conversions. That mount weighs 53 pounds, and supports all the FAA combined loads from a 750 HP, 1235-pound V8 powerplant with a comfortable margin. (The little hose at the front is for purging the moisture-and-oxygen-bearing air inside the tubes and replacing it with the inert gas argon.)

FAA Criteria Mount for 1235-Pound V8