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1. that ecu is very difficult to setup. Its more likely you have a timing issue than anything. Did you verify the ign timing at high rpms? Under load? I don't trust that ecu or anything from MS anymore. Its probably got an inductive delay issue or similar for example.
2. engine being "built" means nobody has any clue now whats inside it now. Could very well be an issue with quench or cylinder head shape or use of valves causing the issue with poor behavior especially if any work was done to the head, or it was machined wrong, etc..
3. tuning that ECU is not simple as a typical stand alone. For example,
Injector phasing is critical to avoid spraying into an open intake valve or you face the issue you are facing. Did you verify the injector end of injection spray setting? I bet not
4. Wrong dyno used. There is no diagnostic info in that dyno graph we can use.
You SHOULD be using a Dynojet, set to smoothing = 0, with an AIR fuel curve present , Along with in-car wideband behavior data to compare with. Also we need to see mph vs torque AND rpm vs torque for max info
5. Big mistake using the stock cams. This pulls about 30-40% of the potential power out of the engine. Not good fighting stock cams on any engine. garbage
6. Need an EGT gauge off the downpipe combined with torque curve & boost pressure log from being on a dynojet to see how expanded exhaust gas volume is before determining its too small. fwiw I've made 350rwhp through an OEM 240sx exhaust system before, complete with OEM muffler and OEM sr20 downpipe neck. 240sx exhaust is identical to silvia exhaust. Silvia uses sr20det making 300rwhp OEM so the small exhaust is not an issue at that level.
My advice is get an sr20 or if you are stuck diagnosing the KA then change the computer to PFC or something with plug and play setting for crank and cam and phase so you don't have these questionable situations with timing or whatever, that will let you narrow down the issue to the engine itself so you can begin replacing it.
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