Two overdrives?

[Frank Lee]

Originally Posted by t vago

They have not responded.

There is info in the FAQs, and scattered about on the site too.

[Joenavy85]

Originally Posted by comptiger5000

Yeah, I was referring to the old 4.0 I6, not the 3.7 V6 you have. Plus, keep in mind, I'm talking about towing close to the max rating, not a 2500lb trailer.

True, but an extra 2300lb (plus the additional rolling resistance) doesn't exactly help your gas mileage

And breaking 20mpg combined is good for a brick. I've only broken 20 on an all-highway tank once

True, that isn't a vehicle that's known for it's fuel economy

[ex-x-fire]

I had a 92 s10 pick-up w/ a 4.3L v6, a 3.08 rear gear, & a th700r4 tranny. I put a gear vendors OD in it, the engine could handle the low rpms ok. With a couple other mods it would get up to 34mpg on a hot summer day. I took it out of the truck when I got rid of it (getting rusty), I did some mock up w/ the GV to put it in my 84 Corvette. It clears the tunnel but I'm working on the mount for it. The C4 corvette use a c beam from the rear diff to the tailshaft of the trans, so you can see what challenges I have.
To handle extremely tall gearig your vechicle needs to be light & areodynamic or have a real torquey engine.

[cleanspeed1]

Originally Posted by ex-x-fire

I had a 92 s10 pick-up w/ a 4.3L v6, a 3.08 rear gear, & a th700r4 tranny. I put a gear vendors OD in it, the engine could handle the low rpms ok. With a couple other mods it would get up to 34mpg on a hot summer day. I took it out of the truck when I got rid of it (getting rusty), I did some mock up w/ the GV to put it in my 84 Corvette. It clears the tunnel but I'm working on the mount for it. The C4 corvette use a c beam from the rear diff to the tailshaft of the trans, so you can see what challenges I have.
To handle extremely tall gearig your vechicle needs to be light & areodynamic or have a real torquey engine.

That 4.3 has plenty of stroke and pokey valve timing to make the torque, plus an S10 is not that heavy.

[t vago]

Originally Posted by ex-x-fire

To handle extremely tall gearig your vechicle needs to be light & areodynamic or have a real torquey engine.

My truck's engine was originally rated at 235 HP and 295 ft-lbf of torque. What with all I did to it, I figure it's closer to 265 HP and 330 ft-lbf of torque. The dyno sheets I've seen so far suggest that torque output is more-or-less constant up to about 2400 RPM, where it rises to a "sweet spot" at around 3400 RPM. Still, it ought to be torquey enough, I figure.



Gear Vendors actually lists ratings for various Dodge truck transmissions, even those with factory overdrives. It's a promising step forward.

That dyno is from another Dodge Dakota owner - he's showing the results of his fuel rail cooling modification which helps ensure that the fuel temperature to each fuel injector remains constant. His modification was the inspiration for my own fuel rail modification that evenly splits fuel across both banks of my engine.

[cleanspeed1]

Originally Posted by t vago

My truck's engine was originally rated at 235 HP and 295 ft-lbf of torque. What with all I did to it, I figure it's closer to 265 HP and 330 ft-lbf of torque. The dyno sheets I've seen so far suggest that torque output is more-or-less constant up to about 2400 RPM, where it rises to a "sweet spot" at around 3400 RPM. Still, it ought to be torquey enough, I figure.



Gear Vendors actually lists ratings for various Dodge truck transmissions, even those with factory overdrives. It's a promising step forward.

That dyno is from another Dodge Dakota owner - he's showing the results of his fuel rail cooling modification which helps ensure that the fuel temperature to each fuel injector remains constant. His modification was the inspiration for my own fuel rail modification that evenly splits fuel across both banks of my engine.

With this guy's combo, it looks like at the beginning of the pull there is enough power to pull gearing like that; you should at least find out what your truck is doing now before you invest the money into a GV, along with BSFC numbers. Then see what can be done tuning wise to trim the fuel without going into misfire in the 1600-2050 range at a similiar loading where you drive.

What is your mpg like at 55-65 mph?

[comptiger5000]

Unfortunately, I can't find any stock dyno charts for my engine, but it certainly has enough low end torque to pull 60mph at 1400 - 1500 rpm. I would expect pretty much any V8 truck / SUV can.

Cleanspeed - Running below peak torque doesn't hurt efficiency. By that logic, I'd need to turn 3200 rpm on the highway, which would be absolutely ridiculous for this engine and vehicle. The goal is to spin the engine as slow as possible without lugging it, or throwing it into fuel enrichment due to too much throttle at cruise.

[cleanspeed1]

Originally Posted by comptiger5000

Unfortunately, I can't find any stock dyno charts for my engine, but it certainly has enough low end torque to pull 60mph at 1400 - 1500 rpm. I would expect pretty much any V8 truck / SUV can.

Cleanspeed - Running below peak torque doesn't hurt efficiency. By that logic, I'd need to turn 3200 rpm on the highway, which would be absolutely ridiculous for this engine and vehicle. The goal is to spin the engine as slow as possible without lugging it, or throwing it into fuel enrichment due to too much throttle at cruise.

Alright, let's make a comparison then; since the variables that affect fuel economy are much greater than engine speed alone, and that your engine is optimized for greatest force generation and lowest BSFC at 3200 rpms, then it would be fair to say that before that point and after that point, unless the cam and induction are optimized for it, the engine is not running at max efficiency. Unless there are devices like variable cam timing, variable induction, variable exhaust, that help to broaden the torque curve and make it more flat, you are inefficient before and after peak torque and hp. That's just the way it is with reciprocating engines.

Your engine has a long runner intake and mild cam timing to make more torque for a truck application. The sheer size of the engine ( 360ci ) means that below that point there will be torque to pull the gear, but with the current tuning and cam timing, do you think that it will be most efficient at 1600 rpms? You can cut the fuel, but you are still not at maximum cylinder pressure (ie peak torque), and that has a greater impact on mileage than engine speed. The motor is still on the way there, going through the rpm range.

Now let's take a Cummins diesel, same size. Peak torque is like 1600-1700 rpms. Compression, cam timing optimized to run at the speed you desire to run at cruise. Difference is at peak torque, stock it has 400+ ft/lbs and a lower BSFC (.33-.35) than your gasser (.50-.55), and because of the long stroke, more torque under the curve so gearing it taller does not affect it so much.
It was built this way to do it. But even as good as this engine is, go too far with the gearing one way or the other will cut the mpg.

Your engine was not designed from the factory to act like a diesel, which is what you want it to do with the stock set up. Will not happen.

To make a vehicle like yours, which was meant to be a performance vehicle for Jeep, to mpg better, it will take a cam swap to put peak torque way lower than 3200 rpms and some judicious calibrating of the electronics.

That and slowing down.

[Joenavy85]

obviously running at an RPM below that at which you make peak torque will be innefficient, this has been a well known fact for some time, but unless you are going to run your vehicle far faster than you should or in a lower gear, then you're stuck with what you have, and should find the best Vehicle speed/GPH ratio that works for you. when we're accelerating we want to be in the optimum spot on a BSFC chart, once we're up to speed, that chart is irrelevant unless you plan on a gear swap.

[Joenavy85]

think of it this way: you have 2 routes to work; the first route is 30 miles but it only nets you 20MPG (1.5 gallons consumed); the second route is 45 miles but nets you 25MPG (1.8 gallons consumed). the question is: is it worth driving a longer distance and burning more fuel just to get a higher MPG average? is it worth running at a higher RPM and lower MPG just to "get the most energy out of your gallon of fuel"?