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Big Power or Low Gears?

Posted in How To on November 1, 2001
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Sand is a whole other ball game. Wheelspin is needed to keep your tires floating across the surface. Lower gears are also needed to keep the engine in its powerband, but too low a gear will cause the engine to wind out before the wheels can get up to the speed they need to be. Sand is a whole other ball game. Wheelspin is needed to keep your tires floating across the surface. Lower gears are also needed to keep the engine in its powerband, but too low a gear will cause the engine to wind out before the wheels can get up to the speed they need to be.
Slow and controlled is what this Bronco II is doing. Too much power and he’d be on his back. Rockcrawling and other trail sports dictate a certain finesse, which can be done with massive motors, but it’s easier with lower gears. Big-blocks can help with the steady amount of torque at most engine speeds, while low gears help even small mills. Slow and controlled is what this Bronco II is doing. Too much power and he’d be on his back. Rockcrawling and other trail sports dictate a certain finesse, which can be done with massive motors, but it’s easier with lower gears. Big-blocks can help with the steady amount of torque at most engine speeds, while low gears help even small mills.
Big power is the name of the game in mud country. The power-to-weight ratio needs to be high enough to overcome the suction of the mud, and wheel speed with high gears can be the ticket. If your tires are taller than what your gears can handle, the ratio should be lowered to find the best combination. Big power is the name of the game in mud country. The power-to-weight ratio needs to be high enough to overcome the suction of the mud, and wheel speed with high gears can be the ticket. If your tires are taller than what your gears can handle, the ratio should be lowered to find the best combination.
Here’s an example of high gears, high power, and not much success. The stock 3.08 gears and tall tires on this Blazer require a momentum approach to the obstacle, where as lower gears and a lighter foot would help. Sometimes a blip of the throttle can ease you over a ledge, but slow and steady crawling from low gears makes it easy. Here’s an example of high gears, high power, and not much success. The stock 3.08 gears and tall tires on this Blazer require a momentum approach to the obstacle, where as lower gears and a lighter foot would help. Sometimes a blip of the throttle can ease you over a ledge, but slow and steady crawling from low gears makes it easy.
Swapping gear ratios for a lower set (higher numerically) can increase the torque multiplication of your rig tremendously. Lower gears also offset bigger tires and weak transmission and transfer case ratios, and can even improve fuel economy. Bigger engines can handle higher gears, but the smaller mills really need the low cogs. Swapping gear ratios for a lower set (higher numerically) can increase the torque multiplication of your rig tremendously. Lower gears also offset bigger tires and weak transmission and transfer case ratios, and can even improve fuel economy. Bigger engines can handle higher gears, but the smaller mills really need the low cogs.
Blasting down fire roads and highway driving requires big power to get you going. Too high a gear ratio and you lug the engine, and too low a ratio will over-rev the engine at high speeds. Plan ahead for what you intend to do with your rig, as swapping gears isn’t cheap. One solution is to run stock or higher axle gears for regular use, and install a crawler box to help you on the trail. Blasting down fire roads and highway driving requires big power to get you going. Too high a gear ratio and you lug the engine, and too low a ratio will over-rev the engine at high speeds. Plan ahead for what you intend to do with your rig, as swapping gears isn’t cheap. One solution is to run stock or higher axle gears for regular use, and install a crawler box to help you on the trail.
Power or gears? Power or gears?

Power, power, we all want more power. Naturally, most people think of the engine when upgrading their 4x4, since that’s where the power comes from. Is your rig a slug in the rocks or on the road? Just bolt on a blower or add some nitrous to your mill and you’ll sure have gobs of power. But is that what you really want, or need, for wheeling?

OK, we admit big power is way cool no matter what, even if you can’t effectively use it. The problem comes about when the power isn’t matched to the terrain, tires, or gear ratios. These factors are different for the type of wheeling you do. For instance, sand or mud running has different requirements than rockcrawling, and regular recreational wheeling is a combination of other needs.

Theory

Regardless of how powerful your engine is, you still need gears to match the relatively small powerband of an internal combustion engine to the surface you’re traveling. The transmission, transfer case, and differentials accomplish this reduction. If you had an engine idling at 600 rpm hooked directly to the axle, that baby would be zipping down the road. However, the engine wouldn’t have the torque or twisting force needed for acceleration, which is why we depend on gears for the reduction. Remember riding a 10-speed bicycle? You have plenty of power in your legs (engine), but you needed to start out in first gear, then work your way up to 10th as the bike got moving. Your legs are just as strong in either gear, but the distance covered on the ground for each pulse of the leg is more in the top gear, just like the pistons of your 4x4 engine.

Working Together

OK, enough of the basics, we need gears and power. It’s the fine line or proper combo that you need and want, but what exactly is best? How do the little four-cylinders and diesels wheel so well when some big V-8s just don’t cut it? It’s all how the components are matched up. An engine delivers torque, or twisting force, to the rearend and out to the wheels. Torque multiplication is achieved through the transmission, transfer case, and rear axle. With enough gears, even your little legs could move a mountain, albeit slowly, but with enough power you can do away with many of those gears.

The mechanical advantage of gears also makes it possible to match the wheel speed to the powerband of your particular engine, which is why Second gear may be too low, Fourth is too high, but Third is just right. The same goes for tires, as in stock applications with stock gearing. If you increase the tire diameter from 28 to 35 inches, your factory gears aren’t low enough to effectively motivate them, and acceleration goes out the door.

Gearing

Let’s take an example of an ’86 Chevy pickup in bone-stock form. The 350 V-8 engine was rated at 160 horses at 3,800 rpm, and 260 lb-ft of torque at 2,800 rpm. A good rule of thumb to figure out your engine’s powerband, or most efficient operating rpm range, is to use 60 percent of the horsepower figure as the low point, and 80 percent as the high point. For this engine, this comes out to 2,200 to 3,000 rpm, which equates well with where the max torque figure is. This pickup came from the factory with 3.08 gears and 235/75-R15 tires, about 28 inches tall. Using the formula to figure rpm (mph times gear ratio times 336, then divide it all by tire diameter) we see that at 60 mph, this combo gave about 2,200 rpm, just barely in the powerband, but just right for best fuel economy.

Off road, even the 2.61 low range is barely able to make up for the 3.08 axle gears, providing less than adequate power for off-roading. Now add a set of 33-inch tires, and even at 60 mph the engine is lugging around 1,800 rpm, way out of the powerband of this engine. In this case, more power can do the trick if you can get the torque and horsepower figures to work at 1,800 rpm. The easier way is changing the axle gears, where 3.73s would be a good compromise, as the chart shows that with 33s and 3.73s, the 60-mph highway rpm would be 2,200. Likewise, choose a set of 4.10s for towing, or 4.56 for better wheeling ratios.

Power

But how about power? Again, couldn’t you just bolt on a blower, add some nitrous, and blast down the road? Sure, but you wouldn’t have the wide variety of choices that gearing gives you. In our Chevy example, 160 hp is definitely anemic. We like to see at least 350 horses out of a mill this size, which equates to quicker getaways and more stump-pulling ability. Through a variety of carbs, cams, pistons, and other items it’s relatively easy to pump up some ponies on most engines, and it’s usually enough to offset the bad choices some people make in the tires/axle ratio department.

We’ll never solve the debate over which is better, gears or power, but we figured you all want to know what the heck is going on. For our money we’d do lower gears first and engine mods later, but without a doubt we want ’em both.

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