American Motors knew that when it launched the Wrangler in 1987 it was playing with fire. Designed to replace one of the most beloved and proven vehicles ever produced, the Wrangler could have been something of a company bomb if it hadn’t been so well received almost universally. It remains a testament to the American Motors Company that, almost 20 years after it ceased to exist, Chrysler still used engines that AMC had designed more than two decades ago.
Horsepower vs. torsion
Engines produce two types of power: torque and horsepower. Torque is the amount of physical force (expressed in pounds per foot) of the engine for each turn of the crankshaft. Horsepower is the amount of torque an engine produces combined with its revolutions per minute; therefore, horsepower is a measure of work done over time. Think of it this way: you have two men carrying buckets of water up a hill. A man is strong but slow, carrying 20 gallons (75 liters) at a time and making the trip 20 times in an hour. The other is weaker but faster, carrying 10 gallons (37 liters) at a time, but makes 40 trips in an hour. They both carried 400 gallons (1.
Power curves and displacement techniques
All engines have revolutions per minute at which they operate most efficiently, or do the best job of converting fuel to energy combined with oxygen. In most cases, an engine uses the least amount of fuel at about 1,500 rpm below its peak torque. Peak engine rpm power (where it does the most amount of work over a given period of time) offers the best acceleration, but also tends to coincide with the point of worst fuel economy. The band of revolutions per minute between maximum engine torque and maximum power is known as the “power band.” Extracting maximum performance means changing what keeps the engine as close to peak power as possible,
Jeep with 4 engines aligned
Most 4-cylinder Wrangler engines do indeed descend from the old four-cylinder American Motors. American Motors acquired the Jeep from Kaiser in 1970 and sold the Chrysler division when it went out of business in 1987. Chrysler used American Motors’ old 2.5-liter four-cylinder engine until the end of 2003, when it went in favor of Chrysler itself. 2.4-liter PowerTech. Jeeps from 1982 to 1986 with carburettors had the best fuel economy at 2,000-2,800 rpm, and had the best acceleration around 5,000 rpm. Later, the 2.5-liter petrol-injected one was most efficient when it ran between 2,200 and 3,500 (using high rpm for longer gears) and the best acceleration when it went between 5,200 and 5,300 rpm. The most sophisticated of 2.
Engines with six-cylinder aligned
The Wrangler has used four different six-cylinder engines: the AMC 4.0-liter and 4.2-liter inline six, and the Chrysler 3.6 and 3.8-liter V-6. While the torque of the 4.0-liter engine did not reach up to 3,000 rpm (depending on the model year) it has enough torque to achieve the best fuel economy, keeping the number of roms as low as possible; between 1,000 and 2,000, depending on the gear. The less sophisticated 4.2-liter AMC produces less torque and spikes between 1,800 and 2,000, so you’ll most likely get the best fuel economy in exchange for 1,200 and 2,000 rpm. The 4.2-liter should return the best acceleration when it goes from 3,200 to 3,400 rpm (which keeps it close to its peak of 3,200 horsepower) while the 4.0-liter will do better when switching between 4,600 and 4,800 rpm.
V-6 engines
The peaks of the 3.8 and 3.6 liter engines are staggeringly high at rpm: 4,000 and 4,800. But they too use modern technology like variable-length intake runners and variable valve timing to increase torque at low revs. With the 3.8 liter, you should achieve the best fuel economy going from 1,800 to 3,000 revolutions per minute, depending on the specific weight and transmission. You’ll want around 2,500 rpm to move the heavy JK chassis. The 3.6-liter dual overhead camshaft is a beast by nature, so it will do best at over 500rpm. The 3.8-liter’s peak horsepower is at 5,000 rpm, and you should aim for this number or a little higher to get maximum acceleration. The one with 3. 6 liters produces 305 horsepower with a peak at a level quite stratospheric for a Jeep: 6,350 rpm; it drifts between 6,400 and 6,500 to keep the 3.6-liter boiling for maximum acceleration.
