To Turbo or Not?

A detailed analysis of the benefits - or lack thereof - of turbocharging on two like model aircraft.

by Paul Bertorelli

Logically, the concept of turbocharging ought to be a slam dunk for piston airplanes. Even modest little cruisers operate up where the air gets thin and where engine performance sags into what can generously be called anemic. Turbocharging fixes that. It stands a downward trending performance graph on its ear and makes high-speed, high-altitude flight where the air is smooth a clear and effortless something-for-nothing tradeoff. Everyone should have turbocharging.

Yeah, right. The ugly reality is that turbocharging-for all its benefits-has the significant drawback of being somewhat unreliable, expensive to maintain and in some models, its supposed performance gains are unimpressive. One reason for that is that, historically, manufacturers have simply slapped a turbocharger on a successful normally aspirated model and crowned it the new King. The results of this approach have been mixed. We cant think of any of these designs that are unblemished winners.

A classic example of this is the Mooney M20K/231, which we recently purchased to replace our trusted M20J/201 lost in a crash in April of 2002. (See Aviation Consumer, July 2002.)

Why not another 201? A good question but the short answer is that the 231 was available, it was clean and relatively low time and contributing editor Coy Jacob, a Mooney dealer, knew the airplane. In other words, we didnt apply the sharp pencil before buying but afterwards, figuring a year of operational experience would reveal if we had made the right choice. (Concession admitted: this is a dumb way to buy an airplane.)

So did we make the right choice? How do the numbers on the two airplanes compare? Would we make the same choice again? In answering these questions, well use the 201/231 comparison as a general analysis of what turbocharging costs and what it delivers.

The Mega View
The overriding reason for turbocharging is speed and the operational freedom to climb into the teens or higher to overtop weather or terrain. For pilots based in areas where density altitude is a worry during the summer, turbocharging all but removes that concern from the equation.

The tradeoff is higher fuel burn-although not always and sometimes not much-higher maintenance and overhaul costs, more operational complexity and, variable with model, less reliability. That last item seems highly arguable in our experience. Owners who have problem turbocharging systems swear at them more than they swear by them. We think its wise to plan on a mid-run turbo overhaul and top overhaul and count yourself lucky if you dont need it.

Lets examine speed first. Mooney had a hit on its hands with the 201 precisely because it was moderately fast on reasonably low fuel flow. Its faster than an Arrow on the same fuel burn but slower than 35-series Bonanza on considerably less fuel flow. Some owners insist 201s are 160-knot airplanes but we think theyre reliably closer to 155-knot cruisers on about 10 GPH.

Flush with success with the 201, Mooney transformed the very same airframe into the turbocharged 231 by replacing the 201s Lycoming with a Continental TSIO-360-GB with a Rayjay/Rotomaster fixed wastegate turbocharger system. It wasnt exactly a marriage made in heaven, for reasons well address later.

What did it do for speed? In the real world, the gains were modest when the two stock airplanes are compared head to head. When taken to mid-altitudes, the 231 can be reliably expected to true between 165 and 175 knots at higher power settings on fuel flows between 11 and 13 gallons. But dont expect to see those numbers below 12,000 feet.

But in exchange for that modest speed improvement, the 231 has an Achilles heel; down low, it is an utter dog. There are circumstances when a trip must be flown low, say for weather or because of short distance. At 3000 feet, the POH says the 231 should true at 148 knots on a standard day at 65 percent power, for a fuel burn in the 11.5 GPH range. In reality, our trials show that the 231 is slower than that, by at least 3 to 5 knots, typically.

Meanwhile, an average 201 will easily top 150 knots on 10 GPH or a bit more at the same altitude and it will climb to that altitude faster. In fact, the 201s climb/speed advantage holds up until about 8000 feet, where it can no longer maintain sufficient power for a brisk climb and cruise. At that point, the curves cross and 231 begins to come into its own, out climbing and outrunning the 201.

Conclusion: if you rarely go high for reasons of preference, terrain or weather, turbocharging may be a distinct liability from the strict efficiency standpoint. Add in maintenance costs and the turbo is a real loser. This low-altitude speed disparity is not typical of every turbocharged model but its generally true that meaningful efficiency gains over a normally aspirated equivalent model lie in middle altitudes, not below 8000 to 10,000 feet.

Going High, Range
Like many owners of turbocharged aircraft, were tickled at having the option of going high, yet we dont do it often. And there are good reasons for this related to trip length, weather and cost. For a trip under 100 miles, its hardly worth a climb into the low teens so we find ourselves mucking along at low altitude, suffering the speed loss. (The 201 wins on that round.)

Beyond 100 miles, climbing is sometimes worth the effort and time. But not always. Into a stiff headwind, the 231s rising true airspeed gradient is often not steep enough to offset higher headwinds. For example, westbound, its not unusual to see winds in the mid-teens that are 20 to 30 knots faster than they are at 6000 feet. At 6000 feet, the 231 can be jollied along at about 152 to 155 knots TAS, about what the 201 will do.

Climbing into the mid-teens will yield true airspeeds nearing 170 knots. But if the TAS gain is 15 knots against a 30-knot headwind, theres a often a wash or even a lower groundspeed at the higher altitude. Absent considerations such as cloud tops, icing or turbulence, theres no compelling argument to go that high just to pay for the oxygen youll need above 12,000 feet. (Score another for the 201.)

In this scenario, the best compromise frequently turns out to be 10,000 to 12,000 feet, where oxygen is optional and the flight can often be made above the tops. The 201 will go to 10,000 feet but not happily and it doesnt have a robust climb rate above that altitude. (Advantage: 231.)

Downwind, its a different story. Eastbound with 40 knots on the tail, clawing up to FL210 is more than worth the effort and time because even though the 231 trues a modest 175 to 178 knots, with a healthy push, the 200-knot-plus groundspeeds we all crave are there for the taking. As we reported in the January 2003 issue of Aviation Consumer, with GAMIjectors installed and the engine running lean of peak EGTs, we can true nearly 160 knots on 9.5 GPH at altitude. In our view, thats transformational efficiency. (Score this round for the 231.)

Which brings us to the all-important range and endurance comparison. If youre a potential buyer of either one of these airplanes-the 201 or the 231-you probably fly longish trips, which is the reason for having it. And if you tend to run legs out to the tank dregs, you probably care about absolute range.

How do they compare? This depends on several variables but the big one is wind. Into a wind, the 231 is hobbled because its additional speed is often not enough to offset the time required to climb to altitude, where its groundspeed will be the same as the 201s at a lower altitude.

In still air, the 231 has a narrow advantage. At 65 percent leaned to best economy-not lean of peak, by the way-its range is about 1000 nautical miles to dry tanks, during which it will burn about 11.5 GPH. Going higher increases the range slightly but so little that pilot technique could easily erase any advantage.

In the 201, also at 65 percent power, still-air range is about 900 miles, burning 10 to 10.5 GPH. In block-to-block time in still air, the 231 wins, since its going a little farther in less overall time.

But when is the air ever still? Theres an unwritten rule somewhere that says if you leave later than you planned or you have to pee, youll be into the headwind from hell. Heres the example of a recent trip from Connecticut to Florida, a typical late winter long cross country with stiff westerlies. Calm winds at the surface, 15 knots off the nose at 2000 feet and 45 knots at 10,000 feet. This is not a good scenario for the 231.

Down low, the book says it should true a bit more than 150 knots on 11.5 GPH. The reality is 144 to 148 knots on the same fuel, which translates to a sluggish 130-knot groundspeed at the lower end of that scale. Climbing higher doesnt help and often hurts when the wind gradient is steep, as it was on this day.

The 201, on the other hand, really does true 155 knots on about 10.5 gallons. Giving up 15 knots to the wind yields a tolerable 140-knot groundspeed. If its bumpy, you either gut it out or climb higher into the greater wind.

Allowing for a reserve of 45 minutes, the 201s range is nearly 760 miles, assuming constant wind. The 231 can manage a shard less, at 720 miles and going higher probably wont help. Bottom line: the 201 will do the trip a little faster and a little cheaper. (Advantage: 201.)

On the return trip, the odds are reversed, especially when the wind gradient is steep. Even allowing for a 35 to 40-minute climb to altitude, the 231s higher TAS and the more favorable winds could give it up to a 25-knot advantage. With the winds reversed, the 231s range swells to a bit more than 1200 miles, allowing for the same fuel reserve. The 201 will have about 1000 miles of range so the 231 wins this round.

Worth noting here is that with GAMIjectors installed, we have typically recorded fuel flows of 9.5 GPH at 155 to 160 knots, running lean of peak. This results in a 5 to 7-knot speed penalty but the additional range more than offsets it. In the conditions described here, the economy lean-of-peak range is nearer to 1400 miles, with the tailwind.

Operating It
The 231 was conceived at a time when light aircraft turbocharging was somewhat of a rarity and sophisticated systems such as FADEC werent even on the table. To describe its design execution as crude is to be generous. In place of the proven Lycoming IO-360, Mooney fitted the 231 with a Continental TSIO-360-GB engine with a fixed-wastegate Rayjay turbocharger.

With no wastegate to dump excess boost overboard, the engine is ridiculously susceptible to overboosting, especially on takeoff. Further, the engine is always under some level of boost, whether it needs it or not. This places stress and strain on the cylinders and valve train and tends to cause the engine to run hotter than it might otherwise, if it had a conventional wastegate and turbo controller. Cooling in the 231 is not the best and theres often a 100-degree plus temperature spread between the hottest and the coolest cylinders. It has cowl flaps, which help, but fiddling with them is just one more thing to do.

Many 231s-we dare not say most because ours didnt have one until we installed it-are fitted with an aftermarket Merlyn upper deck pressure control/wastegate which adds a measure of sophistication to boost management but still doesnt protect against overboosting. You still have to mind the throttle carefully on takeoff.

During climb, the throttle has to be advanced to hold the climb setting and the CHTs have to be eyeballed to avoid overtemping. In cruise, close the cowl flaps-maybe only halfway-fetch the power chart and lean, all the while monitoring the engine analyzer to make sure the TIT and CHTs are in line.

In short, the 231s engine is demanding and finicky to operate compared to the 201, which can be fire-walled on takeoff and all but leaned with abandon at power settings below 70 percent. Like the 231, the 201 has cowl flaps but if the baffling is up to snuff, the engine is considerably overcooled so hot cylinders arent a concern. (Advantage: 201.)

As for reliability and longevity, all we can say is that the jury is still out. At its first overhaul, our 231s troublesome GB engine was replaced with the more robust LB series engine, which is true of many-but again, not all-early 1980s 231s. Owners send us generally positive but mixed reviews on how well this engine holds up to the rigors of typical use. Some owners say theyve made the 1800-hour TBO without a hiccup while others have replaced turbos and/or done top overhauls at mid-time or sooner.

Our view is that the TSIO-360-LB responds to careful operating procedures and preventive maintenance. Avoiding overboosting and managing temperatures with judicious use of flaps may stave off egregious engine costs. The GAMIjectors have significantly simplified engine operating procedures-we no longer bother with the power charts but lean according to target temperatures and acceptable speed. On our to-do list is to add intercooling, which promises to improve performance somewhat at higher altitudes, perhaps bettering the book numbers.

Compared to the 201, the 231s TSIO-360 is more efficient, despite the two extra cylinders, because it can fly high at faster airspeeds on the same or less fuel flow. Advantage 231. But its more demanding to operate and less likely to make TBO without mid-time work. And even if does, its a more expensive engine to overhaul. Advantage: 201.

Buying and Paying
Assuming that turbocharging is done right-alas, it often isnt and that can be said of the 231-it should be highly desirable and thus everyone should want it. Logically, turbocharged airplanes should thus carry a price premium.This is largely true but only by degree and it varies with model. Our 1981 231 is a direct apples-to-oranges comparison with a 1981 201; one has the turbo, one doesnt. According to the Spring 2003 Aircraft Bluebook Digest, the 231 has a value of $109,000 while the equivalent 201 is worth $99,000, a price Delta of 10 percent.

But consider the Bonanza 36 series. The 1982 normally aspirated A36 model lists for $194,000 while the B36TC sells for $234,000, a price premium of 21 percent. Why the disparity? Were not sure but we surmise that it has to do with the 231s checkered maintenance reputation, which is probably perceived as being worse than it actually is. (The B36TC isnt necessarily a prize in this regard, in our estimation.)

If its true that the 231s reliability isnt as bad as nervous buyers might think, its quite a bargain at only a 10 percent premium over the 201. But theres a catch-isnt there always?-and thats operating costs. As turbocharged airplanes go, the 231 isnt vastly more expensive to operate than its normally aspirated counterpart but its still more expensive.

Over eight years of operation, we billed our 201 at $40 per hour dry, allowing $20 for fuel. Within the hourly dry rate, we allowed a generous $12 per hour engine reserve, a rate which would have yielded $24,000 to the 2000-hour TBO, which we fully expected to make. (Never mind that the thing quit and nosedived into a swamp.)

Realistically, we have to budget for a mid-time turbo overhaul and top on the 231, which will add about $6000 to the full-run engine cost. Figuring a basic overhaul cost of $22,000 plus $6000 for potential mid-time work, the hourly rate comes to $15.50 or $3.50 more than we allowed for the 201. Ever the conservative owners, we have boosted our dry hourly charge to $50, plus fuel. This may seem excessive but it has already paid off for the TSIO-360 is a more expensive engine when it comes to fixing incidentals, such as mags, starters and alternators. (A recent alternator drive repair cost $1600-nearly 10 percent of the cost of overhauling a Lycoming IO-360 in the 201.)

There are other expenses, chiefly oxygen, which we find onerously expensive at some FBOs if we can find it at all. Were working on installing our own bottles in the hangar.

So, a year into the deal, is the 231 the right aircraft for us? All we can say for sure is that it doesnt appear to be the wrong aircraft thus far. In reality, the airplane isnt taken high on many trips because theyre either too short or unfavorable winds make the high-altitude schlep a loser. On the other hand, on one out of five trips when the wind is right, life is sweet at 200-knots plus over the ground.

The true attraction of turbocharging, in our view, lies more in its potential than its actuality. Its comforting to know we have the option of going high to overtop weather or terrain, even if we often choose not to do so.

Thus far, we have deemed that worth the marginally higher cost of owning the 231 over the 201. We wouldnt gnash our teeth or kick and scream if forced back into the 201, but for now, well stick with its higher flying cousin.

Also With This Article
Click here to view “Checklist: Mooney 231 vs. 201.”
Click here to view “Engine Costs.”