A cam or “camshaft” is a part that hides in the cylinder heads of your Subaru engine. This part runs the length of the entire cylinder head and is connected to a cam gear at the front of the engine. There are four cams in a Subaru turbocharged engine, two of which control intake valves and the other two control exhaust valves. This type of camshaft setup is referred to as Double Overhead Cam DOHC as the cylinder head has two camshafts running over the top of the valvetrain. Since the timing of the cam is extremely important, the cam gear that drives the cams is connected to the crankshaft by a timing belt, keeping the entire mechanical show running in time.
Camshafts are responsible for operating the valvetrain and opening and closing valves for each cylinder. Subaru engines have four valves per cylinder, two of which are intake valves and two of which are exhaust vavles. The camshaft opens and closes these valves by rotating and oblong shaped lobes on the camshaft runs over the top of the valve stem, pushing the valves which makes the valves inside the cylinder open and close. All this happens incredibly quickly while the engine is operating.
Subaru engines that are equipped with AVCS (or variable valve timing) have a variable cam gear in place of the non-variable cam gear which is connected to a solenoid that is controlled by the ECU. This allows for the cams to be adjusted by up to 35 degrees. Depending on the model of your Subaru, some have AVCS on the intake cams only and some have dual AVCS meaning variable cam timing on both intake and exhaust cams.
The advantage of AVCS is that it allows for adjustments to be made to suit the conditions that the engine is running in. At idle and low engine loads, cam timing is retarded offering a smoother idle. During medium engine loads, AVCS can allow the intake valves to begin opening during the last part of the exhaust stroke while the cylinder is pushing the exhaust gasses out through the exhaust valves that are still slightly open. Some of the pressure created during the exhaust stroke flows back into the intake manifold, having the effect of exhaust gas recirculation (EGR). Due to AVCS advancing the timing of the intake valves allowing them to open earlier, it also means that the they close earlier during the intake stroke. This helps with engine efficiency and fuel economy.
During high engine loads, AVCS advances the timing of the intake valves even further causing them to open sooner during the exhaust stroke. This produces a scavenging effect. Scavenging is when intake airflow helps push exhaust gasses out. It also closes the intake valves sooner on the compression stroke. By doing this, an engine can produce more power efficiently.
Over the years of studying Subaru engines, I have found that the camshafts in Subaru engines can have up to 6 degrees of movement in the camshaft dowels. This results in inconsistent performance from an engine. Not setting camshafts correctly and properly can greatly affect power.
I also discovered that the brand of timing belt used can have an adverse affect on cam timing as they wear out and stretch over time. Even if a timing belt has stretched only ever so slightly, it is enough to throw one of the four camshafts in a Subaru engine out by up to 2 degrees. It may not sound like much at all, but 2 degrees in addition to the extra 6 degrees of unwanted movement in one cam alone will affect overall performance dramatically. After carrying out extensive back-to-back testing with different branded timing belts, the OEM Subaru timing belt and the Subaru STI timing belt offered the most consistent results over the standard 100,000km period.
To emphasise how crucial it is to set camshafts correctly and properly, on many occasions I have been able to gain up to 50kW at the wheels by simply setting the camshafts. So when you think camshafts only make up a fraction of the power to be gained, think again.
Aftermarket camshafts allow valves to stay open longer and can “lift” or push the valves open further. All this is great if you have enough airflow going through the engine to benefit from the extra volume of air the valves allow. What this means is that unless you have a larger turbo to produce enough flow into the engine, aftermarket camshafts do not necessarily offer power gains. I have tested this back-to-back on many engines. Only the cams were changed in these engines. The standard cams still produced more power in the end unless the turbocharger was upgraded also.