8.9L 450HP versus 350HP Version

[RV Randy]

Just curious if anyone has compared what fuel mileage is on a rig equipped with the 450 HP 8.9 versus the 350 HP version, everything else being equal?

Also, what is the mechanical difference between the 450 versus 350 version of the 8.9? Bigger turbo, different injection or just ECM tune?

I know on a Cummins 6.7 you can go to the Cummins dealer and for $500 they will install a different SW tune in the ECM that gives you an additional 25 to 50 HP.

Is the same type of upgrade available for the 8.9? I never did the HP upgrade on my 6.7 because the Allison 2500 tranny was maxxed out the way it was. But the DX3's have the same Allison 3200 in both the 350 HP version and the 450 HP version so the tranny can handle a HP increase.

[LTZ470]

Note the cooling capacities, BIGGEST difference radiator sizes M2 106/ M2 112, that 8.9L cannot make that kind of HP without additional cooling...the M2 106 Frame is also designed for maximum 350 HP 1000 ft/lbs torque, the 450HP 1250 Ft/Lbs could actually twist/distort the M2 106 Frame...the TRV 3200 is maxed out at 1250 ft/lbs TQ...

Different Injectors and Tuning on 450 versus 350 as well...

[RV Randy]

Originally Posted by LTZ470

Note the cooling capacities, BIGGEST difference radiator sizes M2 106/ M2 112, that 8.9L cannot make that kind of HP without additional cooling...the M2 106 Frame is also designed for maximum 350 HP 1000 ft/lbs torque, the 450HP 1250 Ft/Lbs could actually twist/distort the M2 106 Frame...the TRV 3200 is maxed out at 1250 ft/lbs TQ...

Different Injectors and Tuning on 450 versus 350 as well...

All true, but notice my avatar. I am one of those people that can't resist tweaking a bit here and there to get a little more performance and fuel mileage without exceeding safe parameters. There is usually a big safety margin built into these things. True, diesels create power by creating heat, but if you don't push it and over fuel you should be well within the cooling capacity of the system. I mean really, who wouldn't want another 25-50 HP if it only costs a few bucks more and doesn't exceed safe design limits. That sort of increase isn't going to over stress the frame and I don't think Freightliner would offer the upgrade on a unit they have to warranty if it was going to break stuff.

[LTZ470]

Yes Sir, I thought about going that route as well, but wanted the cooling capacity for mountains and long pulls uphill...
I've done a bit of HP adding myself, it's addicting to say the least, even had a Propane setup ready for the M2 106 I just sold, but never added it....

[LTZ470]

One of the best analogies for hp vs tq: "Horsepower is how fast you hit the wall, Torque is how far you take the wall with you after you hit it..."

[LTZ470]

Seems they would at least allow the 8.9L 400hp 1250tq 8.9L in the M2 106, but the 1250tq must be the restricting factor is all I can figure...frame would probably be fine as long as one never got stuck or did actually pull maximum GVCW with it...it probably would never tweak the frame?
Once I get my 450hp XL I will take photos of the engine mounts and framing and then post them to compare...

[mookieblaylock]

This is, by far, the ‘issue’ I struggle with the most. We regularly tow at a combined rate around 29k pounds with a 6.7 which has a factory rating of 370 HP and 800 ft lbs. The engine has no trouble with that load and can easily maintain speed on steep grades. With very little time in an M2-106 truck, and zero time in one pulling a significant load, I have no frame of reference as to how the 350/1000 spec’d motor compares. The GCWR of these rigs far exceeds my towing requirements, so I tend to think the M2-106 would be fine. However, I can’t imagine adding another 15k pounds behind my similarly spec’d 6.7. I know it’s not an apples to apples comparison, but it’s all I have to go on.

What I tend to believe is that the rating on the ‘consumer’ motor in my truck is inflated for marketing reasons and that the rating on the ‘commercial’ motor is more accurate (or, at the very least, more representative of the power available all of the time vs at peak intervals, in certain gears, etc). I can’t find any concrete evidence to support that, though. I also understand, that the transmissions play a significant role in the comparison; how one quantifies that I’m not sure.

All along, I figured that when the time came for our family to take the plunge we’d order a DX3 on the M2-112 chassis. Brian says that option has been discontinued, though. We’re not interested in an XL, so now it’s back to trying to envision what it would be like driving a 350/1000 rig around the country at 45k pounds or so. I have no doubt that it could do it; I just wonder if it will be a dog in the hills and potentially a safety hazard losing speed on the really steep grades.

[FFred]

A modern really sweet well tuned diesel will create 20 HP from a gallon of fuel.

So the question is NOT "how fast can I climb that hill" but how often do I want to stop for fuel?

400HP / 20 = 20GPH , how big is your fuel tank?

[LTZ470]

Originally Posted by FFred

A modern really sweet well tuned diesel will create 20 HP from a gallon of fuel.

So the question is NOT "how fast can I climb that hill" but how often do I want to stop for fuel?

400HP / 20 = 20GPH , how big is your fuel tank?

This is exactly what folks need to understand about diesel engines, it takes a "certain" amount of fuel to pull a given amount of weight including wind push/resistance at a given mph...period...speed is the factor that we control and wind resistance through height/aerodynamics etc, wind factor is left up to weather...rolling resistance is of course another factor keeping tires at correct air pressures...

[Webefine]

We’re not interested in an XL, so now it’s back to trying to envision what it would be like driving a 350/1000 rig around the country at 45k pounds or so. I have no doubt that it could do it; I just wonder if it will be a dog in the hills and potentially a safety hazard losing speed on the really steep grades.[/QUOTE]

Eventually we plan on going full time in our DX3 and the trailer we are going to pull will be around 15,000 pounds. Looking at the Hp/torque to weight ratio I know steep hills and on ramps will be a tough. A tractor trailer weighs 80,000 and a average engine would probably be 450/500 hp which means I would still have a better hp to weight ratio just not as many gears to work with. I don't think performance at 45,000 pounds is going to be great but should be acceptable for a retired couple roaming around the country with no time schedule.

[FOURWHL]

It'll still get up the grades just don't expect to do 50+. Otherwise it'll be fine

Also want to say cooling has got to be a large part of the difference. On a hot summer day going up the steep grades of I70 my fan was cycling probably every 5-10 min. I was only 36k lbs combined. It was then I decided I wasn't going looking for any more horses on this setup. Much rather not having to watch temps like a hawk and worry if I need to back off. Same with over doing strapping on a load, peace of mind goes a long way on long drives.

[mikew968]

While a semi only has 450-500 hp it has 1400-1700 lbs ft of torque

[Webefine]

I agree, going a few mph faster up a hill isn't worth the extra engine heat and possible problems.

[LTZ470]

The Allison 3200 TRV will get extremely hot as well if pulled extremely hard, I have had mine at 260+ with just the 330hp 1000tq Mercedes 926....

[RV Randy]

Originally Posted by mookieblaylock

This is, by far, the ‘issue’ I struggle with the most. We regularly tow at a combined rate around 29k pounds with a 6.7 which has a factory rating of 370 HP and 800 ft lbs. The engine has no trouble with that load and can easily maintain speed on steep grades. With very little time in an M2-106 truck, and zero time in one pulling a significant load, I have no frame of reference as to how the 350/1000 spec’d motor compares. The GCWR of these rigs far exceeds my towing requirements, so I tend to think the M2-106 would be fine. However, I can’t imagine adding another 15k pounds behind my similarly spec’d 6.7. I know it’s not an apples to apples comparison, but it’s all I have to go on.

What I tend to believe is that the rating on the ‘consumer’ motor in my truck is inflated for marketing reasons and that the rating on the ‘commercial’ motor is more accurate (or, at the very least, more representative of the power available all of the time vs at peak intervals, in certain gears, etc). I can’t find any concrete evidence to support that, though. I also understand, that the transmissions play a significant role in the comparison; how one quantifies that I’m not sure.

All along, I figured that when the time came for our family to take the plunge we’d order a DX3 on the M2-112 chassis. Brian says that option has been discontinued, though. We’re not interested in an XL, so now it’s back to trying to envision what it would be like driving a 350/1000 rig around the country at 45k pounds or so. I have no doubt that it could do it; I just wonder if it will be a dog in the hills and potentially a safety hazard losing speed on the really steep grades.

My 2013 class A was a 6.7 rated at 300/600 and I pull a 27' race car trailer that is 8,000 lbs when fully loaded. The 6.7 did "ok" even when pulling over the mountain passes on I-70, never got hot, etc. Pulling performance wasn't spectacular, but it got around OK.

I have to think the 8.9, while only slightly higher on HP, has a huge increase in torque so towing should be better than it was on the 6.7.

My bigger issue was the brakes on that rig sucked. They were drums front and rear, I had a brake controller and brakes on both axles of the trailer. The 6.7 had an exhaust brake (thank God). It was barely adequate for stopping as long as you went really slow and started stopping a looong time ahead of where you needed to stop. If it didn't have the exhaust brake, it would have been too dangerous to tow with. I had to do a panic stop one time while driving on the interstate, nice straight and flat section, while doing 68 mph. A deer ran across in front of me, I stood on the brakes with both feet and sailed right on past the deer, who fortunately kept running. I finally came to a stop several hundred feet later in a cloud of stinking brake smoke. If it had been a car or something I couldn't swerve to miss, I would have ran right over the top of it. So a big factor for me for the new rig is the disc brakes, ABS, full jake brake and factory brake controller.

[RV Randy]

Back to the original question: Any comparison of fuel MPG between the 350 and the 450? Just curious how much difference it makes to have the bigger motor.

[CSWVK]

Read DX3 fuel balancing post for more mileage info

[Webefine]

Originally Posted by mikew968

While a semi only has 450-500 hp it has 1400-1700 lbs ft of torque

I agee but my torque to weight ratio will still be at least equal. Horsepower is what pulls hills and my horsepower to weight ratio will be better than the average semi.

[LTZ470]

Originally Posted by RV Randy

Back to the original question: Any comparison of fuel MPG between the 350 and the 450? Just curious how much difference it makes to have the bigger motor.

Same weights, same road, same weather (wind), same temperature, same aerodynamics, same gear ratios with same tires and same air pressure in tires...at same speeds side x side...

= same fuel mileage

where the 450 is going to use more fuel is getting up to speed faster and pulling hills at a faster speed otherwise if they are both driven same speed they will use same amount of fuel...

fuel mpg = weight x aerodynamics x speed

[Webefine]

Originally Posted by RV Randy

Back to the original question: Any comparison of fuel MPG between the 350 and the 450? Just curious how much difference it makes to have the bigger motor.

I don't have actual numbers for you, but doing research online it seems that if a larger motor is pulling the same weight as a smaller it doesn't necessarily get any worse fuel mileage because it isn't working as hard.