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Tim Sullivan's Subaru-Powered Super Zodiac CH 601 HDS

Part 2 - The Engine | Part 1

Story and photos by Pat Panzera, EAA 555743


In the June 2009 issue of Experimenter, we featured Tim Sullivan’s homebuilt Zodiac in part one of this two-part series. We focused on the airframe, a few of the systems, and many of the custom details, reserving the engine installation for part two.

Stratus Inc., founded in 1992 by Reiner and Petra Hoffman, is known for converting the reputable 1.8-liter Subaru EA-81 engine for use in experimental aircraft. On May 1, 1999, Mykal (Mike) Templeman became the new owner of the company, which is now known as Stratus 2000 Inc.

The converted Subaru engine powering Tim Sullivan’s Super Zodiac CH 601 HDS incorporates a belted propeller speed reduction unit (PSRU) with a 2.2-to-1 ratio, engineered exclusively for Stratus. The PSRU is composed of a 60 mm Gates HTD drive belt coupled to hard anodized drive sprockets. In addition to the reduction drive, Stratus engines are fitted with a lightweight custom intake manifold, dual Bing altitude-compensating carburetors with filters, an ignition coil (an optional second ignition system is available), an electric fuel pump, an engine mount that incorporates Lycoming-style bushings, plus your choice of prop extensions.

The Stratus core engine is “zero-timed” to factory specs, produces 100 hp at 5400 rpm, and has an estimated time between overhauls of 1,000 hours. Before shipping, Stratus engines are completely assembled and test-run.

Although Stratus sells turnkey engines (EA-81 and the EJ-22 Legacy), it also sells the parts to not only convert your own engine, but also to rebuild the donor engine to factory specifications. Tim opted for the do-it-yourself route and purchased parts from Stratus. He chose to stick with the dual altitude compensating Bing carburetors, as recommended by Stratus, because it’s a proven system.

At the time Tim built his plane, the ready-to-run EA-81 conversion from Stratus cost approximately $7,000. Tim has a little more than $3,000 invested in his conversion (the PSRU alone cost approximately $2,500), and $5,000 total invested firewall-forward. The current price for a turnkey EA-81 from Stratus is approaching $9,000, before taxes and shipping, both of which can be avoided by picking up the engine at its Oregon facility.

CONTACT! Magazine, issue #71, contains the most relevant Subaru article published to date. “An Illustrated Guide to the Subaru EA81,” written by Paul Messinger, graphically details the subtle differences from one series of engine to another. If you are considering a Subaru conversion, this article is a must.

Once the engine is removed from the car, it’s extremely difficult to identify which engine it is without dismantling it. The serial number is of no use, and you could end up with a really bad combination of parts if certain precautions aren’t adhered to.

For example, the cam needs to be modified to match the intended rpm range. As a stock cam is reground to net an altered lift and/or duration from the high side of the cam, the base circle needs to be reduced. If the base circle is reduced too much, longer pushrods will be needed, as there’s only so much adjustment available from the rockers. It’s important that at least one full thread be exposed above the locknut after adjustment. The heads are usually resurfaced 0.040 inches to help with the valve train geometry and/or to increase the compression ratio, but in some cases, this is still not enough and the pushrods end up being too short.

Tim is no stranger to rebuilding engines, having multiple VW and Audi projects under his belt. According to him, there’s no need to mess with turning the crank undersized and buying other than standard bearings with these engines. He basically said that should the crank not measure up to spec or be in any way less than perfect, get another. “Toss it; you go to the junkyard and buy another one for 25 bucks. I bought this entire engine from Pick-A-Part on half-price day for $75,” Tim explained. When I reminded him that with the $75, he also has some sweat equity invested since he had to remove the engine, he went on to say, “You just pull it with an axe! Break everything off around it and pull it out, maybe 30 minutes max.”

“I got this engine out of a GL Wagon that had 100,000 miles on it, or thereabouts. When I was rebuilding it, I took the block and a crank to an engine rebuilding shop, and they checked everything to make sure that it was still within tolerances. It was like new. (This is a reoccurring statement I hear from virtually every Sube engine builder I’ve spoken with. ~Pat) Then when I assembled it I used Plastigauge to double-check it with a mechanic friend that came over to help me out.”

Tim was pleased across the board with the service he received from Stratus, in addition to the quality of parts. “You can just buy whatever you want, like this 55-amp alternator. I think I paid $200 for the alternator and bracket, as well as the wiring instructions and the tech support; I could call Mike anytime, and he’s right there to answer any of my questions.”



Pictured above: Bing altitude-compensating carburetor, K&N air filter with custom “p-trap” attachment, and custom intake manifold, all of which comes from Stratus as a complete bolt-on package and can be purchased independent of other packages.

Having a strong construction background, Tim recognized many of the components that make up the Stratus intake system as simple, off-the-shelf hardware. The intake manifolds themselves are pre-bent handrail 90s (used in the manufacturing of custom railing), with simple flanges welded to each end, and the 90s used to connect the air cleaner to the carb are half of a commercial-grade PVC sink drain trap. Other parts that come off the shelf from a plumbing supply house would include the copper thermostat housing shown below.


Tim has an unnatural fear of things overheating. “It seems like everything I’ve ever owned has overheated at some point or another,” he said. So, his system is overbuilt, by his own admission. A custom-built, $500 all-aluminum racing radiator that has the same exact dimensions as the Volkswagen Rabbit diesel radiator (which is specified by Stratus) completely ducted and mounted solo on the belly of the plane is what makes up the heart of the cooling system. “People were amazed I went that far; I know it’s a lot of drag, but I really don’t care about drag. I care about this thing not overheating,” Tim said.


“What other people usually do, they put the small automobile-like air conditioning condensing radiator inside the cowl, but that’s just too scary to me. I’d rather have something high drag that I know for a fact will not overheat, and with this radiator, I can hit it with 120 psi of pressure and it won’t blow.” The stock diesel Volkswagen Rabbit radiator that Tim originally installed began to leak on its own, even though it wasn’t run. “It had plastic ends and a brass core, and just with it sitting under there for six months it started leaking,” Tim said.

Radiator as viewed from behind the plane.

Tim said he used seat-of-the-pants engineering for the location, position, and mounting of the radiator as well as the scoop inlet and outlet sizing. “Well I kind of guessed. I looked at photos of other styles and just kind of guessed at it,” he said. “I just thought it’d look good right there; I wanted to put the weight in between both wheels so it was centered on the airplane.” Tim was certainly happy with his decision when he met a gentleman from the San Francisco Bay area who also had an HDS with a Subaru engine, and sure enough, his radiator was in the same place. “I drove up there and flew with him for a couple of hours, and it just so happens his radiator was in the same exact place. And he said he just guessed on where to put his, too.”

The all-aluminum radiator was built by a local custom radiator expert, known for supplying the off-road automobile racing industry with top-notch products.

Necessity being the mother of invention, when Tim hit a wall trying to locate pre-bent tubes to weld to straight pipe, he opted to chop a few 22.5-degree cuts. A few welds later and his once straight tube now turns 90 degrees.

“I didn’t order any of the cooling from Mike at Stratus or Zenith,” Tim said. “I built this all from raw stock. The piping going to and from the radiator is cut from stock aluminum tubing, mitered with a chop saw, tack welded where I needed it, and then I took brought it to a professional welder.”

Tim had difficulty finding pre-bent aluminum tubes, but it might have been difficult to keep things tight to the firewall and the bottom of the fuselage with off-the-shelf bends anyway. (However, I’ve had very good luck ordering mandrel-bent aluminum and stainless steel tubes from Burns Stainless. It even carries Inconol for our rotary-engine friends. ~Pat)

Being conscious of appearance and weight, Tim opted to go with aluminum, as opposed to the more traditional standard copper pipes and sweating the fittings, as his instincts were leading him since he is a certified plumber.

Since the Bings are altitude-compensating, there’s no need for mixture control. With the air cleaners tucked inside the cowl, hovering over the engine case, Tim didn’t feel the need to install carb heat. But just as a safety precaution, he installed a temperature probe so he can monitor the “ambient” temperature under the cowl, near the air cleaners. As a side note, with his current carb placement, the engine runs poorly when not cowled. “For some reason when I run this thing with the cowling off, it runs really bad,” he said. “I put the cowling back on, changing nothing else, and it runs just perfect.”


Tim had concerns with the stock (Comanche-style) cowling inlet allowing water in and drenching the top of his water-cooled engine, so he blocked it off internally with a baffle just behind the PSRU, which does benefit from fresh air for cooling. With his radiator being located on the belly it doesn’t need air from the cowl inlets, so he figured there would be no harm in essentially shutting off the engine compartment from the outside.



A cabin heat box is mounted on the firewall and is used to divert heated air from the back side of the oil cooler either into the cabin or overboard.

The baffle also gave him a great place to locate the oil cooler, which is fitted for an extra blast tube, though it cools great without one, probably too good as he never sees more than 110°F; a bypass is in order and on the agenda to get the temps up to at least 160°F. On the back side of the baffle is an oil cooler outlet blast tube that leads to a cabin heat box designed to either bring heated air into the cockpit or dump it overboard. Additional blast tubes come off the back side of the baffle and cool components such as the alternator on an “as-needed” basis. The aluminum baffle also acts as a great heat sink for his MSD coil joiner. Tim was cautioned that some form of electrical insulation should be maintained between the joiner and its mount, including the mounting hardware.



The final fit and finish of the cowl is first-rate. Tim was quick to point out that he has 30-some odd hours in detailing the cowl and getting it to fit as it does. Zenith not only supports but also embraces alternative engines. Tim started with a Rotax cowl, and with the addition of the fiberglass parts provided by Zenith, the cowl fits the Subaru installation perfectly.

With the addition of the Zenith-provided blisters and intake scoop, mixed with a little elbow grease, the Rotax cowl is neatly converted to fit the “Sube.”

The installation and tracking of the belt wasn’t difficult for Tim. He’s very mechanically oriented and pays close attention to detail. “Mike sends you special wrenches that turn the eccentric for tracking, and along with a gauge for setting the belt tension, that’s all you need. Although Tim’s intuition told him that he’d have tracking problems, once it was set, it’s been perfect ever since. The recommended interval between belt changes is 300 hours or one year. Tim’s conservative nature dictates that he change his belt well before 300 hours, as the belt replacement is simple and relatively inexpensive at $100 for the belt itself.


Stratus recommends the ground-adjustable Warp Drive, so Tim opted for the three-blade version but didn’t go for the tapered tips. The Warp Drive spinner came with the prop. “I wanted a spinner that would wrap around the prop hub like spinners usually do, but the people at Warp said, ‘Oh, just try this one. If you don’t like it, if it doesn’t look quite right, please send it back; we’ll give you your money back, and you can get another spinner.’ But I ended up liking it,” Tim said. “I thought it would look dorky, but I like the mechanical look of all these nuts and bolts showing.” Before he installed it, Tim spent a few hours on the buffing wheel getting the hub to shine.

The portion of the engine mount that is attached to the firewall came from Zenith and is designed for a Rotax 912. Stratus supplied, in essence, adapter pieces that allow bolting the Subaru engine directly to the Rotax attach points. It’s simple, clean, and clever. Tim had the parts powder coated blue.

The exhaust system and the engine mount brackets provided by Stratus were used to adapt the Subaru conversion to the stock Rotax-to-601 engine mount.

The custom exhaust system, including the muffler, was completely fabricated by hand from stainless steel by a friend of Tim’s who is an expert in thermal research and development. With top-of-the-line equipment, he operates a business where he makes custom mail-order exhaust systems. After owning and operating a muffler shop for several years, he found that fabricating his own bends and mufflers was the way to go. He could make his own supplies for half the price of what he was paying wholesale, so he became a full-fledged manufacturer. Tim is very pleased with the exhaust in general, but is especially happy with the exhaust note. “I was just going to put two peashooters straight out, straight exhaust. I like loud stuff, but I’m so glad I didn’t do that,” Tim said. At one time the exhaust was really polished out but it dulled up after a while. The builder wanted Tim to send it back to him to get it ceramic coated once it was proven, but Tim’s against that idea now. He likes it as-is.

To ease fabrication, Tim had the engine mounted to a mocked-up firewall, affixed to an inexpensive rolling engine stand that allowed the engine to be rotated upside-down.


Stratus offers both single and dual ignition—although it prefers to refer to it as a backup system. Although the dual ignition does not use redundant spark plugs or wires, nor does it use dual distributors, rotor or cap, there are two coils and two sources of timing. The standard single ignition uses an otherwise stock Subaru (but remanufactured) distributor, with the bone-stock Subaru automobile electronic pickup. The optional secondary ignition uses a GM-style sensor, but it’s looking at the flywheel on the redrive. The pickup itself is attached to the side of the engine block as seen below.


The “brain” of the second ignition unit is nested inside the cowl, close to the location of the pickup.

Anyone looking in the engine compartment of Tim’s plane will instantly notice the liberal use of Firesleeve thermal hose protectant on his fuel lines. “All this fireproofing red rubbery stuff, I bought a big roll of it from a place in Burbank that I get all my AN fittings from; it’s like an aircraft salvage hardware store. So I just paid pennies for all this stuff.” Tim once read an article about someone who had an in-flight engine fire, and it had an impact on him. “This stuff gives you a few more minutes of protection. I mean it’d look a lot better if I didn’t have it on there; I have all these nice shiny stainless hoses all over, but I figured I’d be a little safer with it in place.”

Tim didn’t bother to weigh his firewall forward package, but his empty weight is on the heavy side for an HDS. The factory specification calls for 590 pounds, empty, while Tim’s plane tips the scales at a whopping 730 pounds, making it around 140 pounds heavier than the Rotax 912-powered prototype. But how much of that is from the engine and related components, and how much of it is from the installation of the auxiliary wing tanks, full instrument flight rules panel, venturi, electric vacuum pump, and all the other fun things Tim installed? I guess we may find out if and when Tim ever has to remove the engine and remembers to weigh the thing. Oddly enough, with both of the batteries and the electric vacuum pump located behind the seats, and with the radiator located spot on the center of gravity (CG), the weight and balance came in perfect the first time with no need for ballast to offset the weight of the engine. With that, and with Stratus’ claim of 200 pounds for the converted engine, it seems entirely possible that Tim came in at or under Zenith’s requirement of 265 pounds maximum, firewall-forward.

Zenith sells optional wing tanks for the HDS and specifies their weight as a total of 30 pounds. With that in mind, Tim’s wing locker tanks (as mentioned in the previous story) could easily account for 21 percent of his plane’s obesity. As a side note, with his tanks located directly on the CG, the placement and subsequent use of fuel in these tanks has a negligible affect on CG.

The plane was designed around a gross weight of 1,200 pounds. Since the extra fuel tanks are located in the wing as opposed to the fuselage, Tim feels confident that upping the gross an additional 60 pounds to 1,260 is reasonable, and as the builder, it’s his prerogative to do so. 

Technical Specifications for the Stratus EA-81 Engine

Engine type

All-aluminum, four-cylinder, four-stroke, liquid-cooled, pushrod, normally aspirated


1781 cc (109 inches3)

Power output

100 hp at 5400 rpm


92 mm (3.62 inches)


67 mm (2.64 inches)


Unleaded gasoline


200 pounds

Reduction ratio


Direction of rotation


Reduction system

One 60-mm wide HTD belt


Water-glycol mixture


Single electronic, single spark plugs


55 amp


Dual Bing altitude-compensating carburetors

Fuel consumption

3.2 to 4 gallons per hour at 75 percent power


Electric gear-reduction

Stratus 2000 Inc.
5145 SE 3rd St.
Corvallis, OR 97333

Phone: 541-754-4114
Fax: 541-754-4114

E-mail: mjt@camano.net



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