Hannibal wrote:
So then an occasional 5k rpm isn't a problem. I've been saying that for a while now. More peak hp than torque at the flywheel still equates to more torque at the drive wheels. It's also in the gearing.
A boat motor and a pickup truck engine are both internal combustion engines. Your Merc 250 is designed to run 5-6k rpm continuously. The 5.7L Hemi, during R&R, was run at 5400 peak hp rpm full load for 300 hours. When torn down for inspection it showed only normal wear. Nothing melted or broke.
Are you implying the marine diesel unit will run forever?
The difference is that the 5.7 liter Hemi or any motor for that matter that is on the road is tuned from the factory to run best (efficiency wise) at speeds that is will normally see on a regular basis. On a vehicle motor that is going to be in the 1500-2000 RPM range normally. A diesel motor is run at those speeds 95% of the time even when loaded. A gas motor can't run at those speeds loaded because of the torque it takes to keep the load going. Hp is a number and nothing else. It just tells you how much work you can do over time. I don't care how you look at it, it takes torque to move the load. When you run a motor out side of it's optimum RPM range you will have excessive wear.
A boat motor is different. The difference in the spec sheet is that the RPM range that they put on the specs is not the max on a marine engine. It is the optimum range to prop it at. For each inch of pitch change equals around 250 RPMs give or take. You have lower unit gear ratios to deal with also.
What do you think the most reasons that all the big ship use diesel engines for? Fuel usage and durability are the biggest. Will they run for ever? If you don't destroy the block most will. Re-sleeve them install new parts and fire it up.
As far as the OP question, it's a loaded one with not enough info. Same specs? Does that mean same cu/in? Same hp? Same torque? Same speed? Same acceleration? This could go on for ever because you will never get the same specs out of the two engines.
