Josh Oldenburg
Full Access Member
One big think many seem to forget, the IDI era trucks (83-94.5) the national speed limit was 55, everywhere, including the interstates. So when the engineers designed these trucks, they designed them to work their best at 55mph and below. That’s why 55 is in the 2000rpm range, since that is the general area of peak torque (an IDI is peak torque in the 1800ish range if I remember correctly). So the truck being able to maintain 55 while pulling a reasonable load was the goal. Yes, the 4.10’s do help, but, they are still shooting for 55. My 88 IDI C-6 4.10 likes 60 and under, yes it will go faster, but, it’s much happier there.
Now, fast forward a few years, and the interstate speed limits have been raised, and in my part of the country, the highest speed limit we have is 70. I know there are some states that have higher, but 70 is a good number ina good part of the country. Now, the engineers are designed the vehicles for working well to 70mph and below. Looking at my 03 6.0 Excursion, it was designed with 70 as the goal, which is why 70 is approximately 2000 rpm... same rpm sweet spot as the IDI, but is geared differently and has more horsepower and more importantly more braking power to handle the increased demand to get to, and stop from 70.
So, take a trailer and hook it to an IDI and it will be happiest at 55, but hook the same trailer to my 6.0, and it will run 70, and be happy doing it. But it was designed to run 70 to begin with. The IDI was not.
True, but what happens when someone is tired of the gasser in their 2000 SD and puts in an idi they have lying around? 4.10's and OD and plenty of stopping power