With the EA81 we have the good fortune of the engine being very popular with the kit and experimental aviation communities. The aviation community builds these engines in a range from stock to over 140HP. The practical max for non-forced induction seems to be around 120HP. The cost of these modifications is also somewhat in line with the costs of the same work being done on your average non Subaru engine. At least in the Impreza world everything is 3 or 4 times more expensive, cams that would cost under 200 for a Chevy 350 cost us over 400.



The very first thing that you can to help out the EA81 is to help the engine breath better. Take a look at the path of air through the engine. First the air is drawn through the intake, air cleaner and carb. There is allot of restriction here that can be resolved. The best route to take is a bigger better carb complete with a low restriction intake such as a K&N air filter mounted directly on the carb. Next the air flows through the manifold. The problem with the EA81 manifold is that it has a water crossover for the two block halves. This makes modifying or replacing the manifold very difficult. The other drawback is the heating of the air by the hot manifold, cold and dense gives you more power. There are solutions to this although they are not cheap or easy.  Some in the aviation community have made replacement manifolds out of carbon fiber with a water passage separate from the manifold. This is both lighter, better flowing and cooler. Next the air passes through the heads and allot can be gained here. You have several shops that can port the heads or you can do it yourself.  A head porting how to will be coming to this site as soon as possible. Stratus aviation will port heads as well as regrind cams which are the next item. The cams control the operation of the valves and regrinding them to provide different flow characteristics will have the effect of moving the powerband up or down on the RPM range and you can optimize for low end grunt (offroad) or high end HP (road use). Getting the heads ported and the cams reground in combination with the carb upgrade and exhaust, the next step, will greatly improve the performance of the engine. The exhaust on the BRAT is really not all that bad. There are a few problem points though. First there is the CAT, usually somewhat carbon fouled after 20 years of use. Most locations inspect to see if the cat is present and will fail you if it is not. Should it be legal in your location, or if the truck is for (cough cough) off road use only, then you can cut the cat out or bash out it's core and make it hollow to improve flow but retain the appearance. The next problem comes as the pipe nears the muffler. For some ignorant reason the pipe narrows and restricts flow just before going into a highly restrictive muffler that exits in an even smaller highly restrictive pipe. This has got to go. By cutting the pipe before it narrows you can weld or bolt in a same size diameter pipe to go to a free flowing muffler. Here you have allot of choice. If you want something quite there are allot of choices. A sub $50 performance muffler sold at your local auto parts store will keep it stock sounding but provide for much more flow. If you want a louder throaty sound there are just a ton of options. My choice was to go to a Supertrapp muffler that features disks to adjust the backpressure. I ordered this from Summit Racing .


The gains in HP are made here. The simplification of the formula is that a higher CR (compression ratio) yields more power. The limit to this is that the higher you go the higher octane fuel you must use to prevent detonation and you eventually reach a point where you can no longer use pump gas. There are two practical ways and a few complex ones to raise the CR on this engine. First lets get the complex one out of the way. Have a welder that works with aluminum full in space on the heads and therefore decrease the volume in the chamber raising the CR. Difficult, expensive and hard to find a place to do it. The two reasonable ways are as follows. A bolt in solution to get a gain is to use 1600cc Subaru Pistons in the EA81. This is a simple (bolt in)  mod that can be done without machine work but it does demand removal and substantial disassembly of the engine. The other option is to have the heads decked. Any engine machine shop can do this and the amount you shave them by dictates the gain in CR. cuts of .01 to .04 are common. This does affect the valve timing and you should co-ordinate the cam cut with the head decking. Once again a company like Stratus can do a deck and cam grind. I would trust a aviation builder that does this all the time Vs. a local shop that has maybe never seen a Subaru engine.


An easy drop in mod is the replacement of the stock distributor with the 2wd version that has a better advance. Not a huge gain but if you get a deal on one or already have one why not? A Ford escort distributor may also be modified to fit with some effort. More on this as I can find it. The next step is the replacement of the coil. If you gain HP from this your original coil was probably bad. You can expect a improvement in general responsiveness and smoother running from going to a better coil. A good choice is the Accel Super Coil. I got mine from Summit Racing . As far as plugs go, forget the split fork and gimmick plugs and get a good brand name plug. I use the Platinums.



Like I said above, the aviation community has done allot for the EA81. Here are some clips form various FAQs and sites. The author has been contacted for permission when possible and thanked.









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