CONSIDERATIONS

When considering how to modify an engine for competition, an engine builder has to first know what will be expected of the engine. A road race engine will be built different from a rock crawler. When Paul told me that he wanted to subject a Subaru engine to the Mexican 1000, I had a good idea what modifications to do and what not to do. Over several conversations I was to learn what the top speeds are, what the average speeds are, what the elevation and temperatures will be. This critical information was instantly boiled down to one word that guided the whole build: reliability.

NORRA (National Off Road Racing Association)  had one class that this all-wheel drive Outback could be entered in without turbo-charging: Rally Sport Medium. A turbo engine is not as reliable, has terrible low-end low-RPM power but makes lots of power and heat at higher RPM’s which has to be controlled and tailored to fit the type of racing you are doing. So the engine was to be fairly stock but we could modify it to a point. A lot of the modifications were limited by money, not by rules.

Paul’s team named Outta Sight Racing, began with this vintage 1997 Subaru Outback

five speed with a 2.5 liter engine and a very loud rod knock, purchased for $700.

The 2.5 engine was pulled, disassembled and carefully checked for integrity. Every threaded hole was chased with a tap before machine work was started.

This project was an exercise in economy, a nice way of saying we needed to build a competitive engine for cheap, as neither of us had deep pockets. A 2.5 block was decided in favor of the 2.2 for several reasons.

1.The bore to stroke ratio was closer to square.

2. Larger cubic inch which means more horsepower no matter how you look at it.

3. Most important of all: I had a few spare engines laying around!

The twin cam heads were sold off and 2.2 heads were chosen. Several good reasons followed for the head selection. Single overhead cam heads are more reliable because there are not so many moving parts. In addition, the valves are manually and quickly adjustable. Thirdly, if we change cam profile, swapping out the cams is a two hour job not a two day job, plus the cam gears are steel.

COMPRESSION RATIO

Basic physics teach that the more you can compress the fuel air mixture the bigger bang for the buck. Go too high and you get pre-ignition (about the worst thing that can happen to an engine), go too low and you get a wimpy, gutless VW engine. Car manufactures include a warranty with new cars so they play it safe and keep the compression ratio moderate.

 

Subaru states that a 1998 2.5 has a 9.5:1 ratio. I was not surprised to find this was not accurate. Just as the engine itself is not a 2500cc but in fact a 2457cc. Most horsepower figures given are also “Hollywood Horsepower” stats given to impress. What we wanted is real world usable and reliable power day in and day out for one thousand miles. For this reason I never dyno my engines, I test drive the car to see if it will get the job done. Plus dyno testing is expensive.

 

Back to the 2.2 heads: After measuring the volume of both the stock 2.5 heads and a set of 2.2 heads, I found that it will raise the compression ratio to use the smaller heads but not radically. There is also the volume of the pistons and cylinder head gasket to consider.            

 

Our stock 2.5 engine was actually 8.3:1 which is fairly low. With the option of smaller chambers on the 2.2, different pistons and 2 different choices for cylinder gasket thickness, I worked up a matrix of 5 different choices for higher compression ratios from mild to wild. Reliability dictated a ratio in the middle, much better than stock but still be able to burn low octane fuel if need be.

David King using a burette to measure the volume of  chambers and piston dish.

KEEPING IT COOL

The decision was made to ceramic coat the pistons and head chambers as an effective way to keep the heat down. If this race was in the cool northwest, I would not have added this expense. The word from Paul was that even though the race was at the end of April, the temperature in Baja can be over 100 degrees.

 

After a thin layer of ceramic is bonded to the aluminum, only a small portion of heat transfers to the water jacket and oil. In the picture below, the combustion chamber and piston have been coated. It is difficult to see the coating, even when holding them in your hands as the coating is the same color as the aluminum. Even though the coating is only .001-.002” thick it did reduce the volume of the chamber by almost 1cc. Not much, but it does change compression ratio.

Ceramic Coating Applied

The pistons have a tri-coating. Ceramic on the top, Molybdenum on the skirts to reduce scuffing, and a super slick coating on the                                                                         inside to shed oil. The coating gurus did not reveal the make up of this coating. One more word on the cooling safeguard, if you are inclined to make such a Franken-Motor: We had some extra, custom machine work done on the 2.2 heads to open the water passage to match the 2.5 head gasket. Compare the picture above and below.

BALANCING ACT

All engines are balanced from the factory, they have to be. However, close enough for your average car was not good enough for us. Beside we were changing the flywheel and front pulley for lighter weight units. The stock flywheel on this Subie weighs 25 pounds. At 6,000 rpm’s, that saw blade like-thing is sitting down by your ankles rotating at a speed of 60 times every second. Even a couple of ounces off and it could shake the car. Anyhow, our new flywheel weighed 13 lbs and we ditched the 5 pound harmonic balancer for a one pound under drive pulley. My local machinist dynamically balanced the crank, pulley, clutch and flywheel together. I statically balanced the rods and pistons. All the pistons were exactly the same from the factory and only one rod was off by 2.5 grams! American made cars are usually off by ounces. They rely on big soft motor mounts so you don’t feel the engine shaking!

Inside of piston was impregnated with slippery coating.

The Making of a Winner: Building Paul Fournier's Mexican 1000 Subaru Power Plant.

Sept-Dec. 2012