Custom Tune Installed

[Chris74]

Originally Posted by JackandJanet

Darren, unless you know something I don't know (which is VERY possible), the HP and torque are dropping on our engines above 5000 RPM. If you look at your WOT shift points in the custom options menu, I think you'll find that Bill has set the 1-2 and 2-3 shifts near that RPM range. If so, I would not raise them. The 3-4 shift is likely to be lower, possibly around 4500? Again, I would not raise it. I tried raising mine and discovered it seemed to have trouble completing a WOT 3-4 shift, even though my max RPM was 5400.

I also wouldn't raise the max RPM much above 5400 because of TC ballooning, and I like to keep the stress off the transmission as much as possible.

I have my speed limiter set at 110 mph. Net because I really expect to drive that fast, but I DO NOT want it to ever kick in when I'm expecting more speed, as in passing.

I've had my truck dynoed........and with my setup, I was still building power at 5000 rpm's.

My power actually topped out around the 5200 mark.......

[Jackpine]

Hi Chris - The 08's have a somewhat different power curve than earlier models. I think they DO drop off later. I know your engine can develop more power and torque than mine.

Regardless, I really think Bill will set the WOT shifts at the correct points to get you what you want. And, thinking about it a bit more, it makes sense to set the shift point somewhat past the peak HP and torque point because then the shift will bring in the next gear at a higher RPM setting, overall, giving you better acceleration (if WOT acceleration is your bag). This of course means the max RPM has to be set higher too, so that the shift can complete properly.

I DO want the acceleration, but I'm also interested in protecting the transmission, which I feel is a weak link in this truck. (Wish it was a manual).

- Jack

[Jackpine]

Darren, 3.1415 is pi (It's really closer to 3.1416). When you do this calculation: 17 x 25.4 = 431.8, you get the rim diameter in millimeters. Adding in 255 x 70 x 0.02 = 357 gives the additional diameter of the wheel contributed by the tire, so the overall diameter of the wheel is 788.8 mm. Now multiply by pi to get the unloaded circumference, which is about 2478.1 mm. OK, this is how far a wheel will roll in one turn if you take it off the truck and roll it across your garage by hand.

But, the truck's weight "flattens" the tire, effectively reducing the diameter under load. On the truck, one revolution will cover less distance than it did when off the truck. I've found for my tires that about a 3% reduction in circumference is about right. I suspect this will be close for your tires too, so subtract 3% or 74.3 mm from the unloaded circumference of 2478.1 which gives 2403.7 (round that to 2404).

You can avoid all the math, of course, by just entering your tire size in the "Gear Ratio Calculator" that is one of the tools in Pegasus. The "Old Tire Size" you should enter is 255/70-17. The calculator has a default "rubber squish" of 5% which gives a circumference of 2421 mm. The calculator uses higher "squish" values because the rim doesn't deform, only the tire does, so the percentage change is being applied only to the tire's contribution to the diameter. I've found my calculator "squish" needs to be about 7%, and if I use that on your tires, I get 2399. A 6% squish gives 2410. So, you can see that the 3% total reduction I used earlier is between 6 and 7% rubber squish.

Anyway, try 2404 (or maybe 2410, since you inflate your tires higher than mine) as a starting point. I think it will be close. (And I hope I haven't confused you too much)!

By the way, 3.3 recorded miles in 3.0 actual miles is a huge error! (About 10%). And, showing more distance on the odometer than actually traveled says your tire size is too large.

- Jack

[DarrenWS6]

Originally Posted by JackandJanet

Darren, 3.1415 is pi (It's really closer to 3.1416). When you do this calculation: 17 x 25.4 = 431.8, you get the rim diameter in millimeters. Adding in 255 x 70 x 0.02 = 357 gives the additional diameter of the wheel contributed by the tire, so the overall diameter of the wheel is 788.8 mm. Now multiply by pi to get the unloaded circumference, which is about 2478.1 mm. OK, this is how far a wheel will roll in one turn if you take it off the truck and roll it across your garage by hand.

But, the truck's weight "flattens" the tire, effectively reducing the diameter under load. On the truck, one revolution will cover less distance than it did when off the truck. I've found for my tires that about a 3% reduction in circumference is about right. I suspect this will be close for your tires too, so subtract 3% or 74.3 mm from the unloaded circumference of 2478.1 which gives 2403.7 (round that to 2404).

You can avoid all the math, of course, by just entering your tire size in the "Gear Ratio Calculator" that is one of the tools in Pegasus. The "Old Tire Size" you should enter is 255/70-17. The calculator has a default "rubber squish" of 5% which gives a circumference of 2421 mm. The calculator uses higher "squish" values because the rim doesn't deform, only the tire does, so the percentage change is being applied only to the tire's contribution to the diameter. I've found my calculator "squish" needs to be about 7%, and if I use that on your tires, I get 2399. A 6% squish gives 2410. So, you can see that the 3% total reduction I used earlier is between 6 and 7% rubber squish.

Anyway, try 2404 (or maybe 2410, since you inflate your tires higher than mine) as a starting point. I think it will be close. (And I hope I haven't confused you too much)!

By the way, 3.3 recorded miles in 3.0 actual miles is a huge error! (About 10%). And, showing more distance on the odometer than actually traveled says your tire size is too large.

- Jack

Thanks yet again Jack for the information. I will try this again. Hopefully this will be resolved soon. I'll put in the 2410mm size and go back down that 3.0 mile route and see how it goes. I keep my tires at 40psi so that should be a good starting point.

[Jackpine]

Originally Posted by DarrenWS6

Thanks yet again Jack for the information. I will try this again. Hopefully this will be resolved soon. I'll put in the 2410mm size and go back down that 3.0 mile route and see how it goes. I keep my tires at 40psi so that should be a good starting point.

Darren, since the max pressure on your tires is 65, I would not hesitate to inflate them to 45 or 50.

- Jack

[DarrenWS6]

Originally Posted by JackandJanet

Darren, since the max pressure on your tires is 65, I would not hesitate to inflate them to 45 or 50.

- Jack

The max is actually 44psi. I have General Ameritrac's

[Jackpine]

Originally Posted by DarrenWS6

The max is actually 44psi. I have General Ameritrac's

My mistake! Stay with 40 psi. Overinflating invites a blowout, and, premature treadwear in the middle of the tread.

- Jack