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Should I go 2.5" or 3"?
This question seems to be asked daily on GTI forums. I'm going to try to approach the topic from the fluid mechanics perspective.
First, we have to understand that flow velocity in the exhaust is as important as high volumetric flow rate, but for a different reason. Second, increasing pipe diameter reduces velocity (bad) and increases max volumetric flow rate (good). Last, high velocity equals low pressure. That's it - that's the physics.
But what does it mean to downpipe performance?
It's pretty simple:
Obviously, we want both. But we can't maximize both simultaneously, so we compromise. Experience has taught that if we go smaller than 2.5", power at WOT (AKA dyno runs) suffers. If we go bigger than 3", lag rears its ugly head.
So... When folks say they lose power down low with a 3" DP, they are right - low flow velocity creates low delta-p which means lag, even in our small turbo. The flow is slowest at part-throttle, and that's where the lag is greatest.
And when folks point to dyno runs that prove that no power is lost down low with a 3" DP, they are right too. At WOT and steady-state flow, the turbo loves that high flow potential.
Another velocity-related consideration is exhaust gas cooling. We don't want the gas to cool. Cool gas is dense (heavy), and trying to move it creates backpressure. Score another one for smaller pipes.
But, in truth, I doubt that the differences between 2.5" and 3" cause much of a perf diff for the stock turbo.
Finally, why does low-RPM lag appear to increase at Stage II? I'm not sure, but I suspect the issue is turbulence. The Stage II ECU requests lots of boost NOW, creating violence in the flow. Turbulence is another big velocity-killer, and low velocities cause lag.
It's been a long time since I studied Euler and Bernoulli, but hopefully I've remembered enough to make this useful. I have over-simplified a bit. This was a deliberate choice.
Bob
PS Credit to grambles423 for exposing some of the equations in an earlier thread.
This question seems to be asked daily on GTI forums. I'm going to try to approach the topic from the fluid mechanics perspective.
First, we have to understand that flow velocity in the exhaust is as important as high volumetric flow rate, but for a different reason. Second, increasing pipe diameter reduces velocity (bad) and increases max volumetric flow rate (good). Last, high velocity equals low pressure. That's it - that's the physics.
But what does it mean to downpipe performance?
It's pretty simple:
- For max power at WOT, we want high volumetric flow rate. This means a BIGGER pipe. -but-
- For maximum throttle response (minimum turbo lag), we must keep flow velocity up. This creates the max pressure differential to drive the turbine. This means a SMALLER pipe.
Obviously, we want both. But we can't maximize both simultaneously, so we compromise. Experience has taught that if we go smaller than 2.5", power at WOT (AKA dyno runs) suffers. If we go bigger than 3", lag rears its ugly head.
So... When folks say they lose power down low with a 3" DP, they are right - low flow velocity creates low delta-p which means lag, even in our small turbo. The flow is slowest at part-throttle, and that's where the lag is greatest.
And when folks point to dyno runs that prove that no power is lost down low with a 3" DP, they are right too. At WOT and steady-state flow, the turbo loves that high flow potential.
Another velocity-related consideration is exhaust gas cooling. We don't want the gas to cool. Cool gas is dense (heavy), and trying to move it creates backpressure. Score another one for smaller pipes.
But, in truth, I doubt that the differences between 2.5" and 3" cause much of a perf diff for the stock turbo.
Finally, why does low-RPM lag appear to increase at Stage II? I'm not sure, but I suspect the issue is turbulence. The Stage II ECU requests lots of boost NOW, creating violence in the flow. Turbulence is another big velocity-killer, and low velocities cause lag.
It's been a long time since I studied Euler and Bernoulli, but hopefully I've remembered enough to make this useful. I have over-simplified a bit. This was a deliberate choice.
Bob
PS Credit to grambles423 for exposing some of the equations in an earlier thread.
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