This post is in reponse to many others where questions of water pressure have been raised, especially people claiming their water pressure is 30 psi.
First, for the purpose of this exercise, let's say we have 3.0L Merc... an Optimax (although EFI/CARB results are similar). Consider it a perfect world- a brand new, complete water pump, a brand new, unrestricted block, new thermostats, and a brand new poppet valve assembly- seat, spring, diaphram and all, with a verified Pitot Sender operating a Smartcraft Instrument!
The customer states he has 30psi water pressure at WOT, and he is worried about this. Hold on now... before you say his poppet valve is bad.
Going back over the years through the service manuals, most of them simply instructed (and this is commonly missed) to test water pressure on a garden hose flush attachment, or in a test tank. This is NOT the same as the customer's complaint... because his engine is mounted on a bass boat, which is traveling (or better be) at 60-85 MPH while he is looking at his pressure reading. BIG DIFFERENCE!
No-where in any service literature I have read does it indicate that you should take WOT water pressure readings with the boat traveling, let's say in this case, 70 MPH. (Later manuals have a footnote regarding this, which states specifically, "Ram pressure is the water pressure created from water forced into the gearcase inlet holes as the boat travels forward. As speed increases, ram pressure increases. Water pressure may vary as ram pressure increases. Do not exceed specifications."
So, what ALSO needs to be considered is RAM PRESSURE. There are a ton of variables here, which include, but are not limited to hull shape, hull speed, engine mounting height, water temperature, low water pickups, etc. etc. Basic physics tells us that as the pressure at the water inlets increases, so does the pressure on the other end, regardless of how "good" the poppet valve is, period. It's not rocket science, it's just the way it is.
This particular "phantom or hypothetical customer" is not overheating, he has no warning horn, there is no freeze frame history in his ECM/PCM to indicate an overheat. So why is he reading 30psi water pressure? The answer is simply "Too much ram pressure".
In general, the poppet valve in his particular engine should be capable of limiting water pressure to a maximum of 25 psi (let's say it's a 225 Pro XS). That's based on a "mean-average" of boat combinations, but because he is on a jackplate, with a setback (it doesn't matter how much for this excercise), he's got enough "Ram Pressure" (the physics term would be "head pressure") to increase his water pressure to the point that the poppet valve just cannot keep up.
So what do we do? Going strictly "by the book", we need to either:
1. Slow the boat down to decrease Ram Pressure (he won't like this idea!)
2. Change the engine mounting height to bring the water pressure into spec
-or-, we can:
3. Leave it alone... it's not overheating.
4. Put a Hydraulic Jackplate on the boat and inform the customer where to keep
his water pressure.
#2 is a better choice, but may cause top end speed increase (or decrease), as well as hole-shot gain (or loss). So all we can do is try it- and see where he ends up.
#4 is my preferred choice, because it allows the customer to vary engine height based on boat load, etc. But this fellow just spent $50K+ on his boat... and he's not inclined to spend another $1K+ on a jackplate, installed.....
So, NOT IN EVERY CASE, but in some high-speed, high performance applications, the only choice left may be #3. Now I know there are a lot of you thinking "this guy's wrong... I've been to the classes, and I know what the manual says." Take it with a grain of salt here, guys. There is no way, whatsoever, that the spec's given can strictly apply to every single application, from a non-planing work hull to a 70+MPH performance bass boat. And let's face it, most of the folks on these boards are running the later, not the earlier!
So the moral of the story here is: Temper "specification" with real-world physics. If the guy is only 3-6 psi out of range, and he's not overheating, and hasn't been in the last 50-100 hours, odd's are his poppet valve is NOT faulty. He's in a lot better shape than he would be with LOW water pressure!
That being said, WHEN IN DOUBT, change it out. Regular, timely water pump, thermostat, and poppet valve inspection (or replacement) can, and often does, "save the day" when it comes to time on the water!
Just a request now, for my fellow guru's: before you hammer me to death, please go out on the lake and try it. By the time you come back, you'll understand the "ram pressure" theory very well, and have taken a dozen or so WOT readings at different mounting heights, with a fairly wide range of water pressure results!
3/4" mounting height can make more than a 5psi difference in some cases!
"General" Water pressure MINIMUM requirements by engine type:
3.0/3.2L & Verado 1.5 to 3psi at idle, 15 psi at WOT
2.5 Liter: 2-5 psi at idle, 12 psi at WOT.
Remember... these are the "MINIMUMS". Your results may vary.
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Originally Posted by basspro57
