A few questions about speed and gearing

[Burgerboy13]

May I ask why you want to change the gears if its off road only? Stock gears will give you more torque than 4.90 gear ratios. Have you seen how much 4.90's gears cost? They are about $1200. If you plan on tires bigger than 46 inches I see no benefit from 4.90 gears, especially if you only plan on using it off road.

 

[jollyroger]

OK. For mud slinging ability you need HORSEPOWER.

Torque moves the work. Horespower sustains the work. You need both to some degree. If you just have torque and not much horsepower you will be able to move the tires through most anything but not be able to light up the tires to sling mud. If you have mostly horsepower and not much torque you will burn the clutch out trying to get the wheels moving.

Your multifuel diesel just by it's nature and being an inline 6 with a long stroke will make more torque at lower RPM's. A good configuration to move the tires through the mud. Not to do so with mud flinging authority. This is why monster trucks use blown big block style engines with 5 ton axles and big floatation tires. If the diesel was the way to go for that they would have the big Caterpillar 3408 or an 8V92 Turbo Detroit.

Big Block Chevy. Came in everything Chevy made until the late 70's. You already know this. The 454 in my 89 Rinker Captiva boat makes 330 HP. Mild for a boat motor but strong for a typical stock truck/car 454.

500 Cadillac. After 1968 the two engines Cadillac offered were the 472 cid and the 500 cid until 1977. The 500 typically came in the Eldorado. It was an option in the Coupes and sedans. ALL cadillacs in 1976 came with the 500 in celebration of our bicentennial except the Seville which came with a TBI 350 Oldsmobile. The Eldorado's in 1976 were Throttle Body Injected. They all were severely underrated for horsepower and torque. All the ratings were shown at a low RPM. I have not confirmed but have heard that the 500's had a stainless steel crankshaft. I have also heard that the 1970 500 carbureted made 400 HP @ 4000 RPM and 550 lb/ft of tourqe @ 2700 RPM.

Hope this helps.

 

[Flat Black]

ENGINE! -- That's the short and simple answer. An 11-12k pound truck thats basically a brick wall is being pushed down the road by an engine that was 134hp and 330 TQ when it was new. Yes the gears and tires are a part of it, i.e. you can change to 38" tires or numerically higher gears and it will be easier to get up to speed, but your top speed will then be even lower than stock. This is simple math and physics.

The solution is to modify or swap the motor....its what does the pushing, duh! Lets go crazy for a second and say that you put a 1000 hp supercharged big block in your truck. You probably would not do 150 mph but the gearing and tires and aero would not matter driving down the road, bc you have plenty of power to push everything.

These trucks were not made to go fast. They were made to convoy, haul heavy loads on and off the road and be driven and worked on by snot nosed 18 year old kids who didnt know what they were doing.

Big Rig Tractors make tons of torque, 1500-2000 pound feet in new models, but may only make 350-500hp. They dont go "fast" but they can pull a $hit ton of weight. Conversely, a formula one race car makes about 850hp at 18,000 rpm but only 200 pound feet of torque. Great for speed and acceleration, sucks for towing, pulling or "doing work" You need a blend of the two depending on what you are going to do with the truck and you should bias the motor towards that. As mentioned above mud bogging requires HUGE amounts of horsepower and torque. Towing slowly, just lots of torque, so with low hp and high torque you will be the guy going up the mountain at 20 mph in 2nd gear. HP helps you sustain the load. Again you really need both and Math and Physics tells you that 130 hp and 330 foot pounds of torque will only get you so far, just the way it is..... End of science lesson.

Pick your poison (Cummins, Duramax, BBC, Detroit, CAT, 500 Caddy, etc) and repower your truck if you want more power or to go faster. Turning the pump up will get you something and you may be able to upgrade turbos or do some intake work, etc. I dont know the limits of the stock motor, but IMO, you are polishing a turd. Turn the pump up and install some gauges and maybe that will be enough for you.