Fuel Consumption vs. RPM
Q I have an '08 Chevy Colorado with the 2.9L engine and five-speed transmission. At 75 mph, the engine turns 2,500 rpm in Fifth and 3,400 rpm in Fourth. I get 20 mpg commuting in Fifth and 18 mpg commuting in Fourth. So driving in Fourth gear, the engine turns about 36 percent faster, which means 36 percent more cycles of filling the cylinders with fuel/air. But it only uses 10 percent more fuel.
So what happens to all the extra apparently wasted fuel when driving in Fifth gear? I know some goes into producing heat, because the temperature gauge is about two dots hotter cruising 75 mph in Fifth than in Fourth. This is a computer-controlled engine with sensors on the air intake, fuel injection, and exhaust. Shouldn't it be evening out the air/fuel ratio?
A Let me start out with a few numbers of my own. The 2.9L puts out 190 lb-ft. of torque at 2,800 rpm, and 185 horsepower at 5,600 rpm. Now keep in mind that while the frontal area is fairly small-compared to, say, a Silverado-the truck still has to push through a lot of air at 75 mph. This means it is making use of most of its torque and very little of the rated horsepower when in top gear. Torque is what moves the truck down the highway.
While the engine at 75 mph is turning 2,500 rpm in Fifth gear, it is producing much less than the max hp it can achieve, and a bit below the rated torque. Comparing its fuel mileage in Fourth gear at the same vehicle speed, the engine is now turning almost 1,000 rpm faster, which also causes it to make quite a bit more horsepower. Just for giggles, let's say at 2500 rpm the motor makes 75 horsepower and at 3,400 rpm about 102 horsepower. Actually, the latter figure would be a bit higher as the horsepower gain curve rises as the rpm increase. Yes, rpm are quite a bit over the rated torque rpm when in Fourth gear, but the torque curve doesn't descend quite as quickly over the rpm rating as it does when under the rated rpm. This is especially true with a small engine like the Colorado has.
So now, in Fourth gear, the engine is turning higher rpm but making considerably more horsepower and about the same amount of torque than it did at the lower rpm in Fifth gear. This makes it "easier" on the engine to push the truck down the road at the same given vehicle speed, even though the engine is spinning faster. The drawback is that you are burning 10 percent more fuel and producing higher piston and crankshaft speeds that are wearing out the engine faster, hence the Fifth-overdrive gear that the design engineers added. If you had the proper scan tool, you would find that the percentage of throttle opening is higher in Fifth gear than in Fourth. However, it seems to make one wonder why, if the throttle is held open wider, and the fact that the engine is basically an air pump, then it should have more air flowing through it in Fifth gear than in Fourth gear and hence use more fuel. In reality, it is actually flowing less due to fact that the velocity of the airflow is much slower at lower rpm than it is at higher rpm, and the engine's computer compensates part of its fuelflow based on airflow, as well as on engine load or vacuum.
The bottom line? The engine is not working as hard to move the vehicle down the road in Fourth gear as it is in Fifth, but the higher rpm will cause it to consume a bit more fuel.
Best Mild-Lift Setup for Old Chevys?
Q I am starting to build a '77 Chevy Stepside pickup and am trying to figure out how to fit at least 35-inch-plus tires without affecting turning, and avoiding as much lift as possible. What modifications would be the best, with a low center of gravity being the focus?
A Chevy suspensions, with their reversed-arch leaf springs, don't have a great amount of compression travel, so a spring lift up front not only gains more tire clearance but also more suspension travel for better ride quality and articulation. At the back end, I would really suggest a shackle flip kit that changes your spring shackle from a tension shackle into a compression shackle (that is, the spring eye is now below the shackle mount instead of above it). This way, you don't have to use lift blocks or new springs to gain the necessary amount of lift. In reality, your truck's rear springs are most likely pretty fatigued after some 33 years of service, so you might consider some replacements.
Oh, as to lift, I would suggest a 4-inch lift up front, and the rear shackle lift will also gain you about four inches. Yes, you will have to do some fender trimming at both the front and rear of the front fenders and relocate the front fender brace. The steering will be a bit off, so consider an adjustable drag link and/or a dropped pitman arm. Early Chevy trucks are notorious for cracking the frame around the steering box, so be sure to check that area out. There are several companies that offer a reinforcement plate and even braces for that area.