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251 prop help

jammasterj787

jammasterj787

Member
Hey everyone, I have a 86 251 that I just got done doing a full resto on and I am having some major prop blo out issues. Its a 489 bbc world heads/intake JE domes 11.8 compression, fully balanced, cut block, full roller, proform 950 custom race carb, full msd,gill dry exhaust. In the upper 650hp area. It has a bravo one,xr gears 1.5 drive. I ran the boat for the first time this weekend and the boat would not go past 3200 rpm. Soon as would get on plane and hit about 3000 rpm then hit the gas the prop would just wash out and hit the 7k rev limit with a 50ft roost behind me. But if I kept the motor under 3200 I can cruise around at 40 or so. Just soon as the cam kicks in with the intake its a wash out. I have a 25p 3 blade mirage that was on the boat when I got it. Was trying to get a baseline to start with??? now IDK what to do!!! I want WOT to be around 6200-6400. Any thoughts anyone???????
 
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OK, I'll bite. Try a 26 Pitch Bravo 1 and go from there. Some prop shops and CP Performance have a prop loaner program. The way you describe your issues, it sounds like there is going to be more to it then just a prop change. BTW where are you going to find race gas at the dock? 11.8 to 1 is pretty high for a boat engine. Keep us updated.
 
1986 didn't originally have a Bravo drive so it was added later. Check your prop shaft height and compare it to another like unit, like mine. I run about 500-525 horsepower in my 251 and I run a 26P Bravo1 4 Blade at 4900-5000 rpm. If you're really gonna spin that drive to 6000+ rpm then it better be good and use FULL SYNTHETIC OIL in the drive.
 
I also would try a stock unlabbed 26p bravo 1 4 blade to start with. I am also with oldnavy 11.8 on compression is very high for a weekend boater. The recommendations for iron heads now I think is 9.8 and with aluminum heads you can go to 10.8 and maybe push it to 11 flat with octane booster because it dissipates heat quicker. But to run 11.8 on 93 is a melt down getting ready to happen. Compression is your cheapest and quickest HORSEPOWER
 
Thanks for the info. Didnt know cp had a loaner program. :offtopic: And to the compression deal. I just ran the boat this weekend on 93 no oct booster with 34 deg of timing. And can rev to 7k clean and quick No ping or vib at all... If your heads are ported right and bowl work is right with the right cam, tuning. 11.8 is fine with 93 pump gas. Even with 10% eth you just run 1 higher jet size to make up for the lack of btu. For some reason now lots of people think that anything over 10.5 is race gas . But that info comes from people who have never even built a motor before and have only heard what there buddys have told them..... And that motor was built for that boat not a race car motor.
 
jammasterj787 said:
Thanks for the info. Didnt know cp had a loaner program. :offtopic: And to the compression deal. I just ran the boat this weekend on 93 no oct booster with 34 deg of timing. And can rev to 7k clean and quick No ping or vib at all... If your heads are ported right and bowl work is right with the right cam, tuning. 11.8 is fine with 93 pump gas. Even with 10% eth you just run 1 higher jet size to make up for the lack of btu. For some reason now lots of people think that anything over 10.0 is race gas. But that info comes from people who have never even built a motor before and have only heard whats there buddys have told them. And that motor was built for that boat not a race car motor. Tho im sure it would work great in one.
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I dont agree with this a boat is under load at all times like a fat women climbing a mountain wearing flipflops in the rain eating a PBJ in 100 degree weather wearing thick wool socks trying to keep her paints pulled up and sweating like a boxer in the 10th round.

But thats enough about your compression lets us know how the prop does:thumb:
 
Good call Jammster, we were just testing you, and I think you passed! Let us know how the Convincor turns out when you get it dialed in. 4 blade Bravo 1's and 4 blade Hydromotive props seem to work very well on The 251 Convincor Hull.
 
wow and thanks bigsbetter I didnt know that boat came with an alpha!!!!! And the bravo I built for it was not what came on it when I restored the boat. Good Info!!! Thank you:brickwall:
 
my dad just found his spare prop so im gonna give that a shot 26p hydro 4 blade..........the only reason I posted is to find out if anyone else has washed out a 25p 3 blade prop before im kinda shocked thats all !!!! I thought I was gonna get a good base line with it........... o and thanks for the piss me off test good job :cheers:
 
If your cam selection is right and Alumianum heads do help, you could run pump gas on a 11.1 motor, but I' don't know about in a boat. It would take a special ground cam to bleed off compression and a tight LSA, and then it would lope so hard and reversion would be an issue with the tight LSA. It would be fun for a while!! But I'll still race ya with my 500 HP CAst inerds block for a 60 mile run!!!!:bigthumb: Top end don't count when you in it for endurance.
 
thats why im running dry pipes......tho its very loud and msd dist and 6al box to burn it off. I built that motor for a long runs too just that wot deff would not be 6300 more like 5500 for a 60 mile race.
 
Just trying to build a bad ass old school convincor to spank all the bajas on the lake. My dad has had checkmates since I remember boating. Hardcore check fan here.
 
If it has to be detuned so much to run the 93 what good is the higher compression anyway. I would really be intrested to know your cam specs. Bowl and port work have to due with fuel and air delivery and exhaust exit. I dont belive dumping more fuel in the engine will justify anything a rich running motor is not a good one
 
de tuned a lil bit all im doing is breaking the motor in on 93 oct on some lower timing. after about 3 hours of ride time im changing to e85 I built this motor for this boat to run on e85 not 93 . your right about the detune but thats just for the break in. and that is hard to do with a slipping prop at 30mph at 3300rpm. I didnt put all this info in cuz I was looking for some prop info.
 
bowl and port work make a smooth transistion during cylinder fire. More efficient flow moving the high compression !!! also smooth cut block valve reliefs move air. This motor has lots of time spent on it hand building not some stroker kit thrown and a old 454 merc.
 
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