MAXIMUS prop 253 convincer slippage

[glen george]

Im running a 94 checkmate convincer that was 4400 lbs before replacing the heavy exhaust, swim platform and the steel heads. Engine was a 500HP. Stroked now to a 540ci, cmi exhaust,merlin III heads, full msd, 741 cam, isky1.8 rockers, holly marine hp1000 carb with a super sucker pushing over 600hp and cant get over 75mph with a bravo 1 upper and a -2 imco lower. the 3 blade props I blow out easy, hard to get on plane, 4 blade bravo 1 have allways worked well, and the boat is planted in the rough water. I turn a labbed b1 26p around 5350 rpm @ 75mph, im also running eddie marine 24 tabs. full or 1/4 tank, people and stuff have little effect. Top speed is the same, the boat is flying stable at those speeds with the tabs at zero I had the bottom acid washed and checked for a hook, the speedometeres are with in 1mph checked with gps. I thinking about a 24p maximus, less splipage might being me to the 80mph im looking for, any ideals? thanks, glen

 

[ezstriper]

something is not right with your combination ? did you do any dyno runs ? making 600hp but at what RPM ? maybe it's at a rpm higher than you can turn with your prop ? you should be able to get to that 80 mark. My 251, a little smaller and faster hull went 82 with handling issues(no ext steering) before I was forced to back out, with my mild 482 blower motor, which made 615hp @ 5500 @ 6-7 psi. in my testing was turning a stock bravo 1 26 and could only turn it about 4800, needed a 24, but also knew the boat was not safe and changed engines...Rob

 

[glen george]

Rev limiter is set at 5800 rpm msd.I have never seen rpm over 5500 with a 24 hydromotine or a 3 blade mirage, I have imco single external steering, I have had the engine rebuilt twice over the last few years with difference internal parts, cams pistions and rockers, tryed different combonations stock hp500 850 holly , Barry grant 850 carb and now im running a holly hp1000 marine i had built from performance carbs, i have installed large fuel lines from the tank, no fuel starvation here, fuel pressure gauge reads 5-7 lbs. I never had it on a dyno and guesses at the 600 hp i stated. its rock solid up to mid 70s mph and runs with the best of the boats, but I want more, when i went to the -2 imco lower the tracking and hangeling improved alot but i dont really get it aired out like i did with the bravo 1 lower. last spring i had the bottom checked for a hook ,they did not find one and cleaned and polished the bottom, I have purchased allmost everything i can think of to update this boat. when i went with the cmi headers and the merlin III heads i was hoping for a big improvement which was not much over the stock hp500 heads and gill exhaust, something is holding this boat back, not my checkbook.

 

[Big Red]

If you have never seen over 5500 with a 24p and it now turns 5300 with a labbed 26 your problem is lack of torque and HP their is no way around it. If you have 600 hp your torque needs to be way over that. Maybe you have hp and you are light on the torque side.

Whats a super sucker

 

[ezstriper]

I agree, you have the H/P but it's at a level that you never reach, what size intake runners ? what intake, also seems carb to big..seems you have all the componants to make power @ 6000-6800, but less than stock or there abouts in the rpm range you actually run...a common mistake with boaters trying to go fast....automotive reasoning can hurt you some times...Rob

 

[ezstriper]

also, those imco lowers are slower than a bravo...talking about the lower drive right ? I have a friend with a large cat, switched to them and lost 5mph....but live longer

 

[Mr. Buss]

Whats a super sucker

Goes under the carb like 4 hole spacer plate, and supposed to help with air velosity, less fuel fallout, better atomization, improved sex life, hotter women in your life....... yaddayadda,

i barrowed one at the dragstrip once, had no luck with it... but i know people that sear by em'

 

[ezstriper]

What intake runners ? also by looking at the cam..they have several 741 listed, but the one for a gen VI needs to go 6000 to make power, and if you have the 350 intake runners to boot..no wonder it's a dog...sorry you combo is way off..

 

[Big Red]

Goes under the carb like 4 hole spacer plate, and supposed to help with air velosity, less fuel fallout, better atomization, improved sex life, hotter women in your life....... yaddayadda,

i barrowed one at the dragstrip once, had no luck with it... but i know people that sear by em'

I got you, Never herd it called that or maybe I never paid attension. I thought the carb was a bit big myself .

 

[glen george]

merlin III heads with 310 intake runners, stock dart hp 500 manifold , I sould go back to a 850 cfm carb. 741 gen IV cam, backdated the cam button and timming chain+housing set up.

 

[ezstriper]

the heads are fine, which 741 cam ? I saw about 5 listed with that last 3 #'s also what is your total timing ?, yes on the 850

 

[Big Red]

merlin III heads with 310 intake runners, stock dart hp 500 manifold , I sould go back to a 850 cfm carb. 741 gen IV cam, backdated the cam button and timming chain+housing set up.

Make sure its a DP not vacuum.

 

[glen george]

Timing is at 34 degrees at 2700-3000 rpm, msd set up, exact 741 cam part no un known, i did have a 731 gen 5 cam befor this which i could not control at the dock very well compared to this now one installed at rocco marine pa who did my rebuild., idles at 650 and great dock manners, I sold the holly 800 cfm last year that was stock on this motor, when this hp500 was stock, ran only 72mph in my boat with a bravo 1 outdrive. I will down size the carb this spring, with the hp1000 holly the rear transon was alot cleaner after a wot run than the smaller carbs.

 

[ezstriper]

ok, stock 500hp ran 72, now 540 w/good heads etc only 75 right ? something weird here..if it idles around 650 like you say you should be good....hey Red..I know you are watching this to...this is the same boat as yours right ? what's up here ? what engine/combo are you running now ? I know vinny although no longer on here but we still talk runs in the 80's in that hull with a simular engine set-up...

 

[glen george]

see pictures at offshoreonly checkmate photos, I designed in the yellow interior form a group of pictures and had it made, I do run 2 speedometers off one external pickup, I will have to get a gps by spring, mabey get a 2" spacer for the outdrive?

 

[ezstriper]

Glen, another thought...I did ck your pics in OSO..nice peice !, try lifting you engine hatch 4-5 inches..if you can and make a run...see what diff it makes, that much engine may be running out of air..it does happen on these things....the small vents on the sides don't get much "good" air in...here's what I did.....Rob

 

[ezstriper]

the outside

 

[glen george]

Thinking back, i thought it was my rev limiter on the old thunderbolt iginition, switched to the complete msd, box,dist and coil, disconected all previous wires, even if i overtrim it out cant get over 5500 rpms, thats when i started looking at the boats bottom. I have large openings on the sides of the seats going back into the engine comparment that wont restrict the air.

 

[ezstriper]

don't be so sure, you are still forcing a high performance engine to breath that very hot air trapped in there, I know tha are you are talking about, buy next season, run the baot hard for a while stop open the hatch....see hot much heat rolls out...there was a guy on OSO with a chapperal that said he tried it on his and would run almost 5 mph faster...I am even going to make a plate seal off mine so the engine only breathes from the outside air...every race car brings in cool air in some way...Rob

 

[253 SED]

97 convincer 253...496(stroked 454)....crane 741 cam...781 heads with BIG valves...33 degrees timing...blowing 5 lbs into it ..twin 1050 carbs...and running 81.2 on GPS at 5000 revs..is quite a bit more there if i wanna spin the engine to 55-5800 revs...741 cam is good to 6500 rpms...but no need to spin it any harder than you have to...im running a labbed 28p Bravo 1 prop...will someday try a 26p bravo 1 and spin it alittle harder to see what top end realy is but im happy for now ...my wife hates to go any faster than idle...