6.9L IDI and C6, lacking in gears

[351W]

I have read many times that the C6 is lacking in gears for a truck. A 7,000-lb truck hauling up to 15,000-lbs needs more gears. Sometimes, apparently, a 3-speed automatic is not enough. So with the wide ratio gearset, 2.71:1, 1.55:1 and 1:1 gears, where do I need another gear? I can see needing a gear between second and third gear. Do I really benefit from adding a gear between first and second gear? I would thing that down low I want fewer gears spaced farther apart but up high I want more gears spaced closer together.

2.71:1
do I really benefit from an additional gear here? (2.11:1)
1.55:1
1.21:1
1:1
.78:1

Obviously I am going to just have to build a 6.9L IDI and experiment for my self but this should be pretty universal. I have read that the fuel consumption really goes up with sustained cruising rpms over 2,000 rpms. I get 15-mpgs cruising for 100 miles + in my '05 F-150 5.4L 24V a t~3,000 rpms (OD locked out) or ~2,100 rpm (OD on). Now it seems that larger engines really need to be kept throttled down. I am thinking that 1,800 rpm is the appropriate cruising speed for fuel economy but maybe as much as 3,000 rpms is safe.

If the C6 is missing gears when towing than so is the E4OD. I know about Gearvendors but I am not convinced that I need to split every gear.

 

[Dave 001]

A C6 with a wide ratio gear set is still only a three speed. Where are you coming up with 1.21 and .78 in that list of gear ratios?

From what I've been told from reliable sources, you will not be able to get a C6 with a wide ratio gearset to shift at appropriate speeds when used with a Diesel.

A C6 with a wide ratio gear set has the same 1st, 2nd, and 3rd gear ratio as an E4OD. That's basicaly where Ford came up with the E4OD gear train. Plus, obviously, the E4OD has 4th gear (OD) and a lockup converter.

An IDI and a C6 will get the job done, you just have to let the truck work at it's own pace. A buddy of mine drive from south Jersey to Florida for years and years with a 6.9, a C6, and 3.54's grossing 15,000 to 18,000 lbs. He actually wanted to gear the rear UP.....from 3.54 to 3.31 not 3.54 to 4.10. The tach used to sit on 3000 rpm for hours at a time but an IDI can easily handle that.

 

[Wyreth]

Personally on my rig, I'd like the addition of a lower first and an OD.

Hence I got an DNE2 od for now, but will eventually swap out for a ZF.

 

[jayro88]

I have always heard that the C6 is a very reliable transmission for heavy duty towing. Of course you would want to add a large trans-cooler, but that goes for any auto when towing. I would also add a guage so you can keep an eye on the temps. Biggest issues with the c6's are....1. Does not have an overdrive (only a 3 speed) so your MPG is hurt at higher speeds due to the increased RPM's. Never heard of the higher RPM's hurting anyting mechanical though. 2. Non-locking torque converter produces more heat and is not as efficiant at transfering power. A transcooler can fix the heat issue, but the torque converter issue just means it is not as efficient so your MPG will suffer. They do sell different types of aftermarket converters that could help.

A gear vendor would help with the RPM's at the higher speeds and could also make towing a little more driver friendly. By adding gears in between you could stay at the torque peak a little easier. Of couse, this is the theory, and you would want to get some real world thoughts from people with first hand experience prior to spending the money for one.

 

[icanfixall]

Just because you have a gear vendors does not mean you have to split ever gear you have. You can choose which gear you want to split. Thats how I drive my rig. It is interesting when I manually shift thru 8 forwardsgears sometimes. Having many extra gears means you have more choices to choose from. One of our members have a Brownie aux trans in his ZF 5 speed trans.. He has 15 forward grear and three gears in reverse now... does he use every gear driving down the road.. Nope but he has the choice to use what he needs. Maybe you could swap in a ZF 5 speed... Or look on ebay today... These is a gv with the C6 adapter needed to fit your C6...

 

[Dave 001]

I have always heard that the C6 is a very reliable transmission for heavy duty towing.

Very, very reliable. If I told you how many miles without a rebuild were on the C6 mentioned in my post above, you would flat out think I was full of it.

 

[towcat]

I have read many times that the C6 is lacking in gears for a truck. A 7,000-lb truck hauling up to 15,000-lbs needs more gears. Sometimes, apparently, a 3-speed automatic is not enough. So with the wide ratio gearset, 2.71:1, 1.55:1 and 1:1 gears, where do I need another gear? I can see needing a gear between second and third gear. Do I really benefit from adding a gear between first and second gear? I would thing that down low I want fewer gears spaced farther apart but up high I want more gears spaced closer together.

2.71:1
do I really benefit from an additional gear here? (2.11:1)
1.55:1
1.21:1
1:1
.78:1

Obviously I am going to just have to build a 6.9L IDI and experiment for my self but this should be pretty universal. I have read that the fuel consumption really goes up with sustained cruising rpms over 2,000 rpms. I get 15-mpgs cruising for 100 miles + in my '05 F-150 5.4L 24V a t~3,000 rpms (OD locked out) or ~2,100 rpm (OD on). Now it seems that larger engines really need to be kept throttled down. I am thinking that 1,800 rpm is the appropriate cruising speed for fuel economy but maybe as much as 3,000 rpms is safe.

If the C6 is missing gears when towing than so is the E4OD. I know about Gearvendors but I am not convinced that I need to split every gear.

what is your rear end ratio?
if 3.55 you are a freeway cruiser only.
if 3.73 or 4.10 you can tow with and have decent top end power.
if 4.56 you will have a helluva holeshot but will top out at 55mph.
if 5.13 you can climb up the side of a building and top out at 45mph.

my f450's have 4.62's and 5.13's and haul 15k-17k gross routinely but top out at 85mph and 75mph respectively. this is with a ZF5 and bouncing off the governor at 3800rpm.

towing power and fuel economy are diametrically opposite terms. pulling the hills you want the rpm spin above 2500. but your fuel economy suffers above 2500rpm. running below 1800 rpm will get your motor to spin bearings. i tell my guys to run off the governor WHEN YOU ARE LOADED. I'd rather pay for 10mpg fuel than having to do a bottom end.

 

[MUDKICKR]

Very, very reliable. If I told you how many miles without a rebuild were on the C6 mentioned in my post above, you would flat out think I was full of it.

as long as you do proper maintenance to a transmission, there is no reason for it to last many 100,000s of miles. transmission flush machines/services are worth there weight in gold to make a transmission last.

 

[351W]

Towcat:
"what is your rear end ratio?
if 3.55 you are a freeway cruiser only.
if 3.73 or 4.10 you can tow with and have decent top end power.
if 4.56 you will have a helluva holeshot but will top out at 55mph.
if 5.13 you can climb up the side of a building and top out at 45mph.

my f450's have 4.62's and 5.13's and haul 15k-17k gross routinely but top out at 85mph and 75mph respectively. this is with a ZF5 and bouncing off the governor at 3800rpm.

towing power and fuel economy are diametrically opposite terms. pulling the hills you want the rpm spin above 2500. but your fuel economy suffers above 2500rpm. running below 1800 rpm will get your motor to spin bearings. i tell my guys to run off the governor WHEN YOU ARE LOADED. I'd rather pay for 10mpg fuel than having to do a bottom end."

In that case I think I want the 3.73:1 or 4.10:1 gears. I don't require the nth degree in fuel economy and I do pull a trailer or haul a bed full of concrete rubble, blocks or rocks from time to time. These are just personal projects but I do my own labor and I like it that way. I have acquired some IDI parts in my shed, cylinder heads, injector pumps and other parts. For my next project I am going to build a 6.9L IDI to pull my '66 Bronco on a trailer and for my landscaping and residential remodal projects. I have put over 3,000 lbs in the bed of my F-150 numerous times and it will do it but the 6.9L IDI is a better truck engine. I do haul away heavy steel to recycling from time to time.

Dave 001:
"Where are you coming up with 1.21 and .78 in that list of gear ratios?"


(.78:1) x (1.55:1) = 1.21:1

I was asking if I need this gear, between first and second gear?

 

[351W]

I agree that 10-mpg is something worth living with when that is what you get. I am willing to sacrafice fuel economy but how will 3,000 rpms affect longevity?

 

[franklin2]

The c6 has way more gears than that. It has a "virtual" gear that is worth a couple hundred rpm up or down. It's convertor slip. All those ratios listed above are actually lower with the converter slip in this tranny. But of course that's what kills the fuel mileage also.

 

[Hydro-idi]

I agree that 10-mpg is something worth living with when that is what you get. I am willing to sacrafice fuel economy but how will 3,000 rpms affect longevity?

You can keep your idi at the governor (max rpm) all day long if you want. It won't hurt anything but fuel mileage

 

[towcat]

these motors are designed to run pinned to the governor WHILE UNDER LOAD. those are the key words. in my f450 carrier I logged 980k/mi on two motors. the first one died at 650k/mi the second one is still chugging along.

 

[Dave 001]

how will 3,000 rpms affect longevity?

They'll do 3000 rpm for hundreds of thousands of miles. I've seen'em do it with limited maintenance.

 

[351W]

"running below 1800 rpm will get your motor to spin bearings."

Can i lug it below 1,800 when the truck is empty/unloaded, around town at say 25-40 mph?