I have a -89 312 Stinger and this winter a replaced my old 454/330's with new rebuild 502MPI/415's.
Last year I run 61MPH with this old 454 and 23" Ballistic
Now its 60MPH @4850rpm with these f***g 502MPI's and same 23" Ballistic. And they take large # liters of gas
I know this engineshop who did the rebuilding job very well and I know they use to do this kind of job perfectly.

Any clue?


Mange

I think 502's are suppose to be spun around 5000rpm or a little more. You might need to try some different props. Also, I would look at checking fuel pressure and compression check to make sure their are no problems in the engines.

That would be a good start..

If the old 454's spun those props to 4500-4600 rpm before, those 502's should peg 5000 rpm with the 23's. I agree, assure the motors are mechanically sound, compression check, timing and so on.... you should see quite a difference from the old 454's..... the 502's are 415hp at the prop and your old engines 300hp at the prop, adding 115hp per side should liven up your boat..... something definetely not right.... I'd guess upper 60's easily with your setup, then again, not familiar with the stinger and it's weight so I'm just guessing. I've got a 31 sonic 7000lbs with the 350hp 454's and I'm running mid 60's with 23's.... Good luck!!!!

Something's not right. Go to another shop for a diagnostic check - check for codes with scan tool, compression maybe even leak down. With an extra 230 HP you should have picked up at least 6-7 mph if not more.