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Old 04-09-2005, 11:42 AM   #1 (permalink)
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Better mileage with a 4 speed or C6?

What do you guys think? I'd guess that the C6 is better for towing, but what about mileage? [img]/ubbthreads/images/graemlins/confused.gif[/img]
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Truck 1: 1985 F350 dually 4WD 6.9 diesel crew cab auto with 83k original miles. C6 trans, 4.10 rear end. Mods are Banks Turbo, US Gear overdrive, 45gal rear tank, hydroboost brakes, additional leaf springs all around, Hellwig airbags, rear sway bar, GM glowplug controller, fuel Koalescer. 235/85 R16 Michelin XB Rib radials. 11ft.6in Vacationeer cabover camper, TorkLift tiedowns.
Truck 2: 1988 F250 7.3, C6 auto, 2WD w/3.55 limited slip rear axle, Banks Sidewinder Turbo, Gear Vendors overdrive, Utility box, hydraulic lift gate, lumber rack.
Auto: 1987 MBZ 300D Turbo 3.0 Liter 6 cylinder engine. 205K miles. Working on all the "little things" that previous owner neglected. Putting in wvo tank.
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Old 04-09-2005, 12:32 PM   #2 (permalink)
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Re: Better mileage with a 4 speed or C6?

Assuming that both transmissions haave a 1:1 output, the 4 speed manual will will have less loss, better power transfer. Better power transfer = better mileage. At least in theory.

Bob [img]/ubbthreads/images/graemlins/smokin.gif[/img]
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Old 04-09-2005, 08:29 PM   #3 (permalink)
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Re: Better mileage with a 4 speed or C6?

Any automatic has higher parasitic losses than a manual. Plus a non-locking converter slips all the time to some degree.

If you don't mind shifting, a manual is better for most anything besides 4-wheeling. Automatics produce considerably more heat under load than a manual (generated by the fluid shear in the torque converter), which can make them overheat where a manual never will.

C6 compared to T18/19, the T18 is stronger, lighter, and more efficient. But ya gotta shift it!!
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Old 04-09-2005, 08:34 PM   #4 (permalink)
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Re: Better mileage with a 4 speed or C6?

It seems to me that most rigs that were built with the 4 spd, seemed to have 4.10's. And most Auto's seem to have 3.55's. So if we're talking stock trucks????
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Old 04-16-2005, 12:34 AM   #5 (permalink)
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Re: Better mileage with a 4 speed or C6?

Is a T19 really lighter than a C6 MR ?

that T19 weighs 135 lbs....

I'm not sure that parasitic losses in a C6 with ATF are much higher than a T19 full of 80/90w. Maybe a little, but not much.

Number of gear-sets in the power-path is the same, isn't it? Two sets in highway gear? I haven't looked at a C6 in years...

A T19 is constant-mesh, so there about -twelve- gears spinning and churning heavy fluid all the time in there...

With the C6, the torque-converter is a constant 3-4% power-loss at any RPM above stall, in any gear.

Bottom line, with equal RPM's and MPH, the stick will be slightly more efficient.
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There used to be a link here, to some useful IDI-diesel tech info; but it was chopped out by others, in a vain attempt to repress the very essence of the internet: linking!
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Old 04-16-2005, 12:54 AM   #6 (permalink)
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Re: Better mileage with a 4 speed or C6?

Well I'm strongly considering going from C6 and 4.10s to a T19 and 3.55s. Hope I can pull it off. I'm tired of getting 10-15mpg on an engine I know can do a lot better than that. And guess what? I like to shift....
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Old 04-16-2005, 01:09 AM   #7 (permalink)
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Re: Better mileage with a 4 speed or C6?

The auto will lose a fair amount of power simply by virtue of it's design. You've got a hydraliccly run system, and the hydralic pressure has to come from somewhere. I don't know the exact figures, but I would expect 5-10 HP is used by the oil pump. Also, you've got a number of clutches and bands in there, at least a few of which are always going to have diferential rpm acrost them, resulting in losses. These types of clutches always drag to some exent (ever tried turning the output shaft of a an automatic?? It takes a fair bit of torque - and that's all clutch friction) The standard trans will probably actually have more power loss from fluid drag, since the gears are partially submerged, but this is much less than the total loss from the auto. Most autos show 5-10% lower fuel milage in most conditions. (rough guestimant) Modern autos cut that down a fair bit by using lockup converters and being able to select the optimal gearing for fuel milage under that particular condtion. Properly driven, I would still expect a stick to get 5% bettter milage or so.
-John
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Old 04-16-2005, 01:28 AM   #8 (permalink)
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Re: Better mileage with a 4 speed or C6?

I have one of each,actually I gave the 84, C6 to my 16yo nephew, but the C6 gets way better mileage! They both have about the same power, both run good, both have 3.55. WHat up with that?
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Old 04-16-2005, 10:57 AM   #9 (permalink)
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Re: Better mileage with a 4 speed or C6?

OK, I admit that I haven't weighed a C6 and a T19. However, I am pretty confident that the manual trans is lighter AND stronger. I can pick up a T19 by myself no problem. When I had my E4OD (basically the same as a C6) out, it took 2 of us to pick it up, as I could not do it myself.

You have to remember to include the weight of the torque converter and the 15- odd quarts of oil in the weight of the auto tranny.

The manual transmission holds, what 3-4 quarts??

All other things being equel, a manual trans WILL get better mileage, at the expense of having to shift it yourself.

That said, I would happily swap out the 4 speed in my Dooley for an automatic. That and give up the 4.11 gears for 3.55's.
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Old 04-16-2005, 08:54 PM   #10 (permalink)
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Re: Better mileage with a 4 speed or C6?

thanks MR. The two-person requirement answers the question fully for me! [img]/ubbthreads/images/graemlins/biggrin.gif[/img]

And I'd forgotten about the TC and 15 qts....thanks.


jahlun; I'd be shocked if the oil-pump in the auto was anywhere near 5-10 hp. That's close to the the size of a -backhoe- hydraulic pump. I.e., something that's running 10gpm through 1/2" ID hose at 2,000 psi.

Given the low pressure (60-100psi ?), and the tiny lines going to/from the cooler (no way it's more than a fraction of a gpm), I'll guess the C6 pump at more like 1/8 hp...maybe 1/4...if they didn't bother making it an efficient one.
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'71 VanPelt Pumper, Cummins 855 Turbo

There used to be a link here, to some useful IDI-diesel tech info; but it was chopped out by others, in a vain attempt to repress the very essence of the internet: linking!
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Old 04-16-2005, 10:49 PM   #11 (permalink)
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Re: Better mileage with a 4 speed or C6?

I was very surprised to read this!
http://www.hotrodders.com/t30771.html
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Old 04-17-2005, 12:26 AM   #12 (permalink)
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Re: Better mileage with a 4 speed or C6?

Wow, it says 55-60 HP for a C6, that is ALOT. Still, even with all that, the C6 is a very strong, reliable and pretty acceptable tranny. If mine ever takes a crap, I am going to stuff another in there, because with the GV unit, and the C6, I really don't have to worry about my tranny "throwing" a code, or just straight up dieing.. I like the simple stuff and the C6 is just that, simple and strong. Scotty
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Old 04-17-2005, 01:20 AM   #13 (permalink)
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Re: Better mileage with a 4 speed or C6?

yah, that's surprising all right....maybe even nutty... [img]/ubbthreads/images/graemlins/biggrin.gif[/img]

60hp out of 175 total ? 33% power loss in the auto-tranny alone??

sorry...I'm having a little trouble with that one... [img]/ubbthreads/images/graemlins/shrug.gif[/img]
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Hydra 35-ton Excavator, 6V71T; Isuzu C201 genset;
'71 VanPelt Pumper, Cummins 855 Turbo

There used to be a link here, to some useful IDI-diesel tech info; but it was chopped out by others, in a vain attempt to repress the very essence of the internet: linking!
sigh...
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Old 04-17-2005, 01:01 PM   #14 (permalink)
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Re: Better mileage with a 4 speed or C6?

Well It makes sence, an auto takes a lot of power to run. When you look at the guts all the resistance, pump then power loss in the converter. You dont notice it a lot with something that has power . I hd an old honda my mom had just about the same model only difference was one was auto and one was standard. Night and day difference in power, Because the auto drained so much power to run it.
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Old 04-17-2005, 11:26 PM   #15 (permalink)
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Re: Better mileage with a 4 speed or C6?

[ QUOTE ]

jahlun; I'd be shocked if the oil-pump in the auto was anywhere near 5-10 hp. That's close to the the size of a -backhoe- hydraulic pump. I.e., something that's running 10gpm through 1/2" ID hose at 2,000 psi.

Given the low pressure (60-100psi ?), and the tiny lines going to/from the cooler (no way it's more than a fraction of a gpm), I'll guess the C6 pump at more like 1/8 hp...maybe 1/4...if they didn't bother making it an efficient one.

[/ QUOTE ]

Well, mabey not that much, but still a fair bit of power. The only figures I can find for pressures are 60-150psi idleing in reverse, 50-95 in drive. *But* these are firgures sitting idleing and not moving. I am sure that pressure increses signifigantly under normal driving conditions. My guess would be around 600psi max line pressure, mabey more. I know that our books for a laycock 'j' overdrive box, used on alot of volvos (same basic thing as a gv overdrive) has a line pressure spec of 625 psi (I think) when engaged, and this is the same essential thing as a auto trans.

I don't know the flow offhand, but remember that the fluid going through those cooler lines is only the torque converter return, and a signifiagnt of the flow out of the pump is used as lube oil, or leaks past the clutch seals or whatever. Plus whatever spills past the regualtor vavle. From what I recall of the intake size (about 1" square) I would guess the volume at 3-5 gpm, mabey more, mabey less.

The 45 HP figure sounds pretty high, but mabey that's for high rpm racing applications. From what I recall, (or mabey I'm just making this up [img]/ubbthreads/images/graemlins/smile.gif[/img]) typical trans losses in a slushbox are about 10% of the input power. Not sure what a stick loses, but I'd guess under 5%, probably closer to 1%.
-John
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