performance problems
 

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.

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

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.

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

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

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

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!

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...

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

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?????

So, you where personally in this boat when it ran much faster elsewhere?

Yeah good question, lets start with that.

No i wasnt but i bought from a dealer. And i spoke to the original owner about boat props motors. Boat was from oregon with 700 hp. 28 lab would turn 5400 at 5 psi boost with 2 750 holleys jets 73 and 82. 1.5 gears xr outdrive . Boat is solid and motor is strong checked compression is great timing good . I would really like to turn up boost but im not real knowledgeable about blowers boost and hurting motors. Another thing i live in nm and its also been really humid another factor Richness im only getting 4 psi boost right now also

Oh also motor is a merc 500 hp nothing done to it same motor from factory

1 other thing it has a b and m blower not a 671 not forsure with 1 but teague did the work on it

Wish i have efi

1 other thing how about adding a superchiller i have heard good things about them more hp and helps prolong motor. The reason why these posts i would really like to run 75-80 in nm i thought thats what i would have been anyways at least 70.

I'd personally work on engine tuning before messing with props. Get the boost and afr right, then move on from there.

I would ask Teague about your setup. They'll probably be able to point you in the right direction in terms of jetting and props, especially since they set up that motor. Also, they have all the superchiller kits which would definitely add some power and prolong the life of your motor.

OUCH!!

3pointstar

Thanks tomorrow 44. Ill try them tomorrow

Hola NM dude! That's Spanish for you outa stater's!

I have found that the 28 sport cat needs extreme hp to work just ok..... we played with one that made 1347 hp on the dyno here in Albuquerque and would only run 101 mph at ute lake in NM.
Anything over 7 pounds of boost will short live your motor. You will loose 1# of boost for every 2000 ft in elevation. Do you have 1.50 gears or 1.36 gears in the out drive? Wait till you take it to Navajo....that really will disappoint you.
Let me know if you need some help with it...always like helping a neighbor!

Would love some help 1.5 gears

Would love some help 1.5 gears

What do you think of a super chiller and turn up boost a little im only getting 4 lbs

Yo fastboatfella was wondering about the help???? Do u have a shop ???? Im willing to pay. Summer almost over would like to grt this dialed in. Thinking about going to havasu next weekend and seeing if this boat. Really does what they say in havasu going back to original jets and prop and then start all over again

Yep... got your number. I'll call. Yes on the shop.