Also if u run high rpm it’s recomended to swap your impeller...
Never heard that one before. Where did you read that?
Some marine shops have dynamometer’s. We run engines at W.O.T. on it. We use muffs. Depending on the engine this requires special muffs, hose and enough water supply.
The recommendations you see in the owners/service manual to not exceed a certain RPM are there for a couple of reasons.
1. It serves no useful purpose and can damage the engine. Don’t rev a engine with no load, And if you feel you must raise the RPMs, do it slowly both up and especially down. Don’t “chop” the throttle.
2. Some engines at some RPMs will experience water reversion.
In 30+ years as an Marine mechanic the only problem with high water pressure I’ve seen is the muffs have a tendency to slip off. I have seen cheap garden hoses get sucked flat due to not enough water pressure.
The correct way to use muffs.
1. Place muffs over the water intake. Check to see if you have any low water intakes and plug them.
2. Turn on the water just enough to see some water bypassing the muffs.
3. Start the engine and increase RPMs enough to get the alternator charging, don’t exceed the RPMs listed in your owners/service manual.
4. Increase water flow so that a small amount of water is bypassing the muffs.
5. When done running the engine. Lower RPMs to idle and shut off engine. Turn off water and remove muffs and anything you used to plug low water intake.
To make this easier. Make a short hose (like 10ft) and get a in-line shutoff valve. Hang this from one of your cleats so you don’t have to get out of the boat to adjust the water.
