I started losing power once I put my lift and larger tires on. I'm worried I may be harming my engine or tranny. Should I consider putting in a different rear and if so how much am I looking at? Also I'm getting a loud clunking noise when I put her in D or R.
ms_jetset,
your truck most likely came with 3.55 gears. That means that the driveshaft turns 3.55 times for each revolution of the axle. The tire turns one revolution each time the axle turns one revolution. WIth 31 inch diameter tires your truck would move 97.34 inches, with 33 inch diameter tires your truck would move 103.32 inches. I don't recall the formular for torque, but suffice it to say that it takes more engine power to turn those larger wheels.
With that being said, you should have noticed an immediate loss of power when you put the larger tires on, not a gradual loss. That loss should have remained constant. IE if you had a ten percent percieved loss in power with the new tires on, that should not change. The loss is stricktly due to the mechanical advantage loss of the engine to the rotation of the larger tires. I know I wrote that ars backwards, but it is technically correct.
Also, it is my understanding that a lifted truck has worse aerodynamics than a non lifted truck, therefore you have greater drag with your lifted truck than you did before. I believe that drag geometrically increases at higher speeds. But I know nothing of fluid dynamics, however at speeds greater than 30 MPH much of your energy is being spent trying to overcome air resistance (drag). So with a lifted truck you may notice more loss of power at highway speeds, but not at speeds less than 30MPH were there is little wind resistance.
Going back to gears, if you change your rear end, (that big round pumpkin in the middle of the back axle) and hence change the gears, you can restore the percieved power of the engine. The 302 and 351 engines have the most torque in the range of about 2,000 to 3,000 RPMs they idle at about 650 RPM. If you change to 4.10 gears, that means that the drive shaft will turn 4.10 revolutions for each turn of the wheel. That gives a greater mechanical advantage to the engine turning the driveshaft, which will counteract the advantage you gave to the tires when you put larger ones on it.
You may have noticed that your truck when in overdrive at 65 ran at about 2500 RPMs, and now with the larger tires, it runs at 2200 RPMS. At that speed your truck is still within its power range, but if you are at 50MPH and in overdrive, you may be at only 1600 RPMs and your engine does not have as much power there. Your stock tires may have kept the engine at 2000 RPMs at 50, it would have had more pep. I just made those numbers up, but I think you may get the idea.
I have a slightly older truck with a 302 and 33's on it. If you want to come over and take her for a ride to see if there is any difference between mine and yours for a comparison, that would be fine.
Oh.. I went on and on here... but I meant to say that if you seem to have a continued loss of power beyond the instant loss due to the change in tires, then you should check for diagnostic codes to make sure that your engine is running properly.
joe
