|
EA81 PERFORMANCE
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.
BREATHING
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 .
COMPRESSION
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.
IGNITION
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.
AVIATION
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.
HOME
| 
