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Topic Review (Newest First)
01-13-2006 09:27 PM
topfuel Maybe this will help. I had a 330 hp Merc engine in my 27' boat. Stock, with the stock prop, it would turn 4400 at full throttle. I removed the intake and stock cam, timing set and valvetrain and replaced it with a larger cam with matching components and a roots supercharger. I made a few test runs with the stock prop purely to see what the engine would(could) endure rpm and load wise. It was turning 6100 rpm when I decided not to push it further. I was running 12 lbs of boost.
The 330hp engine uses similar components to a truck engine. Cast crank and cast flat top pistons, peanut port heads, cast iron intake w/Q-jet, and small camshaft. Most of the marine big blocks I have seen are 4 bolt main, even the low hp models.

The engine is currently being turned into a 496 with alum heads and I'm adding a Super Chiller intercooler under the blower.
01-13-2006 05:07 PM
Dan B Another thing to remember about boat engines is that the correct propeller must be matched for the engine. If your manual says max rpm is 4400 RPM then the correct propeller would let you run the boat wide open throttle and the engine will not exceed 4400 rpm. Let's say you have a prop with too much pitch (or cup) and at wide open throttle you only get to 4000 RPM... this is very, very bad because you are "overpropped" and the extra load on the engine will shorten it's life. It is just as bad if it is "under-propped" and at wide open throttle you exceed 4400 RPM. The engines are designed to get maximum speed at the stated max RPM, then you should typically run the engines at around 75% of the max (around 3200-3300) when you are cruising. Easy test, just go out and hit wide open throttle for a few minutes and see what it does. You want it to be within a 100 RPM of max otherwise there is either a problem with the prop, motor, or the hull.
If your engine can turn 4700 RPM then it has the wrong prop. Typical boat surveys include wide open throttle runs for about 5-10 minutes to check this and see if the temp and oil pressure meet specs. Some owners almost have a heart attack when we do this but if the combination is set up correct you can run for a long time at WOT with no harm done. This applies to your standard marine engine (Merc or Crusader) and their manual should be followed.

"Captain" Dan B

PS. What kind of boat/drive are we talking about here?
01-13-2006 04:03 PM
gearhead56
boat rpm

from what i have built of boat motors one thing to remeber is theres time the drive actually leaves the water , crossing wakes , getting to plane can be scary on a crowed day!. Unless shes built to kill youl waste the motor easy , i know from experience.
05-07-2002 12:49 PM
middspecauto the reason it says 4400 rpm max is because it is probably a two bolt main. boat engines take more abuse than a regular car or truck engine because they are at sustained high-rpm levels. if the manual says 4400, run it at 4300. better safe than sorry.
05-07-2002 10:34 AM
wakealanche
Stock 454 max rpm

This may be a little different than most of the "hot rod" tech questions but here goes. I have a 454cu in PCM in my boat & the manual says 4400 is the max rpm. There is one other model which has some differences. It has a 4 bolt main, different crank & heads, compression ration etc. It has a max rpm of 5200. My question is, will it be safe to run my 454 around 4700 rpm even though my model is the 4400 max? Am I seriously risking throwing a rod or busting my crank??

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