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Subaru engines (2)

Subaru engines (2)


Subaru engines (2)

The following article appeared in the EAA Chapter 774 January 2000 Newsletter. It is reproduced here with the approval of EAA Chapter 774.

Using the Subaru EA-81

Up until the early 1980's, the Volkswagon engine was the most popular automobile engine conversion for experimental aircraft use. Then, during the 1981 Oshkosh EAA convention, the first Subaru appeared. Since that time the Subaru engine has become the most popular automobile engine conversion with nearly 3,000 engines completed. There are several kit manufacturers who are offering Subaru conversions as part of their packages. Several different models of Subaru engines have been used in aircraft, but I'll focus on a popular one: the EA-81. The EA-81 is a 4-cylinder, overhead valve, horizontally opposed engine with aluminum heads and block. The engine has a compression ratio of 8.7 to 1, and a displacement of 1800cc, or 109 cubic inches. The pistons have two compression rings and one oil ring. It has an electronic ignition system with point dwell 8 degrees before TDC at 700 RPM. Many of Subaru's ignition systems are altitude compensated. Intake valves open 24 degrees before TDC. The engine uses a gear driven camshaft located below the crankshaft, similar to a Continental engine. A normally aspirated, direct-drive EA-81 can be expected to produce about 60-65 HP at typical cruise in the 3400-3500 RPM range, up to 100 HP at about 5400 RPM.

The going rate for a decent, used, low-mileage EA-81 is about $500. Engines with higher mileage can be had for less, but will almost certainly require an overhaul. There are several companies in the United States performing Subaru engine conversions. These conversion shops import low-mileage Subes from Japan and completely overhaul them while adding a variety of modifications and upgrades. Most notable is Stratus in Seattle. Stratus engines are fitted with a lightweight custom intake manifold; dual, Bing altitude-compensating carburetors; and engine mounts that incorporate Lycoming motor mounts. The Stratus engine is zero-timed to factory specs, produces 100 horsepower max at about 5400 RPM, and has an estimated TBO of 1,000 hours. The engine also incorporates a belt reduction drive engineered exclusively for Stratus Inc. It uses two 30-mm-wide HTD belts that provide redundancy. The reduction drive's ratio of 2.2:1 reduces the propeller speed to 2,500 RPM, offering maximum propeller efficiency and quiet operation. All the components of the reduction drive meet aerospace standards. The drive's sprockets are hard-anodized for long service life. Before they are delivered, Stratus engines are completely assembled and test-run. Each engine includes a reduction drive, electric starter, air filters, an ignition coil, a fuel pump, and your choice of prop extensions. The engine costs $6,495 complete with PSRU from Stratus.

The stock EA-81 is a nice solid little engine with great potential for an aircraft if modified. In stock configuration it will make about 75 HP at 4800 rpm and probably never wear out. At 3600 it is down the lower rpm is probably only making about 60 HP or less. By contrast, the iron block Honda Prelude also 1.8 litre, will make 125 HP at 5400 rpm. So, what do we do to get power up on the Subaru?

There is no one answer. A number of things need to be done which all contribute their share of power. The first thing to do is to improve the mass airflow through the engine, which is, after all, just an air pump. This requires first that the cam shaft be re-profiled to allow the engine the "capability" to breath and scavenge better. Stratus also will do this for about $120.00. This alone won't improve it much unless the rest of the induction system is modified.

The intake manifold is replaced with a custom one with larger diameter runners and no coolant passages to heat the charge. This can be done in a variety of ways ranging from mating custom bent pipes to heavy-duty hoses all the up to fabricating an entire manifold from carbon fiber.

Subaru engine

The exhaust system is next. Use 1.75 inch pipe instead of the stock 1.25 inch, which is half the cross section. This is important and accounts for about 10 HP. Run each side back to the exit and couple them together with a cross-over as close as possible to the oil pan to form an ‘H' pattern. This gives two ways out for each pulse and also reduces exhaust noise by a big amount.

The heads are next. Try to get some 1984 heads or later, as they have bigger intake valves than earlier models. Where the valve seats are pressed into the head there will be a lot of excess metal blocking the flow forming sort of an oval pattern. Use an end mill or whatever, and drill or dye grinder and remove this metal to give the port a more round smooth shape. Sand them as smooth as possible to prevent turbulence. Do the same to the exhaust ports, only these won't need as much cutting. Be careful not to cut through into a water jacket. When done, have the heads glass beaded, including the ports, it will make them velvet smooth inside. They don't have to be polished shiny, just smooth.

Finally, have .040 milled off the gasket face. This does two things. It raises the compression ratio to 9.2 from 8.5 and restores the valve geometry that was disturbed by grinding off the bottom of the cam lobes when re-profiled by moving the heads closer to the cam by .040, otherwise you may need longer pushrods. Sounds like a big deal but can be done in one day and is necessary. Get a machine shop to do the beading and milling, about $30.00 per head. If you can't get the big valve heads, do it anyway to the existing heads.

For carburation, use your favorite model but not the stock Subaru since it's far too small and complicated. Some have used a Carter 2 barrel from a 318 V8, which is just right. Remember, it has to flow lots of air now that the engine has the capability to do it.

For ignition, many have used the stock reluctor electronics. Take it apart and open up the slots in the centrifugal advance mechanism to allow more advance, up to 30 crank degrees (15 at the distributor), this is only to ease starting. Disable the vacuum advance and remove the diaphram. Lock the mechanism inside with a screw so it won't move. Set the timing for 37 degrees BTDC on the flywheel with the advance sprung over by hand to full advance. Should be about 7 degrees static. (No advance).

This gets you to about 100 HP at about 5200 rpm. The water pump and alternator will be running too fast and may cavitate, reducing cooling and wearing out your alternator. Make or buy a smaller drive pulley, about half the diameter of the stock one to cure this. I turned one from mild steel and it works great.

Obviously to do all this you have to disassemble the engine, again no big deal, but try to get a book so you know what is what. There are lots of generic ones from auto parts dealers which shows this engine. Once apart, take the opportunity to clean, inspect and measure everything. Easy-off oven cleaner is great for removing baked on carbon, but it attacks aluminum, so don't leave it on for long and don't use it on the outside of the pistons. Get and install new bearings, rings and a gasket kit (about $200.00). Deglaze the cylinders so the rings will seat. Use 240 grit aluminum oxide paper and varsol making a spiral pattern. Don't use a hone unless really necessary as it removes too much metal.

If you like, go ahead and paint it with any good enamel from the hardware store, it won't burn as the engine never gets above about 230F. Break it in with 20 weight mineral oil for about 10 hrs or until the oil consumption stabilizes at almost nothing. Then use Aero Shell 15-50. It's an ashless dispersant high temperature, the best oil there is.

To get 100 to 110 HP this engine has to run fast at 5200 rpm. This won't hurt it at all. Several people report having had it to 6800 and nothing let go. Many people run normal cruise at 5200 rpm. If it won't turn this fast, you need less prop or a higher reduction ratio. It won't give you much at 3600 rpm even modified.

The EA-81 can be run direct or with a reduction. It depends on your particular situation. Remember, for a particular engine, the power produced at 3600 rpm will be less than that produced at 5000 rpm using a reduction. Also, when using the engine direct drive, the prop size is limited to keep the tips from going supersonic. There is a lot to consider based on your particular installation regarding power requirements, space considerations and overall cost.

The following instrumentation should be considered as mandatory for any auto conversion:

1.) Tachometer (obviously)

2.) Water temperature

3.) Water pressure - idiot light and gauge

4.) Oil pressure - idiot light and gauge

5.) Oil temperature - gauge

6.) Manifold pressure (mandatory with turbo only)

7.) Exhaust Gas Temperature

Subaru engine with Stratus redrive

Unless you are exactly duplicating another installation, you should monitor everything you possibly can.

There is a company called Rocky Mountain Instrument (they're on the web- http://www. They make a great engine info kit that will tell you separately. It also saves you a TON of panel space. There is another one available from Grand Rapids Technologies, Inc. 616-531-4893. It has Tach, Dual CHT, Dual EGT, Oil temp &pressure, Coolant Temp. Hourmeter, Voltmeter, Timer, Adjustable alarms, out of range warning light, plus fuel. This unit with all senders is about $650.00 US depending on what inputs you want to switch. Anything with thermocouple inputs can get a little unreliable.

Lastly, some people have used stock Subaru water-cooled turbochargers to increase output to more than 90 hp at the 3400-3500 RPM range. This increase is seen at stock boost levels (maximum 7.5psi, 45 inHgA). The complete firewall foreward weight of this conversion is 215 lbs, ready to run. Turbochargers from EA82 engines have been used, as it suits the same engine displacement as the EA81, but uses a water-cooled center section. The older turbochargers used in 1984 with the EA81 engines require that the engine oil providing lubrication for the bearing also carry away the heat from the center section. In aircraft applications, an oil cooler would be a necessity with this type of turbocharger. With the water-cooled turbocharger, it is possible to operate without an oil cooler. An adapter can be fabricated to mount the Ellison EFS-2 TBI to the compressor inlet. The stock wastegate controller is removed, and a mechanical lever arm and bowden cable attachment can be fabricated to allow manual wastegate control.

The interest in Subaru engines for use as powerplants in experimental homebuilt aircraft has never been greater. They are a rugged, well designed, reliable, light weight, smooth running, powerful and cost effective solution for experimental airplane power.


For more information or comments, mail me at
The photos in this web site are from the excellent book by Xavier Massé,
sent in by happy Jodel owners, lifted from the web or taken by myself.