Will your IDI do 70mph?
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Trailering
Joined: Aug 2019
Posts: 18
Likes: 0
Will your IDI do 70mph?
I recently picked up an 85 F350 roll back. At some point the engine and transmission was replaced with a 7.3 IDI and ZF5. The rear axle is a CC sterling, but it is not original to the truck. I don't know what gears are in it, but I think they must be at least 4.11s or numerically higher.
The truck was owned by a friend, and I had borrowed it a couple of times before I bought it to move some stuff around town. I remember the truck scaling at 8,000lbs empty.
Anyways I know IDIs are supposed to be slow, but the truck really struggles to go 65 mph while empty. Surely that isn't typical for an IDI?
The truck was owned by a friend, and I had borrowed it a couple of times before I bought it to move some stuff around town. I remember the truck scaling at 8,000lbs empty.
Anyways I know IDIs are supposed to be slow, but the truck really struggles to go 65 mph while empty. Surely that isn't typical for an IDI?
Last edited by Jericho86; Jun 12, 2021 at 04:12 PM.
5th Wheeling
Joined: Jun 2021
Posts: 33
Likes: 3
From: New England
I have a 87 6.9 idi with a C6 3 speed automatic transmission without a turbo. On a flat surface I hit 65 without issue and I weigh right around 11,000 pounds. It is a ambulance but I dont think I have anything in it giving me more umpf.
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Trailering
Joined: Aug 2019
Posts: 18
Likes: 0
I know the compression is good, and the truck starts easy.
It does have an older eletric lift pump. How would the IP behave if it was starving for fuel? Could it cause the truck to run with low power or would it cause the truck to surge?
Mountain Pass
Joined: Jan 2014
Posts: 170
Likes: 10
From: South Carolina
I have 3.55 in my 87 with a c6, and at 70 it's screaming at about 3000 rpm. I'd be afraid to run it at 80. Like waiting for a grenade to go off..
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Trailering
Joined: Aug 2019
Posts: 18
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The truck does have 4.10s, so it definitely isn't running out of gear.
Curiously, at low speeds, the truck seems torquier than my 97 CCLB dually with a 351w, E4OD, and 4.10s, but I can drive that truck comfortably down the interstate.
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Logistics Pro
Joined: Dec 2003
Posts: 3,776
Likes: 862
From: Foothills of NC
Jack up one rear wheel, after chocking the front wheels. Make a mark on the tire pointing straight down to the ground. Mark the drive shaft on the side pointing out toward the wheel.
Then rotate the wheel two complete rotations while counting how many revolutions the drive shaft makes. If a little past 4, then 4.10. If 3-3/4 turns, then 3.73.
You can get so extremely close enough with this method that you can know exactly what you have by comparing what available ratios were offered with the axle you have.
Then rotate the wheel two complete rotations while counting how many revolutions the drive shaft makes. If a little past 4, then 4.10. If 3-3/4 turns, then 3.73.
You can get so extremely close enough with this method that you can know exactly what you have by comparing what available ratios were offered with the axle you have.
Tuned
Joined: Feb 2020
Posts: 476
Likes: 91
From: Owen Sound Ontario
I have a 93 F-Superduty. It's got an E4OD in it and it does not like 60 mph. That is about top speed for that old girl. it's got either 5.13 or 5.33 gears in the back of it. Idling around it does feel quite spunky for a 9000lb truck.
My 72 has got 3.55s in it and a ZF5 and it will do 75 mph on the flats with 35's. Its got a turbo but isn't exactly blessed with power. Going to go back to 4.10s in it.
My 72 has got 3.55s in it and a ZF5 and it will do 75 mph on the flats with 35's. Its got a turbo but isn't exactly blessed with power. Going to go back to 4.10s in it.
Senior User
Joined: Mar 2014
Posts: 434
Likes: 2
From: Santa Cruz Mountains
Wow. A c6 pushing 80mph!!! Guess its all about the rear end for sure. Duh.
my 90 supercab 4wd e4od to c6 conversion put me at 55 mph with 2500rpm (per tach and roadside speed signs)
The 7.3 sounds like its screening to me but has lotsa room for more juice...just looking at that yellow part of the tach and hearing/feeling the rpms (and being a little older) keeps me from pushing it to 3000rpms.
my 90 supercab 4wd e4od to c6 conversion put me at 55 mph with 2500rpm (per tach and roadside speed signs)
The 7.3 sounds like its screening to me but has lotsa room for more juice...just looking at that yellow part of the tach and hearing/feeling the rpms (and being a little older) keeps me from pushing it to 3000rpms.
Fleet Mechanic
Joined: Sep 2016
Posts: 1,519
Likes: 224
From: Nevada
My 86 has 3:55 Diff, T-19 Trans an 313K miles, and no tach.
Can`t go by the Spedo as it`s off around 5mph fast. I use my
GPS to see the true speed, fastest speed and average speed.
Took a trip to Reno (60+ miles) to see my friends at the IRS.
They always win.
Thinking of crossing the border into Mexico, and sneek back
into the U.S. to get all the freebies as an illegal.
GPS showed fastest speed 69.4 and didn`t sound like it was
coming apart. If it does, then you will find out pretty soon what
the weak link was....LOL
If you have a lower gear Diff, yeah then the engine will be spinning
at a faster RPM and make you uneasy. Stick in your ear Plugs,
and you will be fine.
Charlie
Fleet Mechanic
Joined: Jul 2010
Posts: 1,753
Likes: 34
From: Los Angeles, Ca.
Jack up one rear wheel, after chocking the front wheels. Make a mark on the tire pointing straight down to the ground. Mark the drive shaft on the side pointing out toward the wheel.
Then rotate the wheel two complete rotations while counting how many revolutions the drive shaft makes. If a little past 4, then 4.10. If 3-3/4 turns, then 3.73.
You can get so extremely close enough with this method that you can know exactly what you have by comparing what available ratios were offered with the axle you have.
Then rotate the wheel two complete rotations while counting how many revolutions the drive shaft makes. If a little past 4, then 4.10. If 3-3/4 turns, then 3.73.
You can get so extremely close enough with this method that you can know exactly what you have by comparing what available ratios were offered with the axle you have.