Background:
Looking over some recent data logs using the stock GTI intercooler brought up some interesting results pertaining to airbox intake conditions
On a few occasions I’ve come across statements related to open and closed air intakes such as:
- Intake Airflow > Intake Temps
- Turbochargers compress the intake air to very high temperatures so it doesn’t matter much how warm the air is going into the turbo.
- The purpose of the intercooler is to cool the intake air “so it doesn’t matter much how warm the air is going into the turbo.”
- My IATs are good.
In the case of the data logs I was looking over I was giving attention to how the pressure drop across the intercooler affects the system efficiency.
Data Analysis:
As expected the stock GTI intercooler outputs charge air with a rising temperature as a pull continues. Of more interest was the pressure drop measured with my IS38+ setup, almost 4 psi of pressure (~3.75 psi) across the stock IC for a significant portion of the pull. Both outcomes are shown on the chart below, IAT in red and pressure drop the difference between dark and light blue lines.
This was compared with the data logged with the do88 intercooler, a product with substantially less pressure drop. The do88 shows a decreasing IAT that flatlines for most of the pull and a much smaller pressure drop, slightly less than 1 psi (~0.85 psi). Shown on the chart below:
With the turbocharger having less restriction to overcome with the do88 intercooler versus stock I was expecting to see a reduction in turbocharger wastegate duty cycle (wgdc), an indicator of the work output required from the turbine. This is labeled Turbine Act. Final Value (%) in the charts.
Surprisingly that turned out not to be the case. The do88 wgdc bottoms out around 70% and holds steady (Chart 2) while the stock GTI intercooler drops past 70% and continues down to 60% at the end of the pull (Chart 1).
Puzzling outcome:
More restriction from the IC correlating with a lower wastegate duty cycle made no sense, the results were the opposite of what is expected. In both cases there was a high flow air intake system being used that should not have caused a large difference in pre-turbo pressure drop. The stock IC was being operated with the Eventuri intake and the do88 with an MST intake.
Checking the notes I had about the MST configuration showed that it was being used without the surrounding enclosure, functioning as an open intake. Recognizing that one setup was a closed intake and the other open I looked at the turbo inlet temperature to determine if there was a difference in this reading. This is the light green line on Chart 1 and 2 above.
It appeared likely that the turbo inlet air temperature was correlated with the wgdc.
Second analysis:
The next data set reviewed is with the Integrated Engineering IC paired with a bicooler using the Eventuri intake normally, and then with the front piece of the inlet removed so that it is similar to an open intake, shown in the picture below:
The results were consistent with what was recorded with the stock and do88 intercoolers, the case with the higher turbo inlet temperature coincided with a higher wgdc.
Shown below is the Closed Intake case; the turbo inlet temperature decreases from 90 degF at the start of the pull down to 80 degF and the wgdc settles around 70% for the extent of the pull.
Shown next is the Open Intake case; the turbo inlet temperature starts out at 85 degF and increases throughout the pull ending over 100 degF. The wgdc starts to descend initially and stops short of 70%, then begins to climb nearing 80% by the end of the pull.
Conclusion:
The temperature of the air entering the turbocharger has a strong correlation with the turbocharger wastegate duty cycle. 20 degF cooler temperature going into the turbocharger compressor correlated with a decreasing wgdc, even with an additional three (3) psi of pressure drop on the charge air side of the intake.
I still get confused here w/r to pressure and WGDC – how does this increase in air temp to the turbo actually impact engine performance when running a larger IC? The simple question is does an open intake in combo with a larger-than-stock IC make more power than a closed/stock intake?
The second test was with the same larger than stock IC and an open vs closed intake. The open intake showed higher temperature entering the turbo and the wgdc was higher. I interpret that as more work required of the turbine and that affects the engine efficiency. I don’t have the equipment to measure power output so I can’t answer what the affect is on power production.
Ah ok, thank you! So the IATs looks pretty similar (around 85 to end) and that was my main question…so the turbo works a little harder/spins faster b/c the air is less dense with the open intake….correct?
IAT doesn’t have a strong correlation with the turbo inlet temperature, it’s more dependent on the IC performance. Yes, the inlet temperature and pressure directly affect the work required from the turbine.
the ecu control strategy is also at play here with an air demand being requested, and measured by map/air temp against a model to achieve a cylinder fill…. Interesting correlation you have observed, in conjunction with how ecu does “its thing”
Yes, compounding my surprise is how the IAT doesn’t seem to make a large difference. Using the stock intercooler the IAT is steadily rising, producing less dense air which would make intake air conditions further from achieving a cylinder fill, yet the wgdc drops with the stock IC, compared to the do88 that produces a relatively flat IAT line.