This year my boat (272 BAJA) has picked up a new ill handling quirk. I freshened notor,changed heads and exhaust from lightning headers and merlin heads to Aluminum stainless high torque 3's and aluminum heads. The motor has approximately 200 more hp than last year(had 725-750 before now has just under 950) and between the exhaust and heads is probably 100 pounds lighter in the stern. The boat would run 82.5-83 with a 30 pitch merc lab last year any day of the week around 5400 rpm's with normal load(4 people/3/4 tk gas,50 lb cooler) and ran a best of 85.1 with virtually no fuel w/1 person. All these speeds were at max trim to get bow up with prop slipping approx 18% by all calculations.I also tryed a non labbed 32 pitch at end of season last year with slightly smaller blower pulley so that it made same boost at 5100-5200 that it normally see's at 5400 but motor was slightly tired and boat ran 84.7 at 5100 (2 people/45 galloms gas no extra weight on board). Finally leaned on boat last night for real short burst after working most the bugs out of it (2 people in front seats,full tk gas minimal stuff in bow) with same 32 pitch prop. It ran 86 mph at 5200 but feels like stern is lifting,bow was digging in and bow steering above 75 mph. Boat has felt like that before when we had 150lbs of camping gear in cabin,full tk of gas and no passengers in back seat.I'm thinking the center of gravity is shifted fwd from less weight in the back,i can only take out another 40 lbs from bow if we leave aluminum anchor in the truck and take the mattress out of cabin so there really isn't any weight we can remove from front that isn't needed. The boat just seems like it won't "air out "and loosen up.The hp peak is at 5600(944 hp/890 ftlbs rorque),I'm going to try the 30 pitch labbed today and run it down to 1/4 tk of gas (holds 95 gallons) but by my calculations if I turn the 30 pitch labbed at 5600 with our normal slip i'm only going to see about 88 mph. Some other additional info is boat makes 910 hp/920 ftlbs tq at 5200, On the dyno we ran cmi elbow tops dry with no accessories,and I'm using ggb extreme 5" clamp on mufflers. I would figure I'm probably making 30-50 hp less as installed but not sure.Also the under hatch heat seems to be at least 50 degrees hotter than before as the aluminum stainless marines and aluminum heads must radiate more heat into engine compartment (my old lightning headers didn't get very hot on the surface,maybe 120 degrees and these new manifolds are so hot you can barely touch them maybe 180 degrees). Any suggestions to get more bow lift or speed,what prop? I'm running std length outdrive w 1.5 gear ratio also. I expected to run mid 90's with additional power,maybe the old motor made more power than we were estimating,Smitty

More info,boat ran 76 mph off dyno 2 years ago with 630 hp/700 ft lbs torque,the additional 7-9 mph it had came from timing adjustments and more boost which is where the 725-750 hp estimate come's from. I'm running that same additional boost and timing already so there's not much more i can get out of motor,Smitty

I assume the dyno run is of your current motor. At 5200 rpm you are making more tq than hp, that's a problem. You have to spin the motor up to a higher rpm and get away from all that tq, your drives will also appreciate it.

I noticed on the dyno run it stopped at 5600 while the motor was still climbing with hp, why?

Yes your cg has moved and that's part of your problem but not all of it, by running a smaller pitch prop you will reduce the stern lift and the slip as well. The smaller pitch prop will allow the engine to spin more rpm thus getting away from the torque peak. I wold find out what the absolute max rpm you can turn your motors and prop it out to that rpm. This is a very common problem with high torque motors propped at a relatively low rpm.

Please call if you have any questions.

Matt

Matt thanks, yes,that is one of my dyno sheets on current motor.The date on top is wrong,tyler's computer printed jan of o3 on top but further down you see where he punched in may 20th of 05. The motor is built to turn to 6000,we pulled it to 5800 on a few runs and still had 852 ft lbs tq and 942 uncorrected hp so at the upper rpm ranges it basically loses 13 ftlbs torque for every 100 rpm's and hp flattens out loosing about 2-3 hp per 100 rpm. Would labbing my 32 get me where i want to be? I'm going to try the 30 pitch labbed today if the conditions are safe to do it( you can't really wring a 272 baja out with this much power unless the water is ideal) Smitty

The first step should be to reduce the slip to an acceptable level. Based on the stock 32 running 5200@86 your slip is at 16%. This may explain why the bow feels heavy. A spacer would drop the drive thus pushing up the bow and reducing the slip which should result in an increase on the top end.
If you could turn a stock 28 Bravo 1@6000 rpm with 12% slip you should see about 90mph.

Do you have a rooster tail when the boat is at top speed and properly trimmed? How does the boat get on plane with both the stock and lab props?

Matt

matt,the boat pop's up on plane effortlesly with the 30 pitch labbed,you can almost punch it to wot from 10 mph without breaking prop loose.With the 32 non labed you have to roll into it a little bit slower or you can make it partially slip.The boats always had the best top speed by trimming it right to the max at wot. Let me give you a example,you punch boat to wot at 70 mph with drive in neutral position,boat rockets to 80 mph then sort of hangs there gaining another 1-2 mph,you then trim the drive to max trim(any higher would be trailer position and uncover gimbal support) the boat then picks up another 2-4 mph as it "loosens up" and starts to throw a small rooster tail. If you drop back a pitch (this was last year) from the 30 to a 28 it would pick up 400 rpm's and run same speed but it still wouldn't get those last 2-4 mph until i maxed out the trim. With the 32 pitch non-labbed (plus the effects of c/g moving forward) i can't get boat to loosen up,it feels like the bow is pitching me around like its digging in with too much weight in the front.I ran boat today w/30 labbed but was unable to get a wot pass in (i was on the lk st claire river at the races) the cross chop and traffic was a night mare,Smitty
Here is a pic from last year at 75 -80 mph with boat trimmed close to max.When its trimmed like that and bow picks up you can feel the thing literally loosen up.

The easiest place to start would be to put some weight on the transom. Salt bags, sand bags, lead, etc. Try 100 libs and see what happens. Put the weight all the way back right up against the transom. Take it for a run and see what happens. Then move the weight forward a few feet, toward the front of the engine and run it again, move the weight forward again and see what happens. This will give you a good idea of whats going on with weight.

If weight in the transom fixes the problem look around and see what you may be able to move aft, batteries, sea strainer, etc to see if you can fix the problem without adding weight. Getting the cg right is important for top end speed. Try removing some of the weight up front for a run and see what happens. With a new motor combination it's not uncommon to have to go through the dial in process all over again.

Matt

ran boat out yesterday,saw 5900rpm's w/30 pitch lab,might have even been touching rev limiter at 6000rpm's,boat ran 91 mph max trimmed in 1 1/2 ft chop,1/2 tk gas,50 lbs gear in cabin,2 people on board,moderate rooster tail. Doesen't seem like a c/g problem as much as I thought,I guess the big diff is the 30 is labbed and 32 isn't. Would like to see boat closer to 5600-5700 wot and more speed if possible,would 32 pitch factory labbed prop be the ticket? Thanks,Smitty