I'm trying to understand the surge problem with the 80 and 88 in the demo you tried. Please explain what is surge. I though it was when the turbo is making boost but the throtle plate is only partially open....like during a light tip in at cruise speed say 45-55mph. But you engine is race only TB is either open 100% or closed 100% so why surge?
If you're familiar with a turbocharger compressor map, you know that there is an airflow axis and an air pressure axis. Within the map is the compressor's efficiency island. Usually, close to the middle of this island is where the output of the compressor is most efficient. Heating the compressed air the least.
I have a much easier time internally visualizing how this works rather than trying to explain it, so forgive me if I confuse you more than make this clearer for you.
If you pick out a particular airflow and air pressure output for a compressor, you can go to this map and pinpoint an area on the compressor map and see how efficient the compressor is at that point. If the airflow demand is too high from a particular compressor, it ends up to the right of the efficiency island where efficiency drops and the compressed air output is heated to undesirable levels. Add to that too high of a pressure demand and you overspeed the compressor risking damage. That is located in the upper right hand region of the compressor map. Generally, you want to pick a turbocharger that will give you no less than 60% efficiency at maximum airflow of your engine setup while under boost.
To the left of the efficiency island is the surge zone. A dashed line separates the surge zone from the efficiency island. As an engine boosts up, the airflow and pressure can be mapped out by a line on the compressor map, starting in the lower left hand corner and angling upward and to the right through the efficiency island. At the point of full boost, that trace line stops moving upward and continues to travel to the right until maximum engine airflow is reached.
If you can match a turbocharger that will skate along the right side of the surge line while the engine is boosting up, you can end up in a higher efficiency zone at maximum engine airflow. Pick a turbocharger that is too small for your engine airflow requirement and the trace line moves upward to the right of the high efficiency region of the island and then moves to the right, well past the 60% efficiency zone. Pick a turbocharger that is too large and the trace line moves upward too close to the surge line and may cross that line to the left of it, causing compressor surge. This will occur as the turbo is boosting up or as the trace line is making its upward and to the right rise before full boost has been reached and the trace line has had a chance to move straight over to the right into a more efficient zone on the compressor map.
The size and shape of the efficiency island and the surge line is different for each compressor.
Some compressors are more tolerant to low airflow and high pressure than others. Those that are not will go into surge easier.
If I just brought up more questions or did not explain it well enough for you, let me know and I'll try again. It's really important that you understand this. It is a very important tuning point.
Surge is when the compressor goes into an unstable delivery of the airflow and pressure. It feels like you've taken your engine by the shoulders and are violently shaking it back and forth. Feels wicked.
I'll get to that comparison sim tomorrow night. Good night y'all.