MattyMacro

We decided to throw the car on the rollers today to get a good baseline number. I am up in the air on a tune at the moment (Not too comfortable voiding the factory warranty on a brand new car) and we wanted to see where everything stood, power and torque wise, prior to doing anything. Now, generally we loose a good amount of power up here; moreso on all motor applications, however, forced induction takes a hit as well, just not as substantial.

Today, in Denver, we had a density altitude of 6338 ft. I was expecting MAYBE 470hp, 500tq SAE corrected (assuming a 15% correction), around 400hp 425tq uncorrected. Needless to say, we were very shocked. The dyno gave us a correction factor of 1.22 and these were the numbers below. Don't pay too much attention to the SAE corrected, as those get skewed on forced induction setups up here.


Uncorrected numbers with the 6338ft. density altitude below:

http://imgur.com/7xbCc7q

Sanch

Wow that looks pretty good

shoe3k

I wonder if the ECU was adjusting for the altitude and increasing boost targets.

Gungaslow

The ECU was adjusting the boost targets because of the altitude..the waste gates were staying closed longer to build more boost in the turbos..also could be the dyno is just a high reading dyno

RDO247

I had my car dyno'd last week which resulted in 329 kw/atw or 440 hp/atw.

This is was done on a Mainline Dyno on the Gold Coast.

MattyMacro

I also had 95 octane in the tank. I know certain ECUs/DMEs will adapt timing and fuel tables depending on detonation/knock and octane. However, I have read that these DO NOT have adaptive tables in the ECU. Is there any information pertaining to this, or the ecu compensating boost targets as well?

Also, just for a nugget of info for everyone, I have heard people ask and state there is a power difference between sport + and race. If you look at the graphs on the video, you'll notice the deltas on both runs are identical; one pull was done in sport +, the other in race. Zero power difference between both settings, and the curves are identical as well. I'm guessing race mode just give us a little more slip in the traction logic.

msd3075

It makes perfect sense that you are making less power at high altitude but not as much of a drop in power as you'd expect compared to a N/A car.

The turbos are technically making additional boost compared to if run at sea level, but the pressure of the intake air should be very similar to what is entering the same engine at sea level (ie. the absolute pressure of the air going into the combustion chamber should be very similar regardless of altitude). The reason you see a drop in air at higher altitude is because the air contains less air molecules per unit volume. This also means that the atmospheric air pressure at higher altitudes is lower than the equivalent pressure at sea level.

There are 3 different pressures you have to understand to see how all this fits together. Atmospheric pressure is the pressure of the outside air (the thing the weatherman is always talking about on the weather report) and is dictated by altitude, weather changes, humidity, and so forth. Gauge pressure is the measured increase in pressure above atmospheric (the 35 psi you measure in your tire is considered gauge pressure since you are measuring 0 psi when the tire is flat). Absolute pressure is the sum of these two and is a measure of the true, actual pressure present. The absolute pressure inside your tire is the 35 psi plus whatever the atmospheric pressure is. What you are measuring with your gauge really only is the difference between the outside and inside of the tire. You have atmospheric on both the inside and outside so that part cancels out.

Standard atmospheric pressure at sea level is 14.7 psi. A quick google search of altitude variations on standard pressure (http://www.engineeringtoolbox.com/ai...ure-d_462.html) shows that at 6,000 ft the same atmospheric pressure would be 10.8 psi.

Now add in the turbos. Say at sea level your turbo setup adds an extra 10 psi of boost (I don't know the actual numbers for the C63S off the top of my head). That means at sea level, the absolute pressure of the air entering the engine would be 24.7 psi (14.7 psi atmospheric + 10 psi gauge).

Take that same car to 6,000 ft altitude. The turbos (theoretically) have the ability to supply extra boost (ie extra gauge pressure) to the intake air so that the absolute pressure of the air is as close to the intake 24.7 psi pressure seen at sea level. Instead of supplying 10 psi of boost at sea level, the turbos will (theoretically) supply 13.9 psi of boost (10.8 + 13.9 = 24.7) so that the engine itself doesn't see a difference between sea level and higher elevation.

Ok, so why does the car not make the same power as at sea level? Why is there still a loss in power? This could be because of several different reason.

First of all, we are assuming that the turbo is able to just as efficiently supply that extra amount of boost as it is able to supple the normal amount of boost as sea level. That's a big assumption and depends on the specific setup. I don't know enough about the C63S' internals to know if this is the case or not.

Second, the lower octane levels that are typically sold at higher elevations could have an effect on power output. Octane level is a measure of how resistant a given fuel is to self-detonation. In other words, it's basically a measure of how high of a pressure/temperature you can bring the fuel to before it combusts on it's own. You can get away with running lower octane at higher elevations in a N/A engine because since you are starting out at a lower atmospheric pressure on intake, your pressure at combustion is lower than it would be at sea level. Add in a turbo setup that is trying to replicate sea level conditions, and you'll need the higher octane fuel to get the sea level power levels.

It could also be that the car is not properly tuned from the factory to efficiently handle all of these additional variables that happen at 6,000 ft.