6 speed in an XJ (who wants to be first?)

[rstarch345]

5-90, I love it when you jump in. It puts all the 'think they knows' to rest. Thank you.

There is a point of dimishing returns to that idea.

The ideal cruising speed for an engine is that speed at which it makes peak torque. This is true at whatever speed (and guaranteed that you will be cruising at 5252RPM or less...)

Peak torque RPM is the crankshaft speed at which the engine is experiencing peak volumetric efficiency, and a relatively low Brake Specific Fuel Consumption (fuel used, in pounds, per horsepower generated - I'd have to look up the unit time, but I think it's per hour.)

If you try to work "away" from the torque peak, you will find that your fule consumption will actually go up slightly due to the decreased efficiency of the engine.

CASE IN POINT - My DD is an 88/4.0/BA-10/3.07. Using fifth gear, I was getting about 18-19 highway MPG. I did a little math, and found that I was cruising on the highway about 1500RPM lower than torque peak. A little more showed that in FOURTH gear, I would cruise at about 2800 RPM, in FIFTH gear I was down around 1600 (which I already knew.)

I stopped using FIFTH gear, and saw an increase in fuel mileage from the 18-19 I mentioned earlier to about 22-23 highway when cruising at about 2800 RPM, which illustrates my point.

This is something I keep telling people when they are trying to select gearing - you have to account for your cruising speeds (which I typically figure around 35/45/65MPH) for daily drivers, as well as tyre size and transmission ratios, before you select axle gearing. I'd have to check my notes, but right now my OD gear is useless until about 90-95MPH (if I want to retain cruise mileage) - or until I put in 4.56 at the axle to complement my 31's

Try it sometime and you'll see similar results - "ye canna change the laws of physics!" I'd be happy to help with the math if you need it, but it's really quite simple (I'd just have to find the equations again...)

For a six-speed gearbox, I'd be more interested in DEEP primary gears (perhaps GRANNY-2 and SUPER-GRANNY-1) than OD and DOUBLE OD. Put 1st down around 7.5:1, 2d around 4:1 (and space the rest accordingly) and I'd probably take about a 28-25% overdrive with matching axle gearing for a good box. With that gearing, I'd also take a flywheel with an inertia ring or loaded with Muntz metal for low-rpm, high-load takeoff. What do you think?

5-90

 

[2001XJeep]

The best way to achieve optimum fuel economy is through the minimum number of rpm's COMBINED with the narrowest angle throttle opening for a given application. Lot's go into that equation though, including vehicle weight, engine displacement, hp & torque peak/curve, terrain, gears, etc. That being said, the lower you can keep the rev's along with the least possible throttle opening that allows the vehicle to smoothly operate (without lugging the engine), the lower you're fuel consumption will be. I've seen this argued to death a thousand times on this site and don't understand why it's so complicated to everyone. The more rev's the engine turns the more fuel the engine burns.

 

[PaulJ]

Try an AW4. The torque converter slip gives you far more flexabilty than any manual.

 

[cosnowrider]

DO NOT BUY OR USE THIS TRANNY !!
We bought a 2005 Liberty in February, 2005 of course. With less than 4000 total miles, this transmission needed COMPLETELY replaced last week. We took it into the dealer due to a notice sent for it having a problem comming out of gear in reverse and 1st. It did this the first day we drove it off the lot. Once the tech tore into the tranny for the staying in gear problem they noticed gear teeth missing from 1st, 2nd, and 3rd gears, and the syncro gears were destroyed. They also found chunks of melted plastic inside the case. Now before you send lessons of driving a manual transmissions to me, I have been driving manuals all my life and so has my wife. The dealership ordered a new transmission and replaced it of course at no cost to me. SO NSG370 USERS BEWARE !!

 

[BrettM]

bummer, are they replacing it with the same thing? any improvements?

 

[cosnowrider]

Yes, Same NSG370 replacement. Although I have allready noticed a big difference in the vibration that used to be in the stickshift its self. I'll have to wait and see if it will last longer than 4000 miles.

 

[88flexj]

unless your girlfriend is 280 lbs. that might cause a dip in the old MPG.

:laugh2: :laugh2: :laugh2:

 

[bajacalal]

Whats the gears in an AX-15? That first gear seems pretty damn low. Do you just start in second most of the time with the 6 speed?

ax15 ratios 1st 2nd 3rd 4th 5th Rev
3.83 2.33 1.44 1.00 0.79 4.22
-courtesy of www.jeeptech.com
So yes thats what I like about the ax15, has a low 1st w/ a high fifth. I always start in 2nd on downslopes. Nice wide range for daily use situations. I really don't understand the point of a 6 speed that has 6th at .8:1 other than getting a granny first (towing?) or close gear ratios for racing purposes. So in a tj, that has 4wd low for crawl, why the low first? I would prefer a .5:1 6th in order to cruise nicely at 115 mph.

 

[PapaPump]

http://www.naxja.org/forum/showthread.php?t=38466

that is a thread i had started when i first heard of the 6-speed wrangler, as you see it kinda fizzled out after people got off subject. the piece of advice about getting an nv4500 is the one i am going with in my current plans. since a stroker motor has more torque than that mercedes tranny is rated to handle, i think i will stick with tried and true. and to all those people who suggested autos...i'm sorry, but no. if i had wanted to swap in an auto, i would have asked about autos...stick shift people have their ways. i personally think autos feel dangerous on the street when they downshift for you while accellerating in a corner. if it were a rock crawling machine, maybe, but probably not.

 

[BrettM]

i personally think autos feel dangerous on the street when they downshift for you while accellerating in a corner. if it were a rock crawling machine, maybe, but probably not.

that's why I swapped in an auto but made it fully manually controlled, best of both worlds

 

[Char-Broil]

that's why I swapped in an auto but made it fully manually controlled, best of both worlds

yeah but there is just something about being able to use that left foot that is fun. :sunshine:

 

[Loose_Nuts_Enterprises]

All of this talk about RPM vs. MPG and not one mention of acetone.....
...amazing.

 

[raneil]

Very nearly tried it. There was a 6-speed still attached to a 4.0L from a Rubicon laying on the floor of the shop that installed my TnT kit and Atlas. The guy had put a supercharger on his 4.0, decided it wasn't enough juice, and opted for a Hemi conversion (don't know what tranny he decided upon). I understand he was willing to sell them seperately.

But.... the XJ is our "family" Jeep, and momma is determined to keep the slushbox in it. At first I was mighty dissapointed (read: :rattle: ), but after driving it for a while, I'm beginning to come around. I think I'm gonna like the auto. Which is a good thing, since I really have no choice. :gag:

 

[bajacalal]

yeah but there is just something about being able to use that left foot that is fun. :sunshine:

:cheers:

If I couldn't find a manual XJ, I was going to buy a Toyota.

 

[Kejtar]

:cheers:

If I couldn't find a manual XJ, I was going to buy a Toyota.

I couldn't find a manual XJ to replace my rolled one, so I bought an automatic and made it a manual: it took less than 2 days to move everything over