Isuzu NRR / Acela Straya High-mobility 4×4 Cab-over Truck

[Andy Hino]

At 3500rpm your 6l was producing somewhere around 250-275 hp meaning it was taking around that to move the truck, trailer. It should be clear that an engine with 215 peak hp would not be traveling at 55mph in the same conditions.

Thats true, but not exactly correct.
The power curve is derived from a full power run, so unless you're sitting at 3500rpm with your foot to the floor, and the engine speed isn't increasing, then you aren't using anywhere near that kind of power.
Up hill maintaining speed, quite possibly, but cruising along, no.

Exactly how much power you would be using is hard to quantify unless you know all the variables (rolling resistance, drag co-efficient, air pressure, etc), but I highly doubt it would be more than 25-30% of that.

 

[Tim K]

Thats true, but not exactly correct.
The power curve is derived from a full power run, so unless you're sitting at 3500rpm with your foot to the floor, and the engine speed isn't increasing, then you aren't using anywhere near that kind of power.
Up hill maintaining speed, quite possibly, but cruising along, no.

Exactly how much power you would be using is hard to quantify unless you know all the variables (rolling resistance, drag co-efficient, air pressure, etc), but I highly doubt it would be more than 25-30% of that.

These are all very rough estimates but provide an exceptional base line as to why a 215hp engine is no considered adequate to push this truck down the highway in all circumstances.

You are correct that we have no idea what the throttle position was at 3500 rpm (note that I used 3500rpm, the low end of the range, and not 4500rpm). At 55 mph 3500 rpm would mean the transmission is not in its highest gear, which it is always looking to be in if load allows it to be, so we know load is significant. Please also notice the estimated range I proposed was 250-275, so not necessarily full power at 3500rpm. Full throttle, probably not, but you bring up a common assumption that is not correct.

The diameter of the throttle body is designed to provide the air required at peak hp (360), with some head room. "Foot to the floor" wide open throttle is not required to generate full power at 3500rpm. You do not need to be anywhere near full throttle to generate 275 hp with a throttle body designed to produce 360hp (and capable of a lot more than that).

For a rough idea of what it takes to push a vehicle down the road. Mustang dyno uses "power at 50" as one of their parameters. I sold my dyno 18 years ago and dont have the list of vehicles they had figures for but it was quite extensive and very interesting. A 2003 C5 corvette, on dead flat asphalt, dead calm 75deg air, requires 12.5hp to maintain steady state speed of 50mph. Not that much. Now consider the myriad of differences between a C5 traveling at 50mph and a truck that is rolling on 40 tires, over 8' tall, and ~ 7'wide traveling at speeds well over 50mph which requires an exponential increase in energy input, weighs several times more, traveling in real world conditions including up hills, headwinds, subzero air, rain etc.

 

[charlieaarons]

FWIW it takes about 102.5kw ~ 137hp to push my 26,000 lb U500 camper with all its frontal area down the road at 57-58mph. The engine puts out 260hp at 2200, rpm at 57 is 1860, hp at 1860 is ~235 on Mercedes’ published graph for my particular engine (OM906LA, model 902.912).
Calculation based on published bsfc of 210gm/kwh for that particular engine at 1860rpm.
According to an easily findable graph on the 4HK-1TC NRR engine, it puts out 140kw=188hp at 2000rpm = 58.5mph with the Straya chassis.
You can be sure that any camper equipped Straya will need somewhat less power to travel at comparable speed due to weight and cross section. Both factory Aisin and Allison have lockup torque converter in higher gears.
Please continue this fascinating discussion.

 

[Tim K]

FWIW it takes about 102.5kw ~ 137hp to push my 26,000 lb U500 camper with all its frontal area down the road at 57-58mph. The engine puts out 260hp at 2200, rpm at 57 is 1860, hp at 1860 is ~235 on Mercedes’ published graph for my particular engine (OM906LA, model 902.912).

Thanks for the post! Good info.

Can I assume that similar to the power at 50 spec for the C5, this is on flat ground in still air at a specified temperature?

According to an easily findable graph on the 4HK-1TC NRR engine, it puts out 140kw=188hp at 2000rpm = 58.5mph with the Straya chassis.

Meaning that compared to the required 137 at a similar speed for your u500, there would be 50hp left over to deal with hills, wind, rain etc before it started to have insufficient power at that RPM and require either a loss in speed or a down shift to move to a different area in the power curve where hopefully it would find another 27hp at its peak of 215.

You can be sure that any camper equipped Straya will need somewhat less power to travel at comparable speed due to weight and cross section. Both factory Aisin and Allison have lockup torque converter in higher gears.

I know very little about unimogs but just a quick search of specs put dimensions very close. The curb weight and gvw are both about 2200lbs (~20% on the curb weight but <10% on GVW) more for the U500. This makes a difference going up hills and accelerating but not a big difference traveling on the flat and zero difference into the wind.

Just to repeat myself, I am not saying and never have that you can not get by with 215hp in this truck. You could get by with less. 215hp in this truck will just leave you wanting more on a regular basis.

 

[charlieaarons]

My GVW is 33000 lb and actual weight (measured recently on scale) is ~27100 lb with 105 gal fuel and water tanks full, with a totally excessive amount of recovery gear, tools, spare parts, 2 winches, 2nd unmounted spare, etc. Height of camper is 3.58m (11’10”) on 395/85R20s, with static loaded radius ~71mm (2.8”) greater. Width of camper 2360mm, cab 2300 not counting mirrors, cab height 2945.
The NRR is certainly narrower, by about 10”, not counting the wide track with new axles.

 

[Tim K]

My GVW is 33000 lb and actual weight (measured recently on scale) is ~27100 lb with 105 gal fuel and water tanks full, with a totally excessive amount of recovery gear, tools, spare parts, 2 winches, 2nd unmounted spare, etc. Height of camper is 3.58m (11’10”) on 395/85R20s, with static loaded radius ~71mm (2.8”) greater. Width of camper 2360mm, cab 2300 not counting mirrors, cab height 2945.
The NRR is certainly narrower, by about 10”, not counting the wide track with new axles.

Is this the same truck: U500? It shows the cab to be a lot narrower than 2300 which is the width of the flairs.

 

[charlieaarons]

Totally different. Mine’s a quite modern North America (faster gearing) UGN. The modern descendants of the one in your picture is UGE/UHE. Either U4000 or 5000. U1550 meant ~155hp (usually); the heavier duty versions went up to U2450/L38 (3.8m wheelbase, higher GVW, more hp). But some U1550s came with up to 240hp, and they made a LWB version. Iain’s U1250 is about 8-10”narrower than mine which is why it was originally used for road/rail on NSW railways. I’ll leave the connection between standard English railway gauge and the width of a horse’s rear end and the Roman Empire (Caesar, Claudius) for another time…unless someone is interested.

 

[charlieaarons]

Width on NRR is 81”, 10% less, so air resistance also less. Presumably one would not fit a 2360mm = 93” wide box, but maybe 84-87” wide. So I’d guess power requirement at 57-58mph would be maybe 94kw instead of 102.5?
Again, 65-68 is your speed limit due to MPT tires. Air resistance ~ square of speed (in this regime of flow velocity, air viscosity, etc), so at 68 that’s 132kw=177hp at 2300 rpm. I doubt hp output at 2300 is very much less than 215 @ 2500. And the other determinants of power dissipation (gear loss, rolling resistance) are more linear (1st power) than air resistance.
Please continue discussion, responses are interesting.

 

[tkj23qe1387]

Width on NRR is 81”, 10% less, so air resistance also less. Presumably one would not fit a 2360mm = 93” wide box, but maybe 84-87” wide. So I’d guess power requirement at 57-58mph would be maybe 94kw instead of 102.5?
Again, 65-68 is your speed limit due to MPT tires. Air resistance ~ square of speed (in this regime of flow velocity, air viscosity, etc), so at 68 that’s 132kw=177hp at 2300 rpm. I doubt hp output at 2300 is very much less than 215 @ 2500. And the other determinants of power dissipation (gear loss, rolling resistance) are more linear (1st power) than air resistance.
Please continue discussion, responses are interesting.

Great analysis on the order of magnitude estimates! Ignoring tire limits, I think the top speed would probably be no more than 90 mph. At that speed, air resistance would be at least three times more than ~55 mph, so the tranny likely must shift to fourth or even third gear. For the Straya, fourth gear at 5000 rpm is 90 mph (60*5000/6.5/1/512), yet it offers merely 1.5 times more torque than sixth gear. Maybe the additional torque is sufficient for the extra resistance, maybe not. 90 seems like a sensible estimate IMO.

 

[Tim K]

Width on NRR is 81”, 10% less, so air resistance also less. Presumably one would not fit a 2360mm = 93” wide box, but maybe 84-87” wide. So I’d guess power requirement at 57-58mph would be maybe 94kw instead of 102.5?
Again, 65-68 is your speed limit due to MPT tires. Air resistance ~ square of speed (in this regime of flow velocity, air viscosity, etc), so at 68 that’s 132kw=177hp at 2300 rpm. I doubt hp output at 2300 is very much less than 215 @ 2500. And the other determinants of power dissipation (gear loss, rolling resistance) are more linear (1st power) than air resistance.
Please continue discussion, responses are interesting.

Im sorry if I missed it but were the calculations based from Mercedes based on flat calm conditions?

Hard to guess on the hp at 2300, it should be pretty close though. Based on the177 @68mph which seems reasonable, squaring up from58 to 68 mph, and assuming these are based on flat ground still air, that would leave just under 40 hp for hills/wind/rain.

 

[yabanja]

Anyone know if the 41" tires they are using are limited to 55 mph like older military rubber?

 

[tkj23qe1387]

Anyone know if the 41" tires they are using are limited to 55 mph like older military rubber?

Continental MPT81- 335/80R20 (41 in.), 68 MPH-rated super singles (6,779 lb. @ 95 PSI)

 

[Tex68w]

I am intrigued by this offering, but like most I am surprised by the lack of a diesel option. Seems like a fascinating option for those who were previously exploring a Fuso build.

 

[adam88]

I was about to pull the trigger on a CORE chassis last year, but backed down. I love the concept of the cab-over as it allows for the maximum habitat length with minimal vehicle length, but ultimately you are paying $150K for a Frankenstein work truck. There will be engineering integration issues, there will be warranty disputes, and there will be repair issues. If you are willing to add another 4 feet to your vehicle, you can get a highly capable domestic heavy duty pickup chassis with tons of options, creature comforts, OEM integration, and warranty support for half the cost. They are not nearly as cool, however.

Very well said. agreed.

 

[CORE_vehicles]

Some comparisons:

View attachment 710420

It is a bit of an apples to oranges comparison. The CORE is the same chassis design as used in the EarthCruiser EXP/FX platform. It is very well suited to a composite habitat of around 14 feet. CORE sells a spring mounted subframe specifically designed for a 14' Total Composites box. The transfer case, axles, and wheels are pretty standard 4x4 stuff with widely available parts and could be serviced by most mechanics. The Acela Straya is a Class 5 truck with much higher payload capacity, more wheelbase options, and a military axle/wheel configuration with CTIS.

A full buildout out on either of these trucks is going to put you in a $250K+ vehicle. I think you really have to ask yourself how much technical terrain you really want to tackle in a $250K truck, and will the truck even fit where you want to go? Also, how much highway driving will you need to do to get to your desired destination? I suspect spending days on the highway in an underpowered cabover truck with 41" mud terrains is not a pleasant experience. The awesomeness of CTIS and the off-road capability in the Acela is undeniable, however.

I was about to pull the trigger on a CORE chassis last year, but backed down. I love the concept of the cab-over as it allows for the maximum habitat length with minimal vehicle length, but ultimately you are paying $150K for a Frankenstein work truck. There will be engineering integration issues, there will be warranty disputes, and there will be repair issues. If you are willing to add another 4 feet to your vehicle, you can get a highly capable domestic heavy duty pickup chassis with tons of options, creature comforts, OEM integration, and warranty support for half the cost. They are not nearly as cool, however.

Thank you for your interest in our CORE chassis. It is a beast for sure. Both Companies are building awesome cab-overs. Each is uniquely different and capable. Check out this video of the Core Chassis in action.