Need help plotting Turbo Airflow Map

[KiK]

Hi folks, I am new to the forum and I have been studying some turbo solutions and since I started doing so I came across some questions regarding the The Turbo Airflow Map and the Turbo options for this car.

I will try to keep this Thread as simple as I can.

First of all I will post here the Turbo Map I got from Garrett.
Disclaimer: I plotted this using paint. The scale I've drawn in the X and Y axis are wrong and inaccurate but shouldn't reflect any real world issues.


First, I got from Garrett ( https://www.turbobygarrett.com/turbo...compressor_map ) information for a 2.0L engine @400hp at the crank. They said that for the 400hp mark at 7200 rpm, for any 2L engine, you need 44lbs/min of airflow. Then, the calculated Pressure Ratio for our 400hp mark on a 2.0L engine would be 3,14.
I poorly plotted this on the graph.

Step 2: Garrett calculated the airflow at 3.14 PR and at 5000rpm, the result was 32,5 lbs/min. Plotted this.

Step 3: by eye I calculated the airflow at 3.14 PR and at the surge line. 3800 rpm +/-

Now, this is where It gets too tricky for my head.

Q1: What happens before that 3800rpm mark? The turbo will just surge? In real would applications we know that doesn't happen. Will the ECU just lower the Boost to lower the Pressure Ratio ?

Q2: This graph tells me that we should be running at full bost at 3800rpm, but according to real world experience the boost only comes later. Why?

Q3: Why we do we need to run a wastegate ? I mean, the engine would run out of RPM before the turbo falls in the stall line.


Q4: If we run a larger turbo like the GT3076R (which comes with the PTuning kit for eg.) For the same airflow (thus the same crank power) we would have to run lower PR, thus the turbo would spool slightly faster and gain more top end end lower end power, right?


Q5: The GT3076R seem a better option for the 400bhp mark since we are running in a more efficient island overall, have more surge and stall margin and can, in theory, spool faster, thus less lag. Also, we would also have a more reliable engine since we are not running a pressure as high. Correct?

GT3076R Map:




Please enlighten me!

[Kiske]

1. Your turbo will either still be building boost or your blow off/bypass valve will be open bleeding pressure so you don't surge.

2. Variables. Target boost is effected by many things, intake, charge piping, intercooler, gear and engine load. Atmospheric (barometric) pressure varies by height (decreases as you go higher from sea level.) It is also effected by things such as wind, density and temperature.



3. Without it your just going to keep building boost and spooling into infinity. All while creating excessive heat until it breaks, your o2's melt or you force enough air into your engine where it cannot keep up with fuel. Lean condition, detonation, knock and all the overboost smiles until shit blows. Excessive heat is bad. Remember power goes down when IAT's go up.

4-5. The main factor if where do you want boost for how you drive and operate. For the street most would want the boost further to the right (come on sooner.) The sooner it's loading, the more faster pull to redline. You'll lose a bit of the topend but, how often are you at redline to make the trade not worthwhile?



Also there are to many variables in an engine build/setup to generalize that 400whp at 14psi is safer than 400whp at 15psi.

[KiK]

Originally Posted by Kiske

1. Your turbo will either still be building boost or your blow off/bypass valve will be open bleeding pressure so you don't surge.

2. Variables. Target boost is effected by many things, intake, charge piping, intercooler, gear and engine load. Atmospheric (barometric) pressure varies by height (decreases as you go higher from sea level.) It is also effected by things such as wind, density and temperature.



3. Without it your just going to keep building boost and spooling into infinity. All while creating excessive heat until it breaks, your o2's melt or you force enough air into your engine where it cannot keep up with fuel. Lean condition, detonation, knock and all the overboost smiles until shit blows. Excessive heat is bad. Remember power goes down when IAT's go up.

4-5. The main factor if where do you want boost for how you drive and operate. For the street most would want the boost further to the right (come on sooner.) The sooner it's loading, the more faster pull to redline. You'll lose a bit of the topend but, how often are you at redline to make the trade not worthwhile?



Also there are to many variables in an engine build/setup to generalize that 400whp at 14psi is safer than 400whp at 15psi.

Got it!

Thanks for answering, definitely helped me a lot

[gtengr]

Originally Posted by KiK

Q4: If we run a larger turbo like the GT3076R (which comes with the PTuning kit for eg.) For the same airflow (thus the same crank power) we would have to run lower PR,

A larger turbo would boost peak power by being less of a restriction once spooled. So in that sense, you could theoretically run a slightly lower PR to achieve the same peak power. However, if you picked your current turbo correctly, the larger one is going to be operating in a less efficient area of the compressor map, so you lost power there, probably more than you gained from the lowered exhaust restriction if you chose the current turbo correctly.

Originally Posted by KiK

Q4: If we run a larger turbo like the GT3076R (which comes with the PTuning kit for eg.) For the same airflow (thus the same crank power) we would have to run lower PR, thus the turbo would spool slightly faster and gain more top end end lower end power, right?

As described above, the lower PR would be marginal, and could go up if your current turbo is a good choice.

Assuming equal materials and tech, a larger turbo = bigger turbine = more resistant to spool = once spooled is less restrictive.