COMMON TURBO MYTHS DISPELLED
Volumetric efficiency / pressure / mass air flow
Choosing a turbo - BIGGER IS NOT ALWAYS BETTER!
Leave it up to the experts. It's like buying shoes, You don't just go to the store and order a pair of shoes that are too big for your feet. The turbocharger has to be the right size for your application. It's true that a bigger turbo will produce more air volume, which translates to higher levels of peak power. But the bigger turbo takes a lot of exhaust energy to make that boost, even more important, your engine set-up must be able to keep it running!
So a wiser decision is to sit down with the turbo expert and go through the engine's parameters.
Questions to ask will be the displacement of your engine, maximum rpm to run, the parameters of your fuel injection equipment, engine modifications like work on valves, camshaft, cylinder head, intake- and exhaust manifold, intercooler and so on.
Of course a lot of development work has been done and experience gained from actual field testing on the pulling strip, so there is no reason to invent the wheel for the second time…!
So, we therefore have NO intention to burn you out with a huge amount of formulas and calculations, plus a load of compressor flow maps and turbine diagrams to fight through…...
A well sized turbo for a given application is a matter of choosing the correct model, compressor- en turbine wheel size, plus the correct nozzle A/R of your turbine housing.
Just picking up ‘a’ turbo from the scrap yard or buying it from E-bay will most probably not bring you what you want.
Anything too big (or small) will give you only problems… not the power and performance you are looking for…
Pictured is a sample HX50 compressor flow map, inducer 67 mm.
Left vertical line indicates Pressure Ratio :1 against atmospheric pressure.
Bottom horizontal line shows Mass Air Flow (divide by 167,5 for kg/s).
White areas are the Efficiency islands with percentages (max 76 %).
Left to the white area rotor speeds have been mentioned (max 115000 for HX50).
Diagonal coloured line shows the ‘ideal running line’, through the efficiency islands.
SURGE LINE AND INSTABLE SURGE AREA
The surge region, located on the left-hand side of the compressor map, is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur.
The most damaging is surge under load. It can be an indication, that your compressor is too large. Surge is also commonly experienced, when the throttle is quickly closed after boosting! This occurs because mass flow is drastically reduced as the throttle is closed, but the turbo is still spinning and generating boost. This immediately drives the operating point to the far left of the compressor map, right into surge.
CHOKE LINE AND AREA
The choke line is on the right hand side of a compressor map and represents the flow limit. Properly sizing a turbo is important to prevent the compressor from operating past the choke line. When a turbocharger is run deep into choke, turbo speeds increase dramatically while compressor efficiency plunges (very high compressor outlet temps). Additionally, the turbo's durability is compromised by the resulting high thrust loads.
BOOST PRESSURE
To make more power, you do not need only more boost pressure!
Boost is only a measure of intake pressure. Everything else being equal and within reasonable limits for the setup, more boost makes more power only if the turbo is operating in an efficient range of performance. If you were to increase the boost to the point where the turbo is trying to move too much air, it actually reduces performance.
This is because past a certain point of diminishing returns, a turbo is basically blowing hot air. This hot air creates intake air pressure and more boost because boost = measure of pressure. To obtain the best and optimum balance between pressure and mass air flow your turbo compressor should run in the right area of volumetric efficiency, because you want the maximum mass of air for the engine. You need the highest possible concentration of oxygen in a given mass flow of air. Higher temperature means lower air density, so less oxygen to burn the maximum balanced amount of fuel. Unless the air can be cooled sufficiently by the intercooler, the density of the air can be less than it would have been at a lower boost level.
It is a common mistake to equate boost, or intake pressure, with denser air. Referring to the ideal gas law as a simplified example, PV=NrT, if you raise pressure, temperature would also have to increase, assuming the other variables remain the same. Also keep in mind the earlier remarks about operating a turbo compressor outside of its islands of efficiency. People often get so caught up with learning about the quick power gains from more boost pressure that they forget. that the ultimate goal of turbocharging is increasing air density, not pressure.
It is the maximum amount of oxygen in the same air mass flow that counts!
So, what do you do when alongside the pulling strip someone proudly tells you, that he had 5-6 bar boost pressure, with an air temperature of 300 degrees C and something more at the compressor, with the additions ‘and he is still alive, so it is possible’..!
Well, what can you do other, than just shake your head, a good bite from the hamburger and a bottle of beer... creates less pressure, more volume and density...