What are the common modifications made to an EA81 to boost its power ?

By : Nigel Field

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 reprofiled to allow the engine the "capability" to breath and scavenge better. I sent mine to formula power to be done but I think 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. I made mine from carbon etc........... deleted...... (see my article on carbon manifold construction....................

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 smooth as possible to prevent turbulence. Do the same to the exhaust ports, only these wont 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 reprofiled 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, its far too small and complicated. I use 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 I use the stock reluctor electronic. 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 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 ant 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. I got a generic one from the auto parts dealer for $15.00 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 do the rings will seat. I use 240 grit aluminum oxide paper and varsol making a spiral pattern. I don't like using a hone unless really nessary 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. Its an ashless dispersant high temperature, the best oil there is.