In Part One of Project Impreza we tried to convince ourselves that making a U.S. Impreza look and handle like a WRX would make it as fun as a WRX. We had ourselves fooled for about a week. Then reality went blasting by and we couldn’t catch up. Reality, if you read last month’s cover story, was the Impreza 22B. Same Chassis, different world. Information available here.
If we really are serious about making our 2.5 RS be all that it can be, then we’re wasting our time with air filters and mufflers. Time is short with our Subaru, as this particular Impreza has to play celebrity and go on display at several major auto shows this year. If a modification doesn’t make a huge difference in performance, we don’t have time to mess with it.
So what can make that huge difference? What else? Subaru has already proven what works, we’re just going to follow its lead and see how hard it is. The plan, of course, is a turbo.
Will It Work?
Before starting an endeavor like this, it’s best to take a long hard look at what you have. Subaru’s powerplant is a unique package. Subaru is much like Porsche in that it began with a unique, but in many ways flawed layout (a flat six all the way in the back for Porsche, a flat four all the way in the front for Subaru). it proved that through solid engineering it could make the advantages of that layout shine through, and the disadvantages disappear.
The horizontally opposed layout of Subaru’s engine is good for chassis dynamics. First of all, it makes the engine lighter since there are no heavy main bearing caps or deep skirted blocks needed to keep the crank from being pushed out the bottom of the engine-the other side of the engine ,case serves that same purpose. The flat layout is also shorter than an in-line layout, minimizing the nose-heavy nature of a very front-engine car, and the center of gravity is lower, since most of the engine is in the same horizontal plane as the crank.
There are disadvantages, though, some of which are not immediately obvious. Because the engine is so wide, fitting it between the frame rails gets to be a challenge, especially as displacement is increased and the crank’s stroke gets longer. All else being equal, increasing stroke by 1 mm on a horizontally opposed engine increases the width of the engine by 2 mm. Push a four-cylinder all the way to 2.5 liters and you’ll have one wide engine if you’re not careful. Subaru engineers are careful, of course, and the 2.5-liter powerplant in the 2.5 RS is no wider than the 2.0 liter turbo in the WRX. That in itself, made us nervous.
To keep the engine compact, Subaru’s engineers gave the engine a huge bore (99.5 mm) and a short stroke (79.0 mm). Compare that to the 89 mm bore and 96 mm stroke on the 2.4-liter engine in the 240SX and you see how disproportionate that is. The problem is that knock resistance under boost tends to be related to, among other things, bore size. A larger bore means it takes longer for the flame front to travel from the spark plug to the furthest reaches of the cylinder. That’s more time for the heat and pressure of combustion to build until the fuel and air that haven’t met the flame front yet get impatient and have a spontaneous and uncontrolled explosion party of their own. ping! ping! Ping! Bang! End of Project car.
The bigger bore, also means there is more surface area for combustion gases to dump heat into, reducing the amount of energy left for shoving on pistons. of course, there is a lot more to knock resistance, and power output than bore size. The big bore, short stroke engine can be made to work just fine, thank you. Can You say 400-hp Porsche turbo? We thought you could.