Found a path to Decrease EGTs! Reliable and works!!!

[CreepyJeepy]

I finally bit the bullet and geared my 3.0 on 37" tires from the stock 3.73 Ratio to 4.56 Ratio.

Besides having a small axle seal leak (Right Side) which I discovered during the regear (like most of the 21 gladiators) everything went really great!

I wanted to share a few datapoints post the re gear that I'm noticing.

1. Around town hilly conditions Much Less EGTs
2. Hearing the turbo a lot more (S&B Intake) ~ This is due to not being at higher boost pressures as often as the engine does not have to work as hard. Under higher boost you can't hear the turbo... Honestly unexpected but loving this.
3. Throttle hesitation has been improved, much more than a pedal commander can offer. Still present, will likely get a GDE tune somtime.

People say 3.73's are enough for diesel. I agree... They are... For around town...

But no mater how you slice it... 4.56 is better..... (assuming 37" tires)... 20+% ish more wheel torque means less throttle required, and just overall much better to drive.

Anyways Really stoked to get through break in and evaluate how much better it is in the mountains towing my camper. I can already tell, this is the best improvement I've made so far....

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[kb5zcr]

Sounds great, thanks for the opinion. Let me ask you, can you tell me what rpm your now running and in what gear (60 mph in 5th, 60 mph in 6th, ect).

 

[VA6489]

Running BFG 39MTs on 4.56 gears. Yes I concur the rig runs much better and more efficiently. On 37 I think I would have opted for the 4.10 gearing. 39/40 inch tires boarder line 4.56 /4.88. You will give up some top end as to run higher speeds you will need more RPM but that is not a bad thing.

Towed an 18 ft box trailer at highway speeds for a few hours each way. The deeper gears made a huge difference in drivability. And despite the big Box and running big tires and lift I got a respectable 18 mpg.

One thing I keep in mind, this is a 3.0L power plant making less that 1/2 what pickup trucks make with the 6.7L motors. Dropping a gear on hills and easing off the throttle will help keep temperatures under control.

 

[Zachanadandy]

I finally bit the bullet and geared my 3.0 on 37" tires from the stock 3.73 Ratio to 4.56 Ratio.

Besides having a small axle seal leak (Right Side) which I discovered during the regear (like most of the 21 gladiators) everything went really great!

I wanted to share a few datapoints post the re gear that I'm noticing.

1. Around town hilly conditions Much Less EGTs
2. Hearing the turbo a lot more (S&B Intake) ~ This is due to not being at higher boost pressures as often as the engine does not have to work as hard. Under higher boost you can't hear the turbo... Honestly unexpected but loving this.
3. Throttle hesitation has been improved, much more than a pedal commander can offer. Still present, will likely get a GDE tune somtime.

People say 3.73's are enough for diesel. I agree... They are... For around town...

But no mater how you slice it... 4.56 is better..... (assuming 37" tires)... 20+% ish more wheel torque means less throttle required, and just overall much better to drive.

Anyways Really stoked to get through break in and evaluate how much better it is in the mountains towing my camper. I can already tell, this is the best improvement I've made so far....

Not to open the whole can of worms on regearing, but you only have 20% more wheel torque at the same engine output, ie the same amount of throttle and rpm. And even that only math's out if you're holding the trans in the same gear. For instance 4.10 gears in 7th will have identical wheel torque at a given speed as 5.13 gears in 8th because they have the same final drive ratio. Aside from 1st gear everyone is simply a downshift away from the same wheel torque. Now if you're cruising around in the exact same gear at every given speed (obviously you'll be manually shifting to make this happen) the engine will see less load but much higher rpms. The auto would never let this happen as it downshift when it needs more torque...automatically. With the torque of the diesel it typically holds 8th pretty well on the freeway even with larger tires. Now there after the regear you're just running higher rpms than you need. Not to answer extreme by any means with the smallish gear change, but likely a noticeable reduction in fuel economy on long trips.

 

[CreepyJeepy]

It’s a very complex math issue to calculate heat generation under load in a multi gear transmission.

The core benefit here is the overall mechanical advantage, that’s gained via a bigger final drive ratio, while reducing engine load to move the same amount of mass.

Folks are not wrong, for me I’m seeing 360RPM higher than stock (same gear, same speed)… but the reduction in required engine load means less heat generation.

I'm seeing 21MPG now city driving (this number was lower before for me. 18.1 previously

This is due to the fact the engine and transmission can stay in its peak / efficiency curve better at these more normal driving speeds.

I agree, if I bought a highway cruiser and that was its intended purpose 4.10s would have been better.

But I bought a diesel to tow my 3600lb camper to the mountains. Then enjoy crawling in the rocks like a tractor….

4.56 for towing , mountain conditions, on 37’s all day.

Down side here is I bet I see more regens, due to the reduced engine load requirements, but time will tell…

 

[JTdiRtyD]

Post back with an update on towing up grades. I'm curious if you see a decrease in oil/coolant temps at all, even if marginal.

 

[CreepyJeepy]

Post back with an update on towing up grades. I'm curious if you see a decrease in oil/coolant temps at all, even if marginal.

I have a trip coming up in a few weeks. I'll track EGT's, Oil, Coolant, up Kinosha pass and report back. 100%

I commonly hit 258F Oil temps on that hill, as well as watch EGTs at 1500 (which is way to hot for most diesels).

 

[CreepyJeepy]

I have a trip coming up in a few weeks. I'll track EGT's, Oil, Coolant, up Kinosha pass and report back. 100%

I commonly hit 258F Oil temps on that hill, as well as watch EGTs at 1500 (which is way to hot for most diesels).

I’m honestly not sure how I won’t see a reduction.

Heat generation is a result of Engine Load (Torque generation).

Heat dissipation is another topic, but obviously with less generation less heat needs to be shed.

Assuming I’m not 100% into the throttle when I tow (which I’m never because that’s terrible on any diesel) the me mechanical advantage will help keep loads (thus heat) lower as I climb hills, thus reducing the need to dissipate.

But I’ll data log and report back!

 

[VA6489]

With the torque of the diesel it typically holds 8th pretty well on the freeway even with larger tires. Now there after the regear you're just running higher rpms than you need. Not to answer extreme by any means with the smallish gear change, but likely a noticeable reduction in fuel economy on long trips.

What I have noticed is re-gearing helps fuel mileage and with this rig and bigger tires the transmission is always shifting down even with slight grades in Cruise. Cruise off yes it hold a lot longer but still down shifts.

Something of note, with a small area radiator running low RPM and more throttle relying on torque build heat quite rapidly. The small radiator and oil cooler have no way to disperse the heat. the only way to deal with the heat build up is higher RPM and or lower road speed.

 

[Zachanadandy]

What I have noticed is re-gearing helps fuel mileage and with this rig and bigger tires the transmission is always shifting down even with slight grades in Cruise. Cruise off yes it hold a lot longer but still down shifts.

Something of note, with a small area radiator running low RPM and more throttle relying on torque build heat quite rapidly. The small radiator and oil cooler have no way to disperse the heat. the only way to deal with the heat build up is higher RPM and or lower road speed.

That sounds like 2 opposing arguments? Gear lower so it doesn't downshift, but run higher rpms so it doesn't heat soak? The quickest way to higher rpms is to downshift and it's far more effective than a relatively small gear change like 3.73 to 4.10 or even 4.56. I don't own a diesel, but in our JLUR we lost fuel economy on the highway by gearing too low. A lot of the gassers recommend gearing too low because the want to hold 8th on inclines, but it's not supposed to. It didn't stock. 7th is still overdrive. The majority of JLs from the factory have 32" tires and 3.45s. That's the same gearing as 4.10s and 38s... and those are gassers with low torque compared to the diesel.

 

[CrazyCooter]

Honestly whatever makes you feel better.......

I tested 37's vs stock tires back to back next day with the same load, same hill, and almost identical temp conditions the following day with nearly identical outcomes. I did however prefer the gear spacing better on the 37's for descending the 9% grade to enable engine braking

I think the biggest factor where gearing is concerned is how the spacing between the transmission ratios puts you at the desired operating speed and load.

Spacing in the 8hp75 between gears:
8th-7th is 20.5%
7th-6th is 16.1%
6th-5th is 22.2%

3.73-4.56 is 18.2%

You pretty much just made the bottom range of gears more unusable while increasing the loads on the OD gears which are weaker and probably generate more heat? 1st is already too deep in low range on Rubi so no gain there.

Someday people might start to understand that it takes "X" HP to do "work" load/speed and that generates a certain amount of heat within a powertrain. Only way to reduce heat internally this is more cooling capacity to shed the heat, improve efficiency so less fuel is burned, reduce load by lightening, reducing frontal area, or slowing down?

Horsepower (HP) = (Torque x RPM) / 5252

In theory........If you run the numbers through the calculator (before and after gear ratio changes)it takes more hp to do work at a higher rpm...that takes more fuel yes? Example: A 20% increase in operating RPM requires 7.5% more hp. More fuel = heat? Then your trans generates more heat because it's pulling a light grade in 7th instead of 6th(direct)? This generated heat goes directly into the cooling stack?

I settled on 38's and 4.10s so that I can spend most of my time while towing under a load in 6th(direct) and only running 7th when it's flat, saving 8th for unloaded for fuel economy.

 

[CreepyJeepy]

Honestly whatever makes you feel better.......

I tested 37's vs stock tires back to back next day with the same load, same hill, and almost identical temp conditions the following day with nearly identical outcomes. I did however prefer the gear spacing better on the 37's for descending the 9% grade to enable engine braking

I think the biggest factor where gearing is concerned is how the spacing between the transmission ratios puts you at the desired operating speed and load.

Spacing in the 8hp75 between gears:
8th-7th is 20.5%
7th-6th is 16.1%
6th-5th is 22.2%

3.73-4.56 is 18.2%

You pretty much just made the bottom range of gears more unusable while increasing the loads on the OD gears which are weaker and probably generate more heat? 1st is already too deep in low range on Rubi so no gain there.

Someday people might start to understand that it takes "X" HP to do "work" load/speed and that generates a certain amount of heat within a powertrain. Only way to reduce heat internally this is more cooling capacity to shed the heat, improve efficiency so less fuel is burned, reduce load by lightening, reducing frontal area, or slowing down?

Horsepower (HP) = (Torque x RPM) / 5252

In theory........If you run the numbers through the calculator (before and after gear ratio changes)it takes more hp to do work at a higher rpm...that takes more fuel yes? Example: A 20% increase in operating RPM requires 7.5% more hp. More fuel = heat? Then your trans generates more heat because it's pulling a light grade in 7th instead of 6th(direct)? This generated heat goes directly into the cooling stack?

I settled on 38's and 4.10s so that I can spend most of my time while towing under a load in 6th(direct) and only running 7th when it's flat, saving 8th for unloaded for fuel economy.

It’s crazy to me that there is a jeep that can tow a trailer on 38” tires at highway speeds.

Thinking back to my jeeps growing up, 65mph was a pipe dream, without towing, on 33” tires…..

 

[CrazyCooter]

It’s crazy to me that there is a jeep that can tow a trailer on 38” tires at highway speeds.

Thinking back to my jeeps growing up, 65mph was a pipe dream, without towing, on 33” tires…..

And get 14+mpg towing.

I love this truck!

 

[CreepyJeepy]

Horsepower (HP) = (Torque x RPM) / 5252

In theory........If you run the numbers through the calculator (before and after gear ratio changes)it takes more hp to do work at a higher rpm...that takes more fuel yes? Example: A 20% increase in operating RPM requires 7.5% more hp. More fuel = heat? Then your trans generates more heat because it's pulling a light grade in 7th instead of 6th(direct)? This generated heat goes directly into the cooling stack?

Remember though this is full throttle situations. Without throttle there is no torque.
The equation result changes with different torque levels being applied at the crank.

I agree with the lower RPM shift points around town resulting in better MPGs at lower speeds. I suspect that’s why I’m seeing an improvement.

My camper is not the most aerodynamic, I expect higher loads and higher rpm shifting.

 

[BlueScapegoat]

The way I look at it, it takes the same amount of energy to move an object up the same grade at the same speed, everything else ignored.

If you gear down manually or regear to a numerically higher gear, each revolution of the engine will be doing less work but moving substantially more air. Less work per rpm, and air moves heat out. Next time you're idling down and watching your EGTs just rev it up for a bit and see what happens.

It's of course more nuanced than this, more fuel can aid in cooling. And there's brake specific fuel consumption to consider. You might not see better mpg. I doubt you'll see measurably worse. But overall I've been a huge proponent for regearing diesels as much as the gassers and this is why.

I'm still running 3.73s with 40s right now and it scoots just fine. But you can tell it's working harder. I've got 4.88s and rebuild kits on the bench and ready to go. Just because the power is on tap doesn't mean you're not asking more from it at even your normal street-driven rate of acceleration.

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