Hi Guys, I'm a newbie from South Africa.
You have a great forum! I've been following the threads for quite awhile.
This a lengthy one but bear with me! I’m up for experimentation and this solution might change JK’s performance issues all over the world.

I received my black 3.8l 4dr auto Rubicon with 32’ (Goodyear Wrangler silent armor) tires in august 07. Paid 51000 USD for it. The only mods thus far are a hard top (with no water leaks), tow package, central locking, alarm and tyre pressure monitoring system (TPMS).
I am very pleased with the overall package.

I’ve done just more than 14000 miles (2 services) and about half of that was done on nasty dirt roads at speeds of 60+ mph. This is where the stability control really saved my ass a couple of times, I just love it! I’ve also taken her off-road onto the low range tracks for weekend outings and the JK is definitely the off-road king here in SA compared to the other standard vehicles available (Toyota land cruiser, Nissan Patrol and the H3).

We live at an altitude of 1700m (5100ft) above sea level. Most of the country is high lying and we regularly travel to our beloved mountain ranges at 3000m (9000ft).
I do 95% of my driving between these altitudes and I must admit that engine performance really sucks compared to sea level altitudes.

Now comes the problem…! Speed….i need more performance.

When the jk carries only me behind the wheel then the performance is OK.
But as soon as I attach my off-road trailer at the back or the jk gets fully loaded for a weekend trip and we meet a hill on the highway then the performance really sucks badly.

If I don’t approach the hill at 80+mph the speed will drop to 40mph in second gear at 3000rpm and stay there. If I lift my foot a bit it will gear back to 1st at 5450rpm and cycle between 2’nd at 3000pm and 1’st at 5500rpm. The speed at this point is +-40mph. I’ve even pulled over and tried reaccelerating but it just doesn’t have enough power to increase the rpms to over 3000rpm in 2nd gear.

Here in SA the speed limit is 75mph and nobody keeps to the limit so every one else races by at 75+mph while my throttle is fully depressed at 40mph.

Like I wrote previously this happens at an altitude of 5000+ft ASL and it is repeatable. (In other words: if you don’t believe me, come and visit and I’ll show you) I have been to the dealer and they say the car is fine and that other customers have complained about similar experiences.

I can get rid of the jk and buy a cruiser but I love this car so I’m going to make her more powerful myself!!

The options:
Performance exhaust: it’s already on its way
CAI….yep I’m up for it although I am concerned about all the dust that i encounter on the high speed dirt roads ….not ordered yet.
Unichip or JET performance chip: I’ll even order both although the performance gain doesn’t seem earth shattering one of them might help.
Supercharger: a bit pricey for me but if all else fails I’ll get one off those as well.
Differential gearing: Now this is affordable and might just do the trick.

I might need some help or corrections with my assumptions in the following part…

I’m thinking of ordering 5.13 crown and pinion gear packages with my standard 32inch tires. The thinking behind it is simple: The ratio will increase by a factor of 5.13/4.11=1.248 or 24.8%. This equates to 24.8 % increase in torque at the wheels for the same rpm as before the upgrade. It also means an increase in rpm of 24.8% for the same speed and gear compared to the standard 4.11 ratio.

The rpm increase might however not be a problem because currently I almost never use 4th gear (Over Drive) because of the low torque at low revs. When the jk switches to 4th at 75mph the speed immediately drops down and I have to reaccelerate, this action then toggles the gearbox back to 3rd and increases the rpms by 1/0.69 = 45% just to keep a steady 75mph.

After the upgrade 4th might even be useful highway gear and fuel consumption might not even increase at all due to 25% higher rpms in all gears for the same speed.

I worked out the reduction in speed for every gear.

The gearbox ratios are as follow: 1st=2.84, 2nd=1.57, 3rd=1, 4th=0.69.
The current theoretical top speed for every gear as calculated at 5500rpm and 32’ tires with no compression is: 1st=44.8mph(72km/h), 2nd=81.1mph(130.6km/h), 3rd=127.4mph(205km/h), 4th=184.5mph(297kmh)

What I’ve found is that my 32inch tyres perform more like 30inch tyres when I take 1st gear at 5500rpm my speed is about 40mph and not 44mph as calculated. The difference is mostly due to tire compression.

If I compare the top end speeds to my other vehicles that have manual gearboxes it is clear that the JK gearing is just tooooo long.

My 5 speed Toyota max out at top revs in 5th gear at 121mph (195km/h). My wife’s freelander 2 TD max out in 6th gear and top revs at about 118mph (190km/h).

With a 5.13 differential gear ratio the jk’s equivalent true top speed in every gear will be remapped as follow(again with 32 inch tires assuming no tire compression): 1st=35.9mph(57.8km/h), 2nd=64.6mph(104.6km/h), 3rd=102mph(164km/h), 4th=147.8mph(238kmh)

The above is a (100% - (4.11/5.13)%) =19.88% or rounded to 20% reduction in true speed

If I could change my differential ratios to theoretical 5.96 then I would have remapped the gearbox 4th gear top speed to the current 3rd gear top speed. This gear rig will create even beter kick ass performance than my wife’s new Landrover freelander 2….heeha:-)

What do you guys think? Will it work? It’s all theoretical but I’m actually remapping a 3spd auto to a 4 speed auto by changing differential gearing. We can do the same thing by reducing tire size, but I wouldn’t like a JK on small 26inch tyres:-((.
Maybe someone can try this if they’ve got 26inch tyres lying around. I would like to hear what you think about the performance, especially the usefulness of 4th gear.

My biggest concerns:
Will the OD gear work like a normal 4th gear?
Will the 25% increase in rpm on the driveshaft cause problems?
Can the side shafts and diffs handle the 25% more torque?
The speedometer will show a speed that is almost 20%higher than actual speed. Is it possible to reprogram the ECU to a tire size that has a 20% smaller diameter? In my case I’ll have to configure the ECU so that it thinks I’m running on 25.6inch tires. How small can we go?

I need to go sleep now...have to work in 3 hours..
Thanks guys and cheers here from SA

 

Your going to get bigger tires eventually anyway.

Go for it.

Besides, I was going to get the gears before the tires for the same reasons you have. So you might as well. Just be sure you go to somebody with experience installing them in JK's.

 

Question about octane rating for your gas....

In the states...regular is 87 octane.

If you gain altitude though...you need LOWER OCTANE for more power....many people assume the higher octane will make up for lost power at altitude/in the mountains...but, its the opposite....higher octane burns too slowly for proper flame propagation when the air is less dense.....so, either changing the timing, or using lower octane gas is the solution.

In some places (NOT all), the gas companies lower the octane for "Regular Gas" if the stations are in the mountains....but, if you fill up in the valleys.....and then drive up to 9,000', etc....you might be over octaned for the altutude.

So - just to see if we have a free (Actually, cheaper than free, lower octane gas is cheaper than premium, etc...) fix for you....tell me what gas you're using.

 

Octane ratings vary between 93-97 in SA.

regular around here is 93 octane, I usually fill up with 95.

Thanks for the info...i'll give it try.

 

Hey! nice to see a fellow South African on this site.
I'm from Cape Town and my JK Rubi 2 door should be here by the end of March, I placed my order 9 months ago!
Looks like you could be from JHB (GP)?
As far as I'm concerned this is the best JK forum site out there and forks here should be able to help you out on all things Jk.
Don't be shy and please let us all know what you have to say. It's a give and take thing and we all learn from each others experiences.
Hope you come right on getting a little more thump out of your 3.8.
Welcome and good luck.

 

RubiconSA, interesting concepts you posted, will need time to think about it, but yes, the OD on the auto will work like any other gear with lower differential gears. Basically what you are trying to achieve is something similar to what you'd have on a 1:1 normal highest gear.

You might want to think about what you are doing now, the KPH/MPH and equivelant RPM.

For instance your current 2nd gear might be what your 3rd gear would be doing after regearing. It would have the effect of faster starts, longer 2nd gear, etc. and would keep your momentum going till you hit 4th. BUT and this is a bit but, if your current 3rd gear is already similar to what your 4th will be then you can just plain figure that your HP/torque is lacking for the altitude and angle of ascent.

Basically regrearing is just reodering what you already have, hence you will need the extra power. Off the top of my head I'd say you need a combination of both. Not so radical on the gearing and a big boost in power.

Definitely a lower pressure exhaust, CAI (dirt roads won't make that much of a difference over stock in regards to getting dirty, a reprogrammer for your gears and possibly a chip.

I'd email the manufacturers with your dilemna of altitude and mountains climbs and see if they can benefit you.

Or...........buy a hemi chrysler, swap engines with your Jeep and sell the Chrysler.