big motor low performance

need some direction to help this new motor, the boat is a 21 ft. bow rider, the old motor was a 305, this motr was replaced with a 350 with a 400 crank i was told that this is known as a 383 stroker motor and is known as a good set up for a boat motor. we just finished installing motor and seems to do fine in the drive way with water hose hooked to lower unit. placed boat in the water and was disappointed. the old 305 seemed to perform better. could there be a problem in using the original lower unit and propt set up.
the current lower unit set up is a merc cruiser alpha 1 (1998 model) 1.62 gear ratio. i think the propt is a 21 degree 14" aluminum. do we change the gear ratio and prop? if we do how do you select the correct set up. if my gauges are correct i was traveling about 22 mph at about 3200 rpm
any helpful info would be greatly accepted
thanks
ronnie

 

You have to supply us with some additional info; first reasonable figures on max power @ what rpm:s. Otherwise we have to guesstimate based on engine displacement, and there are far too many variables to justify any ideas on appropriate medicins!

 

It is a 383 if what they told you is true. The camshaft is really critical for the application. If it is for automotive use, the low end torque may be low. What is your specific complain? Hard to get on plane, low top end....?

 

seems to be running out of rpms on the top end, the oil pressure seems to drop as the rpms increase only when the boat is under power, setting still and increasing rpms oil prssure seems to hang around 40 and no noise, under power oil pressure is 20 or below and a rattling noise occurs in the motor area as rpms increase above 3200, seems that about the max rpms is about 3200 with a speed of 22 i was thinking that the max rpms would be around 4500, i am going to get a mechanical gauge and install where the electronic gauge is connected to see if the electronic gauge is correct

the man that built the motor knew it was for a boat i would think that that he would have used the correct crank. i will check with him what kind of answer should i be looking for
as far as the boat planing it seems to come out of the hole o.k. but i am having to limit myself on the rpms due to the noise and oil pressure

ronnie

 

Those engines need 10PSI of oil per each 1000RPM. If the oil pressure drops at high revs, something is really wrong. The rattle you hear is because the crank and rod bearings are running dry. They may already be damaged. The minimum max RPM should be 4200. The oil pump may be automotive instead of marine. Also, the oil pan mey be automotive and not have enough oil capacity. At high revs the oil accumulates on the top and may starve the pump. Also, it should have an oil scraper to get the oil on the crankshaft to fall into the sump.

 

Check that your oil level isn't too high , if the crank webs are hitting the surface it will cause foaming and the oil pump will ingest air. You may have to alter the level on the dipstick to allow for this crank. perhaps even change the sump .

 

Yes, the longer reach of the crank may be foaming the oil and add to the problem.

 

.....and with an auto sump, these problems will get worse in a boat at high trim angles as well.

 

Your next problem will be with the alpha 1 outdrive. The stroker 383 has the power to tear it up quick.

 

A windage tray and deeper pan can solve this issue, though I'd want to take the engine apart and find out what's really up and the damage, as the engine builder should have known about this issue, which isn't a common, but it does happen. The stroke of a 400 crank is 3.48", the 305 is 3". It's possible the last couple of crank throws are hitting the oil, but again a windage tray and deeper sump will fix the problem. If the builder missed this issue, what else did he miss? Take it apart and find out now, while the parts are salvageable. You might have to turn things true again, but it's cheaper then a new crank, rods and/or block.