Speed, torque, r/a ratio, tire sizes

[kieth]

I was never able to post my Excel Spreadsheet for usable torque to the ground on this forum (maybe one is limited in file size if you do not pay ?) Anyway over on the GMC Club forum I was able to post up the spreadsheets in a PDF format. So all you guys have to do is click here, go to post #6 and post #7 and you can see information on how fast different height tires run with different transmissions and rear axle ratios.

Torque within a usable range of speeds is what I call it and you need to keep in mind these things:

1. look up the torque values which currently match your trucks set up. Now you have some idea how the ft/lbs of torque feel and at what rpm your truck now runs.

2. Compare that given number with the torque value that corresponds to the changes you are about to make. (If you are changing from 4:10 to 3:73 and then a 700r4 you can see how your vehicle is going to perform before making the change)

3. Keep in mind that one should use 460 ft/lbs of torque to the ground as a minimum (my trucks torque with 700r4, 3:73, tbi350 200hp, 30" tire diameter.) It is not a race truck but it is a nice cruiser ( 68mph 2000rpm)

Here is the link, if you have questions Pm me here on this board....Kieth

posts #6 & #7 tire diameter is shown in file name.....
http://6066gmcclub.com/forum/showthread.php?t=48090

[scottofksu]

I too have been spending an inordinate amount of time considering these issues. I am determining the variables that will comprise the drivetrain for my truck with an eye towards roll-on power and fuel mileage. My interest is to keep cruising RPM's as low as possible (fuel efficiency) without having to downshift or floor it just to get up a small hill (drivability).

My understanding is that you want the rpm at which you typically cruise in your vehicle to correspond to either just past the peak of the torque curve or an area where the torque doesn't drop off dramatically if rpm drops by 200-500 revolutions.

Further, as I understand it, horsepower is essentially the amount of torque that can be applied at a given speed/rpm. As I explained it to a friend, torque is how hard we can push my truck and horsepower is how hard we can still push it after it starts moving at a given speed (i.e., HP = (RPM X Torque)/5252). That is, my understanding is that HP is at least equally important as torque when we talk about how a vehicle operates at speed. Hence, I have heard that you want your cruising RPM to be noticeable below where peak HP is generated, but at a point on the curve that will ensure smooth acceleration. Thus, I have heard it recommended that you want your cruising rpm to be situated between the RPM at which peak TQ and HP are generated. That way, if you encounter a hill, torque increases as speed (RPM) decreases. This is one of the reasons why they typically say you need a lot of low-end torque for heavy vehicles (i.e., the heavier the vehicle the more TQ you need to keep it moving at speed).

Now that I got that out of my head... :0)

Can you give me some more specifics on your engine (Size, MPG, etc.)? The reason I ask, is that I am looking at a cruising rpm of around 1,800-2,000 RPM, with slightly higher geared torque ratings (i.e., torque "to the ground").

[kieth]

Scott, it will be easier if we use examples of what you have in mind, then we can try and get the best possible combination of torque and speed. I will send you a PM and give you my phone # where we can talk about this......

I will need, rear tire diameter, engine, transmission, rear axle ratio, of what you currently have and what you are thinking of changing to.......Kieth

[scottofksu]

Thanks, Kieth!

I have some extended family medical stuff to deal with this weekend, but maybe we could arrange a call next week...

In the meantime, here is what I am planning on:

383 Chevy
Dynamic Compression = 8.02; Static Compression = 10.00 (Plan on running on 91+ gas)
Roller Cam: 232/242 @ .050; .507/.507 Lift; LSA = 112, installed 4 degrees advanced = 108 LSA (Wanting to start making power around 1500- 2000 to 6000)
Vortec Heads (Intake cleaned up; exhaust ported and polished; 1.45 springs good for .58-.60 lift; screw in studs)
Edelbrock Air Gap Intake
Looking for around 450-475 lb/ft at the engine

Transmission
TKO 600 with the .64 overdrive

Final Drive = 3.73

Tires = ? I have something like 245-75/15's on there now. I plan on buying new wheels and tires, but first I am going to see how this combo drives and then adjust my wheel size up/down to improve power/rpm at cruising speed (approximately 65-75 mph).

Cam Link: https://www.lunatipower.com/Product....d=2548&gid=337