4x4ord wrote:
ShinerBock wrote:
4x4ord wrote:
SweetLou wrote:
Cummins12V98 wrote:
"When towing, whatever gear keeps engine rpm between 1,800 and 2,000 rpm will give the best fuel mileage and power because that is the rpm window where the Cummins 6.7-liter makes peak torque.
I tow at 60 with rpm's about 1,750. Just right! That's with 4.10's. At 70 I am about 2,000.
With my last DRW I had 3.42's. I would tow in 5th at same rpm's as above, just right!
Exactly right
I’ll expect to be getting the 3:31s in my next truck. Whether I’ll have to lock out 9th or tenth will depend on the load I’m pulling but with 1050 lb ft of torque most my loads will be considered light.
It not a factor of engine power. I can and have towed my camper many times on my level three tune which puts me at around 470 hp/1,020 lb-ft at the wheels which is about 540 hp/ 1,175 lb-ft at the crank with the standard 15% loss from the drivetrain. These were my dyno numbers with my stock turbo and they are probably 30-50 hp more than that with my current turbo, but I haven't put it on a dyno yet to verify. I still have one more level on my CSP switch that will put me well over 500hp/1,100 at the wheels.
So as I have been saying over and over and over..... my truck has more than enough power to tow in 6th and would blow that 1,050 lb-ft PSD out of the water. It just gets better fuel economy in 5th than 6th towing my 13k RV since it puts me at a more efficient rpms (@ 1,800) at the speeds I tow which is backed up by the BCFC map I posted earlier. I feel sorry for you that you refuse to believe that.
I will also say that 6th gear in my trans is the same ratio as 10th in the Ford trans. So if I did have a hard time holding 6th with the power that my 3.42 rear geared truck makes, then you would definitely have a hard time holding 10th in a 3.31 geared truck with less power.
What kind of fuel economy do you get towing your trailer with the custom tune? If you look at the BSFC map you posted you see that the stock Cummins is similar to most diesel engines in that when you are only requiring a fraction of peak HP you get better fuel economy running low rpm and higher torque. If an engine is capable of 1050 ft lbs of torque at 1800 rpm and only 380 lb ft are required to tow the trailer at 1800 engine rpm and 60 mph, it would be running way off its peak cylinder pressure at that rpm. Better to slow it down.
You got any maps or manufacturers data to gob with that?
I am at 1775 rpm at 65 mph in 5th not 60 mph. At 60 mph in 5th I am at 1660 rpm and 1270 rpm in 6th. At either speed, I am closer to my most fuel efficient rpm while towing my trailer in 5th gear rather than 6th.
