Maxwells Subaru-based FWF package evolves.


Its only been five years since Gwen and John Maxwell acquired the assets of the Subaru conversion concern NSI, and created Maxwell Propulsion Systems. The company has made several significant improvements to its line of aero engines, recently boosting power output beyond the original 165-horsepower MX1 to the new 205-hp MX2. Both engines are based on the EJ25 four-cylinder Subaru engine-normally aspirated and electronically fuel injected-the extra power for the MX2 coming from a new long-stroke crankshaft. But the big news is that Maxwell is working hard on a turbocharger package for each engine.

Maxwell goes well beyond simply bolting a few things to an auto engine, proclaiming it an aero engine and then putting it in a crate for shipping. Every MPS engine is completely rebuilt from the crankshaft outward, blueprinted and carefully assembled using only the highest quality components available. This is all done with the realities of continuous, high-rpm airborne operation in mind. Gone are the stock Subaru ECU systems that feature a limp home mode thats unsuitable for aviation. Gone are the automotive style throttle bodies, replaced by a simpler manually controlled version. Also gone are any other components that just wont stand up to the unique rigors of flying.

Many builders who recognized the promise of the Subaru years ago have been frustrated in getting their engines to perform reliably in an aviation role. Sometimes the packages fail-though, usually, the core engine is fine, it has been the added components that let pilots down-and other times its support thats sorely lacking. Maxwell Propulsion has heard these all-too-common tales of woe and taken them to heart, and its engines seem to confirm the company’s dedication to getting it right.

The MX2 engine is externally similar to the 165-hp MX1, but packs 40 more ponies thanks to a longer-stroke crankshaft.

Favoring the four-cylinder, single overhead cam EJ25 series over the longer and heavier H-6 Subaru, Maxwell builds both the MX1 and MX2 engines from a carefully selected group of parts. For example, the new 205-hp stroker MX2-NA starts with the stronger EJ257 block originally designed for the STi Turbo; a billet crankshaft is mated with forged 4340 connecting rods and pistons, all rotating on ACL racing bearings. The proprietary camshafts are custom-made for Maxwell, as are the higher-output 10mm oil pump and the throttle body. The stock 79mm stroke is increased to 83mm, flat-top JE pistons are used with valve relief, and the compression ratio is increased from 9:1 to 10.4:1. The increased stroke raises the displacement from 2.5 to 2.6 liters; the 99.5mm cylinder bore remains unchanged.

Given the absolutely crucial dual role of lubrication and dissipating heat, significant attention is paid to the oil system. The oil capacity is increased with a new oil pan, and proper internal lubrication and cooling is assured by increasing main bearing clearances from the automotive minimum of 0.0007-inch to a more acceptable range of 0.0018 to 0.0020.

Meet Dominic

Why do the MPS engines need larger bearing clearances? Maxwells head engine builder, Dominic Acia, has spent years building Subaru engines for high-performance automotive racing use, and he sees many similarities in aviation applications. People always underestimate the cooling function of oil, Acia said. Thats why we increase the oil capacity so much, and make sure the bearing clearances are big enough to allow for proper cooling. In the automotive world, the engine cruises along, producing about 35 horsepower, so you don’t need much oil flow through the bearings. But when you’re consistently operating above 4000 rpm, you need to make sure you have enough oil flowing constantly to prevent the rod bearings from welding themselves to the journals.

The larger clearances allow up to 250% increased oil flow through the bearings, which is critical to engine reliability and longevity. Acia also noted that the bearing material itself has been upgraded to withstand continual high-rpm service-overkill for highway cruising but essential for continuous airborne operation.

The PSRU is another item that has been vastly improved over the last five years. Using a clean-sheet approach, MPSs spur-gear system hardly resembles its NSI predecessor. Maxwell has performed exhaustive torsional vibration testing to confirm it would stand up to the rigors of both engine and propeller loads. Using eight elastomeric bushings and helical-cut spur gears, the MPS PSRU appears to shrug off the varying power pulses from the engine, and showed no discernible wear after 75 hours of operation. The company anticipates a TBO of 1500 hours, the same as the MPS engines.

Shown here in mockup form, the Garrett turbocharger will reside behind the engine and be managed with an auto-derived wastegate.

So High

Acia recently confronted the typical performance loss with altitude found in any normally aspirated engine. His solution is to offer two different turbo packages, one for the 165-hp engine and another for the 205-hp version, raising power output to 195 and 240 hp, respectively. There are some minor differences between the two, Acia said. The 195-hp version doesn’t have an intercooler, produces 3 psi of boost (in cruise) and has slightly different CPU mapping. The 240-hp version uses a Maxwell-designed, Bell-manufactured intercooler and has 6 psi of boost. They both use the Garrett GT-32 turbocharger with a 44mm wastegate. Although the Garrett turbo is rated for airflow of up to 49 pounds per minute, the Subie needs only 19 to 26 ppm. Both systems will produce up to 36 inches of manifold pressure for takeoff.

Turbo response time in aviation also differs from whats considered optimal in automobiles, and Acia factored this into his turbo selection. I wanted a large turbo housing to give a slow turbo response time, Acia explained. The last thing I wanted was a turbo running all over the place whenever the power setting was changed. The instant throttle response auto engineers work so hard to achieve is not necessary in a steady-state application such as aircraft.

The whole goal of any turbo installation is increased engine power output at altitude, and here Acia has struck a balance between reliability and maximum performance. On the test stand, we could get up to 26 pounds of boost resulting in well over 300 horsepower! he said, laughing. But that just doesn’t work for long in real life. Initial tests indicate that the 195-hp turbo package will produce sea-level performance up to 15,500 feet density altitude, and the 240-hp version will maintain sea-level power up to 11,500 feet.

Acia said they considered using a liquid-cooled turbo unit, but the added complexity and plumbing required wasn’t worth the effort, so they settled on a version that uses simpler oil-cooled journal bearings. We found that the turbos have plenty of time to cool during taxi after landing, so water cooling just isn’t needed, he said. Both turbo installations run at speeds from 96,000 to 128,000 rpm, depending on the power setting and engine version.

I asked company pilot Ephraim Carter what the practical service ceiling would be with the turbo installation in the company Sportsman, and after sharing a knowing glance with Acia, he said, With the normally aspirated 205-hp engine, we’ve been up around 12,000 feet, but we still had plenty of climb performance left, around 500 to 600 feet per minute. Thats one thing we still need to do with the turbo, is go out and see how high we really can go. I just need to get some oxygen aboard first.

There’s always a price to pay for anything worth having, and the Maxwell Propulsion firewall-forward packages are no exception. There’s the initial investment, of course, and the added weight of the turbo system and the intercooler (with the 240-hp version). The 195-hp turbo system will add 25 pounds over the normally aspirated model, and the 240-hp system adds another 10 pounds, 35 total, to engine weight. Without the turbo system, the complete FWF packages weigh about 407 pounds.

Final costs are ultimately determined by whatever options the customer chooses, but its estimated that the 195-hpr turbo engine package will cost about $33,000. (The $27,000 base price does not include the Maxwell developed propeller speed reduction unit [PSRU], the dual-ignition option, the motor mount, the propeller or spinner kit or several more options designed to provide cockpit heat, enhance engine cooling or to provide 60 more amperes to the electrical system.)

The 240-hp version will likely be about $36,000, Acia said. If this seems high, consider that these prices also include the Whirlwind electrically adjustable (though not constant-speed) propeller and spinner, the engine mount, modified cowling, wiring, all cooling and engine accessories, and even a test run and proper dyno break-in on the company engine stand. You can even be part of the engine blueprinting and building process, which takes 10 to 14 days, for an additional $1500. You can pay us to slow us down, Acia joked.

Maxwell Propulsion touts the promise of increased efficiency of a modern, fuel-injected powerplant, one that uses carefully mapped electronic ignition precisely controlled by a CPU. According to Acia and Carter, cruise fuel consumption in the normally aspirated 205-hp Sportsman now averages 7.4 to 7.9 gph at between 8000 and 10,000 feet. This works out to around 55% power, similar to the fuel consumption of a properly leaned Lycoming IO-360-but there’s the benefit of the computer doing all the work. Speeds are comparable to a Lycoming-powered Sportsman. If the turbo performs under flight test as expected, however, the MPS Sportsman test bed may turn in some impressive high-altitude performance numbers. Rough calculations indicate that a 165-knot TAS would be possible at 15,000 feet, and 175 knots is likely at 20,000 feet.


The MX series engine FWF package has an acquisition cost close to what a certified aircraft engine-based installation of similar power might command, but the company urges potential buyers to consider some other factors. While the anticipated TBO of 1500 hours is lower than a typical Lycoming, the estimated overhaul cost is just $5000. The normally aspirated engines can be operated on 87-octane regular auto fuel, possibly saving some money. Thats assuming you live where ethanol isn’t a mandatory auto-gas additive; the engine wont mind but components in the airframe fuel system might.

Other considerations: Any auto conversion-equipped airplane is best served by having some mechanically inclined person nearby, in case of a malfunction where repair expertise would be needed. Ideally, this would be the owner/builder/pilot of the airplane, as many A&P mechanics are understandably hesitant to provide service on an ad hoc basis. However, the Maxwells have steadily worked toward improving the state of the art of Subaru-based conversions, and have brought decades of combined experience in aerospace and business management to the company. They have also recruited the engine design expertise needed to improve the product line and increase reliability.

The Maxwells faith in their FWF packages can be demonstrated by the number of flight hours that Gwen Maxwell has racked up in the company’s Subaru-powered Sportsman, as she and Carter travel to aviation events. Apparently, there’s little hesitation to fly from home base in Arlington to all corners of the country to showcase the MX series of engine packages to potential customers.

Given the significant number of improvements that MPS has made to the Subaru conversion over the last few years, and the company’s constant drive to increase the engines performance, reliability and efficiency, there’s a good chance the Subie four-banger will eventually be found on the nose of more projects. Currently targeted primarily at RV and GlaStar/Sportsman builders, these engines may well find their way into additional Experimental designs soon.

For more information, call 360/474-8118 or visit www.maxwellpropulsion.com.

Rick Lindstrom has been fascinated with motorized devices since the disassembly of his moms Kirby vacuum when he was 3, predictably followed by record player motors and lawnmower engines. After he learned to fly, it was only natural that he gravitated to the world of alternative aircraft engines. He currently pilots a Corvair-powered Zenith and is undecided about what will power his GlaStar.

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Rick Lindstrom
Rick Lindstrom has been fascinated with motorized devices since the disassembly of his mom’s Kirby vacuum when he was 3, predictably followed by record player motors and lawnmower engines. After he learned to fly, it was only natural that he gravitated to the world of alternative aircraft engines. He currently pilots a Corvair-powered Zenith and is undecided about what will power his GlaStar.



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