Aircraft fuel injection is supposed to provide even distribution of our beloved power- producing fluids for greater efficiency and smoothness. With a supposedly equal volume of air (and, in the right proportion, fuel) for each cylinder, each jug's power output should be about the same, improving the balance of combustion events and resulting in a smoother-running engine. Likewise, by seeing to it that each cylinder operates at the same fuel/air ratio, fuel injection would theoretically make it possible to use a leaner mixture before "lean misfire" sets in.
Since fuel injection became widely available to general aviation in the 1950s, we've assumed that the extra cost and complexity of this system was outweighed by its performance and operational advantages. Besides, it can't develop icing in the same way as a carburetor, so you need not keep one eye on the carb-temp gauge when in weather conducive to formation of ice. And fuel injection is not subject to the vast differences in mixture distribution at various throttle settings. (A slightly cocked throttle in a carbureted setup can create a mixing of the fuel and air that may help or hinder inter-cylinder distribution.)
Although fuel injection has by and large delivered on the theoretical promises, it's not correct to assume that each cylinder receives fuel and air equally — at least not in the majority of the installations. Sometimes the results of the maldistribution are slight — showing up in the inability of the engine to run smoothly at lean-of-peak exhaust-gas temperature, for example. At other times, the differences appear in high cylinder-head temps and recurring mechanical distress in certain cylinders. With the advent of multicylinder EGT and CHT instrumentation, pilots began to realize that not all cylinders' EGTs peaked at the same fuel flow. On a large-displacement engine, for example, the difference between the first cylinder to peak and the last can be 1.5 gph — although some installations display a greater spread. (One reminder here: The absolute value of EGT is unimportant with regard to determining mixture distribution. Rather, the relationship of the EGT peaks relative to fuel flow is what bears scrutiny. More later.)
Now combine that with the idiosyncrasies of the various installations and the mixture maldistribution takes on greater meaning. In many Bonanzas, for example, the number two cylinder (left, aft) is most often the hottest-running, CHT-wise. It also turns out that number two is often one of the first two cylinders (number one being the other) to reach peak EGT — meaning also that anywhere in the fuel-flow range, this cylinder runs leaner than the majority. For some time, owners have tried to mitigate the high CHT by swapping the stock fuel injector nozzle in number two for one with a greater flow rate, thus bathing the cylinder in fuel in the hopes that it would run cooler and last longer. (Other big-inch Continental installations — particularly those with forward-mounted oil coolers — have a different CHT spread and different hottest cylinders, but the fuel imbalance remains.)
Unfortunately, this practice of swapping injectors, while often successful, is illegal. The FAA and the engine manufacturers frown on home-brew injector juggling- -Continental offers the nozzles in a variety of flow ranges — or, worse, backyard nozzle modifications, saying that if the engine didn't come with different-sized injectors, it isn't airworthy in that configuration.
While the idea is sound — alter the fuel flow of individual injectors to suit both the intake-air flow and the cooling-air flow — there was no reasonable way to make the change legally. It took General Aviation Modifications, Inc., of Ada, Oklahoma, to make the procedure legal. Company principals George Braly and Tim Roehl had heard plenty about the advantages of tailoring injectors for better performance and surmised that getting supplemental type certificate (STC) approval should be relatively easy. More than a year later — and after countless hours of labor; design and implementation of a sophisticated 64-channel, PC-based data logging system; and numerous test flights — GAMI's first product is here. Called GAMIjectors, the kit includes six injector nozzles that replace the stock Continental pieces. No other fuel- system changes are necessary. Not only do the GAMIjectors optimize fuel distribution, but the nozzles themselves are calibrated in-house to a standard twice as exacting as Continental's. GAMI's STC covers 28 models of big-bore Continental engines, including most in the 470, 520, and 550 series. Although testing was performed on a Bonanza, the FAA issued the STC for all related engine families because of intake-system similarities; if you have an engine listed in the STC, the GAMIjector mod is approved regardless of the airframe. Introductory price is $649; the regular price, to take effect after AOPA Expo, is $749.
To comprehend the improvements brought by the GAMIjectors, it helps to understand how the updraft runner intake system on these large-bore Continentals works. In the Bonanza, intake air comes through a filter at the front of the cowling, passes through the throttle body assembly, and then splits off for the two banks of cylinders, left and right. Each bank is supplied intake air by a tube that joins with short risers to each cylinder; this tube is the same inside diameter along the length of the bank of cylinders. (Other installations are substantially similar.) Fuel comes in from the top, injected directly into the intake ports immediately above the intake valve. All aircraft injection systems are of the continuous-flow variety, meaning that fuel flows through the nozzles whether the intake valve is closed or open.
Because the intake tubes for each cylinder on a bank draw from a common conduit — and the length of the individual risers is short — there are some unwanted interactions among cylinders. (Long, tuned intake runners used on other Continentals get away from this.) For the home-brewers tinkering with injector sizes, the conclusion has been that the first cylinders in the line (being the rear pair in the Bonanza) "get more air" and so, to make the fuel/ air ratios balance, they would need more fuel.
Turns out, however, that there's something else entirely going on here. According to GAMI, after a tremendous amount of testing and many discussions with its hired designated engineering representative (DER) — who had been head of engineering for Teledyne Continental — the air's not the thing. Instead, it's a fuel imbalance.
Consider this: As the fuel stream continues from the injectors for the three- quarters of the time the intake valve is closed, some of this fuel migrates downstream, following the intake-air path to the next set of cylinders. The next cylinder in the line (for our purposes, the center set) receives both its intended air flow and a misting of residual fuel from the previous jug; combined, its mixture will be richer than that of the jug ahead of it in line. Finally, the last jug in the row gets a bit of fuel from both the first and middle cylinders, making its fuel/air ratio richer still. To further complicate matters, the design of the induction system on most Bonanzas places the richest cylinders in positions of the best cooling-air flow. So the jugs that require the least cooling — those running richest — get the best, and vice versa.
Knowing this, it's easy to understand how well-meaning owners intent on cooling the back cylinders inadvertently enrich the forward four, which don't really need the extra fuel.
Moreover, if you watch your multicylinder EGT display — or better yet, graph the data — during leaning, you'll see the results of this intake system's quirks. The accompanying graphs, from GAMI's flight-test runs in a Beech A36 equipped with a stock 300-horsepower Continental IO-550-B, tell the tale. The data point is 75- percent power at 5,000 feet pressure altitude. Gradually leaning from a full-rich mixture, the first cylinders to reach peak are numbers one and two, the back row opposing pair. Cylinders three and four reach peak next, about 0.5 gph after the first peak, with the forward pair (five and six) hitting peak about 1.1 gph later. By the time the last cylinders reach peak EGT, the first (number two) is running 40 degrees Fahrenheit lean of peak.
From an operational standpoint, this imbalance means that pilots operating at a setting rich of peak are pouring excess fuel through the front four cylinders. Lean in cruise for 25 degrees F rich of peak for number two and you'll be burning 15.9 gph (compared to 14.5 gph at number two's peak); one cylinder will be running 80 degrees rich of its peak. Try operating at 50 degrees F rich of peak and the disparities are even more pronounced; the last cylinder to peak is running nearly 100 degrees F rich of peak.
Now check the chart with the GAMIjectors installed. Same airplane, similar conditions and power settings. Cylinders three through six peak very close together, at about 14.2 gph. At this setting, the hot, back pair of jugs are running about 20 degrees F rich of their peaks, which comes at 13.4 gph.
What does all this mean? For one thing, you will save fuel. Without the need to douse the majority of the cylinders just to keep the first-to-peak happy, you can expect an overall reduction in fuel flow of about 1 gph in cruise. (An owner flying 150 hours a year might save $300 in fuel with no decrease in performance.) Moreover, the GAMIjectors should help keep those back cylinders cool — the time-honored tradition of cooling with fuel applies here. GAMI reports a CHT drop of as much as 17 degrees F on the number-two cylinder. For some operators — particularly those for whom the hottest cylinder runs more than about 425 degrees F — this CHT reduction may make the difference between recurring heat-related problems and longer cylinder life.
In testing, GAMI also discovered that the modified engines were quite willing to run smoothly well lean of peak. For most stock installations, roughness comes with running much more than 25 degrees F lean of peak. Several operators with the GAMI modification have reported smooth powerplants at 50 degrees F lean of peak, and a few others claim smoothness well beyond that. (Engine manuals for the older big-inch Continentals do not recommend running lean of peak, although the fundamentally identical later engine models are allowed to do so.)
You would think that operating an engine so far lean of peak would result in significant roughness, because of what has long been attributed to lean misfire. But the GAMI crew believes differently. It's possible, they say, that what pilots have been experiencing on the lean side of the mixture curve has been power-pulse imbalances, not the repeated misfiring of one or more cylinders whose fuel/air ratios were too lean to support combustion. This situation was exacerbated by the qualities of the standard fuel distribution — that is, the cylinders to run leanest weren't physically opposite each other — and the steep drop-off in power on the lean side of peak. (Graph power output relative to fuel flow and you'll see that it is relatively flat from well rich of best-power mixtures to peak — but that the power decreases fairly sharply lean of peak.) But with the GAMI setup, opposing jugs reach peak in near lockstep, which the company believes is why a modified airplane will, with all apparent happiness, run well lean of peak.
Attempting to balance the fuel/air ratios among cylinders is not new thinking, of course. A relative handful of Continental's engines were designed with balanced induction systems — you'll find them on the Mooney Ovation and 252, the Piper Malibu, and a handful of kit aircraft using the IO-360 and IO-550 engines. These models use equal-length intake runners and top-mounted intake ports that, in theory, help equalize intake-air distribution and reduce fuel-carryover characteristics. In many cases — notably for the Malibu — operation lean of peak was recommended, even at high power settings. And, miraculously enough, the engines would perform smoothly well down the back side of the fuel curve. Food for thought for the GAMIjector buyer.
Anxious to try a set, Associate Editor Pete Bedell installed a set of GAMIjectors on the family Beech D55 Baron, which has IO-520 Continentals. After gathering data from the onboard Insight Instruments Gemini 1200, it was clear that the GAMIs did what they were supposed to. The spread in fuel flow between first-to-peak and last- to-peak fell from 1.8 gph to 0.6 gph. Because the Baron installation is already well cooled, there was no big reduction in CHT. Bedell says that while the improvements are quantifiable, it's unlikely that he and his brothers will fly the Baron at or lean of peak. "We've had excellent luck with these engines, so we're hesitant to fly any differently." He admits to looking forward to the 2.4 gph fuel savings, though. We also plan to install GAMIjectors on the company A36 Bonanza; we'll report on the long- term effects.
Are there any drawbacks to the GAMI system? So far, none from the operation side. Maintenance-wise, you must make sure that any injector removal is followed by replacement of the correct nozzles in the correct cylinders. To make matters easier, the GAMI STC calls for applying placards to each valve cover, denoting the correct injector number for that cylinder.
Work is under way at GAMI to gain approval for turbocharged Continentals (an STC is expected late in the year), and the staff predicts that injectors for other fuel- injected engines are on the horizon. At the least, GAMI has legitimized and vastly improved upon a longstanding (and oft misunderstood) shade-tree modification. Consider it the end of don't-ask, don't-tell injector swapping.
For more information, contact General Aviation Modifications, Inc., 2800 Airport Road, Hangar A, Ada, Oklahoma 74820; telephone 888/359-4264 or 405/436-4833, fax 405/436-6622.
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