Two overdrives?

[t vago]

Originally Posted by cleanspeed1

I'll answer, it's not as if I'm afraid to, because despite you getting personal, I'm going to keep it technical.

Once you show signs of actually reading more than 1 or two sentences of my post, maybe I'll take you more seriously.

Originally Posted by cleanspeed1

If you are getting 20-23 mpg in a heavy pickup with 4x4 at 70+ mph, congratulations and more power to you.

Your statement above is illustrative of my point. Typical mechanic mentality right there. You jump straight to a conclusion without bothering to pay attention.

Originally Posted by t vago

Notwithstanding the fact that I have a 2WD truck, why would I want to cripple a 4x4 vehicle in this manner?

Originally Posted by cleanspeed1

Are you using the stock programming in the motor?

SuperChips FlashPaq 3815 performance tune.

Originally Posted by cleanspeed1

Are you using the stock wheel and tire sizes and the same ride height?

Yep. P235/75R15s on stock 15 inch alloy rims, and the only change I did to my suspension was to add polyurethane lower control arm bushings.

Originally Posted by cleanspeed1

If someone else were to drive the truck, could they produce the same mpg result as you?

As long as they didn't "baby" the truck coming out of a complete stop, and as long as they didn't excessively use the brake pedal to slow down, then yep.

Originally Posted by cleanspeed1

Did these results come from using standard E10 pump gas fuel?

Standard 93 octane at the neighborhood Circle K.

Originally Posted by cleanspeed1

Were there any fuel additives added?

Nope. Don't believe in them.

Originally Posted by cleanspeed1

The numbers generated, were they from a Scan Gauge or some other device that can measure the fuel consumption or were they done by hand, and what proves their accuracy?

Again, your statement above is illustrative of my point. You must get a lot of exercise jumping to conclusions.

Originally Posted by t vago

First and foremost, I take readings with my ScanGauge II on the drive back from my work

And besides that, I checked the commute return path with Google Maps and with my GPS. Fuel consumption is checked by filling up to the first shutoff click at the same exact pump at the same exact gas station, and as close to the same exact time of day as I can manage. What, you thought I was just pulling numbers out of thin air?

Originally Posted by cleanspeed1

I can get behind Big Dave because I have owned Powerstrokes and with the type of powercurve they have, the fact that it's a diesel ( and a big one at that ), their tuneability and the fact that if you slow them down they won't lug.

I can listen to him because he's already experimented with an overdrive unit! Have you?

Besides that, how do you know that a gasoline engine such as mine will lug at 1600 or so RPMs? Isn't that the same as saying that it'll lug at 2000 RPMs? And what of all of the cars out there that have gasoline V8 engines and a T-56 with a 0.50 6th gear? Are you saying they're lugging their engines by cruising down the highway in 6th gear?

Originally Posted by cleanspeed1

So, back to the original topic. You should go ahead and spend the money and get that GV installed.

I'll do the installation myself, if it comes to that.

Oh, and since I took the time to answer your questions, which were related to my original topic, you can answer mine (which are also related to my original topic).

Approximately how much loss am I currently experiencing in my driveline?

How much more loss I could expect with installing an overdrive?

What happens to bsfc when you choke the living hell out of a "high powered" gasoline engine, such as driving down the interstate at 72 MPH and getting a manifold vacuum of around 7 or 8 psig?

Why should I downgrade my towing/hauling capability by going to a taller FDR?

Notwithstanding the fact that I have a 2WD truck, why would I want to cripple a 4x4 vehicle in manner that you described?

[cleanspeed1]

Typical auto trans equipped vehicle experiences 25-35 percent loss in the drivetrain because the transmission absorbs most of the power. Adding the overdrive adds weight and drag to the system ( the mfg can give the number )

BSFC before or after peak torque goes up substantially, but since each engine is different and the conditions it operates in vary, only testing will give an accurate number.

Going to a lower numerical gear with the stock combination will not make sense if you tow all the time, but boosting bottom end torque with a cam swap and a recalibration to keep the torque level the same with a lower numerical gear increases efficiency ( stock Metro vs Metro XFi as an example ).


I don't have to experiment with an OD unit; I own a Volvo Class 8 style expediter. Has a VED12 ( 335hp / 1300 ft/lbs of torque, 33000 GVWR ). Averages 9-11 mpg loaded @ 63-65 mph. Compared to a smaller engine straight truck ( 210hp/ 600 or so ft/lbs and 7-8L ), gets a good 2 mpg better. Why? More torque at a lower rpm with a more favorable bsfc. The torque band goes from 1100-1300 rpms, then falls off a cliff. Running the engine in this range gives the best mpg. 9 speeds with OD.

Your last question brought the reason for free wheeling hubs, parasitic losses. Since you clarified that you have a 2 WD that is a moot point.

I will emphasize again, since you are making it personal, I am going to keep it technical, and as far as jumping to conclusions, you've done the same.

You are trying to prove something beyond the machinery and the parameters around the question that were presented in the beginning. If it's going to make you feel better, then so be it. But in the end, it has not affected me one bit. I only have done what you are trying to do, just with much larger vehicles in gas and diesel, and with a much higher pricetag.

[Big Dave]

The GV overdrive is a single planetary spur gear setup. Expect a 3% loss in driveline efficiency, but this will be more than offset by the lower engine RPM resulting in lower engine frictional HP.

What concerns me about t_vago's setup is reduced cooling in the automatic transmission due to the big overdrive ratio.

I had a GV on a 1983 GMC K2500 with a TH400 tranny and the wimpy old 6.2 engine. I destroyed not one but two TH400s in front of the GV. Once I raked up the parts from the Interstate, I took them to a transmission guy and he said excessive heat buildup.

[cleanspeed1]

Originally Posted by Big Dave

The GV overdrive is a single planetary spur gear setup. Expect a 3% loss in driveline efficiency, but this will be more than offset by the lower engine RPM resulting in lower engine frictional HP.

What concerns me about t_vago's setup is reduced cooling in the automatic transmission due to the big overdrive ratio.

I had a GV on a 1983 GMC K2500 with a TH400 tranny and the wimpy old 6.2 engine. I destroyed not one but two TH400s in front of the GV. Once I raked up the parts from the Interstate, I took them to a transmission guy and he said excessive heat buildup.

I would think synthetics and a huge by large trans cooler with a fan would help with that, but you are talking more about pump speed, correct?

[Frank Lee]

It's pretty standard to assign 3% loss to a gear set... but if it's on huge gears it could be more. For example if I put a GV on my bicycle I'm sure it would feel like more than 3% more effort to pedal through it, ratio change notwithstanding.

[t vago]

Originally Posted by Big Dave

The GV overdrive is a single planetary spur gear setup. Expect a 3% loss in driveline efficiency, but this will be more than offset by the lower engine RPM resulting in lower engine frictional HP.

What concerns me about t_vago's setup is reduced cooling in the automatic transmission due to the big overdrive ratio.

I had a GV on a 1983 GMC K2500 with a TH400 tranny and the wimpy old 6.2 engine. I destroyed not one but two TH400s in front of the GV. Once I raked up the parts from the Interstate, I took them to a transmission guy and he said excessive heat buildup.

Hm... Gear Vendors claims less than 1/400 of a HP for frictional loss. That sounds reasonable.

Certainly better than 25% through a transmission. Even granting hydraulic fluid at 100 psig flowing at 4 gpm with 50% pump efficiency, that's 1/2 a HP taken by the automatic's pump. Ecomodder's tools tell me that I need about 38 HP to push my truck down the highway at 72 MPH. This is supposed to represent the 75% of the engine output that isn't sucked up by an automatic. If this truly were the case, then that means that 12 HP is being sucked up by the automatic while cruising. If we were to assume that 1/2 HP is from the pump, that still leaves 11.5 HP (or about 8.5 kW) being consumed by the transmission. I can't think of anything other than heat frictional losses that could consume the remaining 8.5 kW during a steady cruise, and that amount of heat being lost would bring a gallon of transmission fluid from room temperature to boiling in about 45 seconds. So either the planetaries within the automatic are heavily dragging down the geartrain, or the clutch surfaces are doing a really lousy job of locking down the required combination of planetaries (as well as doing a truly hideous job of locking the torque converter). As we both know this isn't the case, I come to the conclusion that that 25% is not a valid figure to go by to determine losses through a transmission.

And before somebody mentions "radiator," let's run through the same set of numbers, using a 5-speed manual that was also offered for the 2000 Dodge Dakota, 15% losses, NO oil pump, and NO radiator. If 15% losses were truly what I could expect, I could also expect to have the transmission burst into flames 5 minutes into my 72 MPH cruise, or seize up from oil cookoff much sooner than that. This is what I mean by theory. Theory states 25% losses through an automatic transmission, while real world observation suggests something far less. The trick is to separate frictional losses from losses associated with angular acceleration (which is what you'd see on a dyno).

Big Dave does makes a valid point about reduced cooling, though. It would seem that the TH400 did not provide enough flow at reduced RPMs to carry away the heat that was being generated. I might have the same problem with a second overdrive.

[comptiger5000]

Keep in mind, his TH400 may not have had a lockup TC. In that case, it would have been slipping the TC more, making more heat. Provided you have a big tranny cooler (and you could always look into modding the valve body to increase fluid flow for cooling / lube), it should be fine.

[t vago]

Originally Posted by comptiger5000

Keep in mind, his TH400 may not have had a lockup TC. In that case, it would have been slipping the TC more, making more heat. Provided you have a big tranny cooler (and you could always look into modding the valve body to increase fluid flow for cooling / lube), it should be fine.

That was part of what I was thinking, too. I was also thinking that maybe his TH400 oil pump simply was not pumping enough to properly get rid of the heat, and maybe the bands in the TH400 (I'm assuming it uses bands) were slipping from this unintended extra load.

Not sure about the oil pump in my 45RFE, but I think it can handle the flow requirements, as it is a truck transmission. It does have a lockup converter, so that heat source should not be present during cruise. The clutch packs should also stand up to the extra loading, as the 4.7L / 45RFE combination is rated at a maximum towing capacity of 6100 lbs (of course, with upgraded cooler).

I simply don't know right now. I currently have a 0.67 5th gear. If I were to remove my 545RFE computer and revert back to a 45RFE with a GV overdrive, that'd be a 0.56 overdrive.

[Big Dave]

As a general rule manual overdrive transmissions are about 96% efficient.

Non-overdrive (1:1 top gear) aproach 99% efficient.

TC lock-up automatics can get to 89% efficiency.

Because my engine is capable of protracted operation at high power and the heavy chassis allows for big loads without wheel slip, my ZF6-650 has an oil cooler because of the overdrive sixth. Some T-56s (Corvette Z06, Viper) come with an oil cooler pump.

[Joenavy85]

Originally Posted by t vago

The clutch packs should also stand up to the extra loading, as the 4.7L / 45RFE combination is rated at a maximum towing capacity of 6100 lbs (of course, with upgraded cooler).

seems kinda low for the 4.7L, my jeep has the 3.7L and is rated for 5000lbs. with the tow package. maybe the truck itself is the limiting factor instead of the drivetrain??