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If the internal diameter of the up-pipe is larger than the original this will cause the velocity of the gases to decrease thus slowing spool rate and the opposite with a smaller ID up-pipe. Although I would assume the mid to top end power delivery will be higher with the larger ID up-pipe
The up pipe which we sell is exactly the same size as the entry to the turbo, so there is no turbulence as the gas enters the turbo housing. This actually helps the gas speed as the standard design will cause the gas to be very turbulent at the final stage before it hits the impeller. Turbulent gas is less dense so gives less push.