Wanted to post this in the dyno section also.




http://blogs.cobbtuning.com/?p=212

not too shabby.

keep posting as you guys try to crack the 'uncrackable" ECU coding.

Damn nice power figures. ;)

I thought boosted cars create their own atmosphere and are therefore immune to altitude since boost is being regulated.

That is a true statement IFF the turbos have enough capacity to compensate and the ECU will try to compensate. Here is a small mathematical example to illustrate.

-Atmospheric pressure at sea level = 14.7 psi
-Atmospheric pressure in Colorado Springs CO (11.4 psi - you lose approximately 1.1psi for every 2000ft you go up from my observations in CO)

It seems that at sea level the GTR is running around 11.5psi. So the PR (pressure ratio) the turbo must work at at sea level is the following:
PR = (turbo pressure generated + atmospheric pressure) / atmospheric pressure

sea level PR = (11.5 + 14.7) / 14.7 = 1.78

Colorado Springs PR = (11.5 + 11.4) / 11.4 = 2.00

So at sea level the turbos have to run at a PR of 1.8 and at 6000ft a PR of 2.0.

Looking at this compressor map you can see how the PR comes into play on turbo efficiency (this is a compressor map of a mitsu 16G)
Click to view attachment

On the GTR I would guess in the PR range of 1.8-2.0 each turbo would be flowing around 375cfm (to provide 500hp crank). So if the GTR was running 16g turbos they would be spinning at about 110,000 rpms and be in the 75% efficient range which is pretty good.

Now so far the above example has only taken into account how much harder the turbos must work to NOT maintain the same atmospheric pressure. The above example on encompasses what happens with the turbos if they make the same amount of boost at all elevations (Subarus actually make less boost the higher you go up [oppositte of what you want] since the turbos are sized to be nearly maxed out at sea level].


The following example shows how much harder the turbos would have to work if you wanted them to create the same atmosperhic pressure for the engine (same absolute manifold pressure).

sea level PR = (11.5 + 14.7) / 14.7 = 1.78 (total manifold pressure of 14.7 + 11.5 = 26.2 psi)

To get the same absolute/total manifold pressure the turbos have to produce an extra 3.3 psi to compensate for atmospheric pressure losses so:
Colorado Springs PR = (14.8 + 11.4) / 11.4 = 2.3

So now the turbo is running at a PR of 2.3 which means it is now spinning at 130,000 rpm and its efficiency, with this map, is still around 75%. Typically this is not the case for factory turbo cars. Usually your efficiency would drop by 5-10% with that kind of jump in PR. It's just that 16G turbos with this low boost fair pretty well in this example.


Keep in mind the above example doesn't take into account the extra EGBP (exhaust gas back pressure) that the hotside/turbine side of the turbo is creating from spinning faster, the later onset of boost due to the turbos having to spin up to a higher rpm, and the efficiency drop for the intercoolers since there is less cooling medium for them to use.

From seeing how much boost Cobb is seeing on their dyno run (roughly 13.0-13.6) it seems encouraging, if stock boost levels really are around 11.5, that the Nissan engineers have put in some headroom so the car can compensate for altitude up to a point. It also seems like they might have since typically with a turbo car when you start maxing out turbos you usually end up with a torque hump/spike that only lasts in the midrange and drops down towards redline and you end up with a higher torque peak than hp peak on a dyno graph. Where as the GTR seems to be oppositte of this with 430ft lb and 480hp = lot of head room in the turbos for altitude compensation or more power from tuning at sea level ;).

One more fun example to show how altitude affects turbos. Pikes Peak (14,500 ft asl) vs. sea level

Pikes peak atmospheric pressure ~ 14.7 - 7.7psi = 7.0 psi atmpospheric at the top (think Pikes Peak Hill Climb race...)

lets shoot for our target absolute manifold pressure of 26.2psi. So that means the turbo has to produce about 19.2 psi to make up the difference.

PR at top of Pikes Peak = (19.2 + 7.0) / 7.0 = 3.75

Looking at the same compressor chart that value puts the turbo WAY 'off the map' on the vertical axis. And probably at an efficiency well below even 50% assuming the turbo won't die from overspin.

Now for fun lets take the PR (how hard the turbo is spinning/working) that the turbo is running at up at Pikes Peak and see how much boost that would give us if we ran the turbo as hard at sea level.

(boost + 14.7) / 14.7 = 3.75

Solving for boost gives boost = 40 PSI!!

So you can see that at the top of Pikes Peak the turbo has to work almost twice as hard to make 1/2 the relative boost!! Lose lose situation overall the higher you go up in elevation.

im pretty sure the GT-R uses a MAF sensor and not a MAP sensor thus adjusting for altitude or temperature shouldn't be a problem.