Indmar 325 to 383?

I recently bought an 06 Supra 21v. It’s got 1900 hours on it. I bought it very cheap with intentions of rebuilding the motor. Aside from the internal parts needed to build a 383 stroker, will the computer need to be reflashed for correct timing and fuel curves? It’s not a cat engine so I don’t think there are 02 sensors for feedback to adjust fueling. And then my next concern would be the transmission (replaced in 2015 already). Any input would be awesome. I have built a few small blocks in my life, but they were all carb motors not EFI.
thanks in advance!


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Why are you changing the stock design ?

For the same reason boats come with different engines. More horsepower, I hear lots of people saying the boat doesn’t have the power for a good surf wake when loaded down. A friend of mine with a MC X45 told me he wishes he had a big block. If I’m rebuilding the engine I see no reason not to build it into a 383. It’s all internal items to do it and looking at it you would never be able to tell. (Aside from adding 75 to 100 horsepower)

Back to my original question. Does anyone know if the ECM would need to be reflashed or swapped out?


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You will absolutely need more fuel and timing to feed 33 more cubic inches and make 75-100 more HP.


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Originally Posted by Larry Arizona

You will absolutely need more fuel and timing to feed 33 more cubic inches and make 75-100 more HP.


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I’m just not familiar with the ECM on these, if it were a car I would be in my element. I think I’m going to call Indmar and ask. I just don’t want to build the engine and then not be able to get it to run correctly and give it the fuel and timing curves it needs.


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You may want to reach out to blueprint engines. They make a drop in 383 for marine application.

Today's boats are big and heavy even before we put 3000lbs of water in them. Might as well throw all the cubic inches at it that you can. Displacement is where torque comes from and torque moves boats. I have a 24SSV and couldn't imagine an Assault motor in it. Every time we surf or run wide open I am again thankful for all 450HP and 525 LB torque making things happen at the stern. With all the tuning options i am sure you could make a built 383 work.