Sorry if you already are aware of such things, but additionally to what I’ve said earlier (which is of most help to your issue), despite being a big fan of these short block v8 mpi and gxi engines…having one myself, I and others who own them and run them, will tell you that they can be a little lazy down low in the rpms…compared to a carb version of the same engine, and that’s despite the carb (factory engines) engine having a lower HP rating than the gxi/mpi. They can be forgiven for such a behaviour, by making up for it, when they find their legs higher up the rpms though. At your size of boat, this will be probably emphasised. So what will help things along a little is if you allow her to gradually build speed. Not only is it a good idea in terms of mechanical sympathy, it helps the engine find its stride easier too. Try and get her up to a speed just short of her wanting to lift the bow and planing. Let her own momentum help things along by allowing her to sit sustaining at that pace for 10 seconds or so…easing the load on the engine. Tabs and trim down…then open her up from there. If not…the lazy reluctance behavioural trait I describe above, will be at its worst. Let’s face it…she’s not intended to be a wee speedboat, or even designed/required to be a rocket ship from a standstill.
I think you’ve got the best possible set up there, as you are. Would a big block 7.4 or 8.1/8.2 have more punch out the hole…yeah. Definitely…would it be much quicker top end ? Marginally…would your fuel costs be higher ? Without doubt…shockingly so. The big block engine is also exponentially more heavy, than the small block…the last thing you need is hundreds of lbs more weight in the stern. Clever fuel/water tankage thinking, well maintained engine (with particular reference to the health of the distributor cap in mind) and technique like I’ve suggested above, is your best bet.
Finally…just also make sure some idiot hasn’t been maintaining her previously, making the common and very detrimental mistake of putting normal spark plugs in the engine. Very typical to find that a less than experienced/knowledgeable maintainer has been naive enough to presume that these take the same plugs that the rest of the 5.7 more numerous versions of these engines use. These gxi/mpi are the exception to the usual rule here. They absolutely require the plugs stated clearly in the manual. Minimum spec would be the platinum ac delco. But ideally and preferably, the ngk iridium plugs. If you find yours with the usual bpr6efs or something like that…I can tell you, that this will hugely worsen or even be largely to blame for your hindrance. Hope some or all of that helps…apologies for the biblical length of the reply!
