I have a 6.9l diesel 1986 f350, and it has a c6 3-speed. Sadly, this just kills fuel and still can't keep up with traffic at all. I have to get better economy asap, and I'm leaning towards a zf6. Does anyone have experience with this stick shift? Or any other suggestions?

 

zf6 won't fit. There was one guy that made it fit but it required a lot of modification. A zf 5 speed will fit, that is what I put in my truck and it's wonderful, with no other changes my mileage jumped 3mpg. And it is so much quieter going down the highway at speed.

You will need a flywheel kit, and the trans. If you have 4x4 the zf will bolt right up to your original transfer case. You should get the floorpan cover that has the shifter hole, I have heard that pan is higher up to clear the zf trans anyway. And you will need to get a clutch pedal setup from a 86 dwn truck. The 87-up trucks is where you are going to find the trans you need, but you can't use their pedals, they won't bolt in 86 dwn trucks. And you do need to get a zf from a 7.3 diesel, the zf has the bell made into the trans housing like a automatic, so you can't swap the bellhousing only like you could on the 4 speeds.

P.S. You will also need the short upper trans crossmembers. The long lower c6 crossmember will work on the zf, it just needs to be moved back 1 inch. My truck already had the holes drilled for the lower crossmember. I used the zf driveshaft also, but I have heard of people being able to use the old c6 driveshaft with no problems.

 

Your 3mpg jump, was that going from a c6 to the zf5? Or from a different trans? My mostly highway mpg is sitting at 14.5 average right now, and I can't even hit the speed limit. My biggest goal with the manual swap is simply to get the mpgs up to roughly 20 at the lowest price I can.

Also, what's the biggest difference between the 5 and 6 speed zf trans? They cost roughly the same to get the trans, and I have seen someone at one point say the zf6 from a 99-03 7.3 would work. again, the goal is the best bang for the buck mpg improvement I can do asap.

To be honest, I want the tci 6x, but that's 8k by itself, which will take me way too long to afford. Hense the stick shift.

 

The 6.9L IDI is known to be lacking in power with only 170 hp and 315 ft/lb of torque. I have a 1985 6.9 with the T19 manual transmission and it, too, barely keeps up with modern traffic speeds with any type of load. After all, here in the states, before 1987, the national speed limit was 55 and that's what vehicles were built around.

Your best bet is to add turbo to the 6.9L or get a different, more modern truck that was built after 1986. Also, the C6 is a strong transmission and shouldn't be simply tossed away.

 

Another option to free up a little more power from your C6 is to replace the thrust washers with needle bearings.

 

I got a buddy that wants my c6 since it's a k-code. It's not a bad trans, but my local speed limit on the highway is usually about 70, and my 6.9 just can't keep up with that.

And my only real concern with turboing the truck is the fact that it's a tired old engine, and I dont wanna blow the engine.

 

Yes, I had a 1986 F150 with the C6 transmission. It, too, had problems with 70mph speed limits for extended periods of time. Sounds like you need a different truck.

 

I love my truck. It just needs work since it's my daily driver right now. That's why I'm trying to boost fuel economy with more gears

 

^^^^ +1; the most powerful turbo available won't bring down the RPMs one bit on a truck with no overdrive. Personally, I'll take a more sane RPM at highway speed and a higher potential top speed over a faster way to get there.

Re. ZF5 vs. ZF6, everything I've read is that the ZF6 is just closer jumps between gears, not a (significantly) wider ratio range. So for the purpose of getting the R's down, a ZF5 will do just fine, and will be a simpler swap. And in the used market, I'm sure you'll find them cheaper than the available ZF6s.

 

Ok, so the zf5 would just be simpler, then? And I agree, better rpm control is more important than turbo.

 

Size. From what I've read the ZF6 is huge, and fitting it in requires moving a bunch of sheetmetal. The ZF5 is big, but the new floorpan cover Dave suggested is all you need to make room for it.

 

Could keep the c6 and stick on an aftermarket overdrive, like a gear vendor unit. They are pricy new, but you could get lucky and find a used one

 

You could swap to taller tires (that wont help your low power complaint) but it will reduce RPMs at highway speeds and save some fuel. But it may take a little longer to get up to speed

 

ZF-5, ZF-5, ZF-5, ZF-5,
I did this too, Bolted up to the 351 AND T-case!

For Diesel, you would need the bigger model ZF-5

 

x2


My 86 f250 with the 6.9L IDI is sort of a dog on the highway. Most of the driving I do is highway so I should consider a swap as well.


Unfortunately, the GV units are so expensive. I know someone mentioned an E40D swap in a recent thread. Of course, you aren't going to get quite the mpg boost you would with a manual ZF5. Plus, I recall someone saying that it needs an aftermarket controller.


Just throwing it out there since nobody has mentioned it yet.