Dyno Testing of GT-R

2 messages, Last post on Mar 29, 2008 at 7:25 AM

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#1 of 2 Dyno Testing of GT-R by jkavanagh Mar 29, 2008 (7:25 am)
I've received some feedback on the dyno testing we performed on the GT-R a few weeks back. The article is located here: http://www.edmunds.com/insideline/do/Features/articleId=125172 Specifically, there were questions regarding why the data was left uncorrected for weather or altitude. The reason is actually quite simple--modern turbocharged engines like that of the GT-R inherently compensate for variations in ambient pressure. Here is why. Let's say we dyno test a normally aspirated car like a Corvette at sea level. Then we dyno the same Corvette in Denver, where the ambient pressure is roughly 20% lower. Measured power will be roughly 20%, too, because the normally aspirated engine is at the mercy of the elements (literally). It has no means to compensate for the loss in ambient pressure, and its power at altitude drops accordingly. So to get a sea level-equivalent result on a Denver dyno, we "correct" its Denver performance by multiplying by 1.2. Now we do the same tests on a turbocharged engine. The trick with the turbo engine is that it will simply increase the boost pressure to compensate for the reduced ambient pressure. Modern turbo engines have controls that allow them to run the same absolute pressure in the intake manifold, irrespective of the ambient pressure. Taking this one step further, the GT-R, like many other turbo cars, actually runs whatever boost is required to achieve a predetermined amount of air mass flow. As a result, the actual, uncorrected power of the GT-R on the Denver dyno will be much closer to its sea-level result than our Corvette. For this reason, it is actually incorrect to correct this turbocharged engine for ambient conditions. Doing so will artificially inflate the result measured in Denver. Furthermore, the dyno's correction usually states "corrected to SAE J1349 standards" or similar verbiage. Here's the catch, though: SAE J1349 states in Section 5.5: "... boosted engines with absolute pressure controls shall not be corrected for ambient barometric pressure." Engineering Editor Jason Kavanagh

#1 of 2: Dyno Testing of GT-R by jkavanagh; Mar 29, 2008 (7:25 am)

I've received some feedback on the dyno testing we performed on the GT-R a few weeks back.

The article is located here:
http://www.edmunds.com/insideline/do/Features/articleId=125172

Specifically, there were questions regarding why the data was left uncorrected for weather or altitude. The reason is actually quite simple--modern turbocharged engines like that of the GT-R inherently compensate for variations in ambient pressure.

Here is why. Let's say we dyno test a normally aspirated car like a Corvette at sea level. Then we dyno the same Corvette in Denver, where the ambient pressure is roughly 20% lower. Measured power will be roughly 20%, too, because the normally aspirated engine is at the mercy of the elements (literally). It has no means to compensate for the loss in ambient pressure, and its power at altitude drops accordingly. So to get a sea level-equivalent result on a Denver dyno, we "correct" its Denver performance by multiplying by 1.2.

Now we do the same tests on a turbocharged engine. The trick with the turbo engine is that it will simply increase the boost pressure to compensate for the reduced ambient pressure. Modern turbo engines have controls that allow them to run the same absolute pressure in the intake manifold, irrespective of the ambient pressure.

Taking this one step further, the GT-R, like many other turbo cars, actually runs whatever boost is required to achieve a predetermined amount of air mass flow. As a result, the actual, uncorrected power of the GT-R on the Denver dyno will be much closer to its sea-level result than our Corvette.

For this reason, it is actually incorrect to correct this turbocharged engine for ambient conditions. Doing so will artificially inflate the result measured in Denver.

Furthermore, the dyno's correction usually states "corrected to SAE J1349 standards" or similar verbiage. Here's the catch, though: SAE J1349 states in Section 5.5: "... boosted engines with absolute pressure controls shall not be corrected for ambient barometric pressure."

Engineering Editor Jason Kavanagh