28ftsportcat

I just bought a 28 ft 1999 sport cat 500hp 671 blower dual 750 holleys 5psi boost 700 horse they say. The boat came from Oregon ran 80-85 with 28 lab prop.I purchased from havasu ran the same, Carbs had 73 and 82s. I live in nm 4000 ft, I went to 70 and 81s with a 26 pitch bravo 1 boat ran 60 at 5000rpm not impressed. I do have a hole shot problem boat does not plane very well . I give throttle and have to back off then it seems to grab then I gun it. boat seems very responsive with jetting maybe a little rich but safe. I think im getting prop slip any help or suggestions would be great . Im thinking boat should run 70-75 but it didn't.

3pointstar

With a hole shot problem - are you sure that your tabs are correctly positioned and outdrives down-- it could be something as simple as the "gages" are not giving the correct information. I know once and a while I forget to lower the engine and then "try" to get up and go - the bow rises and can't get the boat on plane. Then when the brain cell kicks in - lower the engine and all is well. I have a Bravo III so prop slip is "almost" impossible - it really grabs.

I can't comment on the altitude issues and the pitch of props -- I'm not that smart.

Safe boating

3pointstar

SB

iTrader: (1)

Figure out how much more boost you need to run for the elevation change..ie.so you make same hp now as it did at low elevation....swap pulleys for that...and leave everything else alone.....assuming everything else was right on before.

28ftsportcat

If i turn up boost to 8psi that should be the same but im affraid to hurt motor, i did think of that solution but i thought i will hurt motor

Tinkerer

You hurt the engine with HP not boost. The increased boost will only add the oxygen you lost with the increase in elevation.

28ftsportcat

So change pulleys to get 8 lbs to compensate and i should be ok with out hurting motor

ToMorrow44

By going up in elevation, you probably lost boost. If it was running 8lbs at Havasu, its probaby only 5-6lbs at 4000ft. So by putting the smaller pulley on it, you would really just be getting back to 8lbs. The blower has to move a greater volume of air in order to make the same boost due to the decreased air density up there.

If you're running a stock 26p prop, thats almost the same as the lab 28 in terms of rpm. You may need to go down even further in prop size or lab the 26 to pick up more rpm.

Honestly what I would do is run it with the same setup as Havasu, jetting prop everything. Then change one thing at a time. Like try changing the blower pulley, then run with the 28p prop. Then try changing the jetting. If that works, then play around with props from there until you get it set up the way you want. Its hard to tell if maybe the motor or the prop change is causing the blowout while planing. Keep us updated!

ToMorrow44

Lol sorry to say that your 22 Cobalt with a Bravo 3 is a far cry from a 28ft Blown Cat. I'm sure he had the drive trimmed down when putting it on plane...

28ftsportcat

true that my prop is down but my boost was at 5 in havasu now at 4000 it is at 4psi so I was wondering leave everything prop jets and just boost up to 8 lbs and that should compensate for the lost of elevation and put me up to the closer hp from havasu .Now with the elevation change im really loosing about 200 hp and obviously cant push that 28 . so I jetted down to 69 and 80s it sounds pretty good gonna try a 24 4 blade and see what she does this weekend

28ftsportcat

So i tried a 24 pitch 4 blade bravo 1 picked up some holeshot ran same speed but picked up 300 rpm only ran 60 like before now im more confused i ran 5000rpm at 60 now 5300 at 60?????