Such thing as Perfect HP for off-roading?

[Travis Craig]

I had a 'disagreement' with someone today who was firm on their opinion regarding the power needed for off-roading. We aren't talking any specific truck, I even brought this up. They had a set HP number that they claimed was the perfect number.

Do you believe there is such a number?

 

[smritte]

NO.
If the person actually understood what they were arguing, they never would have made that statement.

Its all about math. Modern, stock vehicle HP/TQ numbers are good for general off roading. It's when you start building for competition or serious toys is when these numbers change. As we go away from stock, there are things we need to do. 4WD's tend to need less HP and more torque. If you freeway drive mostly, you need more HP to offset the weight and tires you added. Bigger tires and armor need more torque at lower speeds. If you amplify torque, you loose HP (to a point).

Here's my last Jeep. This will give you an idea what happens to the numbers when you build. I actually needed more power but I didn't want to change the drive line.

My TJ was rated at a whopping 190 Hp. Unfortunately that number is way over rated and most likely from the back of the crank, not to the wheels.
When I put it on the dyno (before the build), it was about 140 hp and 165 torque. With that low of HP/TQ, I need lower gears to multiply Torque (not HP).
In order to turn the 39" tires and all the extra weight in armor (5300#), I needed more torque. I got this from lower gears. Doing so I sacrificed HP. I didn't care because it was a toy. If I wanted to go fast, now I need more HP.

I don't know what the actual numbers out the crank are so I'm going to use the "ground" TQ. These numbers wont be exact. Were also not calculating HP loss in the driveline or tires.

165Tq X 2 (torque converter) = 330. Most automatic torque converters give X2 (ish)
330 X 2.84 (1st gear) = 937,2tq
937.2 X 4.0 (transfercase low)= 3748.8 Tq
3748.8/2 (split front to rear)= 1874,4tq
1874.4 X 5.38 (gear ratio)= 10084,27Tq
10084.27/2 (split to each tire) = 5042.13

With the stock numbers (sort of) I had a bit over 5k torque to each wheel. That was great as a toy but freeway I couldn't go over 65 mph. I didn't have enough HP. There's formulas to calculate speed based on HP and weight (put simply)

When you design a vehicle, you plan the chassis around what your going to do with it. Once you have general weight numbers you start calculating HP/TQ numbers.
My first desert car was a Baja Bug. I probably put less then 100hp to the ground but it was enough for the chassis. One of my buddies runs about 900hp in his desert truck. Top speed is about 130 mph. The more power you have, the faster you go. Their cant be a "Number". Its all based on what your doing and what your driving.

A piece of advice I give my students. Never argue mechanical things. I will debate to a point and thats it. Sometimes I say "lets do the math", all that does is make the other person angry.