Power mods for the I-5

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Ah, but GM got the 20+ HP from boring the engine out to a 3700. The engine is capable of obtaining quite a bit more HP with a turbo or SC, but unless GM offers it, it would void the powertrain warranty.
Later engines, or those with the new cylinder head, should be able to take the extra HP, as long as you keep it low, around 270. This is not from any GM official information, but from what I know someone did with this engine.