A
Anonymous
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Most recommendations on picking a turbo are based on max CFM and the expected Pressure Ratio (PR) of the turbo. Plot the point on the map and see where it sits regarding efficiency and distance from the surge line. Ok, fine. A too-small turbo will spool up quickly and hit maximum boost at relatively low CFM -- but the trade off is that it will cap total HP fairly low as well, and will run hot most of the time. A too-big turbo will take forever to start producing boost, but will continue to increase HP until you blow something up. You can guesstimate the speed of spooling up and whether the turbo can take maximum CFM from the standard recommendations
But there is much more to having a good turbo system than that.
It seems to me that outside of WOT racing, a turbo spends most of its time _not_ at full throttle (therefore not at max CFM), making the standard recommendations irrelevant. In a street car, it spends most of its time sitting doing nothing, and then adds less than maximum boost for short periods when you pass another car or are entering a freeway. It'd be nice to be able to add those less-than-maximum points to the turbo map to see how it'll work under more normal conditions.
Corky Bell's book shows a number of turbo maps where he seems to draw a straight line from when the turbo starts to spin and produce boost (well below max PR), and when it reaches max PR. If you knew those two points, you'd have a good idea how the system would respond through shifts and the like. Unfortunately, Mr. Bell doesn't say how to get those lines.
Is anyone in the group aware of any way to calculate [1] at what CFM the turbo starts spinning up, and [2] at what CFM a turbo will reach full PR? It'd seem that CFM, A/R ratio, vane type, etc., ought to tell you enough to make some rough calculations?
Thanks!
--- Barrett
But there is much more to having a good turbo system than that.
It seems to me that outside of WOT racing, a turbo spends most of its time _not_ at full throttle (therefore not at max CFM), making the standard recommendations irrelevant. In a street car, it spends most of its time sitting doing nothing, and then adds less than maximum boost for short periods when you pass another car or are entering a freeway. It'd be nice to be able to add those less-than-maximum points to the turbo map to see how it'll work under more normal conditions.
Corky Bell's book shows a number of turbo maps where he seems to draw a straight line from when the turbo starts to spin and produce boost (well below max PR), and when it reaches max PR. If you knew those two points, you'd have a good idea how the system would respond through shifts and the like. Unfortunately, Mr. Bell doesn't say how to get those lines.
Is anyone in the group aware of any way to calculate [1] at what CFM the turbo starts spinning up, and [2] at what CFM a turbo will reach full PR? It'd seem that CFM, A/R ratio, vane type, etc., ought to tell you enough to make some rough calculations?
Thanks!
--- Barrett
