Good afternoon. Im from New Zealand.
I have just bough my very first "F truck" - a 84 f100 XLT (aussie assembled factory right hand drive). ( will put a pic up, when i work out how!)
Its running a 3.5 LSD in the rear end, in a 8 3/4 diff.
Now, ive always like the look of 78-82, and 82-86 shape trucks, hence why i went for this one - also, someone had the brainwave of shoe-horning a navistar v8 diesel into it, and running what we believe to be a c6 3 speed auto behind it.

My first questions are - how the heck do we tell what size navistar it is??
How do we tell what trans it actually is?

My first problem with the package is that at 100kph (60mph), the motor appears to be pulling 3000 rpm. What do they rev out to? and what sort of rev's should she be doing at 60mph? - i was told 2200-2400 rpm??

Also, running on 5 stud rims, we have 31x10.5x15lt rubber on all corners.

To correct the rev-ing motor, what ratio diff should i run?? someone suggested a 2.75, but i was also told that it would be too tall for reverse.

All Help, and input appreciated, and i certainly aim to be a long serving "F truck" owner, for year to come.....

Its great to drive, grunty, and still has that nice v8 rumble....

 

In the USA/Canada, Ford offered a 6.9L and later a 7.3L diesel, both were Navistar engines. The C6 was also used behind the diesel's. These engines were only offered in the F250HD and F350 chassis, but they could be stuffed into a F100/F150 chassis.
I can't say what the max RPM is, but I believe it's around 3500 RPM where it tops out.
8 3/4? Sounds like perhaps the 8.8" that started showing up in some 83 model 1/2 tons. Most notable for having a rear cover, while the 9" had a removable center that came out the front of the housing.
Being an Aus/NZ model, anything is possible.......

 

At the top of the page is a drop-menu 'Articles/Specs'. If you click on Automotive Calculators, you can get to the MPH-RPM calculator which will let you try any set of combinations to get your rpm range.

Also, on the Models drop-menu, is access to the diesel engine section.

 

Does it have a tach or are ou just guessing the RPM? With 3.55 gears you should have no problem going the speed limit. Many of the trucks came with 4.10's. (Diesels)

If it's making it to 3rd gear there shouldn't be a problem.

 

Hey, thanks guys for the input....

Im informed that its a c6 trans, and an 8 3/4 diff, apparently factory fitted to the aussie f100's.

Yeah, im just trying to get an idea what rpm a v8 diesel should be doing at 60mph/100kph, as (running off the factory tacho) it appears to be, and feels to be reving quite hard at 100kph. if it were meant to be 22-2400 rpm, then that would dictate what Diff ratio to try to switch to.

 

Ok, here goes...


I jacked it up, and spun the driveshaft. it appears to be an 8.8 diff (with removable rear cover) and at a 3.5 or 3.55 ratio. It appears to be a c6 trans, as its 3 speed auto, and 17 bolt pan.
The tyres are 31x10.5x15, so any online calculator indicates the diff ratio should be 4.4. what am i missing?? the dash (factory rev counter) is the one im working off of, and appear to have been made to work thru some box unit, off the diesel - however, the motor does sound to be reving quite hard at 60mph/100kph.

Is there some factor that i/we havent thought of?

 

"Revving hard" depends on what you are comparing it to. Compare it to a more modern car or truck with overdrive, then yes it will sound like it's revving hard. But like was said in a previous post, it won't hurt the engine, just your wallet with fuel consumption. The engine is governed to 3300 rpm.

I had the c6 in my diesel, and yes I got tired of the rpms. What I did was swap in a zf 5 speed tranny with overdrive. It runs much quieter down the road and I get 3 more mpg.

If you are not going to pull any loads, you could certainly swap in a 3.08 or some lower gear, but it will kill your pulling power if you haul something on a trailer.

 

With 31" tyres you should be at 2300 ( plus a little extra for the non-lockup converter) ......if definitely a 3.55 rear.
You could use a strobe-type rev counter on the engine & compare to the trucks own tach.

 

strobe type rev counter??/ Hmmm... ok - how do they work?
But yes, thanks 2300 rpm at 60mph/100kph is around what i had been told it should be doing. trans feels like its shifting properly, and not slipping etc... So, looks like i will have to try a 2.7 or 2.9 ratio diff to get them revs down. Nah, not carrying any great loads, or pulling anything too heavy....

 

They are light sensitive & 'count' anything revolving that they are aimed at.
If necessary you can put paint or reflective tape on the turning part (shaft, flywheel etc).

You would need to determine your actual revs before you can make any diff changes.

Are you 100% sure that you have a 3.55 axle now?

 

well, first up, yeah. jacked it up, spun the drive-shaft. both wheels turned. 1 rotation of the rear wheels equaled 3 1/2 turns of the drive-shaft. reading up on the ratio's, 3.5 is open, 3.55 is LSD.

Now, an interesting thought - this gearbox - wasnt there a 4 speed auto made, based on the C6 for the later powerstroke diesel's? I wonder if thats what the trans is, and not hooking into 4th?? It does however have 17 bolt pan on this one....

 

The C6 pan looks like a square with a corner missing, like an "L", the OD transmissions pans are a long rectangle shape.

 

Yes there was, it's called the E4OD. It's a fairly good tranny if you upgrade it to the 95-up internal parts. The big drawback to it is it's electronic and requires a computer to run it. Ford had a stand alone computer that ran it on the old diesels like you have, and there are companies like Baumann that sell a controller for around $500.

 

ok, side thought. from the engine to the trans. although they list the final drive ratio's, is there any loss in revs in the trans, with the likes of the torque converter, etc??

I dont know - it just doesnt make sense - the engine certainly doesnt want to rev over 32/3400 rpm on the dash tacho, the manual count of the rear diff comes up at 3.5(ish), any all the maths ive done, all the input from you guys, all doesnt add up in the proven maths.

 

some good info here on the trans ID's Technical Video - Rebuild Your Transmission, Differential, Transfer Case