Six Star Magazine Six Star Magazine Spring/Summer 2005 | Page 24

TECH TALK power and grace The new 6-cylinder horizontally-opposed SUBARU BOXER engine I t used to be that you had to choose: either you had a smooth engine with perfect idle qualities and good fuel economy or you had a powerful engine that gave you quick response and enough power to pass with confidence. With the technology available today, you don’t have to choose anymore. The proof is in the new H6 SUBARU BOXER engine found in the Outback 3.0R and 3.0R VDC and under the hood of our new flagship – the Subaru B9 Tribeca. It has been said that an engine is like an air pump; you need to get air into it, mix it with the proper amount of fuel and ignite the mixture at the right time to produce combustion and get things moving. This might sound simple, but things need to happen rather quickly. An engine at idle, say 700 revolutions per minute (RPM) or 11.7 revolutions per second, needs to get air in and out of the combustion chamber every 0.17th of a second (it takes two revolutions to complete the process). Minimum Overlap Maximum Overlap BDC BDC Intake valve timing TDC Exhaust valve timing BDC Intake Valve lift Small overlap improves timing This position offers excellent idle quality combined with improved fuel economy. *European model shown. 24 TDC BDC BDC Intake valve timing TDC BDC Intake Valve lift Valve lift TDC Valve lift Exhaust valve timing At 6500 RPM, that’s every 0.018th of a second. Much of the secret is in getting the right amount of air at all engine speeds. To get this done in the most efficient way, the new H6 uses two technologies: variable cam timing and variable cam lift. One of the first things technicians learn to recite in school and the basis for how an engine works is “intake, power, compression and exhaust.” As mentioned above, all this needs to happen at engine speeds ranging from idle to the red line. Variable cam timing allows us to vary the overlap between intake and exhaust valves (varying the time they are both open at the same time). This needs to be very little at idle for smooth operation and good fuel economy but needs to be quite substantial at higher RPMs to allow burned gases to be evacuated from the combustion chamber and allow clean air and fuel to enter. The mechanism that allows this to happen is controlled by the engine computer. Overlap produces Scavenging effect (due to exhaust pulsation) Inertia supercharging effect produced by advancing the intake valve closing time This position offers excellent combustion chamber scavenging to improve power output.