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I usually in my workshop remove the 2 original turbos and replace them with a big one (turbo + headers + wastegate).
I still make the engine run on the original ecu and the MAF sensor without having issues
Is this conversion healthy without an after market Ecu?
Let me ask it this way. Is there any reason NOT to run an after market ECU besides cost?
Yes for the price!
At best you're not going to see the full potential of an upgrade like this on the stock ECU> At worst you could end up damaging the engine by running it too lean or suffering from knock. If you're making a dramatic change such as fitting a single turbo then I would strongly recommend an aftermarket ECU.