Some questions for y'all about the 6.9...

[Goat]

Howdy,
I've got some questions for y'all 'bout the 6.9 idi that I've had trouble finding answers for elsewhere on the interwebs.

So here they go...

1. What's the top speed of an '86 F-250 (or similar year) with the 4 speed manual transmission and 4x4?

2. What's the average RPM of a manual 6.9 running at when going 65 mph?

3. What's the average MPG going 65 mph?

4. Would it be a good freeway commuter? (I'd be driving back and forth to college classes daily an hour and 15 mins each way, but I also need a truck on the weekends)

5. How good are the 4x4 capabilities of a 1986 F-250 for basic rock crawling, difficult dirt roads, and climbing rocky hills?

6. How reliable is the truck body itself? I've heard fantastic things about the 6.9 and 7.3 idi, but not much about the body.

Thanks for responding! I'm seriously thinking about buying one, but I just wanted to know a bit more about it before I commit.

 

[Oledirtypearl86]

To answer the first depend on the gearing my 86 f350 will do 65 mph but it's turning pretty hard i have 410s in mind it sits around 3k rpm and the mileage is garbage tone it down to 55 and I'll see 14 if I'm not pulling as far as commuter I ran mine 35 miles one way to work off and on for a bit and actually enjoyed it. Off road min is a crew cab long bed with a Dana 60 out front it really doesn't turn very well but iv had it places people would **** also when you get it stuck it take a bit of planning and little more help than a Jeep and body wise mine is a tank and i use the hell out of mine it pulls most the summer and is hauling fire wood or hay the rest

 

[david85]

Howdy,
I've got some questions for y'all 'bout the 6.9 idi that I've had trouble finding answers for elsewhere on the interwebs.

So here they go...

1. What's the top speed of an '86 F-250 (or similar year) with the 4 speed manual transmission and 4x4?

2. What's the average RPM of a manual 6.9 running at when going 65 mph?

3. What's the average MPG going 65 mph?

4. Would it be a good freeway commuter? (I'd be driving back and forth to college classes daily an hour and 15 mins each way, but I also need a truck on the weekends)

5. How good are the 4x4 capabilities of a 1986 F-250 for basic rock crawling, difficult dirt roads, and climbing rocky hills?

6. How reliable is the truck body itself? I've heard fantastic things about the 6.9 and 7.3 idi, but not much about the body.

Thanks for responding! I'm seriously thinking about buying one, but I just wanted to know a bit more about it before I commit.

1. Depends on gearing and tire size. With 4.10 gears and stock tires, 76 MPH is the theoretical top speed if we assume no slip (stick shift). With 3.55, the top speed increases to 87.7MPH. Both assume 3300 RPM governed engine speed, but some have reported higher.

2. At 65 MPH, the numbers are 2820RPM for 4.10 gears and 2450RPM for 3.55. High RPMs won't really hurt these engines, but the fuel economy may hurt you. Older engines may also see higher oil consumption at these speeds.

3. That one really depends on many factors. But I wouldn't expect much better than 16-18 MPG without overdrive even with 3.55s. My truck was factory equipped with 4.10 and a C6. I hand calculated 10 MPG on the highway. In less than 3 months I had 3.08s and could get 18-20.

4. In a word, no. But if you can tolerate lower cruising speeds like 55-60 instead of 65-70, then you can save some money, but a 4x4 without overdrive is going to burn fuel at that speed. If you want an economical and reliable daily, look for an older corolla (or celica, if you're a Scotty Kilmer fan)

5. The 4x4 setup is decent in stock form, provided you don't go nuts with oversized tires and lift kits. I'm of the opinion that the Dana50 TTB can be an adequate setup if you don't abuse it. But all TTB setups will need work by the time you come along. Most go the SAS route and never look back. I would avoid a D44 TTB for anything other than getting to work in the snow. It worked well on the F150s with coil spring setup, but it was too fragile for work under a diesel engine. The transfer cases and transmissions available in this era are comparable to most any other truck of the era. Tough, simple and cheap to replace if something does break.

6. The 6.9 IDI is the truck's worst enemy because it just won't die. Some get hard starting and burn plenty of oil between changes, but if it still runs, owners will still thrash them until they won't run anymore. That can take a while, so the body keeps falling apart in the mean time because most don't bother trying to keep an old beater pretty. Most IDIs were straight up slave horses with a VERY small number being used as toy haulers. You're in California, so you might be able to find a 4x4 IDI that hasn't been thrashed or rusted out. Up here in Canada, they're pretty much all gone now. That's the only reason my truck is 2wd.

 

[Goat]

1. Depends on gearing and tire size. With 4.10 gears and stock tires, 76 MPH is the theoretical top speed if we assume no slip (stick shift). With 3.55, the top speed increases to 87.7MPH. Both assume 3300 RPM governed engine speed, but some have reported higher.

2. At 65 MPH, the numbers are 2820RPM for 4.10 gears and 2450RPM for 3.55. High RPMs won't really hurt these engines, but the fuel economy may hurt you. Older engines may also see higher oil consumption at these speeds.

3. That one really depends on many factors. But I wouldn't expect much better than 16-18 MPG without overdrive even with 3.55s. My truck was factory equipped with 4.10 and a C6. I hand calculated 10 MPG on the highway. In less than 3 months I had 3.08s and could get 18-20.

4. In a word, no. But if you can tolerate lower cruising speeds like 55-60 instead of 65-70, then you can save some money, but a 4x4 without overdrive is going to burn fuel at that speed. If you want an economical and reliable daily, look for an older corolla (or celica, if you're a Scotty Kilmer fan)

5. The 4x4 setup is decent in stock form, provided you don't go nuts with oversized tires and lift kits. I'm of the opinion that the Dana50 TTB can be an adequate setup if you don't abuse it. But all TTB setups will need work by the time you come along. Most go the SAS route and never look back. I would avoid a D44 TTB for anything other than getting to work in the snow. It worked well on the F150s with coil spring setup, but it was too fragile for work under a diesel engine. The transfer cases and transmissions available in this era are comparable to most any other truck of the era. Tough, simple and cheap to replace if something does break.

6. The 6.9 IDI is the truck's worst enemy because it just won't die. Some get hard starting and burn plenty of oil between changes, but if it still runs, owners will still thrash them until they won't run anymore. That can take a while, so the body keeps falling apart in the mean time because most don't bother trying to keep an old beater pretty. Most IDIs were straight up slave horses with a VERY small number being used as toy haulers. You're in California, so you might be able to find a 4x4 IDI that hasn't been thrashed or rusted out. Up here in Canada, they're pretty much all gone now. That's the only reason my truck is 2wd.

Thank you so much for your super detailed response! You gave me a lot of of super useful data there that'll help a ton when looking for one to purchase.

 

[IDIBRONCO]

Just don't forget the most useful tool that you can have for an IDI-Oilburners.net!

 

[saburai]

Just don't forget the most useful tool that you can have for an IDI-Oilburners.net!

I'll second that