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Subaru EA Engine Performance Page

This is a reproduction of The Subaru EA Performance Page which was published by SubieMech...
The server where this resided went away and never came back, but this information was saved so that others may benefit from it's great content...

The Subaru EA engine is the ultimate workhorse. It's simple in design, dependable, and extremely durable. The purpose of this page is to show you how I drastically improved the performance of my 1986 4WD GL Wagon through simple but effective modifications that the average ''backyard mechanic'' can do in a few hours.


The Modifications described on this page are generally safe and should not cause any harm to your Subaru engine. However, I cannot be held responsible for any damages that occur from the use, or improper application of these modifications. I have safely performed these modifications on my own Subaru and have experienced excellent results. The changes involved can cause non-permanent changes in the emission characteristics of your Subaru. I would advise that you check with your local authorities before making any changes that would increase emissions from your vehicle. These mods apply to the carbureted versions of the EA series engines. I'm not sure how these mods will react on the fuel injected versions

Getting Started

You should have a set of basic handtools such as a metric socket set, metric wrenches, a pair if Vice Grips, screw drivers etcA jack and pair of jackstands are also helpful. Purchase al1 the required parts ahead of time or have another car on-hand in case more parts are needed. Above all, be careful when working on your car. Make sure that the car is supported by jackstands when working underneath. NEVER work under the car when it's supported by the scissors type jack that came with the car. These jacks are for temporary use only and should not be trusted to support the car for extended periods.

Exhaust System

The factory exhaust system fine for it's suited purpose, however it is extremely restricted. The focus of the restriction is the muffler. The exhaust pipe from the engine to the muffler is l 7/8, which is well-suited for the EA series engines (see Flow Velocity below). The restriction is the muffler itself. Notice that the tailpipe, which is welded to the muffler, is actually smaller than the inlet pipe. The pipes inside the muffler are this small as well. At least this is the case on my 86 4WD wagon. This causes back pressure which robs the engine of efficiency and power. Some backpressure is desirable to control the temperature of the exhaust valves. But too much is not good.

I replaced my muffler with a ''turbo muffler'' from a local parts store. Turbo mufflers reduce back pressure, flow freely, and aren't too noisy. The muffler should have a centered inlet and offset outlet. The dimensions should be 3 7/8''x7'' oval. The shell length is 13, and the overall length is 19. Try to get one as close to these specs as possible for to fit properly. The inlet and outlets are 2 in diameter. You'll also need two 2'' muffler clamps and one 2 x 24 extension pipe. If your original muffler is shot, you can cut off the muffler from the pipe and reuse the pipe. If not ' get a muffler from a salvage yard and cut the front pipe off of the muffler. The pipe will tit snugly into the muffler after removing the old welds and old muffler inlet. I cut the hanger off of the old muffler and welded it to the extension pipe. You cat: do the same or use a muffler clamp to hold the extension pipe to the hanger. I paid $17.99 for the muffler $3.49 for the extension pipe, and $0.99 each for the clamps. A great deal considering the lowest price for an aftermarket ''stock'' muffler was $100!

The result is a noticeable power increase and better fuel economy. The throttle response is ''snappier'' and that dead spot right off idle disappears. The sound is barely louder than stock with most of the sound being very low pitched. You can ''feel' it more than you can actually hear it. It's more noticeable when decelerating, but there's no popping or cracking as associated with ''glass packs'' or straight through mufflers. I find it a pleasant sound. If you're looking for something a little louder you can use a 2 extension pipe in place of the 2 extension pipe. This pipe will slide over the outlet on the muffler (instead of inside) and will require a 2 1/4'' clamp to hold it in place. The larger pipe has a slight ''megaphone'' effect deepening the exhaust sound and adding a little more volume.

A word on performance exhaust systems. These systems replace the exhaust system from the catalytic converter to the tail pipe. Some systems actually replace the entire exhaust system. They also increase the size of the exhaust pipe up to 2 1/2,' These systems are fine if you plan to run the engine above 3500 RPM all the time. They actually reduce horsepower and torque at lower RPMs. The reason:

Flow Velocity & Scavenging

On a stock system, the pipe is smaller and the velocity or speed of the gases in the exhaust pipe is high. As a cylinder expels it's exhaust into the pipe at low RPM a wave of pressurized gas travels down the pipe at a high velocity. This is called the exhaust pulse. These pulses consist of hot exhaust gases at high pressure and velocity (speed). After the exhaust pulse starts to travel down tile exhaust pipes a partial vacuum or low pressure area forms behind it Kind of like a piston moving down a cylinder in this case the exhaust pulse is the ''piston.'' The ''piston'' draws even more exhaust from the cylinder, leaving a partial vacuum in the cylinder as the exhaust valve closes. This is called the ''scavenging effect ''

This partial vacuum remains in the cylinder as the intake valve opens. The air/fuel charge comes rushing in packing the cylinder with a dense air/fuel ''charge.'' More air/fuel in the cylinder means more power and torque. That's why a turbo charger gives you so much power. It packs air and fuel into the cylinder under pressure.

The problem with larger diameter exhaust pipe is it has lower flow velocity, thus a lower scavenging effect.

Your best bet is to leave the exhaust system at 1 7/8'. and change to a better flowing muffler. The difference in power at higher RPM is negligible but the gains at lower RPM is very noticeable. We all know Subaru engines need a little help with low-end torque.


The thermostat is one of the most overlooked performance enhancements. By reducing the operating temperature of the engine a number of things happen.

First, volumetric efficiency is increased. That is when the engine runs cooler the air/fuel mixture entering the engine is also cooler. A cooler mixture means the mixture is more dense. The higher the mixture density the more that can be packed into the cylinder. You would be surprised how much more responsive the engine becomes just by changing the thermostat from 195F to 180F.

The next step would be a 170F thermostat. 170F thermostats for the EA series engine are hard to find however almost any 170F thermostat with a 54mm diameter will work. The little 'jiggler'' valve is not required but its preferable. The valve helps to purge air in the system and allows a slight flow of water around the thermostat to eliminate any temperature differentials between the area around the thermostat and the engine itself.

Second, with the combustion clambers running cooler a lower octane fuel can be used. Or if you're looking for better low-end torque, you can advance the ignition timing a few degrees without having to worry about detonation. (Author's note: After installing a 160F thermostat the performance increase was negligible. However, I was able to advance the ignition liming another 2 and use 87 octane[R + M/2] gasoline with NO detonation at all!)


Gasoline? Who cares! Just put premium in the tank and everything will be tine right? Wrong! You should ALWAYS use the lowest octane fuel that your car can tolerate without detonation. Why?

Well of course the lower octane fuels are cheaper, but there's another reason. Lower octane fuels burn at a faster rate than higher octane fuels and contain more BTU'S (British Thermal Units) per equal measure. In short it takes less 87 octane gasoline to produce the same power of a 93 octane gasoline.

'I'he octane number is NOT a measure of the power-producing ability of the gasoline. The octane number is simply an index to relate the anti-knock (anti-detonation) properties of the gasoline. The higher the number, the more resistant the gasoline is to pre-ignition. (ignition of the fuel before the spark is applied) or detonation. The flash-temperature, or temperature the fuel will spontaneously ignite, is much higher for a high octane fuel. So a higher octane fuel takes more energy to make it ignite and releases less energy as it burns.

Since the lower octane gasolines contain more potential energy, it takes less to make the same power in your engine. The result? slightly better fuel economy and better torque.

Want more information on fuels? The Federal Trade Commission has an excellent page on gasoline, octane, and what fuel to use based on octane requirements.

Hot Air Pipe

The hot air or stove pipe is the length of flexible pipe that runs from the bottom of the air filter snorkel to the metal sleeve wrapped around the exhaust pipe. It provides hot air to preheat the air entering the air filter. Preheating the air helps promote complete combustion and reduce emissions.

Heating the air also makes the air less dense thus less can be packed into the cylinders. Removing this pipe from the air cleaner allows the engine to ingest cold air helping to make the mixture even more dense. The only drawback is on cold rainy days, ice can form on the inside of the carb and keep the engine from idling properly. l fixed this problem by placing an electric vacuum switch between the vacuum source and vacuum motor connected to the hot air flap in the air filter's snorkel. When current is on, the vacuum switch is open allowing the hot air flap to operate normally. Turn it off and the vacuum is cutoff keeping the hot air flap closed and allowing only cool air into the intake.

Ignition System

The ignition system on the EA series engine is a good design, however as with all ''good designs'' it too can stand a few enhancements. If you have a 4WD car l recommend getting a distributor from a 2WD car. The 2WD distributor has a different advance curve and provides full advance ''sooner'' than the 4WD version. The difference means better low-end torque. Also bypass the vacuum valve between the distributor and curb. This will assure that the vacuum advance on the distributor always gets a vacuum signal from the carb when you press the accelerator.

Ignition Coil

The stock coil puts out about 30,000 volts with a fairly short spark duration. l replaced my coil with an Accel ''Super Stock'' coil. The Accel coil puts out 42,000 volts and has a longer spark duration. The higher voltage helps assure that the spark plugs fire and allows you to run a slightly wider spark plug gap. Why a wider gap? The wider gap exposes mere air/fuel mixture to the spark. This combined with the longer spark duration assures that the cylinder will fire EVERY time. Stock coils allow the cylinder to misfire an average of 8% of the time. This translates to about 80 misfires for each 250 crankshaft revolutions. The result? Lower power output and fuel economy. You can experiment with the gap on your plug, but l use .045-.050.

Ignition Wires

A good quality set of wires is essential to insure that the spark makes it to the spark plugs with as much energy as possible. Wires with silicone jackets and a minimum of 7mm diameter are highly recommended. Anything less will reduce the spark voltage at the plugs Avoid plug wires that use a Hypalon jacket. Hypalon does not last as long and tend to ''leak'' at higher voltages. Hypalon jacketed wires are usually packaged as ''economy'' wire sets.

Spark Plugs

With so many spark plugs on the market how can you choose the one that works best? Well one rule of thumb that will help... NEVER, that's right NEVER buy a set of plugs from a department store. I have researched this and spoken to representatives of numerous manufacturers. This is what I have found

1. In most cases spark plugs sold to department stores are ''seconds.'' Meaning they did not pass (lack the minimum quality control standards of the manufacturer. There may be a blemish on the plug or the plug may not be able to handle the voltages required by the manufacturer. In any event, the plug is not the best you can buy.
2. With hundreds of shoppers hand ling these plugs daily, how many times has the package been dropped? One good impact is all it takes to crack the insulator or break the seal around the insulator.
3. Prices. Yes, the prices are low, but why are they so low? Factory ''seconds'' are routinely sold at 50% off the normal wholesale puce of the plug.

ln short, ALWAYS buy your plugs from a reputable automotive supply store.

Always buy a name-brand plug. Some of my favorites are NGK, Bosch and Accel. I have found that most others simply don't perform as well or last as long. For example brand C (what plug name starts with ''C''?) only lasted about 8000 miles before they needed to be recapped and/or replaced. The problem here is the center electrode doesn't seem to be hardened as well as other brands. The plug itself is fine however it seems to wear much faster than all it's competitors.

Brand A doesn't have a broad enough selection. They have three stock numbers that ''fit'' 99% of Japanese imports. By comparison, NGK has over 10 different plugs for the same 99% coverage. It seems that Brand A is making some serious compromises in plug design to save themselves some money. Who ends up paying for these compromises? YOU! !

Specialty plugs, such as the one with the split electrode are a source of much controversy. The theory of it's operation is fine however in real world condition' s the dual electrodes rarely fire at the same time.. I have watched how these plugs work and the spark almost never jumps to both electrodes at the same time.

Cooling Fan

The cooling fan mounted to the water pump, is another item that will draw horsepower and torque from your engine. The fan driven by the water pump pulley, can use up to 10 horsepower just to keep it spinning. The clutch that disengages the fan when there is adequate cooling air flowing through the radiator helps reduce the amount of horsepower that is used by the fan but it still draws up to 5 horsepower even when disengaged! For an engine that only makes about 87 horsepower, that's a big chunk!

Removing the fan and replacing it with a quality electric fan will give that horsepower back to the engine. l highly recommend purchasing a temperature activated switch to have the fan operate automatically.

You can also wire the fan to activate when the air conditioner is running. Use the wiring diagram below to insure that the cooling fan is isolated from the AC system. This will keep the cooling fan from activating the A/C compressor.