Engine Theory: Modifications

It’s been a year since Engine Theory debuted as a series, and in it we’ve considered the basics of internal combustion aircraft engines, starting with the fire triangle and moving on to such complexities as supercharging. Now, as something of a review in this final Engine Theory article, we’ll—hopefully—bring much of what we’ve learned to some practical fruition. That is, what can we experimenters realistically do to make our engines more efficient, powerful and long-lasting?

Good Candidates

Before we mentally turn wrenches, let’s begin by saying our advice is intended for new or rebuild engine projects that are still in the planning or early machining stages. If you’re already flying, or nearly so, it’s late to be modifying your already operating engine, especially internally. Tearing apart a running engine and its installation in the airframe doesn’t pay. But you can read up for your next project.

Furthermore, are you personally a good fit for engine modifications? Many, perhaps most, pilots are not, and there’s certainly no shame in that. Anyone looking for the quickest build time, lack of construction headaches, maximum operating convenience, ease of repair, longest engine life, running mogas, and espousing a practical, “right down the middle” philosophy should stick to a bone-stock aircraft engine and not look back. Stock engines are designed to excel in general use, don’t ask for special consideration, and are highly tolerant of unconscious or boneheaded operation. There’s much to be said for a well-built, stock engine.

If, on the other hand, improved efficiency or higher aircraft performance is what centers your course deviation indicator, then you’ll enjoy making a few well-considered changes from stock.

Many object to engine mods, saying they cut TBO. Certainly that’s true of hard-core modifications that substantially raise cylinder pressure—super-high-compression pistons, sea-level supercharging, nitrous oxide injection, and so on—but it’s not a worry with the mild upgrades we’re addressing here.

Even if you are an inveterate hot rodder and build a fire-breather, are the reduced TBO margins disqualifying? That’s impossible to say as there are far too many variables, but let’s pessimistically postulate an engine originally rated for a 1600 hour TBO “only” goes 1200 hours. At 50 hours per year that’s still 24 years of flying. Most of us are going to reach our personal TBO before our engines, or move on to another airplane or repair as necessary.

We must also say our guidelines apply to familiar gasoline-fueled Continentals and Lycomings (especially). There is little practical experience with hot-rodded Rotax engines, as the Austrian powerplants are already well optimized from the factory. Likewise, auto conversions are too specialized for our general overview and should be considered on an individual basis.

Finally, what is our goal of increased efficiency? It’s either better fuel economy or more power. Most engine modifications give either result depending on how you operate the engine. Full throttle means more power, part throttle means more fuel economy at the original power level. Don’t count on it working both ways at the same time unless there was something truly wrong with the engine to begin with.

To get right down to it, making our engines more fuel efficient, easier to manage in flight, and less heat limited are smart goals. Of course, more power is always nice in the bargain, too.

What We’re After

When improved engine performance is the goal, recall that piston engines are air pumps, and efficiency improvements are all about letting the engine breathe more air with less effort, then expand that air to do work. A few such improvements are fundamental gains, such as increasing the compression ratio. Many other hot-rodding tricks are really just tuning, that is, engine efficiency at one rpm is traded for efficiency at another rpm range. Opting for a different camshaft grind is a good example of tuning, as is fiddling with intake or exhaust systems. The goal of tuning is to get all the compromises in an engine optimized in the same rpm range—and in a range you actually use! The fundamental improvements are the foundation of efficiency and should always be optimized to get the most from tuning.

Some Favorite Things to Do

Below is a list of improvements the industry finds useful. Best bang-for-buck improvements are almost always a quality 3-angle valve job and higher compression. After that the return on investment slows; work with your engine shop to choose the options right for your flying, and don’t be afraid of a second opinion if you think you’re being sold a bunch of propwash.

Three-Angle Valve Job/Porting

Engine breathing is greatly affected by the size and shape of the valves plus the intake and exhaust ports leading to them. Furthermore, the shape of the valve and valve seat are critical in this, so it’s a surprise the big engine manufacturers use a simple single angle to provide the sealing between valve and seat, but nothing more. Performance rebuilders make two additional cuts, one before and one after the standard single angle. These two angles more smoothly bend the airflow over the valve seat and past the valve. Engine breathing improves measurably.

Auto engines went to 3-angle valve jobs sometime before the flood, as have all the good aircraft engine rebuilders.

The only advantage to single-angle valve jobs is lower cost, the only disadvantage to a 3-angle valve job is increased cost, and in fact many shops install 3-angle valve jobs as a matter of course because it is such low-hanging horsepower fruit. Many times the 3-angle valve job is combined with porting—hand or CNC detailing of the intake and exhaust ports. Because the majority of Continental/Lycoming ports are too large from the factory, the result is general aviation porting is more akin to polishing rather than reshaping or enlarging the ports. The majority of the efficiency improvement comes from the 3-angle valve job.

Increased Compression Ratio

Lycoming and Continental aren’t crazy about this mod as it fundamentally makes more power and increases stresses just about everywhere in the engine, but the verdict has been in for some time now on bumping compression up to the mid 9s or 10:1 range. Shops such as Barrett and Ly-Con have collectively done thousands of such engines, and properly installed and operated, they make stock TBO.

The benefits of a 10:1 engine are real. There’s more power everywhere, so sea level climbs are faster and cruise a touch better, but the superior fuel economy is the best reward. This is especially true cruising at 5000 feet and higher. High-compression engines definitely “hang in there” better at typical naturally-aspirated altitudes. In fact, any high-density altitude scenarios are more sprightly with some compression on your side.

On the flip side, if your typical flight sees high manifold pressures at low altitude on hot days, go easy on raising compression because the detonation margin is reduced.

Compression is raised by fitting taller pistons. By keeping the weight of the high-compression piston the same as the stocker, the engine’s balance is not changed. That means if there’s ever a need to reduce compression, swapping pistons in the field is all that’s necessary. That’s not a small job, but it beats a full overhaul—comforting should the 100LL replacement somehow not replicate the current fuel’s anti-knock performance. There are no weight, packaging, or complexity penalties for high compression, and the mod is relatively affordable. And because the throttle works both ways, the bigger bang compression gives can be taken as either power or fuel economy. The option is open to the pilot at any time simply by moving the throttle.

In fact, should low-octane fuel become an issue, simply limiting manifold pressure via the throttle at lower altitudes will continue to allow operation. This is exactly what classic warbird owners have been doing for ages with 100LL. In the meantime, raised compression is a good fit with 100LL, but not mogas. If you insist on mogas, stick with stock compression.

Roller/DLC Lifters

Roller lifters were introduced by Lycoming to reduce cam and lifter wear, and they worked. But it turns out diamond-like carbon (DLC) coating applied to a flat tappet lifter does the same job for less money, and this is what the rebuilders and performance shops are mainly doing today.

The DLC-coated lifters are great for longevity, which is reason enough to get them. For the power-hungry big spenders, shops such as Ly-Con package DLC-coated flat tappets, a more aggressive re-ground camshaft, and advanced cam timing for a bump in power. Such packages are good hot-rodding for the tundra-tire crowd, but less fiscally appealing to the optimized RV builder.

Increased Displacement

This is an easy one. Larger engines make more power, so fitting a larger bore cylinder, longer stroke crankshaft, or both makes for greater displacement without materially affecting the engine’s overall packaging dimensions. Usually.

Like higher compression, this definitely increases stress on the rotating assembly (crank, rods, etc.) and crankcase. Also, the reciprocating weight is greater and the rod/crankshaft geometry less favorable, so reciprocating loads go up. Piston thrust loads against the cylinder are often raised and, all told, a bit of extra displacement works well, but returns diminish if carried too far. This mod is best for increasing power but not fuel efficiency unless the airplane was desperately underpowered to begin with. This mod also gets increasingly more expensive after the first easy steps upward.

Worth mentioning, noted stroker crankshaft supplier ECI is apparently more open to selling individual parts lately (kit-only sales were the previous policy), making a stroker crankshaft fiscally comparable to a yellow-tagged original, or Lord help you, a new replacement crankshaft. If your project needs a new crankshaft anyway, a stroker is worth considering.

Electronic Ignition

Going electronic on the ignition is a good thing. It saves weight and money in the long run, and can spark a dramatic improvement in fuel economy via ignition advance. The higher and leaner you fly the more you need e-ignition. Our only admonition is the airframe must be electrically robust, with sanitary wiring and a strong ground system. And that’s something we should all be building into our planes these days. If your mags are tired, electronic ignition should make sense.

Cold/Ram Air Induction

A small but useful increase in manifold pressure is possible on medium to faster homebuilts when the engine air inlet is forward-facing and designed to capture ram air. An air filter negates the ram effect, so this either means going without a filter (not a great idea) or adding the complexity of a moveable door or flap to the inlet. Then air routes through a filter on the ground or bug-strewn low altitude and bypasses the filter at altitude (if the pilot remembers to properly configure the inlet). Ram air ducting can also complicate cowling removal and replacement, but the faster the airplane the more valuable ram air is.

Tuned/Cold Air Intake Manifolds

Intake manifolding on popular Lycomings is compromised by running through the hot oil sump, along with too-small plenum areas, and non-optimal runner lengths. Readily available—for a price—aftermarket cold air sumps go far to correct these ills on fuel-injected engines. Because the inlet side of the engine works with so much lower energy than the massively energetic exhaust, optimizing the intake takes priority with hot rodders. This is a great mod, but pricey.

Fuel Injection

Speaking of fuel injection, it is an important step up from carburetion. Not that it makes more power, but rather it is the prime enabler to making all the cylinders run as much alike as possible. Even power cylinder to cylinder facilitates aggressive lean-of-peak operation and can help ultimate power. There’s no carburetor ice, either.

Optimizing the injector nozzles on mechanical injection systems (GAMI injectors) takes this philosophy another step forward.

As for old-school mechanical versus aftermarket electronic fuel injection, the practical advantage of electronic injection is greater bandwidth in fuel delivery accuracy (from idle to big power), plus the data logging and computer control that reduces pilot workload. Don’t expect a meaningful power difference between mechanical and electronic injection.

All the above applies to Lycomings with Bendix or aftermarket injection; Continental uses their own mechanical fuel injection system that’s already on most engines of interest and can be pumped up to handle moderate power increases.

Free-Flowing Exhaust

Most experimenters buy their exhaust systems these days, at least for the hyper-popular kits such as Van’s. That’s a huge improvement over the kinked, mild-steel kluge jobs that once predominated. Those lousy exhaust systems snuffed power and raised CHTs.

Assuming natural aspiration, an uncomplicated but well-executed manifold-style exhaust system is almost always the best compromise among cost, power, back pressure, packaging, under-cowl heat rejection, noise, and fitting a heat muff. Crossover systems on 4-cylinders are a good step-up, but if exhaust optimization is a serious thought, consider going all the way for headers. They’re a financial and sometimes packaging bloodbath, but build torque, are quiet, and are the last word in back-pressure reduction.

Getting back to practical, the real-world exhaust goal is to not pinch gas flow with dumb bends and confused merges. Reducing back pressure is the real prize, as it reduces cylinder-head heating at high power (takeoff and climb).

Reduce Weight

Every pilot’s and designer’s primary directive, reducing weight is always a win, but there are diminishing returns at some point. Firewall-forward weight is clearly and logically saved with lightweight starters and alternators (a given these days), plus smaller batteries. Beware of heat issues with AGM and lithium batteries; these are finicky beasts and should be mounted outside the engine compartment at nearly any cost.

Hot rod engine shops know where to shave pounds off the engine itself, milling off unused lugs, scalloping cylinder base flanges, and so on. This works, and is a favorite of weight-crazed back-country STOL birds where a few pounds of engine weight is a significant percentage of total aircraft weight, but is too expensive and specialized for the average enthusiast.

Install It Like a Pro

We’ll close by saying the integration of the engine to the airframe is a real lottery in the Experimental world; each experimenter is free to install his engine according to his personal needs, preferences, and ability. And yet the installation is one of the most important—and often overlooked—parts of engine hot-rodding. As we improve engine breathing and thus power output, the need to shed heat increases, as can torque reaction forces, vibration, the need to flow sufficient fuel, and provide foolproof electrical grounding among other things. As the engine is more highly tuned the more detail oriented the installation must be. Close all air gaps and consider plenum cooling. Finally, save some coin for an engine monitor—it’s the only way to get the most from an engine.

Closing the Flight Plan

We hope you found the Engine Theory series useful. Suggestions for additional articles should be sent to editorial@kitplanes.com with Engine Theory in the subject line.