Torque curve and efficiency

arengant · **Posted:** Thu Apr 11, 2013 3:11 am

My current set-up is lacking, I am running 32.1 tall tires, 3.73 gears and the GDE TCM tune. I drive uphill to work, and downhill home. When going uphill, I lose momentum and power when I get locked into 4th and 5th, the RPM drops down to about 1500 rpm (which would be roughly 40 ft/lb less than peak). I feel like I am lugging, really working the engine and increasing EGT, and often I have to nearly floor it (unlocking the TC) to get up the hill. 4.10 would put me of course closer to stock, but then my high speed driving mpg would suffer.

I am overthinking this I know, but some quick internet research with google has me thinking. The best fuel efficiency for our diesel is at peak torque RPM (2000 not 1800 rpm according to GDE table http://www.greendieselengineering.com/files/GDE%20Power-Torque%20Jeep%20Liberty%20data.pdf)

So I should try and have my gear/tire combo put me at that RPM the most often right? The curve is slightly flatter after peak Torque, so I would have thought going 2200 rpm would give me better MPG than 1800 by just looking at the numbers, however in real life 1800 rpm does seem to be my MPG sweet spot.

Anyone else experience lower than expected economy at sub 1800 rpm?

Glend · **Posted:** Thu Apr 11, 2013 6:02 am

I know your pain. I run the GDE full-torque tune and the GDE TCM. I run 255/70 tyres and 3.73 final gears. The problem is the TCM tune and the massive torque hole when it shifts down to 1400rpm. It might give good fuel economy in the city but towing up hill sucks. I'd say get rid of the TCM tune before you try anything else. I think GDE has options regarding TCM setup, including custom setup. Even the stock TCM would be better, wish I still had mine.

striperman36 · **Posted:** Thu Apr 11, 2013 7:41 am

look at getting a tcm from a ram 1500 to tow.

tcoilburner · **Posted:** Thu Apr 11, 2013 11:07 am

striperman36 wrote:

look at getting a tcm from a ram 1500 to tow.

What will the ram Tcm do? What years?

I have the full torque eco tune. I find running p235/75r16 tires, that my mileage at 55-57 mph is better leaving it locked in fourth gear. That is hand calutated mileage. The trip computer shows running in fifth gear is better. :2cents:

arengant · **Posted:** Thu Apr 11, 2013 1:58 pm

Glend wrote:

I know your pain. I run the GDE full-torque tune and the GDE TCM. I run 255/70 tyres and 3.73 final gears. The problem is the TCM tune and the massive torque hole when it shifts down to 1400rpm. It might give good fuel economy in the city but towing up hill sucks. I'd say get rid of the TCM tune before you try anything else. I think GDE has options regarding TCM setup, including custom setup. Even the stock TCM would be better, wish I still had mine.

This is what I have been thinking about. I will talk with Keith, but I would still like to hear a little more about optimum rpm. I do not tow a lot, but with taller tires, armor, and camping gear I feel like the I would get better MPG running at a higher rpm band range that a stock set up.

I would rather work on custom tuning my trans through GDE over the Ram TC, that set up isn't quite what I am looking for. I will have to run some numbers, but if I can get 3rd Lockup and hold off on shifting into 4th and 5th until I reach a higher RPM than that would be best. However, 4.10s would completely through off the goal of a custom program so I need to decided for sure what set up I like. I am 100% set on my tire size, so I am going to use a gear calculator and work backwards to find some good shift points... After I decide on the optimum RPM for my set up.

So pretending we had 5 speed manuals, and I could choose my own gears, I am thinking that putting running between 1700-2100 Rpm would give me most power. Any theories?

papaindigo · **Posted:** Thu Apr 11, 2013 3:43 pm

I'll certainly admit I'm not an expert but I strongly suspect you need to look at more than just the torque curve when trying for efficiency (presumably mpg). Generally speaking mpg goes down as speed goes up in any gearing and certainly the KJ has all the aerodynamics of a medium large brick. Focusing just on power and speed assuming roughly stock tires I suspect the best mpg is going to be achieved locked up in 5th at ca. 1800rpm or about 62 mph although I suppose in theory you might do better at the lowest speed (if you can stand it) you can hold locked up in 5th. Once you push on up toward 70mph mpg begins to drop fairly rapidly. However, if you are on a long downhill run (say from Flagstaff AZ to Tempe AZ) speed and rpm have less influence on mpg.

All I'm trying to say is what the setup and rpm/speed for efficiency will vary from KJ to KJ in large part depending on terrain, towing/not towing, relative amount of hwy/city, etc. Bottom line is there will not be 1 answer for all KJs or even a single answer for "your" KJ.

Glend · **Posted:** Thu Apr 11, 2013 5:49 pm

There is a VM graph around here somewhere which illustrates the engine fuel volumetric efficiency point as being around 2000rpm. This point is also in the peak torque area of the torque curve.

jlgail4309 · **Posted:** Thu Apr 11, 2013 5:51 pm

GDE has a tow tune that holds 4th gear up too 75 miles per hour. That must be a big hill your driving. Mine holds 5th up most any thing at 1800 rpms. But when I'm towing my travel trailer I could use some help with 4th gear. Hope some people with the tow tunes will respond. I would have aready bought but if you have to send it back for a change from GDE it 100.00 every time. Wanting to get right the first time.
Thinking 4th to 73 miles per hour then to 5th back to 4th at 63 miles per hour. I don't want to shift to 3rd when I floor it. I want it to drop to 4th only.

arengant · **Posted:** Thu Apr 11, 2013 11:30 pm

jlgail4309 wrote:

GDE has a tow tune that holds 4th gear up too 75 miles per hour. That must be a big hill your driving. Mine holds 5th up most any thing at 1800 rpms. But when I'm towing my travel trailer I could use some help with 4th gear. Hope some people with the tow tunes will respond. I would have aready bought but if you have to send it back for a change from GDE it 100.00 every time. Wanting to get right the first time.
Thinking 4th to 73 miles per hour then to 5th back to 4th at 63 miles per hour. I don't want to shift to 3rd when I floor it. I want it to drop to 4th only.

The tow tune is too aggressive for what I want, so I will drop the 100.00 once I get the RPM figured out. My HOT tune kicks in when I get on the pedal, dropping out of TC lock and letting a lot of black smoke out

I think I will try and get 3rd lock up again, and hold that till 2100 RPM, and same with 4th (I am not using MPH but that is based on your gearing/tire size, where as I am trying to find best efficiency/torque/power band).

Right now, I am thinking holding those gears down to 1700 RPM, not the 1400-1500 RPM that it holds now.

arengant · **Posted:** Thu Apr 11, 2013 11:38 pm

papaindigo wrote:

I'll certainly admit I'm not an expert but I strongly suspect you need to look at more than just the torque curve when trying for efficiency (presumably mpg). Generally speaking mpg goes down as speed goes up in any gearing and certainly the KJ has all the aerodynamics of a medium large brick. Focusing just on power and speed assuming roughly stock tires I suspect the best mpg is going to be achieved locked up in 5th at ca. 1800rpm or about 62 mph although I suppose in theory you might do better at the lowest speed (if you can stand it) you can hold locked up in 5th. Once you push on up toward 70mph mpg begins to drop fairly rapidly. However, if you are on a long downhill run (say from Flagstaff AZ to Tempe AZ) speed and rpm have less influence on mpg.

All I'm trying to say is what the setup and rpm/speed for efficiency will vary from KJ to KJ in large part depending on terrain, towing/not towing, relative amount of hwy/city, etc. Bottom line is there will not be 1 answer for all KJs or even a single answer for "your" KJ.

I agree with your statement about KJs having different set ups giving different results.

This was more of a question of diesel theory I guess. I will use efficiency for the engine, and economy referring to vehicle specific MPG. I will look for the VM graph Glend posted, that should give us the most accurate efficiency range, and I will research more on my specific economy.

Bottom line, the more I push the pedal down, the worse economy. Sometimes when I am in the lower RPM I have to push further than if I turn OD off...

arengant · **Posted:** Thu Apr 11, 2013 11:47 pm

jlgail4309 wrote:

GDE has a tow tune that holds 4th gear up too 75 miles per hour. That must be a big hill your driving. Mine holds 5th up most any thing at 1800 rpms. But when I'm towing my travel trailer I could use some help with 4th gear. Hope some people with the tow tunes will respond. I would have aready bought but if you have to send it back for a change from GDE it 100.00 every time. Wanting to get right the first time.
Thinking 4th to 73 miles per hour then to 5th back to 4th at 63 miles per hour. I don't want to shift to 3rd when I floor it. I want it to drop to 4th only.

I shift at about 1750-1800 RPM, which is too soon for my set up (weight, tire size, drag from bumper/lift)

jlgail4309 · **Posted:** Fri Apr 12, 2013 8:14 pm

Mine seems to tow better above 2000 rmps. If I drop below then down shift and start all over again. If I run in 3rd the all the time my miles per gallon drops. My rpms are at 2600 all the time. GDE can set your rpm shift point were ever you would like. Call Keith

arengant · **Posted:** Fri Apr 12, 2013 10:51 pm

jlgail4309 wrote:

Mine seems to tow better above 2000 rmps. If I drop below then down shift and start all over again. If I run in 3rd the all the time my miles per gallon drops. My rpms are at 2600 all the time. GDE can set your rpm shift point were ever you would like. Call Keith

Have you tracked enough miles/gallons with towing to find your best power/rpm range? Before I drop $100 on modifying my tune, I want to narrow down the RPM band.

jlgail4309 · **Posted:** Sat Apr 13, 2013 12:09 am

The tow tune is 300.00 then any changes are 100.00 and mail back and fourth. I leaning to 2200 rpms in 4th gear for my best towing. I towed to Destin Fl from Tulsa at Christmas time. My tank were all over the place. From 12 to almost 17 with a 3000 pound dry weight travel trailer. The big think was holding the speed so close to hold 4th gear. But then shift to 5th going down hills then back to 3rd on the next hill. Maybe next will be that new turbo from GDE really need more power to hold speed,

arengant · **Posted:** Sat Apr 13, 2013 1:26 am

jlgail4309 wrote:

The tow tune is 300.00 then any changes are 100.00 and mail back and fourth. I leaning to 2200 rpms in 4th gear for my best towing. I towed to Destin Fl from Tulsa at Christmas time. My tank were all over the place. From 12 to almost 17 with a 3000 pound dry weight travel trailer. The big think was holding the speed so close to hold 4th gear. But then shift to 5th going down hills then back to 3rd on the next hill. Maybe next will be that new turbo from GDE really need more power to hold speed,

Yeah I am on the list for a turbo as soon as they make another batch.

fool4wheelin · **Posted:** Sat Apr 13, 2013 9:07 am

I have a stock CRD with the GDE ECO-Tune. I was pulling a trailer last weekend on rolling hills trying to go 70 mph against a 20 mph headwind. I ended up putting it in 3rd and leaving it there. It seemed like 4th gear would have given me sufficient torque to maintain 70 mph, but I couldn't get it to hold in 4th, it would shift to 5th everytime I would be going slightly downhill; then when going on flat or uphill grades, the RPM's would drop way down before even trying to shift. Mileage was pretty bad, 12-13 mpg. I really need to find a way to manually select 4th gear over 5th.

If you're driving at sea-level, then the elevation isn't hurting you. Are you running #2 Diesel Fuel? How much does your rig weigh? My stock rig weighs ~4500 lbs with a full tank of fuel, which is heavy for a little SUV. What kind of ambient temps are you running in? Stock tires on the CRD are ~29", so you're losing about 10% of your torque there. You mentioned not wanting to swap your rear end ratio because of high speed driving, how fast do you drive? With my stock setup, I'm only at 2250 rpm going 75 mph. I would say either get a TCM-Flash that holds 4th gear for a long time (running in 4th gear for you would be the same as a stock setup in 5th) or put the 4:10 gears in.

Here are my 2¢ on diesel theory. Diesel engines are more efficient the slower you run them, partially because of friction and partially because the more time you give the fuel to mix with the air the more complete combustion will be. Peak torque is a little different because the faster you run the engine, the more boost your turbocharger is going to give you. More boost means more air which means more fuel which gives you a bigger "boom". In theory, an ideal engine that has the same air charge every cycle would have a "flat" torque curve across the entire speed range. Its things like the combustion efficiency, turbocharger, and internal friction that give the diesel engine its unique torque curve.

arengant · **Posted:** Sat Apr 13, 2013 2:18 pm

fool4wheelin wrote:

I have a stock CRD with the GDE ECO-Tune. I was pulling a trailer last weekend on rolling hills trying to go 70 mph against a 20 mph headwind. I ended up putting it in 3rd and leaving it there. It seemed like 4th gear would have given me sufficient torque to maintain 70 mph, but I couldn't get it to hold in 4th, it would shift to 5th everytime I would be going slightly downhill; then when going on flat or uphill grades, the RPM's would drop way down before even trying to shift. Mileage was pretty bad, 12-13 mpg. I really need to find a way to manually select 4th gear over 5th.

If you're driving at sea-level, then the elevation isn't hurting you. Are you running #2 Diesel Fuel? How much does your rig weigh? My stock rig weighs ~4500 lbs with a full tank of fuel, which is heavy for a little SUV. What kind of ambient temps are you running in? Stock tires on the CRD are ~29", so you're losing about 10% of your torque there. You mentioned not wanting to swap your rear end ratio because of high speed driving, how fast do you drive? With my stock setup, I'm only at 2250 rpm going 75 mph. I would say either get a TCM-Flash that holds 4th gear for a long time (running in 4th gear for you would be the same as a stock setup in 5th) or put the 4:10 gears in.

Here are my 2¢ on diesel theory. Diesel engines are more efficient the slower you run them, partially because of friction and partially because the more time you give the fuel to mix with the air the more complete combustion will be. Peak torque is a little different because the faster you run the engine, the more boost your turbocharger is going to give you. More boost means more air which means more fuel which gives you a bigger "boom". In theory, an ideal engine that has the same air charge every cycle would have a "flat" torque curve across the entire speed range. Its things like the combustion efficiency, turbocharger, and internal friction that give the diesel engine its unique torque curve.

Good thoughts! I agree with the slower speeds give you better efficiency in theory, and for real world the slowest speed that still provides enough HP to get the job done will give you the best economy. So, depending on the hill/load, economy varies at any given RPM. I weigh ~5000lbs, the slight lift, roof rack, and design of the ARB all increase drag.

I am up in the air about 4.10, I want to do it as it will help with take off power and city driving economy (which is over half of my current driving) but I do not have the free time for that big of a project currently. Dropping $100 to increase my 4th gear range would help a lot now, but I would want to plan it so after I regear I am not holding it too long, lowering MPG even more.

Torque curve and efficiency

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