68.9 mph to 79.8 mph
 

Very happy with the new engine in my chase 800. Removed the 496ho and put a hp500efi with eldlebrock heads and reprogramed ecu.Gained 11 mph on radar.Salesman who sold it to me told me it was 75mph boat.I'll never buy another boat without trying it first.Learn from your mistakes I guess.

Same thing happened with my cousin's 26 Donzi. Salesman said it was only 3-4 MPH with a 454 Mag then a 502. Said it would run high 60's. It will only run 63-64 on a good day with a tailwind. He ended up getting a 500EFI and put it in yesterday. Should be testing it today.

Welcome to the wonderfull world of boating. I do not care if you are talking to a sales man or some people out on the lake, when you ask the speed if they do not say radar or GPS along with the speed subtract 10mph from what they said. Then you will be a lot closer to what boats top speed is. Another unsaid disclamer is the top speed the boat did once. That is, with 1 - 60lbs person in boat, 1 life jacket, no other seats, 5 gallons of fuel, cool/cold low/no humidity day with a 50 mph tail wind.

You forgot it was tested on a trailer on I-70 at 3am. Or on a river at flood stage 15 mph current and a tail wind and the old motor before it blew and the 330 got put in. Buy a GPS and take it for a test drive the 10 mph rule is pretty close.:cool:

Something sounds fishy ... Assuming both engines are pretty much stock, going from 425HP to 470HP will not give you 11 mph. If the numbers are gps numbers it sounds like there was something wrong with the 496 or you had the wrong prop etc, etc.

Will- I think its entirely possible,although some boats wouldnt see that kind of gain. I went from a very similar motor to Bill's current setup to a new stock 502 Mag MPI and lost more than 10 MPH!! A couple of years ago, I got the chance to talk to someone "in the know" and was told that Merc rated their production motors +/- 10% and if you had a 502Mag that would dyno at 400HP with accessories, that would be "a pretty good one" , while motors out of the high perf. division come real close to their rated gross output with accessories. If this is the case, then Bill would be getting pretty close to a 100HP difference, which on some hulls would make a 10 MPH diffrence feasible.
Take Care

Also remember back at the watering hole the speed of a boat goes up 5 MPH per three drinks

This motor I put in has 500hp at the prop.It's similar to the new hp525 from mercury.They also modified the trim tabs that were acting like brakes when the boat was trimmed up.This alone was good for almost 3 mph.All you guys with trim tabs mounted with the edge to the bottom of the boat ,take them off, if your boat is trim sensitive and try it you never know they might be mounted wrong from the factory like mine.

Im going through this with tabs right now. If there Bennets, they make a retrofit top hinge that brings the tab up a full inch further. Ill find out if it helped tomorrow.

Intolerant1,
so....did it help?