Question: I have a '91 Nissan Hardbody King Cab 4x4 with the 2.4L four-cylinder, five-speed, basically stock drivetrain, K&N air filter, and torsion-bar crank (1- to 2-inch lift) in the front, 31x10.50x15 BFG A/Ts and straight-pipe exhaust with 20-inch glasspacks and 16-inch echo tip that it had when I bought it. The engine was rebuilt about 25,000 miles ago.
Mostly it is a daily driver and weekend warrior going on just mild trails and mud running. The biggest problem I am having is a lack of power. While I do not expect a lot out of a four-cylinder, this just stinks. I've heard that backpressure is everything for a four-banger and I need to put a muffler back in and to add new gears. I would like a deeper, throatier sound anyhow, rather than the bumblebee it is currently. I can't run much over 70 mph and the acceleration sucks, especially on the top end, but it lacks power all around. Hills and wind are horrible and interstate (75 mph) travel is nearly impossible. It struggles sometimes climbing steep slopes in low gear.
I had almost given up and figured it was hopeless, but I really like the truck. I see Toyotas run bigger tires and seem to do fine. Nissans, especially Hardbodys, are not very popular here. My goal is to improve its trail capabilities with a lift and 33- or 35-inch M/T tires, but I'm afraid of making things worse.
The gears are about $1,000 and I am kinda avoiding that unless that is what it has to have. There are also a few upgrades like cam/rockers, ECU upgrade, performance header, performance intake, but these are also approximately $1,000, and turn only about 20 hp more. I'd like to keep my four-cylinder to avoid doing a swap and the downtime of a daily driver. Can you help?
Answer: I doubt that the lack of "backpressure" in the exhaust system just because you removed the stock muffler is causing a loss of torque. I have no idea where this myth came from, but in your case it's totally false. Yes, in some instances a better-flowing exhaust can cause a lean fuel condition, and maybe a loss of power, but your fuel injection should handle the minor change in air/fuel ratio without a problem.
Yes, you could put the money, like you said, into the motor and make about 20 more horsepower. In reality, that is a pretty big percentage boost, but it is not going to get you the performance that you want. In fact, it may kill the performance. Why? Because all the horsepower gain will be in the upper rpm range, not down low in the powerband where you need the extra power to get and keep those large tires moving.
I would really recommend that you invest some money in some lower gears. Automotive Customizers (954/971-3770, 4x4parts.com) has the ring-and-pinion gears you will need. They are available in 4.375:1, 4.625:1, and 4.875:1 ratios. The price is in the $450 range for each gearset, so by the time you pay someone to set them up, you're going to be over the $1,000 mark.
In all reality, you should also go for a locking differential-at least in the rear-while you're doing the gear swap. You will be totally amazed at what it will do for your Nissan's off-highway ability.
What gear ratio to go with? I am leaning towards the 4.625:1 gearing with the 31-inch tires, which will buzz your engine at about 2,600 rpm at 65 mph in top gear. The 4.875:1 gears will bring the rpm up to about 2,900 or so, but it would be a much better choice if you plan on going to a larger-diameter tire in the future. Just keep in mind that the lower gearing means less gear strength because the tooth contact area of the pinion gear goes down with the smaller tooth count. With the four-cylinder engine I doubt that this will be a problem.