Maximum Horsepower ??

[Venturer]

On our recent trip to ID and back to WI, I made note of the different percent of power vs turbo boost. At lower altitudes, the Alladin would indicate 60-70% power @ 32 psi boost and 100% @ 40-42 psi. At 7000' elevation, it would show 100% power @ 30 psi boost which was the maximum boost attained. I'm trying to understand what these numbers mean as to actual HP produced at different altitudes. Does it mean that the engine will only produce 60-70% of its maximum @7000' altitude? I had thought the variable turbo would account for the thinner air by producing more volume thus maintaining close to maximum HP.  The overall performance of the ISM is great. My inquisitive mind just wants to know why.      

[CAT Stephen]

@Venturer,

 

Rated advertised maximum HP is measured by the engine manufacturers at the crankshaft at a specified RPM at sea level atmospheric pressure and at a specified temperature.

Your actual horsepower measured at your drive tires can be measured on a dyno and will be lower due to drivetrain losses.

As elevation increases, your maximum HP generally starts decreasing because maximum HP is almost always at maximum boost.  Turbocharger & supercharger boost is the gage pressure that is above current absolute atmospheric pressure.  Engines generate less HP as the total absolute pressure (i.e. Atmospheric Pressure + Turbocharger/Supercharger gage pressure) decreases. 

[Venturer]

I understand what you are saying and agree, however, I always thought the variable turbo on the ISM would compensate for the thinner air by increasing speed to keep the boost similar to what it would be at sea level. Isn't that the reason for a variable turbo?? The Alladin system indicates 100% power at 1000' with 42 psi boost and 100% at 7000' with 30 psi boost.

[timaz996]

The more the turbo has to work, (higher alt) the hotter the induction air will be. The less HP you will make. The density of the air is what makes power, not just volume. I would (guess) the ECU on the engine is derating because of high temp induction air. 

[Benjamin]

The variable turbo can only vary within the designed range, and that fits most conditions well. A "larger" turbo would not be ideal for most conditions, so design is always a compromise even with a variable turbo.  I'd say that yes, the engine is really producing only 60-70% of max hp at that altitude, because the turbo is operating at "max" and the fuel is "max" allowed for that air volume and temp.  Here's some more reading if you wanted to pursue further:     https://www.scannerdanner.com/media/kunena/attachments/13193/AdvancedEnginePerformanceTroubleshooting.pdf