So I've been reviewing the ISV 5.0 and wondering how much headroom they have built into this motor.
After a bit of research, I'm guessing not much. While I'm no mechanical engineer or diesel expert, I've been looking at all the info flowing around on the ISV.
You'll find quite a bit of the thought behind the M2("M Squared") Turbocharger here:
As a reference this is how the M2 is laid out:
For Source, see Note 1
Light Green=Rotatory Turbine Control Housing and Low Pressure Turbine, Exhaust Side
Blue=High Pressure Turbine, Compressor Side
Yellow=Low Pressure Turbine, Compressor Side
Dark Green=Rotatory Turbine Control Valve Controller and Valve
Brown=High Pressure Compressor Bypass
Grey=High Pressure Turbine, Exhaust side, Outlet Piping
Firstly, I do believe they have done a very good job optimizing this engine.
The real challenge is meeting stringent emissions requirements while meeting goals for durability, power delivery, packaging and fuel economy.
If you have looked at the M2 ("M Squared") compound turbocharger, you will find quite an innovative bit of engineering. It's not a twin turbo, but two turbos in one assembly, working at times independently and in concert to produce boost, reduce lag, facilitate EGR functionality, DEF regeneration and perform waste gate functions. They are basically doing with two turbos what a single variable geometry turbo can do. They claim packaging and efficiency advantages. The interesting part is the rotary turbine control valve. It is essentially a diverter valve that controls where the exhaust stream goes. It's able to split the flow between the high pressure (smaller) and low pressure (larger) turbos. Additionally, it can restrict exhaust to facilitate warm-up, EGR and regen functions and it can act as a waste gate. I also believe it could be programmed to act as an exhaust brake, but for some reason, Nissan and Cummins chose not to do so.
So looking at what is available, I have done my best to recreate the HP and Torque curves, just to see what is really going on here. Cummins gave us a look at the torque delivery several months ago. From that we can extract HP.
While not exact, it's quite close. What is shows is a very flat torque curve across a broad range staying in the 550lbft range from 1600 to 2800 RPMs. Peak HP is at 3200 RPMs but is above 290hp from 2800 to around 3500 RPM.
So while diesel folks make a big deal about torque, the ability of the engine to actually do work is measured in HP. What makes a diesel so special is its ability to make lots of torque quickly, thus producing meaningful hp before it hits peak hp.
In this engine, what is remarkable is the range across which it maintains torque, equating to steadily rising hp from peak torque to peak HP.
So why is that interesting? For towing, it means the engine remains capable of delivering near peak power early and across a broad band. This reduces shifting and allows for a more relaxing experience at peak power levels. In fact, the trend is towards larger number of gears, to keep power delivery consistent and flat, eliminating lagging and peak areas of power delivery for a given engine. Thatís why you generally see the HD diesel pickups lagging in gear count over gasoline engines. Today's light duty diesels just donít see as much benefit for the additional ratios as gasoline engines do. Even Ford EcoBoost engines, which are a model of gasoline power delivery, are getting more ratios. Again, this is about deeper overdrives for unloaded cruising, while allowing the engine to remain in an optimized power band during high load situations. GM's 6.2 saw significant improvements in loaded performance from the addition of an 8 speed. But there is a cost, and that is in the transmission's propensity to hunt gears as loads change. With great transmission tuning, it can work, but if things are less than optimal, there can definitely be a franticness to the transmission's performance.
That said, having seen the videos of the XD at highway speed, I'm a bit surprised that there isn't a higher overdrive or lower rear axle ratio available. Obviously Nissan is trying to allow the XD to tow in top gear. They could add a 7th gear and probably drop RPMs at 70 from nearly 2000 to around 1600 and still produce adequate power (about 160hp) for light load driving. There would be a jump in economy, but it would be at the cost of either gear hunting, or locking out the final OD when towing. My guess is that we will eventually see seven speeds in the diesel XD when Nissan finds the right transmission.
Now back to power. How much is left is the ISV? My guess is that Nissan and Cummins left some headroom. If you look at the M2 modes of operation, you'll see that they start modulating the waste gate at high engine speed and high torque operation. This suggests they are dumping exhaust to either limit peak boost or LP turbine rpm. Either way there appears to be some more boost to be had at higher RPMs (Green area of the graphic below). It could be at the cost of durability, or Nissan/Cummins could have chosen to keep some boost in reserve for high altitude operation. If that can be tapped, you could potentially see a bit more torque and power from 3000 to 3400 rpm.
For Source, see Note 1
And here's how the modes work:
For Source, see Note 1
But if you are thinking that there will be a 400hp version or even a 400hp tune, I think you are wrong.
Overfueling this engine will not be an option due to the emission controls. So fueling and boost will have to go hand in hand. The only real benefits will be had at or near peak HP or around 3200rpm. I donít think that they will try to extract more rpms from this engine given its lofty 4000 rpm redline. As is, torque drops off dramatically after 3600 rpm and the extra revs donít accomplish anything as output above 3800 rpm drops below 290hp as is.
If they can extract 10% more torque from the engine at 3200 rpm, by reducing and delaying wastgate operation, then you would see peak torque move up in the powerband, and HP increase. But I donít know if the engine or other components would be up to the additional boost or if the LP turbo is capable of it. If this is possible, I can't see hp rising much above 340hp or so at 3200 rpm.
The point is that increasing torque significantly is not really going to be possible, particularly in the lower power band as the HP turbine is doing all it can. You may be able to get more out of the LP turbine which would significantly increase torque in the peak HP range, but peak torque would only increase modestly.
To get larger gains, you would have to start modifying the M2 turbo. The real issue is the LP turbo packaging. It's not easily replaceable as some of the larger diesel turbos are. Maybe modifications in materials and impeller design can yield some additional performance, but more boost will require larger turbines, which will require a new and unique housing. I don't see the aftermarket clamoring to do that given the limited production. Maybe Cummins will tweak the design down the road.
So for tuning, there may be some wiggle room in delaying waste gate operation and adding fuel with the additional boost, the gains won't be as dramatic as diesel tuners usually claim. And given the delicate balance of turbo, rotary turbine valve and emissions operations, there may not be anyone lining up to try it.
The only aftermarket engine part so far is the Banks intake. I can't see that really doing much for power as the intake is not restrictive to begin with. The more critical airflow areas are contained in the mess of tubes that is the M2 turbo.
What I'm really hoping for is the addition of the exhaust brake programming, continued refinement of the M2 turbo and, potentially down the road a 5.5 to 6 liter variant of this engine. NV2500 anyone?
Note 1: Source: "Development of Cummins Turbo Technologies Integrated M2 Two-Stage Architecture using Rotary Turbine Control (Holset RTC) Technology for the Cummins 5.0L V8 Turbo-Diesel Engine"
D. Brookshirea, N. Manoharana, J. Lovea, N. Khannaa,
and Cummins Turbo Technologies