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Double Overdrive??


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A great many decades ago, I had an MGB-GT with an electric overdrive.

Loved it! I now have this lust for something similar to hang on the back

of an NP242 T-case. Trouble is, it seems that the only company still

standing (Gear Vendors) that makes such a critter cannot, does not, will

not, make one for Jeep. I've had some discussion with them, and they say

the reason is because the D-shaft will reduce in length for ~42" to ~31"

and create a too severe angle. I told them I could probably deal with

angle correction (spring perches and T-case mount) but got nowhere.

Has anyone else ever tried to cross this bridge? What I'm wishing for is

a sixth gear for high speed highway travel.

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Show them a stock rear driveshaft for a 4wd YJ. It's like a foot long. Cutting an MJ's driveshaft down by 11 inches, even starting at a 4wd swb, would put it in the XJ length range. They obviously have no clue what they're talking about, just using it as an excuse.

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In JP magazine they put a newer 6 speed in an 89 MJ. :wrench:

 

If it's the 6 speed from the TJ, it's not a double overdrive trans. It's got a grannied first, 5th is 1:1 and 6th is overdrive.

Yeah, but then if you run tall gears (like 3.08) then it becomes the equivalent. Don't get stuck focusing just on the transmission -- what counts is the overall final drive ratio.

 

Say you run a 5-speed with 4.11 gears. 4th gear final ratio is 4.11, and 5th gear is 4.11x.75 = 3.08

 

So if you run a 6-speed with a 1:1 5th, run 3.08 axles and 5th gear is the equivalent of what your OD is now, and the 6th gear becomes a second overdrive.

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The NSG370 six speed manual from the 2005-2006 Wrangler (last year for I6?)

has a final ratio of 0.84. The AX-15 final ratio is 0.79. So, for me, with

29" tires, it works out like this:

 

At 80 mph with 3.54 and 0.79 I will turn 2600 rpm

 

At 80 mph with 3.08 and 0.84 I will turn 2400 rpm

 

Doesn't seem like enough gain for the cost/effort.

 

The Gear Vendors unit final ratio is 0.58 (if only I could have one)

If I could have one, my rpm with 3.54 at 80 mph becomes only 1910

 

Or viewed the other way, for 2600 rpm I could be doing 110 mph!

(totally ignoring wind drag)

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You're forgetting that lower rpm =/= better mileage. Even a 4.0 is not gonna want to do 1910 rpm at 80 mph. Sure, it most likely will, but your gas mileage is gonna tank. Many people have seen improvements taking the 3.07 axles out of a 4.0/5 speed and replacing them with 3.55.

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On a flat highway at 70 mph, I got slightly better mileage with 3.08 than 3.54,

provided I didn't stuff my foot into it when loaded (like a slight grade). The RPM

with 3.08 was just under 2000, and the RPM with 3.54 is just under 2300. From

that, I concluded that 2000 RPM was a more efficient point for "low load" cruising.

Add load and all bets are off. Knowing that Ad is going to kick me at higher

speed, I was hoping to maybe just break even if could achieve ~2000 RPM at 80 mph.

Yes it's 20 years old, but yes I've made new most all of the age-related risk points

I can think of, and handles quite well at that speed.

 

It's past the limit of my understanding whether the diff capability of the NP242

could tolerate that much difference in ratio between front output and reduced

rear output, but would not be using 4WD at highway speeds, and very unlikely to

be accidentally shifted there. Maybe once I add the cable shifter it won't be

quite so stiff.

 

Is it reasonable to expect that lower rpm's equate to less wear/longer life? I know

the I6 lasts forever, but 1.15 x forever is even better.

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  • 1 year later...

I have not given up this quest, and just recently came across some info

about a mechanical OD unit made decades ago for high performance cars,

called a Hone-O-Drive ( http://www.stangerssite.com/honeodrive.html).

I think the universal model could be mounted between T-case output

and diff input, with some creative yoke mods. Until I learn the length,

won't know how badly it would crew up D-shaft angle. Opinions?

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A great many decades ago, I had an MGB-GT with an electric overdrive.

Loved it! I now have this lust for something similar to hang on the back

of an NP242 T-case. Trouble is, it seems that the only company still

standing (Gear Vendors) that makes such a critter cannot, does not, will

not, make one for Jeep. I've had some discussion with them, and they say

the reason is because the D-shaft will reduce in length for ~42" to ~31"

and create a too severe angle. I told them I could probably deal with

angle correction (spring perches and T-case mount) but got nowhere.

Has anyone else ever tried to cross this bridge? What I'm wishing for is

a sixth gear for high speed highway travel.

 

Why would you want another overdrive? Your info says you have a 4.0L with an AX-15. That means you have 3.08 gears. Even with stock tires, in 5th gear your engine is only turning 1960 RPM at 70 MPH. That's below the torque peak of the engine. In my '88 Cherokee 5-speed, I have verified that I get the same or better gas mileage in 4th as I do in 5th, because the engine is operating near it's torque peak in 4th gear on the highway.

 

What would the reduction ratio of an additional overdrive be, maybe 0.75:1? That would reduce your 70 MPH engine speed to 1470 RPM. Your gas mileage would suck, and you'd have to downshift for even a modest upgrade.

 

A long time ago a prestigious British motorsports magazine had an article by an automotive engineer who cautioned against being seduced by the term "overdrive." He said the proper engineering definition is that an overdrive ratio is any ratio that results in the vehicle being capable of LESS top speed than the next lower ratio. I have never attempted to push either an XJ or an MJ to flat-out maximum speed in either 4th or 5th gear, but based on other vehicles I HAVE done such with, I have no doubt that our fifth gear is already a true overdrive and that our trucks can probably go faster in 4th than they can in 5th. And that means that a further overdrive would be worse than useless, it would be counter-productive.

 

You would do better putting in one of the new 6-speed trannies, and going back to 3.08 gears (or even 2.87 if you can find 'em).

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I agree that it would be useless weight with a properly geared MJ w. a 5-speed. However back in the day I had several Volvo 544s and 122s. They all had 2.0L Volvo tractor-like engines, low revving with tons of torque, 4.56 diffs, and 4-speeds. There was a factory overdrive option, but it was very rare. The solenoid activated overdrive units were mounted to the output shaft of manual transmissions. Volvos used the British Laycock de Normanville box, also used on some Triumphs and MGs. A version of this box is still being built and supported by Gear Vendors (http://www.gearvendors.com) in the US today. I installed a few of these units on my Volvos and loved them. It allowed a comfortable cruising speed of 75MPH at around 3300 RPM. W/o the overdrive unit you were straining at 65 on the freeway.

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My Renix 4.0 when stock, the best mileage I got (24mpg) was cruising 60mph which was right around 2000 rpm. That is with the automatic transmission and 3.55 gears. Any faster than that and the mileage went down a lot.

 

On a 400 mile trip to Badlands Offroad Park a few years back, running 70 almost the entire way) I only got 19mpg, while a buddy in a 90 Cherokee, same engine, transmission and gearing but on 31s, still got 22.

 

So I would say between 2000-2200 rpm.

 

Also from experience, the HO engines seem to be better at higher rpms, like around 2400 or so.

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To mvusse, I have located three of these units, they are findable.

 

To Eagle, I have 3.55 gears (I changed them from 3.08). Above

80mph I'm turning more RPM's than I wish to. I drive this truck

regularly at highway speeds where 80-90mph (and up) is common.

I'm not chasing mpg miracles, as the wind resistance at the higher

speeds will chew that up fast. I just want to run at lower RPM on my

highway trips, and keep my 3.55 gears. My objective is to be able

to run 90+ with RPM's just above 2000. The (0.71) Hone unit is all

mechanical, and I think could be made to fit. My biggest concern

is the resulting driveline angle. I'm waiting for dimensions to

figure out what that might be, but it may be too steep. Gear Vendors

told me "no" for just that reason.

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Have you talked with "Herm the overdrive guy" ? He may know of a way.

 

I will add that my 06 rubicon (stock 4.10s' and 6 speed) gets better mileage in 5th gear running 67 mph, that it does in 6th running 72. I know you said you didn't really care about mileage, but it does give you a good indication of that "happy" speed vs rpm range that you want to shoot for.

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I have an old solenoid operated Borg Warner OD and D18 T case, name your price I'll sell it to you. The problem with those is they are terribly complicated and essentially integral with the the TC. So you must use the D18 which requires offset axles with the diffs on the right side.

 

To have an OD on the back of the Tcase is a nifty idea but if its on the back and not inbetween the transmission and the TC then you could only use it in 2WD.

 

This is why the Warn/Saturn overdrive which mounted on the PTO opening of the D18 and ran off the tranny output quickly made the Borg Warner OD obsolete. I've spoken with Herm about these. He's got plenty of parts to keep them on the road but he recommends the Warn/Saturn overdrive.

 

With the warn/Saturn overdrive on a D18 you can use it as a gear splitter in 2WD or 4WD. But then you've got 4 shifters comin through the floor. But it allows you to shift to 1st, then 1st - %25, 2nd, then 2nd -%25, and so on.

 

Now the next problem. Due to the inverted "Texas" bolt pattern of the Tcase, your transmission selection is limited. Your looking at either a T84, T90, T14, T15. Unless you find the holy grail of bell housings to run a T18. Imagine that, a four speed with a gear splitting OD, that means you could have an 8 speed jeep.

 

Oh but wait, none of those bellhousings will bolt to an AMC or modern jeep engine. So in less you're willing to run an L or F head 4cyl or a flathead 6 in your MJ, then you're stuck with modern stuff.

 

I'm trying to think of a way to do it,,,a hot rodded flat head injected 6 cyl with 8 speeds and 4wd would be pretty awesome, even in a more modern jeep and even though it's a huge step backward in technology.

 

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I know this is way off topic, but i'm taking a mental break from "coding".

 

In 72, 73ish, IH installed the AMC 258 into the new Scout II model. They ran a T-19 4 speed which had fully syncronized gears and came with an optional wide ratio. You can bolt up a D18 and the warn/saturn overdrive. If you twin stick the D18, then you can have a total of 4 shift levers.....COOL! :laughin:

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To Eagle, I have 3.55 gears (I changed them from 3.08).

 

I saw that after I entered my post. My bad.

 

Above 80mph I'm turning more RPM's than I wish to. I drive this truck regularly at highway speeds where 80-90mph (and up) is common. I'm not chasing mpg miracles, as the wind resistance at the higher speeds will chew that up fast. I just want to run at lower RPM on my highway trips, and keep my 3.55 gears. My objective is to be able to run 90+ with RPM's just above 2000.

 

I think you're making a serious mistake. I think your gas mileage will be worse, not better.

 

Let me explain: Completely aside from the fact that the most efficient engine speed is the speed at which it produces maximum torque, you should also take into account the heritage of this engine design. The injection dates to 1987, but the basic engine is an AMC engine that was first introduced in 1964. I grew up in a Hudson ==> AMC family, and we owned a number of different cars with earlier versions of this engine. Ours were all 3-speed manuals, and they all came with small-ish tires and 3.08 gears. The overall drive ratio worked out to 24 MPH per 1000 RPM, which conveniently came to 2500 RPM for a 60 MPH cruise. That's what the engine was designed to run at.

 

But we didn't cruise at 60 MPH. My brother and I were gearheads. We raced ... any kind of racing you could think of. And, being young and immortal, we drove perhaps "a bit" in excess of posted speed limits. This is New England, however, so prolonged cruise at 90 MPH wasn't viable, but 75 was pretty customary. Another neat round number from that final drive ratio: 3000 RPM gave us exactly 72 MPH. And cruising at 3000 RPM or a bit more still gave us decent performance and very respectable fuel economy ... although my Rambler American wouldn't get 28 MPG at 75 MPH, it was still in the mid-20s.

 

I had the American while I was in the Army and stationed in Maryland, and I got it in my head that it was important to be able to break 100 MPH in a box on wheels. I tweaked the tuning, installed a straight-through glasspack muffler, and converted to an open air clear ... and very late one night on I-95 somewhere a bit north of Edgewood Arsenal, MD, I pushed the box-on-wheels up to a calculated 104 miles per hour. I was a happy camper.

 

Winter arrived. I needed snow tires, and in 1967 the pay for a PFC-E3 wasn't anything to get excited about. My brother at the time was driving a bigger Rambler with a V-8, and he had just switched over to Michelan radials. So he gave me his bias-ply snow tires from the previous winter. I don't remember the sizes, because they were in one of the old tire sizing schemes, but the bottom line is they were a couple or three sizes larger than what came standard on my American, and probably about two inches larger in diameter. And with those snow tires on and no other changes, my top speed DROPPED from 104 MPH to 80 MPH. The gas mileage took a dive, too, but I don't recall the exact figure. The speed, though, is permanently etched in my brain because I remember being out there on an empty stretch of I-95 looking at the speedometer reading 74 with my foot pressed all the way to the floor, thinking "WTF?"

 

Bottom line, the power curve crossed the drag curve. The engineers knew what they were doing when they chose the gear ratio and tire size, and they had optimized the drive train. 104 MPH in stock configuration was 4333 RPM, which was right about the horsepower peak of the engine. What a surprise -- once you reach max power, you can't go faster! I was shocked, I tell you ... shocked!

 

My best guess is that the larger snow tires were about 2 inches taller. Assuming that the originals were approximately 25" tires, which I think is about correct, that's about an 8 percent difference. Doesn't sound like a lot. But with the stock tires, 80 MPH would have been 3,333 RPM. Cut that by 8 percent for the bigger tires, and that's 3,067. That's where the drag curve crossed the power curve.

 

And you want to cruise at 90 MPH at 2000 RPM?

 

NOT ... GONNA ... HAPPEN. Not in this world.

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