In previous posts I had suggested that 5.13s with a 37" tire (true 36) was not necessary and that I could have gone with 4.88s and been just as well off but with better mileage. I may have said this here as well, not sure. And that is at 70mphish, which is my normal freeway speed. Just so happens, my current drive to work speed limit is 45-55 along the route I take, thus those pics.. I only posted those pics of 50ish because I was told the JK didn't have enough power to be able to do that. Well, those that say that are simply wrong. So, the 50ish thing just came up recently. So did the green markings on the tach that I pointed out that everyone wants to ignore.

I am curious to see how a stock motor turning 2500 rpm making 184 ft/lbs of torque is so much superior to a modded JK pulling 2000 rpm and 190 ft/lbs of torque. (I was actually pulling a bit more I figure since I also had a K&N filter and a Viper TB. For those that don't know, those both allow more O2 into the combustion chamber allowing for a more complete burn which = more power. Sooooo, the modded JK has more power yet some here will argue to no end because they simply do not like to be proven wrong. I have taken what Planman so graciously provided and shown where he is wrong yet he and others still continue to aregue and imho, give bad advice. Then some here start making absurd comments that are totally off the wall with no basis to back it up. I have backed up what I have said and shown with Planman's own numbers the err of those ways.

So, I too , like planman, am curious to see how Jpops trys to explain this. I imagine it will have to do something with hp. That is where he is wrong, as I have stated before. He likes to use hp as the number needed and that is where he is misleading. The true figure needed is torque. Use some critical thinking skills on this. A 90sish F350 with the 7.3 IDI deisel was rated to tow 17,000 lbs. That is a significant amount of weight, yet that motor only made 190 hp. Yes, that's it. Yet it made 388ft/lbs of torque and that was at 1400 rpm. Do you really think that 190 hp is capapble of pulling 17,000 lbs at freeway speeds, not to mention having to overcome what ever wind resistance there might be with that monster 5th wheel behind it? The reason that truck was rated to pull so much was because of the torque, plain and simple. Oh, and that was with and AUTOMATIC transmision! Now I know many will want to just plain ignore valid evidence, facts, and most importantly, common sense, but I try. Why do I try? Because don't want others to make the same mistake I did.

 

Yes, hilly terrain and 6% grades the lower gears would work better for you. I'm sure it would work better for me ifn I lived there as well. It's pretty flat here in TX and speed limits up to 80 that's why I suggest not quite so low gears are ALWAYS needed.

As Jpop has said, it's not a one size fits all yet continues to tell me I am wrong. In the flatlands at lower elevations, we can get away with less gears, to a point. Like I said earlier, I am not suggesting you can run 40s on 3.21 gears by any means. I am just asking folks to be reasonable.

BTW, the OP is in Houston.

 

Quote:

Originally Posted by planman

Actually, I'm very interested in a discussion as to why HP matters and how much HP and TQ is needed to push a 5500+ lbs JK at 70-75 mph.


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Torque is momentary rotational force. When I'm twisting a wrench to place 125 ft lbs on a track bar bolt or the crankshaft of a vehicle. It has absolutely no element of time, just how much is applied. Take a slow moving river and the waterwheels of more than a century ago. The water isn't moving quickly, and therefore the water wheel, but they could provide tremendous amounts of torque used to grind wheat into flour, or wood into pulp to produce paper. If we could replicate this low rpm power in a combustion engine, it might make a great low rpm bulldozer or dump truck. Not so great for a passenger vehicle.

Horsepower is simply torque over time and applied. Take a thousand pound boulder and two vehicles with identical torque that can push it. One achieves peak torque at 2,000 rpm and the other at 4,000 rpm. Torque alone has no measure to determine how far we can push this boulder, again because it has no element of time. This is where power comes into play, as the vehicle spinning at 4,000 rpm will push the boulder twice as far as the engine rotating 2,000 times per minute.

Torque certainly has an important roll in moving a vehicle. It's what gets you rolling from a dead start, the power to pass someone at highway speeds or in racing the power to accelerate off a turn. It's basically the hammer that gets you moving while horsepower is what keeps you moving at a given rate. Take for example an engine from an F1 car. They don't have an incredible amount of torque, only around 200 ft lbs, but they can apply that torque at 19,000 rpm. It's a quite different animal than a pick up truck that might have 400+ ft lbs of torque when speaking of how they maintain speed at much lower rpms. Again, it's the frequency at which torque is applied, and why torque alone doesn't tell the whole story.

Here's how torque and horsepower were explained to me by my father (an engineer) some 30+ years ago after having a debate with a neighbor kid over the big torque of a big block buick engine vs a higher revving small block chevy. He had me flip my bike over, asked me to firmly grasp the rear tire and proceeded to move the pedal. He stated the amount of force that I was feeling trying to hold my hand in place was torque, momentary rotational force. He then asked me to hold palm loosely on top of the tire as he began to slowly spin the pedal. As he increased the speed, my hand would get warmer. The heat generated by friction was power. Lots of different ways power is measured, BTUs, Watts or for our purposes horsepower.

Looking at torque numbers at 2,000 rpm and 2,500 doesn't tell the whole story because one has torque applied at a rate 25% higher. When we look at a vehicle maintaining speed at a given mile per hour, we can't do it with torque alone and need to include the measure of time. Not that torque is totally removed from a horsepower rating, as we can't deduce what horsepower is without knowing what torque is and the rpm at which they are spinning. In other words, let me know the either the horsepower or torque at a given rpm and I can let you know the other with some simple math. It's not fuzzy or convoluted, it's just math.

Now we come to the next part of getting our JKs to move at a given velocity, aero drag. You can look at a JK and tell that it has aerodynamics similar to a barn, and a high drag coefficient. Then we have the drag equation at velocity, quadratic drag, which states that when we double speed we quadruple drag. Not only do we have 4 times the drag at doubled speeds, we are asking the engine to provide the power to do this work in half the time. What we find is the aero drag on our JKs increases from near 90lbs at 50mph to 360lbs at 100mph. Certainly no one is driving their JKs at 100mph, but from 50mph to 70mph the aero drag nearly doubles.

Why is aero drag important and what about drag from tires, rolling resistance and weight? At 50mph more than 70% of the power needed to maintain velocity is combatting aero drag. At 70mph more than 80% of the power needed to maintain velocity is combatting aero drag. Weight, rolling resistance and tire drag are basically flat from 10-70mph because once they're rolling or spinning, it take very little energy to keep them rolling because of inertia. For instance, we have all rolled a tire by hand to move it somewhere. It takes some initial effort to get it rolling (torque), but very little effort to keep it rolling. Things don't change much until we hit around 80mph which may be important for racing, but makes little difference for passenger vehicles and our JKs.

Where does weight come into play? Weight has it's biggest effect on grades or slopes. Not only does the engine need to provide the power to combat aero drag and rolling resistance, but it also needs to provide the work of lifting the vehicle to the top of the grade. Now the engine must provide the power to do the work and additional power to lift the vehicle up 20' to make it over an overpass. I often refer to 3 degree grades as that are what most of the overpasses are. So even if you're running on flat highways, you still need to contend with the grades of overpasses. The easy thing with incline drag is that it doesn't change as long as the grade doesn't but a simple 3 degree grade at 70mph more than doubles are power requirements to maintain velocity.

So there you have it, what goes into determining the power needs of our JKs to maintain velocity and how through those power needs the appropriate gear set can be determined. That's the thought process in place when building out the gear charts on this site, and why there is a pretty big window for what's appropriate. I apologize in advance to those taking college level fluid dynamics courses as I'm certain a 30 year spread since I made that effort has left out some explanation and probably didn't use the correct verbiage all the time. The knowledge presented comes from my 20+ years drag racing, countless spreadsheets and calculators. It also comes from an array of people on these forums and their real life experiences, as the JK has been the first underpowered vehicle I have owned in the 30+ years since I had my drivers license.

If you want to shoot tiny arrows my way, you're more than welcome to but please back it up with the math and science that supports it. Real world targets such as 70mph velocity the majority of us experience every day and the moderate grades that accompany them. Then we can jump to the inefficiencies of running in open loop when the throttle is engaged at 75%+.

 

What was that I said before about dazzle and baffle?

You have proven nothing. I have shown with the charts so kindly provided by Planman that torque can be obtained at a lower rpm and that will do the same work but at a lower rpm.

Please explain away my example I gave regarding the ford 7.3 deisel with 190 hp being able to tow 17000 lbs. If I were to blindly follow you and what you say like so many do, that would lead one to believe that since our 3.8L JK motor stock makes 202 hp we would be able to tow more than that 1 ton ford truck.

Planman posed a question earlier in this post about power needed at 70 mph in a 5500 lb JK. Since you claim to have such advanced knowledge and resources I was also anxiously awaiting that since you undoubtedly know that. Still waiting I guess because maybe that knowledge and those resources simply aren't there.

 

At issue here, is that you can run out of power before you run out of gear. What may well work at 50mph, including moderate grades does not work at 70mph. A 40% increase in speed and hence a 40% increase in rpm does not account for the need of more than double the amount of power. This is exactly where I have needed to back planman off of the people who want to roll at 80mph needing less gear, because again, they will run out of power before they run out of gear.

 

Reading is fundamental. You don't want to believe me, read a definition of torque. Then read a definition of horsepower. Perhaps even read an article that explains the difference.

The difference in a low rpm diesel motor is that it makes peak horsepower under 3,000 rpms. Specifically, the engine you mentioned makes peak power at around 2,650 and peak torque at under 1700rpm. You will never get the same moment of force (Torque) out of a JK. If you want the same amount of power, you would need to twist the 3.8L near 5,000 rpm.

As for planman and a 5,500lb JK, it takes a rounded 35hp to maintain 70mph velocity on flat roads and more than 80hp to cope with moderate 3 degree grades. In case you didn't understand, planman's question was rhetorical as he already knows the answer and it just didn't fit into the box of your 50mph flatland jaunts.

 

Yeah, let's get a diesel JK!!

 

Just for the heck of it, I tried a little experiment, I am not advocating this was moral, legal or whatever else, but I did it anyway.
I was going up the same 6% grade and since traffic was almost non-existent, I gave it the beans. At approximately 90-95 mph the JK will pull the hill in overdrive. Normally have to turn OD off to maintain the speed limit, where the engine is turning more than 4k, approaching 5k.
So, at 95mph and nearly 5k rpm, in OD, was it torque or horsepower that allowed me to crest the hill?

 

Apparently I am not the only one who feels you do not need to go with stuoid low gears.

https://www.jk-forum.com/forums/show...ith-3.21-gears

 

I was trying to remain a silent voyeur to this thread and hoping for an answer to arise on 5.13 final gears. Disappointed and rather irked by the ridiculous bickering has lead nowhere, I want to try to refocus this thread. Factually a taller final gear has certain impacts upon a vehicle. Taller gears, larger numbered, WILL increase the torque of said vehicle via the amplification of a mechanical force. While avidly searching for a clear and concise means to articulate the principles of automotive gearing I found one that satisfied my needs. Unfortunately my source is a Wikipedia article. I know this will bring scrutiny and cause some prejudice but it is the only thorough description that is easily understood. An in-depth engineering analysis would be too confusing and lengthy for anything but a text book or an engineering forum. Anyway, you must first know your desire for your vehicle and the stresses your will impose upon it, ie steeper hills, larger rocks or softer sand/mud. Based upon this knowledge and the gearing of your stock jeep coupled with the size of the original tires and your satisfaction with its performance abilities. If your stock set up didn't have the torque for what you need then you must account for that with a taller gear. If you had a 3.21 and you feel as though the 3.73 would have been better than factor that in your calculations. From this you can get a fair estimate of your the gears you would like. Here is a link for a gear calculator . As for the principles behind it you'll be interested in the "automotive applications" and "Wide-ratio vs close-ratio transmission".