Relocated inlets- sizing theory

proboner

Specialist
Sep 13, 2020
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This is kind of a one-off question, but would love to hear the community's thoughts. My Z4 came with Pure DD turbos with 1.75" inlets and the previous owner "custom made" relocated intakes for it. I was doing some work recently and realized that the piping for the intakes is 2", while piping for most aftermarket relocated intake kits is 2.5".

This is my first turbo car, and in the NA world I'm used to, I've always lived by the idea that it's useless to increase the diameter of the intake/exhaust larger than it's smallest diameter, because that will be the bottleneck anyways. In this case, logic suggests to me that since the inlets are 1.75", it isn't beneficial to have 2.5" diameter piping upstream because it will all be bottlenecked at 1.75" anyways. Curious what you all think about this?

There seems to be weird rules these turbo cars follow that don't jive with my NA experience, like having dual 3" downpipes but dual 2.5" exhaust. Do you all think I'm leaving power on the table by not having 2.5" intakes on my 1.75" inlet turbos?
 

wheela

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Jun 4, 2021
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I've always lived by the idea that it's useless to increase the diameter of the intake/exhaust larger than it's smallest diameter, because that will be the bottleneck anyways. In this case, logic suggests to me that since the inlets are 1.75", it isn't beneficial to have 2.5" diameter piping upstream because it will all be bottlenecked at 1.75" anyways. Curious what you all think about this?
In general, I'd disagree with this. Length plays a factor in the overall resistance to flow, as does flow rate. For example, a 1.75" pipe 4 feet long will have greater flow restriction than a 1.75" pipe 2 feet long. I'd say a combination of 1.75" orifice with 2.5" diameter upsteam piping would present less flow restriction than a combination of 1.75" orifice with 1.75" diameter upsteam piping.

The big "it depends" comes in with flow rate. The effect will be more pronounced as flow rate increases. For low flow rates, there would be a point where the benefits are negligible. But as air flow demand increases, the benefits of the larger inlet pipe will become more pronounced.
 

Torgus

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Nov 6, 2016
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Inlets are cheap(all things considered) but a PITA to install. I would just leave what you have. Performance wise you are not going to pick up anything noticeable. The money would go better to other places to make your car 'faster'