"Water Pressure 101"- Poppet Valves and Variables with Water Pressure

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.

I understand it if no one else does... Good post Don..

Great post, I run by water pressure, when making a speed run adjust the plate height by keeping at least 12psi. Its amazing how much that last bump on the plate makes.

Don: Excellent read http://xs-s.com/zf/images/smile/emthup.gif http://xs-s.com/zf/images/smile/emthup.gif http://xs-s.com/zf/images/smile/emthup.gif BTW: In my opinion if you spend as much as we do on motors your best bet is to spend the extra 1K and get a hydraulic plate.

As you know water may look the same but every time based on conditions your optimal engine height may vary a little and a Hydraulic plate is the only way to achieve the best height for all conditions http://www.bassboatcentral.com/smileys/smile.gif

+1 http://xs-s.com/zf/images/smile/emthup.gif

Thanks Don .
Great post .
Doug http://www.bassboatcentral.com/smileys/mug1.gif

Excellent post Don. I posted awhile back {in Jay's forum} about the "ram pressure theory" on my Mirage. I actually cut off half of the impeller blades {yes I know, but I was experimenting!} and even though there was little if any water pressure at lower rpm's, in a drag-race application there was plenty {20+} at 110+...........and yes it did work....honestly, don't know if I gained any performance though http://www.bassboatcentral.com/smileys/devil.gif ....... The actual pressure may vary boat to boat, trim angle, speed involved, and motor height.

Thanks for posting that, Bob. I was hoping you'd share that experience with people here....

Great example of the theory... and as we see everywhere else, I'll add the following disclaimer:

"This stunt was performed by an expert in his field, who has spent many years preparing for this excercise, and taken adequate precautions to lessen risks. You should not try this at home, under any circumstances, at any time, as it may cause personal injury or property damage." http://www.bassboatcentral.com/smileys/bows.gif

Great to see you in the new forum, Redline!

+1, thanks for the "disclaimer", in this day and age everyone needs to be cautious of risks associated with the operation of hi-perf applications....and, you are entirely welcome!!!! :)

Not to jump your post EuropeanAM, have you seen the new face seal for the 43026t2 impeller housing,saw my first one last week,its alittle wider than the old one.Had a psi problem with a 115 Opti that would over heat in a left turn every time but would not in a right hand turn,temps would soar turning left and set the alarm and go into guardian,9.5 was as high a psi I would see.Changed the impeller and checked engine height all looked perfect and lake tested while on the DDT and still the same problems.Got my hands on the new style face seal and it cured the problem and also raised water psi to 15 psi on top end.the guys who change your own water pumps the 43026T2 housing takes a rubber seal on top of the housing around the drive shaft to keep air and exhaust from entering the water pump housing and causing a over heat problem,it takes a cheap little tool to set the seal to the housing,if you change your own impeller get one from your dealer he can order one.Bottom line this impeller fits a bunch of Mercurys from 3 liter down.Also good read on water psi 101. http://www.bassboatcentral.com/smileys/bows.gif

All this sounds interesting, but for me, I have no issues with WP at WOT with my 3.0L EFI (20-25+ lbs @ WOT). I do have the Poppet Valve kit from Eric Simon, and it will pump a 1" stream of water out the back of the motor that looks like a fire hose. I have had no issues with overheating, and with this kit, I get an increase in performance from having the water go out the rear, rather than through the exhaust column. http://www.bassboatcentral.com/boardstuff2/beers.gif

Blackmax...

If you mean by "new" face seal- the one in current production, then "yes".

Replacement seals usually come with the little "plastic cup install tool". Allows you to properly set the face seal. We do see problems with the face seal, especially in high-mounting height applications... occassionally, when the boat it "bounced" out of the water, the pressure is so high (ram pressure) on "re-entry" that the face seal is driven anywhere from 1/2-5" up the driveshaft.

That's a "dead-giveaway" as to what's "going on" http://www.bassboatcentral.com/smileys/wink.gif

Of course, we don't see too many of the 1.5 Liters this way... mainly 3L's.

The new ones are not out yet,it is wider at the base than the old one,a part number from what I was told will be issued soon and dealers will be able to order them,this from our TAM. http://www.bassboatcentral.com/smileys/USA1.gif

That would be good to see... especially as it appears most of the ones we've been getting "barely" seal to the top of the housing http://www.bassboatcentral.com/smileys/eek.gif

Mr. EuropeanAM,

I cant seem to get a solid answer out of anyone when it comes to my motor. It is a 1992 Mercury 200 XRI EFI serial #0D097779. My water pressure (on the garden hose in the driveway) reads 3psi at idle and when you give the throttle to it 10psi. Is this normal for this motor? I have replaced my water pump, (impeller, upper housing, metal plate and gaskets), thinking that was the problem. Still nothing. I then took the poppet valve off and looked for trash or cracks in the diaphram, and put it back together. Still no changes in pressure. I then did a compression test (engine was warm) to see if i could notice a head gasket leaking and i was 125psi on all cylinders. The reason i did a compression test was i noticed the water pressure gauge was jumping from 8psi to 10psi once i gave it a little throttle. Is this the poppet valve or something major? PLEASE HELP!!!

Mark

Other than "idle water pressure"- ALL water pressure tests should be taken "in the lake- with the boat in forward gear". It's not possible to properly determine water pressure at WOT on a garden hose- the engine should NEVER be accelerated over 1300 RPM on the hose, anyway. http://www.bassboatcentral.com/smileys/wink.gif

Can you get a pressure reading in the lake at WOT? It sounds like your idle pressure is good... and it sounds like the poppet valve is properly working at mid-range RPM's (it begins to open around 2500 RPM). WOT pressure should be 12 PSI mininum... but we prefer 15 psi.

Thanks for the info Don,I have a 225 optimax and it runs in that range.But my temp has been low.Around 116 degrees WOT.It could be that the lake water temp is low I guess.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 198elite &raquo;</TD></TR><TR><TD CLASS="quote">Thanks for the info Don,I have a 225 optimax and it runs in that range.But my temp has been low.Around 116 degrees WOT.It could be that the lake water temp is low I guess.</TD></TR></TABLE>

WOT water temps will often be SUBSTANTIALLY lower than "thermostat temperature".

This is simply because the Poppet Valve controls water pressure at higher RPM's... and once the Poppet Valve has opened, the thermostats are essentially "non-functional". http://www.bassboatcentral.com/smileys/wink.gif

Thanks for getting back to me.What do you think the temp should be at idle?

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 198elite &raquo;</TD></TR><TR><TD CLASS="quote">Thanks for getting back to me.What do you think the temp should be at idle?</TD></TR></TABLE>

AT LEAST 120 degrees, for ALL Optimax product. Thermostat temps vary on the Optis... but ALL should be able to attain "at least 120 degrees" at idle speed. http://www.bassboatcentral.com/smileys/wink.gif