Driving the US specification WRX engine, or more accurately the 'world motor' used also in Australia and Europe, shows that it lacks any real power below about 3500 rpm.  It seems very sluggish below 3000 rpm. Why is this so?

The 'world' WRX engine is basically tuned as a normally aspirated engine below 3000 and as a turbo motor above 4000 rpm. Most significantly is the fact that its cam timing is optimized for emissions and part throttle, low boost running while the Japanese standard WRX engine has different cams optimized for mid-range power  and variable intake timing to provide quite a different feel to the engine. And, much more midrange torque, might I add.

When we compare to the Japanese engine, not only does it have a better midrange punch, but there is vast difference in the 1500-3000 rpm throttle response between the two engines as well. With the slightly relaxed Japanese emissions regulations ,the Japanese engine runs different cam profiles and variable cam timing. With the better octane fuel in Japan, the Japanese WRX runs much more timing advance, not so much "more boost".  The engine is able to make more boost down low because of timing and cam factors, not the other way around. Drivability is much better as a result. Or, as a friend said of the low speed running of the Japanese WRX, "It feels like a V-8" in comparison to the US car.

Again by way of comparison, the American car as a throttle than can only be described as 'wooly' to use the old British tuner expression.  It does not want to go below 3000 rpm, and the motor just does not feel happy at all below 2000 rpm. The Japanese WRX runs on light throttle at 1500rpm, pulls cleanly and smartly from 2000 rpm and is making good power by 3000 rpm. While we might want to say this is because of the variable valve timing, in fact the presence of variable valve timing  and the improved drivability is directly attributable to the difference in the fuel the engine is being fed and also the effects of the stringent emissions and fuel economy regulations in the US.  

The "world motor" we get does not have the Japan variable intake cam timing, which oddly enough can be used to boost low to midrange torque capability as much as it can be used to extend top end power. A variable intake cam timing engine can use a bit more intake/exhaust valve overlap in the low rpm regions for better off boost throttle response and then close down the overlap and at the same time extend intake closing time, which lets the cylinder fill better for more power. But, it is only effective with higher octane gas. So, it becomes a chicken and egg type question. Did we lose the variable valve timing because of our lower octane gas or for some other reason?

I can imagine this variable timing was not included on our motor for a couple of reasons. Cost is certainly one of them, but there are other factors. I would have hoped Subaru would have used variable valve timing to help achieve emissions and mileage goals, but motors tend generally to detonate in the 3000-4000 rpm region where the cylinder pressures are highest and variable valve timing does no really help this problem. It is very tricky to produce highest possible torque at low rpm with our 91/93 octane. This fact is perhaps overshadowed by the EPA nitrous oxide output limits at full throttle and the need to keep the car out of low rpm boost so that it can run the EPA mileage cycle with minimum fuel consumption. These regulations are an ugly fact of life for US specification engine designers these days. Subaru engineers reasoned, and In all honesty, what was there to gain by adding expensive technical wizardry if it cannot be exploited on 91/93 octane gas available world wide? (For the record, the European 95 RON standard is roughly 92-93 US octane these days, although they also have a "97" octane super)

Instead of a torque optimized engine such as the 250hp Japanese EJ205, the engineers needed an engine to meet strict emissions and economy regulations. This required a different set of cams, a different combustion camber shape and revised engine management tuning.  In the end, our WRX engine can run on just about any gas it is given, get good gas mileage and be stone reliable. This is not the end of the story however.

In direct contrast to the high power capability of the STI EJ207, the Japanese and world version the  EJ205 'tumble generator' injector housings are cleverly disguised intake restriction plates that limit the ultimate upper rpm air flow of the EJ205 engine. (Just like NASCAR restrictor plates, cool!)  The EJ207 has much larger 'tumble generator' ports. Both engines have these secondary throttle plates to promote excellent lean burn characteristics at part throttle, low rpm running and warm-up. They are the main feature that permitted the WRX engine to make it to the US for 2002. A 1993-6 WRX engine would have had no problem meeting US regulations in 1996, but the regulations became ever tighter, sending Subaru design engineers back to the drawing table to come up with this solution.

In the EJ205 'world motor', Subaru engineers have made a turbocharged engine that runs more like normally aspirated engine below 3000 rpm with a transition range to 4000rpm where it behaves like a turbo motor. It has only the quality of materials internally needed to stay alive at modest power levels. As a result it is not fitted with forged pistons, exotic valve materials, light weight valve train, nor the reinforced block of the STI engine. The EJ205 cannot take the long term effect of heat and torsion stresses of 300hp with the same ease as the STI EJ207 engine, never mind the need for better gas to achieve these power levels. There are a lot of WRX owners who find this out the hard way with broken ring lands or scuffed pistons. Ask around....