(I posted this in the transmission section, but didn't get much response. Figured I would re-post here, since the years are closer here.)

I am considering converting a '66 F-100 352 from a 3 speed to a C-6, using an FE pattern from a '66 Galaxie. I'll have it rebuilt to bring it up to the modern standards for the clutches and fluid, but I am attempting to ID the differences between the C-6 truck tranny (used in the late '60s on) and the car version.
I'm interested in details such as the following:
1. Yoke size and the size of the U-joint in the car and truck yokes, and what years were changes implemented, if any;
2. Gear ratios;
3. Gear hardness, strength, durability (again, car vs truck if any);
4. Tailshaft/tailhousing differences;
And anything else which may impact the conversion.
Recommendations for the rebuild.

Currently, I have a 3.25 9" rear, but I'll change it out to a 2.75 when I make the transmission swtch. I'll use the truck for occasional light towing or Home Depot runs, but mostly for freeway driving. Hence the need to learn about gear ratios if possible.
Thanks!

 

I wish I could be more help on your specific C6 questions, but I can relate my experience with a 2.73 rear gear in a van. I'm almost positive any C6 is 1 to 1 in the top gear and with a 2.75 rear gear it ain't gonna take off too fast! On the flip side that van wouldn't get outa 2nd until 55-60 and then it just kept pullin' and pullin' until aerodynamics got in the way.

Easy swap on the 9" pumpkin tho. If ya don't like the 2.75 I think the most common is a 3.00 that shouldn't be hard to find.

Tire size has some lesser affect too of course. That van had 235/75/15's and I'm assuming your F100 has 15" wheels like most 1/2 tons.

Good Luck on it.

 

Hey Banjopicker;
My opinion if any; I would stay with the 3.25 rear gear; there's not a better combo for initial get up and highway driving; dropping down to 2.75 ? is there a major difference to show over-all preformance change and mileage etc?

Mitch

 

"BP". . .

As you suspect there are subtle differences between P-Car and Trux. But you know, nothing is cast in concrete or cut in stone within FoMoCo family. There are so many variables it could fill a book in itself.

As a generality, Trux & Vans, in heavier vehicles used higher numerical/ lower mechanical low & intermediate gear ratios. P-Cars & Light trux/ Vans had lower numerical/ higher mechanical low & Intermediate ratios. This more or less holds true regardless of which A/T is used.

Another consideration is engine size in relation to vehicle size & application. For example I use a C-5 from a late 70's E 300 Van that had 4.9L (300 Cid) engine and was used to transport people, it was a 14 pass model state Van.
It has a 1.87 or a 1.87 :1 low gear and a 1.10 or 1.15 :1 intermediate, while most other C-5s are more like 1.65 :1 Low & 1.20-.25:1 Intermed'. All still do wind up at 1:1 in high or drive gear however.

So in theory anyway, C-6 from a F350 with 352 or 360 engine should have a lower ratio low & intermediate gear set. One from a F100 or light F250 2Wd should be slightly higher. I realize FE's are fondly referred to as a "BIG BLOCK' but it's wishfull thinking if big blocks start over 400CID in terms of mechanical output. Although in terms of mass and weight FE's are truly "big" blocks. But in terms of pure power when OEM stock FE's don't produce the base power the technically "BIG BLOCK" engines produce.

I mention this because C-6 has a Loss Coefficient around roughly 20%, while other trannies, like C-4's and C-5's have about an 11% " L-C". An average for 352 FE's is somewhere betwixt 200 and 235 HP in most OEM stock, slicks. The
median number will be about 217.5 HP for a 352 CID/ FE truck engine.
That means a C-6 will take 43HP off the top of an average Truck FE's output at max output. Due to that a 2.75 : rear gear is way too low unless all your driving is down hill or with a tail wind. 3.25's will work a lot better than 2.75:1.

As for shaft splines, yokes, U-Joints, Slip Joints et al, the "plate is full" since there is a great commonality among all C-6's in general. The same goes for internals & Tail shaft/ output shaft "details" etc. There always seems to be a way to combine and adapt, generally with OEM-Script parts too. So fitting a C-6 behind your FE in your Slick is very do-able in many different ways from a drop in / bolt in all the way to adaptation and Modification.

Drop in / bolt in is obviously desirable, but total rework is not impossible or difficult either. . . .

I have been talking with Garbz about aftermarket AODs & O/D Units that bolt onto the back of a C-6, and others that bolt up to it, but remain remote, or un-attached directly to the C-6 case. Doug Nash makes one that stood up behind a rather "Snotty and Snorty 460" for 150K miles trouble free.

I do not know what they go for now, but a few years ago they ran around $750 installed. With todays' fuel prices right at $2.00 / gal , and if I did the math right, $750 <$2= $375. $375 X 10 mpg = 3750 Miles (or $750) Assuming max OD efficiency is 20% or less that means the Doug Nash O/D Units should pay for itself in 18,750 miles of useage. [20% = 1/5th and 3750 X 5 - 18,750]

With the Doug Nash set up the O/D Gain is switchable, and happens at the top end, or at final output where it has the most gained. The other trannies I mention have their advantage in low & intermediate and that's no help cruisin' along the Boulevard in high gear, or drive. . . .

I know you'll check all your options before you make a move, I just wanted to throw this data/ info into the pot for your consideration too John.

CIAO' FBp

 

you are talking a relatively easy swap here, as far as differances in trannys the only thing i have noticed after 30 years of this is the shift arm. the shift arm is differant from pickup to car. i beleive pickup is longer and is turnered almost 180 degrees from the car arm.[ i.e. one is pointed up, the other is pointed down.] your 3 speed may have the yoke bolted to the output shaft, the c6 will have a slip yoke. if you have the shaft from the car the yoke should fit your u joint. i cant rember if the overall length is going to be differant or not, but if it is, a one peice driveshaft should be easy enough to find.

my experience is based on what i have done over the years, and since my father in law owns a small salvage i have a pretty good outlet for parts swapping/scrounging.

hope this helps

jim barry