Turtle n Peeps wrote:

Put the thing in TH and drive. Forget about shifting and trying to out guess a computer.

eHoefler wrote:

The Cummins is happiest between 1800 and 2100. Pulling our 18000 ish fifthwheel, will only go above 2300 on extremely hard pulls.

Me Again wrote:



And if it is hunting between gears, then lock it in the lower one. I lock out 6 gear when towing a 16K 5th wheel. It will drop of 4th on mountain passes. RPM go to 2100-2200 if I remember correctly.

It is at 1750 at just over 60.

Love it.

Chris

bbaker2001 wrote:

So running in TH will change gears automatically and downshift for me?

My gasser ran about 3500 going uphill. So 2000 for a diesel is not a problem going uphill. I dont think I could get it to 2000 on straight ground.
When cruising at 60 it is so quiet, I dont want to lug if, but don't know how to tell

bbaker2001 wrote:

So running in TH will change gears automatically and downshift for me?

My gasser ran about 3500 going uphill. So 2000 for a diesel is not a problem going uphill. I dont think I could get it to 2000 on straight ground.
When cruising at 60 it is so quiet, I dont want to lug if, but don't know how to tell

Turtle n Peeps wrote:



Yes it will do all of that and then some. All modern engine auto tranny combo's will not lug. It's impossible. It knows how much your pulling and what kind of strain is on the engine. It's all programed into the computer. You're along for the ride; just enjoy it.

One more time. Put it into drive; put your TH on and drive.

Turtle n Peeps wrote:



Yes it will do all of that and then some. All modern engine auto tranny combo's will not lug. It's impossible. It knows how much your pulling and what kind of strain is on the engine. It's all programed into the computer. You're along for the ride; just enjoy it.

One more time. Put it into drive; put your TH on and drive.

RCMAN46 wrote:

With my Duramax / Allison I put it in TH and never have to worry about the RPM up or down hill.

VoodooMedicineMan wrote:

Next time I am going to try that. 3300 seems awful high.

Redwoodcamper wrote:

If you have the aisin tranny, then that is correct. 68rfe is different. There are hundreds of tranny rebuilds because of that thinking.

Turtle n Peeps wrote:

Put the thing in TH and drive. Forget about shifting and trying to out guess a computer.

4x4ord wrote:



There is nothing really wrong with what your doing it just means that you will go slower up the hill. Anyway the way to maximize your speed up a long hill is to start by anticipating the hill. By that I mean downshift before the truck begins to slow down.

So if you are going 60 mph in high gear with 3.73 gears your engine will be turning about 1713 rpm. Drop two gears and you should be travelling 60 mph and revving 3117 rpm. If the truck is pulling a long steep hill it can slow all the way down to 42 mph before you can shift again. At 42 mph your engine will be revving 2183 before the shift and 3500 after the shift.

Redwoodcamper wrote:



If someone has an aisin, then the above advice is great. But with the stock 68rfe it will bog down to 1100-1200 Rpms at 50-70 percent throttle and not downshift. Egts go crazy, tranny clutches get cooked. Companies tune things with mileage and the EPA too close in mind. So many posts on here from people who have never driven or towed with a 68rfe. Let alone tear one apart or have to rebuild one. Which I have done. It isn't a bad tranny, just has some weak points.

rhagfo wrote:



Well don't know the listed red line on the Cornbinder 6.0. but 3,300 rpm would be 100 rpm over my CTD 5.9's red line.
I don't like taking it much above 2,500 as that seems to be the point power flattens out.

Wild Card wrote:



Thats cause the Cummins is most at home at 2100rpm. Peak torque. Everyone gets caught up in the HP craze. Horsepower doesnt do crapola for pullimg grades. Gear it for peak torque and let it pull.

4x4ord wrote:



Comments like this are what confuse people. Here is the math:

If you have a 2015 Cummins/Aisin it makes 865 lb ft of peak torque at 1600 rpm. If it is run through gears to slow the rear axle down to 507 rpm (5th gear with a 4.10 rear end) the torque on the rear axle will be 1600/507 x 865 or 2768 lb ft of torque.

Now rev that engine up to 2800 where it makes 385 HP and the torque drops down to 722 lb ft. Drop down to 3rd gear. Your rear axle will be turning 509 rpm. The powertrain will be able to deliver 2800/509 x 722 or 3972 lb ft of torque to the rear axle.

So in 5th it can put 2768 lb ft to the rear axle and in 3rd 3972 lb ft to the rear axle. HP is king ...... if you have HP, torque can be generated with gears.


These numbers assume no power loss through the gears and so are to demonstrate the physics rather than be absolute values.

Wild Card wrote:



Thats cause the Cummins is most at home at 2100rpm. Peak torque. Everyone gets caught up in the HP craze. Horsepower doesnt do crapola for pullimg grades. Gear it for peak torque and let it pull.

rhagfo wrote:



Yep, that is why high HP gas engines, can't tow what a diesel. More energy per gallon. I will take a Low HP/High torque over a High HP/low torque gasser any day. Diesels don't need to spin fast to move a load.

rhagfo wrote:



Yep, that is why high HP gas engines, can't tow what a diesel. More energy per gallon. I will take a Low HP/High torque over a High HP/low torque gasser any day. Diesels don't need to spin fast to move a load.

bbaker2001 wrote:

When towing with my ram diesel how do I tell when I should downshift.
it seems to tow so easy, but slows when going uphill
at 60 what should my rpm show.

4x4ord wrote:



Here is some facts for you:

Gasoline has more energy per pound than does diesel.

Diesel engines need more air to burn their fuel than do gasoline engines. So when comparing a gasoline engine to a diesel of the same displacement the gasoline engine will make more torque and because it can be designed to rev much higher it will make much more Hp than the diesel.

The designer of the diesel engine (Rudolph Diesel) predicted that due to the slow burning of the fuel, the diesel engine would never be able to maintain operating speeds above 700 rpm. High pressure fuel injection has changed this.

Unlike gasoline engines, Diesel engines have been well suited for turbocharging for decades. The high torque of the diesel stems from turbo charging.

The reletvely recent development in direct injection gasoline engines is allowing gasoline engines to be effectively turbocharged. For instance the Honda 1.5 liter turbocharged gasoline engines make 108 lbft of torque per liter from 1700 rpm all the way to 5500 rpm. The HO Cummins makes 134 lbft of torque per liter at 1700 rpm but by 2800 rpm it is down to the same 108 lbft that the Honda makes. Peak HP of the Honda is 116 HP per liter. The HO Cummins is capable of only 54 HP per liter.

Although Diesel fuel has less energy than gasoline per pound it has more energy per gallon than does gasoline. This and the high compression ratio of the diesel engine contribute to its better fuel economy.

A high HP gasoline engine will beat a lower horsepower diesel up the hill everytime if the gasoline engine has the necessary gears to keep it in its peak hp rpm range.

wilber1 wrote:

All these numbers are at WOT so it is just as important to have a gear which allows you to maintain WOT without either over reving or losing speed.

My Cummins 650 has a flat torque curve, it peaks at 1500 RPM and stays flat until 2900 RPM. At 1500 RPM at WOT it can make only 186 HP. At 2900 RPM it can make 350 HP.

Another graphic example of what a turbo DI gas engine can do is the new Audi RS3 engine. It is a performance engine but it also comes with a four year warranty.

2.5L 5 cylinder
HP 400 hp at 5850 RPM 160 hp per litre
Torque 354 lb ft from 1700 to 5850 RPM

That is 141 lb ft of torque per litre at 1700 RPM compared to the Cummins HO's 134 lb ft per litre at 1700 RPM.

ShinerBock wrote:


Some of those aren't facts.

Diesel torque stems from its more explosive combustion which pushes the piston down with more force the fact that diesels generally have longer strokes than gasoline engines which creates more low end torque, but also limits max engine speed. Turbos only make the output greater just like they do with gasoline engines.

Comparing the the torque output of a Honda 1.5L to Cummins pickup engine is just plain silly. They are built and rated for two different things. The Cummins is de-rated significantly to keep exhaust and other temps in check for towing while the Honda is not. If you put that Honda engine in a tow vehicle application then I can guarantee that the torque rating will go down. The Cummins also has more restrictive emissions equipment limiting that power a long with turbos and cams made for two different applications.

If you want to make both engines that you used an example for a more even playing field, then you would use the QSB 6.7L which is not de-rated for towing and has about the same emissions equipment as the Honda. The QSB 6.7L has a max output of 542 hp @ 2,900-3,300 rpm and 1,250 lb-ft @ 2,000 rpm. It is down to 890 lb-ft by the time it hits max hp at 2,900 rpm. That would put it at 187 lb-ft per liter at peak torque and 132 lb-ft per liter when it hits max hp at 2,900 rpm.

4x4ord wrote:



Torque comes from the amount of energy in the way of fuel that can be burned in an engine per revolution. Larger displacement yields more torque. Higher boost pressure ... more torque. Diesel needs more air to properly burn than does gasoline so more energy can be burned into a set volume of air if gasoline is used as the fuel as opposed to diesel.... therefore gasoline has a higher potential for making torque than does diesel. You are right that the use of an engine has a lot to do with its design. It won't surprise me if the newer gasoline DI/turbo engine technology and the fact that gasoline requires less restrictive measure to be used to meet emission requirements causes engine builders to turn toward designing gasoline engines to be favoured over the diesels for towing.

ShinerBock wrote:

If you want to compare a diesel and gas engines with similar displacement, bore/stroke, duty cycle, and technology then you can compare the Audi 3.0L turbocharged diesel and gas engine.

Audi 3.0L Diesel
240 hp
428 lb-ft


Audi 3.0L Gas DI
354 hp
369 lb-ft

ShinerBock wrote:




What each fuel does outside of the engine is one thing. How each engine uses that fuel is another. Gasoline engines are regulated by air and add a certain amount fuel to stay around a stoichiometric air/fuel ratio (14.7:1) while diesel engines are regulated by fuel and uses all the air available to burn lean most of the time. The more air you add to a diesel engine, the more efficient it becomes. The more air you add to a gasoline engine, the more fuel it consumes to stay around stoich. This is one of many reasons why diesel are way more efficient than gas engines under load and are used for towing applications.

Yes, with the same compression ratio gasoline engines would be more efficient, but I don't live in unicorn would-a could-a should-a world so I go by what the real world has. Diesel fuel itself allows for much higher compression ratios making the diesel compression ignition more efficient at utilizing more out of a drop of fuel than spark ignition.

I disagree on the emissions thing. As technology increases, I see less emissions devices on future diesels. Cummins has already deleted the EGR on their current ISL engines and only utilizing SCR making the engines more efficient and better performance. they already have plans to do the same for other engines on their line up including the ISB 6.7L.

4x4ord wrote:



The DI gasoline engines are no longer regulated to a 14.7 air/fuel ratio nor is the compression ratio limited. Preignition can't happen without fuel being injected.

wilber1 wrote:

Mazda Skyactive gas engines are using a 14:1 compression ratio. The are planning on going to 18:1 with gas engines.

Skyactive 2

ShinerBock wrote:



I haven't heard about DI engines no longer being regulated to 14.7:1? What engines are these?

Every DI I have hooked my OBD Torque App up to from my Ecoboost truck to my wife's old Audi(both turbocharged) have been regulated to an A/F of around 14.7:1 with a very narrow range. The only time either engines ran lean was the brief instant after going WOT it would add too much air before the injectors could compensate. It would go no more than 15.0:1 though. After that it would run rich at 13.2:1 if you left your foot into it and then start to go back to stoich as you picked up speed. In normal driving or idle they always stayed around 14.7:1 +/- .5. My diesel on the other hand will go anywhere from 20:1 to 80:1 depending on engine load. At Idle, it will go even higher than that to upwards of 100:1.

I know some DI engines have high compression ratios like the 6.2L Ecotec with 11.5:1(probably why 93 octane is recommended), but I have not heard about any mass produced DI engine getting to the 15:1 and up compression ratios of diesels or the 17.3:1 of my Cummins.

4x4ord wrote:

Wikipedia explains how the air fuel ratio in direct injection gasoline engines can vary from ultra lean (as lean as 65:1) to stoichiometric to slightly richer than stoichiometric during full power.

Click


Edit: I copied and pasted the part that of the write up that explains ultra lean operation:

Ultra lean burn or stratified charge mode is used for light-load running conditions, at constant or reducing road speeds, where no acceleration is required. The fuel is not injected at the intake stroke but rather at the latter stages of the compression stroke. The combustion takes place in a cavity on the piston's surface which has a toroidal or an ovoidal shape, and is placed either in the center (for central injector), or displaced to one side of the piston that is closer to the injector. The cavity creates the swirl effect so that the small amount of air-fuel mixture is optimally placed near the spark plug. This stratified charge is surrounded mostly by air and residual gases, which keeps the fuel and the flame away from the cylinder walls. Decreased combustion temperature allows for lowest emissions and heat losses and increases air quantity by reducing dilation, which delivers additional power. This technique enables the use of ultra-lean mixtures that would be impossible with carburetors or conventional fuel injection.