Inlet tract throttling Turbonetics T72 turbo?
 

Preamble
Firstly, let me apologize for my selfishness in joining the forums only to get help on some of my issues with my turbo set up. I hope to put something back in later, but for now please bear with me.

As a former military aircraft maintenance engineer who has had quite a few dealings with aircraft and missile jet engine design and maintenance, I understand the subject fairly well and, given the time, can do just about all car turbo calculations with a fair expectation of success.

Alas, I've lots of theory but little experience on car turbo installations and I'm on my own a bit here . . .
. . . . . particularly as I bought the set up as a 10% unfinished project and the previous owner and the engine builder are of little if any help (long story, sorry)

My car and turbo set up
My car is a Mazda RX8 fitted with a bespoke turbo-charged RX7 FD3S2 13B rotary engine. More specifically: Turbonetics T72 Turbo, Custom Turbo Manifolds, RacingBeat Dual Resonated De-Cat Centre Pipe 3" Bore, HKS HI-Power Cat Back (Single Exit, 3" Bore), APEX-I Power FC ECU, Custom Map, Custom CAI, Greddy Front Mount Intercooler, Mishimoto Racing Radiator, Mishimoto Upgraded Fan Shroud, Greddy DumpValve, Tial 60mm External Wastegate, Custom Screamer Pipe.

https://www.homemadeturbo.com/attach...ine=1384035215

The builder and previous owner both claim 400+ BHP. However, I don't believe what I'm told when I buy cars and won't be able to put her on a dyno for a few weeks as I'm still running her in.

My concerns
The inlet diameter of the T72 turbo is, I'm told, 72 mm. The inlet tract is around 55 mm internal diameter, leads to an unbranded foam air filter and, in the opinion of both myself and another [but different] turbo 13B owner, throttled by the air intake.

I'm concerned that increasing the inlet tract diameter may cause more harm than good as:
(a) the T72 turbo is big for the engine size and the inlet tract may have been chosen to reduce max power and hence strain on the engine and powertrain.
(b) the custom fuelling map may not cater for the increased flow and I can't ask the builder/mapper (long story)

Help!
My head is reeling from trying to get info from the net.

Please let me know what you think - all advice appreciated.

I suspect the smaller tube is for packaging reasons instead of an intentional restriction. If you want to reduce power on a turbo build, the correct way is to limit boost, not throttle the turbo inlet.

The inlet isn't likely to affect the tune that much; the ECU should detect there is more airflow. It may not compensate for it perfectly, but it should do something. And there's a good chance by the time the project is finished, it'll need to be retuned anyway.

I'd put a bigger inlet on there and not worry about it.