C6 auto..reliability?

oilburner22

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85' 6.9 automatic...getting a factory turbo kit, bull moose pump, and moose mister injectors after I finish this head job/reseal. I don't know much about these transmissions other than I've heard they are stout...debating putting the ZF5 out of my 89' 460 gas rig that don't run(I've read up on all that jazz of converting)...that'll be down the road tho but I wanna know how long this C6 will last under my lead foot and this power range.
 

franklin2

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Put a aux oil cooler on it and it will last a long time.
 

FarmerFrank

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It depends on what your definition of "tough as nails" is.

My first IDI was an 86 6.9 with a c6. When the engine locked up I put a 7.3 with a hypermax kit and the c6 never complained pulling a 10k trailer. Guy I sold the trans to is still running it in a plow truck and can't believe how good it shifts. I put 30k miles on that truck and who knows how many was on it before that.

I tore up 2 back in my younger days. Junk yard unit I put behind a solid running 351w. A dozen passes at the drag strip with one foot on the gas and the other on the brake in one night burnt it out. Actually it was the truck pull the next night that finished it. Still made it home 70 miles and it wouldn't move with less than 3000rpms. Had a reputable guy rebuild it and I said make it stout.

$800 later I had a solid rebuilt c6. Built a 408w stroker pushing 450hp/450ft lbs and it held up great behind it. 80mph at 6000 RPMS in 2nd gear and it would all but snap your neck when you jammed 3rd.

I often wonder why I could never keep that engine in 1 piece :) :) better question is how I'm still alive.




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D_Fresh361

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I have a C6 in both my truck and my gasser. Mustang trans was rebuilt in 07 and was my daily driver for almost 3 years. I also have one helluva lead foot! They have both held up quite well.
 

oilburner22

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That's what I like to hear. Supposedly the 69k on the odometer is original but the more I tear into this thing the less I believe it. This is all good news to me tho...thank y'all for the input.
 

david85

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The C6 was designed for the first horsepower wars (60s-early 70s) so it was meant for big power and torque. It was also fairly well laid out having learned many lessons of older 3 speed slushboxes like the FMX. Shifts were meant to be crisp, but not harsh. Design and reliability was based more on over-engineering, than precision engineering.

Simply put, its about as good a 3 speed auto that you will find anywhere. By the 1980s, the bugs (not many) were pretty much worked out. Even if it does fail, they are cheap and easy to overhaul.

The only reason it was phased out was to make way for the E4OD and later 4R100...which were still based on the C6 architecture.
 

oilburner22

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So my question is since it is not a lock up torque converter, how well is it gonna get power to the ground? It seems like a lot of power is lost in that thing and that is kind of why I want to do a ZF5 swap later down the road...
 

FORDF250HDXLT

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So my question is since it is not a lock up torque converter, how well is it gonna get power to the ground? It seems like a lot of power is lost in that thing and that is kind of why I want to do a ZF5 swap later down the road...

exactly.it's a dino trans.tough as nails and cheap to buy/repair but,very very expensive to operate at today's fuel prices.
fantastic trans for just a toy,off road,occasional use vehicle.not ideal for a dd.

edit; just noticed your location. and far from ideal for Texas flat,straight and high speed hwy's.she'll eat fuel worse than trying to keep hay fed to a horse.:D
 

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My 86 4x4 with 3.55's weighed in at 6k lbs with the fuel tank and tool box combo in the bed. Daily drive was 120 miles, all 4 lane highway with typical PA rolling roads. I averaged 15 mpg. 1 time I got 20 mpg on a trip up north with no tool box.

I was happy without a lock up converter. Sadly ford didn't offer an overdrive transmission for our trucks I like as much as 4 speeds and c6's in the case of reliability.


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david85

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Here's what my MPGs where over the years of owning my truck:

4.10 axle and C6: 9-13MPG @ 60mph (OUCH!)
3.08 axle and C6: 15-18 MPG @ 70 mph (Whew...)
3.08 axle and E4OD: 18-22 MPG @ 70 mph

All time best:

3.08 axle and E4OD: 24 MPG @ 55 mph (3 consecutive tank average)

My C6 was perfectly reliable but long term fuel cost forced me to toss it. Even after I turbocharged the truck, the trans never complained.

You'll find plenty of good writeups on the C6 to ZF5 swap since its by far the cheapest and most popular way of gaining overdrive in these trucks.
 
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oilburner22

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Mine never got better than like 11-12 MPG...I wonder if I don't have the 4.10's. I have an 89' F-350 with a 460 and a ZF5 and that is the one I had planned on using in the future but as low as the gears are in those I wonder if highway speeds are attainable without too high of RPM's for comfort...a buddy of mine texted me today saying he had a T-19 with a transfer case sitting out back. I wonder if that would be my better route...what do yall think? I think the ZF5 needs some work as far as synchros and maybe more if I remember (hasn't moved in 3 years...) and that T-19 has been sitting on a pallet uncovered for no telling how long.
 

franklin2

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The zf will do fine at highway speeds with 4.10's. And it is well suited to running larger tires if you want. I run the zf with 4.10's and 35 inch tires. It's perfectly drive-able, but I do have to down shift on some hills when running empty on a 55mph roadway. If I had smaller tires I would not have to do that (or a turbo which I do not have).
 

david85

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If the truck tops out at around 70-75mph indicated, you have 4.10s. As for MPG numbers, it depends on the driving cycle. All my above numbers are for highway cruising at operating temperature. Short trips around town were always around 13 MPG.

Going to a 4 speed stick will help, but not by very much since all you're eliminating is converter slip.

My suggestion would be to use the truck as is and keep an eye out for a diesel spec 5-speed. You're not likely saving much if you have to tear it apart, then modify the bell housing before you can use the big block spec 5-speed. They're pretty common since that was the standard trans from 1987 up. A lot of IDIs are getting parted out in my neck of the woods lately.
 
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