By Jim Kerr
Over the past two weeks I have been driving through the mountains enjoying the beautiful scenery and winding roads. Both weeks, I was between 1,500 and 2,500 metres above sea level. At those altitudes, the air starts to thin out. You notice it when you try to hike a mountain trail, gasping for that next breath. You notice it even more when trying drive up a steep road or pass another vehicle on a level stretch of road. The engine runs out of air, in the same way the human body does. The difference is, the human body can adapt, while the engine needs to be redesigned before it will become effective in the thin air.
You don’t have to be driving in the mountains either to feel the performance difference altitude makes. For every 100 metres you climb above sea level, your vehicle will have about one per cent less performance, so with my home about 500 metres above sea level, I have about a five per cent loss of performance every day. That affects pulling power, responsiveness and fuel economy.
There are ways to overcome that performance loss however, and one of the most effective ways is with turbocharging. During my first week in the mountains, I was driving Ford’s new Flex EcoBoost. The EcoBoost V6 uses twin turbochargers to pump air into the engine. It is very effective, as this 3.5-litre V6 engine puts out 350 ft-lbs torque at all speeds above idle, even at high altitudes. In comparison, the 5.7-litre V8-powered truck I was driving the following week has 330 ft-lbs of torque at sea level. At 2,500 metres, if feels like half of that torque evaporated!
There is more to look at than just peak torque figures, however. For normally-aspirated engines, peak torque is achieved at the engine rpm with the highest volumetric efficiency, which is typically in the 2,200 to 2,600 r.p.m. range. Volumetric efficiency is the percentage of the engine cylinder that is filled with air. Torque will increase as engine rpm goes up and then gradually decrease at higher rpm after peak torque is reached. When graphed, this produces a smooth curve shaped like a mushroom top.
Turbocharging changes all of that by changing an engine’s volumetric efficiency. Turbochargers can actually pressurize the air in the cylinder above atmospheric pressure, so volumetric efficiency rises even above 100 per cent. This produces more torque and over a longer rpm range. In the EcoBoost engine, the torque curve could more accurately be described as a torque “square.” It climbs rapidly to peak torque at 1,500 rpm and stays there continuously until engine redline. That means performance is strong at any rpm, and especially at common engine speeds. You don’t have to downshift or race the engine to get a lot of torque.