Turbo Compressor Maps

The GT3 Esprit uses the following Turbo :
Lotus P/N - A920E6002
Garrett P/N - 465133-(000)3

From the turbo's ID plate, it is known that the GT3 has a 50 trim compressor wheel in a T3 housing. With that information, the compressor 'map' can be obtained from the manufacturer to observe it's performance.

The 'map' is an area graph, covering a complex set of variables, the main culprit being the constantly changing heat generated by compressing air. This is represented by the coloured efficiency 'islands' - the more efficient areas the turbo operates in, the less heat is generated.  In the above case, the T3 is actually pretty good across the board, only deviating 65%-75% over the map.

Pressure Ratio.
The Pressure Ratio (PR) can be basically defined as the boost pressure. This turbo's maximum boost pressure on the GT3 Esprit is capped at 0.97 bar (14.06 psi) above atmospheric pressure.  Pressure ratio is the absolute pressure at the turbo outlet, divided by the absolute inlet pressure, in this case the pressure ratio = (14.06 psi + 14.7 psi) / 14.7 psi = 1.95

However, no induction system can ever be perfect and will always encounter losses.  Whilst the pressure ratio is regulated at 1.95 at the cylinder head, the pressure ratio measured at the turbo outlet itself would be read higher due to these losses, in this example 5% loss gives a PR of 2.05 at the turbo.  This represents the fact the turbo has to spin faster to generate the required PR minus the losses.

Therefore, if the engine requires 15lbs of air / minute, it can be seen that the  turbo would need to spin faster (by a few thousand rpm) to generate the same air flow.

This clearly sets out the case for an efficient induction system which would reduce turbo spool time (lag) and temperature.




Air Flow.
Air flow rate is rated in pounds of air delivered / per minute (lbs/min), but can also be expressed as cubic feet per minute (cfm).

The graph now shows the approximate air consumption rates for engine speeds of the GT3's 1997cc engine at 3,000 and 6,000 rpm.  As can be seen at 3,000 engine rpm, the turbo needs to spin about 116,000 rpm to achieve the demanded pressure ratio.  If the induction was more efficient it would need to spin less fast.

At 6,000 engine rpm, the turbo races at nearly 140,000 rpm to maintain the pressure ratio.


Surge Limit.
The last part of the graph is the surge limit line.  This dictates how quickly the turbo may accelerate before the blades on the compressor start to encounter turbulence and potentially stall.  This can cause the compressor catastrophic damage so it is important to consider this especially as some smaller turbos can accelerate very quickly unless checked.

At idle the engine needs only approximately 4 lbs of air a minute.  If the accelerator is smashed, the engine will bolt and the turbo will accelerate and rapidly build up pressure towards the cap point.  This rate of acceleration can be contained by the car's ECU, and indeed the Esprit uses a special valve to control this performance called the 'wastegate solenoid'.