Water Pressure vs. Cylinder Head Temperature Gauge

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Author Topic:   Water Pressure vs. Cylinder Head Temperature Gauge

cmarques posted 07-08-2005 09:37 PM ET (US)
What difference or benefit is there between having an engine cooling system water pressure gauge versus having an engine cylinder head temperature gauge? My Dauntless has a temperature gauge installed and when I test rode on a new Montauk recently it had a pressure gauge instead and no temp gauge. I thought about a pressure gauge but really don't have room for another gauge. If a temperature gauge shows a normal temperature reading, does that mean you have enough water pressure? Whereas a pressure gauge shows water flow but won't indicate overheating?
Inquiring minds want to know!
Chris
where2 posted 07-08-2005 11:42 PM ET (US)
A pressure gauge will indicate a loss of pressure the moment you suck a bag onto the lower unit. A temperature gauge will react more slowly since it will require loss of cooling water to heat the head enough to indicate a higher temperature reading.
Get a pressure gauge, and carry an IR thermometer in the toolbox. The IR thermometer is all sorts of fun around the house on non-boating days. Hey, how cold is that Soda I put in the fridge an hour ago? How cold is that air coming out of the air duct? How warm does the top of my monitor get?
jimh posted 07-09-2005 12:02 AM ET (US)
Many installations have both a cooling system water pressure gauge and a cylinder head temperature gauge.
I have a water pressure gauge. When I asked my master mechanic about installing a engine temperature gauge, he said it was not really necessary. Usually there is an alarm sensor to warn of high engine temperature.
Jerry Townsend posted 07-09-2005 02:21 AM ET (US)
There are pros and cons to both - but when all is said and done, I want to see the temperature.
The temperature gauge senses the parameter which we are concerned about. As such, we know the temperature of the water discharging from the engine - the fact that there is flow of coolant and heat removal - or the lack there-of. In the engineering and control world, such a parameter is denoted as a feedback signal - required for stable control. But, a temperature indication will not be as fast in detecting a flow blockage/stoppage.
A pressure guage only provides a 'possible' indication of flow. But nothing regarding the heat removal process. And in the engineering world - this indication would be denoted as a feed-forward signal and 'may' be used for control - but must be used with a feedback signal. And one should realize that you can have pressure - but no flow! - simply block the discharge.
There is not a big difference in the cost of either measurement/indication.
Therefore, I fail to see the rationale for using a pressure gauge instead of a temperature indication.
Answering Chris's question - if the water temperature is normal - there is, by definition, water flow which requires adequate water pressure.
Some refer to cylinder temperature - however, I suspect that the temperature indications are of the temperature of the water being discharged. Measuring cylinder temperatures and displaying the peak or maximum temperature would be, in my view, ideal - but expensive. --- Jerry/Idaho
jimh posted 07-09-2005 09:30 AM ET (US)
Jerry--I love your engineering analysis!
My engine is a V6, and it has two temperature sensors, one on each cylinder head. The temperature sensors are located at the top of the block, which probably tends to be the hottest location. If I installed a temperature gauge to replace them, it would seem appropriate to install two gauges, one for each head. Or perhaps there is another located provided on the engine where a single temperature sender could be installed.
Also, I like the water pressure gauge because it can show you incipient problems. If your water pump is beginning to weaken or if the cooling inlet is clogged, you will see reduced pressure before the engine overheats.
You can get a visual indication of cooling water flow from the output of the cooling system aspirator (or tell tale or confidence stream). Depending on the model and brand of engine, the amount of water in the output flow can be fairly significant. On some engines it is just a little needle of a stream, while on others it is more like a faucet flow. My engine produces a significant flow, so this also provides an indication of proper cooling system operation.
kingfish posted 07-09-2005 11:07 AM ET (US)
I really like to know *both* temperature and pressure, so I have installed a temp gauge system to augment the pressure gauge system that was OEM on my boat. I am a little disappointed in that the normal operating temperature is so far at the low end of the scale on the only gauge face available on the particular teleflex gauges I have (Amega), that the needle hardly moves from rest when the motor is at 140º. But I am more confident in the data I'm getting, and that's a good thing.
I have picked up an IR hand held thermometer (to check differential head temp from one side to the other on a Yamaha 225 that seems to keep giving phantom overhest alarms one one side), and it really is fun to mess with. The most fun thing is how the laser pointer light drives my cat crazy.
John
Clark Roberts posted 07-09-2005 03:26 PM ET (US)
Like Kingfish, I prefer both temp and press meters on big V6 engines. A press meter is helpful when a jackplate is used to assure that engine is not raised too high. I have one engine with a power jack plate and when I must raise engine as high as possible as I traverse oyster and sand bars I can just watch the press meter and stop raising engine when press starts to rapidly drop off. On my 60hp engines I have only the temp gauge and on my little 20hp I have no instrumentation at all... works for me...Happy Whalin',,, Clark.. Spruce Creek Navy
PS>jimh, I had twin temp gauges on my 135 OptiMax and thought it was a good idea. THere are blind holes on each cyl bank for the sensor installation and that may be the case on your engine also.
where2 posted 07-09-2005 10:39 PM ET (US)
Kingfish, If you'd bought an IR thermometer w/o the laser pointer, you could measure the cat's temperature. I point it in their ear (not good to do with the laser aiming version since the cat will look at the laser). The IR thermometer is definitely more fun than a 163° Markall thermo-melt stick for measuring engine head temps, and troubleshooting. I'm always finding new uses for my IR thermometer.
kingfish posted 07-09-2005 11:39 PM ET (US)
Still can take Porter's temp - the laser light is an elective button on the IR thermometer I got at Sears (and I'm careful about his eyes when we're playing!) I am still amazed that for less than 50 bucks you can lay your hands on such a sophisticated and handy tool.
LHG posted 07-11-2005 02:39 PM ET (US)
Getting back to Chris' question, an engineer at BW told me that for the 4-stroke engines, MERCURY told them to switch from the CYLINDER HEAD (not water) temp gauge used on the 2-strokes, to a water pressure gauge. Obviously, this would only affect the Yamaha produced powerheads, as all of Mercury's other engines have Smartcraft, which gives you everything.
Assuming you were looking at a 170 with a 4-stroke, that is what you saw.
Evidently on this Yamaha produced 4-stroke powerhead, knowing that you have water pressure (water intake) is a better indicator of impending trouble than circulating water temp, since the engine is not recirculating water like a car does, and gets a constant flow of new, cool water. I don't know if these 4-strokes have a temp warning horn.
As mentioned, for conventional 2-strokes, water pressure has always been recommended for hi-performance installations.
Jerry Townsend posted 07-11-2005 05:04 PM ET (US)
Larry - something does not compute - Why would MERCURY be telling BW to switch from a temperature sensor to a water pressure sensor? MERCURY is the outfit making the engines - not BW. What cog have I slipped? ---- Jerry/Idaho
LHG posted 07-11-2005 05:57 PM ET (US)
Jerry - I don't really know the answer, but we are talking gauge rigging by the boat builder, and Mercury wanted the 4-strokes rigged with the water pressure gauge, and the 2-strokes rigged with temp gauges.
From my experience with water pressure gauges, there actually is no "sensor" on the engine, only a plugged threaded hole where the brass take-off fitting can be installed, connected to a tube up to the dash gauge. These are mechanical gauges, and only use current for the lighting.
Maybe someone with more experience than I with 4-stroke outboards could help out as to why Mercury and Yamaha want water pressure gauges instead of temp gauges on these engines.
jimh posted 07-11-2005 10:21 PM ET (US)
Good point about the temperature gauges usually measuring the cylinder head temperature (at the top of the block) and not the cooling water temperature.
[Changed TOPIC to reflect this.]
Clark Roberts posted 07-13-2005 06:34 AM ET (US)
My Merc 115 four stroker has only one threaded hole into the water jacket, located on starboard side of engine low on the block, and is the only place to tap in for water press gauge or temp sensor. I chose to install a water press gauge since I have a power jack plate and for the temp sensor I formed a bracket and installed the sensor pressing against the block high on starboard side. I also installed an oil press gauge to augment the low oil press audible warning. Yes, the engine has a high temp audible alarm. My engine does not give an audible warning check when ignition is turned on but all my other Mercs do! This must be a Yamaha feature. Happy Whalin'... Clark...Spruce Creek Navy
DaveH posted 07-13-2005 10:04 AM ET (US)
Larry (LHG):
A few theories. I suspect that the nature of the cooling channels in the 4 stroke are more critical than in the 2 stroke:
First, remember that "real estate" is at a preminum with 4 strokes. Size and weight being the critical measure on the boat versus power, the engineers created a very intricate lost-foam casting which allows tight cylinder spacing. The casting on a 4-stroke allows for very small cooling channels over the entire engine which are required to bring coolant into very critical areas around the cylinder to prevent exposure of overheating. The 2 stroke channels were huge in comparison.
When coolant flow is lost, air against metal is exposed in its place in these channels and the heat will rise accordingly. The ability of the coolant to reach these areas has been compromised and the increase in temperature will not be recognized in the outgoing flow. It will take time for the excess cylinder heat to travel to the sensor and the problem to be identified.
Liquid pressure loss however, is immediately felt in the entire system and a very good indicator as well. In a pure mechanical system, the small pressure tube leading up to the dash will recognize the pressure loss and flex the internal bourdon tube, rotating the dial on the gauge. It is cheap, quick to respond and very accurate. The accuracy is dependant on the gauge itself, but even the cheap Chinese-made 1" gauges are within 5% accuracy.
Both of these methods together are the best way to protect yourself. I believe Smartcraft (as well as other engine manufacturers controllers) uses electronic sensors to measure these parameters which are fed to a computer (PLC). The electronic sensors are very accurate and quick to respond as well. But remember, electonics hate vibration and heat. They may fail without notice. I do not know if the PLCs in these engines do a sweep of the sensors on start-up. Maybe Seahorse will read this and give us some more background on this subject.

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Author	Topic: Water Pressure vs. Cylinder Head Temperature Gauge
cmarques	posted 07-08-2005 09:37 PM ET (US) What difference or benefit is there between having an engine cooling system water pressure gauge versus having an engine cylinder head temperature gauge? My Dauntless has a temperature gauge installed and when I test rode on a new Montauk recently it had a pressure gauge instead and no temp gauge. I thought about a pressure gauge but really don't have room for another gauge. If a temperature gauge shows a normal temperature reading, does that mean you have enough water pressure? Whereas a pressure gauge shows water flow but won't indicate overheating? Inquiring minds want to know! Chris
where2	posted 07-08-2005 11:42 PM ET (US) A pressure gauge will indicate a loss of pressure the moment you suck a bag onto the lower unit. A temperature gauge will react more slowly since it will require loss of cooling water to heat the head enough to indicate a higher temperature reading. Get a pressure gauge, and carry an IR thermometer in the toolbox. The IR thermometer is all sorts of fun around the house on non-boating days. Hey, how cold is that Soda I put in the fridge an hour ago? How cold is that air coming out of the air duct? How warm does the top of my monitor get?
jimh	posted 07-09-2005 12:02 AM ET (US) Many installations have both a cooling system water pressure gauge and a cylinder head temperature gauge. I have a water pressure gauge. When I asked my master mechanic about installing a engine temperature gauge, he said it was not really necessary. Usually there is an alarm sensor to warn of high engine temperature.
Jerry Townsend	posted 07-09-2005 02:21 AM ET (US) There are pros and cons to both - but when all is said and done, I want to see the temperature. The temperature gauge senses the parameter which we are concerned about. As such, we know the temperature of the water discharging from the engine - the fact that there is flow of coolant and heat removal - or the lack there-of. In the engineering and control world, such a parameter is denoted as a feedback signal - required for stable control. But, a temperature indication will not be as fast in detecting a flow blockage/stoppage. A pressure guage only provides a 'possible' indication of flow. But nothing regarding the heat removal process. And in the engineering world - this indication would be denoted as a feed-forward signal and 'may' be used for control - but must be used with a feedback signal. And one should realize that you can have pressure - but no flow! - simply block the discharge. There is not a big difference in the cost of either measurement/indication. Therefore, I fail to see the rationale for using a pressure gauge instead of a temperature indication. Answering Chris's question - if the water temperature is normal - there is, by definition, water flow which requires adequate water pressure. Some refer to cylinder temperature - however, I suspect that the temperature indications are of the temperature of the water being discharged. Measuring cylinder temperatures and displaying the peak or maximum temperature would be, in my view, ideal - but expensive. --- Jerry/Idaho
jimh	posted 07-09-2005 09:30 AM ET (US) Jerry--I love your engineering analysis! My engine is a V6, and it has two temperature sensors, one on each cylinder head. The temperature sensors are located at the top of the block, which probably tends to be the hottest location. If I installed a temperature gauge to replace them, it would seem appropriate to install two gauges, one for each head. Or perhaps there is another located provided on the engine where a single temperature sender could be installed. Also, I like the water pressure gauge because it can show you incipient problems. If your water pump is beginning to weaken or if the cooling inlet is clogged, you will see reduced pressure before the engine overheats. You can get a visual indication of cooling water flow from the output of the cooling system aspirator (or tell tale or confidence stream). Depending on the model and brand of engine, the amount of water in the output flow can be fairly significant. On some engines it is just a little needle of a stream, while on others it is more like a faucet flow. My engine produces a significant flow, so this also provides an indication of proper cooling system operation.
kingfish	posted 07-09-2005 11:07 AM ET (US) I really like to know both temperature and pressure, so I have installed a temp gauge system to augment the pressure gauge system that was OEM on my boat. I am a little disappointed in that the normal operating temperature is so far at the low end of the scale on the only gauge face available on the particular teleflex gauges I have (Amega), that the needle hardly moves from rest when the motor is at 140º. But I am more confident in the data I'm getting, and that's a good thing. I have picked up an IR hand held thermometer (to check differential head temp from one side to the other on a Yamaha 225 that seems to keep giving phantom overhest alarms one one side), and it really is fun to mess with. The most fun thing is how the laser pointer light drives my cat crazy. John
Clark Roberts	posted 07-09-2005 03:26 PM ET (US) Like Kingfish, I prefer both temp and press meters on big V6 engines. A press meter is helpful when a jackplate is used to assure that engine is not raised too high. I have one engine with a power jack plate and when I must raise engine as high as possible as I traverse oyster and sand bars I can just watch the press meter and stop raising engine when press starts to rapidly drop off. On my 60hp engines I have only the temp gauge and on my little 20hp I have no instrumentation at all... works for me...Happy Whalin',,, Clark.. Spruce Creek Navy PS>jimh, I had twin temp gauges on my 135 OptiMax and thought it was a good idea. THere are blind holes on each cyl bank for the sensor installation and that may be the case on your engine also.
where2	posted 07-09-2005 10:39 PM ET (US) Kingfish, If you'd bought an IR thermometer w/o the laser pointer, you could measure the cat's temperature. I point it in their ear (not good to do with the laser aiming version since the cat will look at the laser). The IR thermometer is definitely more fun than a 163° Markall thermo-melt stick for measuring engine head temps, and troubleshooting. I'm always finding new uses for my IR thermometer.
kingfish	posted 07-09-2005 11:39 PM ET (US) Still can take Porter's temp - the laser light is an elective button on the IR thermometer I got at Sears (and I'm careful about his eyes when we're playing!) I am still amazed that for less than 50 bucks you can lay your hands on such a sophisticated and handy tool.
LHG	posted 07-11-2005 02:39 PM ET (US) Getting back to Chris' question, an engineer at BW told me that for the 4-stroke engines, MERCURY told them to switch from the CYLINDER HEAD (not water) temp gauge used on the 2-strokes, to a water pressure gauge. Obviously, this would only affect the Yamaha produced powerheads, as all of Mercury's other engines have Smartcraft, which gives you everything. Assuming you were looking at a 170 with a 4-stroke, that is what you saw. Evidently on this Yamaha produced 4-stroke powerhead, knowing that you have water pressure (water intake) is a better indicator of impending trouble than circulating water temp, since the engine is not recirculating water like a car does, and gets a constant flow of new, cool water. I don't know if these 4-strokes have a temp warning horn. As mentioned, for conventional 2-strokes, water pressure has always been recommended for hi-performance installations.
Jerry Townsend	posted 07-11-2005 05:04 PM ET (US) Larry - something does not compute - Why would MERCURY be telling BW to switch from a temperature sensor to a water pressure sensor? MERCURY is the outfit making the engines - not BW. What cog have I slipped? ---- Jerry/Idaho
LHG	posted 07-11-2005 05:57 PM ET (US) Jerry - I don't really know the answer, but we are talking gauge rigging by the boat builder, and Mercury wanted the 4-strokes rigged with the water pressure gauge, and the 2-strokes rigged with temp gauges. From my experience with water pressure gauges, there actually is no "sensor" on the engine, only a plugged threaded hole where the brass take-off fitting can be installed, connected to a tube up to the dash gauge. These are mechanical gauges, and only use current for the lighting. Maybe someone with more experience than I with 4-stroke outboards could help out as to why Mercury and Yamaha want water pressure gauges instead of temp gauges on these engines.
jimh	posted 07-11-2005 10:21 PM ET (US) Good point about the temperature gauges usually measuring the cylinder head temperature (at the top of the block) and not the cooling water temperature. [Changed TOPIC to reflect this.]
Clark Roberts	posted 07-13-2005 06:34 AM ET (US) My Merc 115 four stroker has only one threaded hole into the water jacket, located on starboard side of engine low on the block, and is the only place to tap in for water press gauge or temp sensor. I chose to install a water press gauge since I have a power jack plate and for the temp sensor I formed a bracket and installed the sensor pressing against the block high on starboard side. I also installed an oil press gauge to augment the low oil press audible warning. Yes, the engine has a high temp audible alarm. My engine does not give an audible warning check when ignition is turned on but all my other Mercs do! This must be a Yamaha feature. Happy Whalin'... Clark...Spruce Creek Navy
DaveH	posted 07-13-2005 10:04 AM ET (US) Larry (LHG): A few theories. I suspect that the nature of the cooling channels in the 4 stroke are more critical than in the 2 stroke: First, remember that "real estate" is at a preminum with 4 strokes. Size and weight being the critical measure on the boat versus power, the engineers created a very intricate lost-foam casting which allows tight cylinder spacing. The casting on a 4-stroke allows for very small cooling channels over the entire engine which are required to bring coolant into very critical areas around the cylinder to prevent exposure of overheating. The 2 stroke channels were huge in comparison. When coolant flow is lost, air against metal is exposed in its place in these channels and the heat will rise accordingly. The ability of the coolant to reach these areas has been compromised and the increase in temperature will not be recognized in the outgoing flow. It will take time for the excess cylinder heat to travel to the sensor and the problem to be identified. Liquid pressure loss however, is immediately felt in the entire system and a very good indicator as well. In a pure mechanical system, the small pressure tube leading up to the dash will recognize the pressure loss and flex the internal bourdon tube, rotating the dial on the gauge. It is cheap, quick to respond and very accurate. The accuracy is dependant on the gauge itself, but even the cheap Chinese-made 1" gauges are within 5% accuracy. Both of these methods together are the best way to protect yourself. I believe Smartcraft (as well as other engine manufacturers controllers) uses electronic sensors to measure these parameters which are fed to a computer (PLC). The electronic sensors are very accurate and quick to respond as well. But remember, electonics hate vibration and heat. They may fail without notice. I do not know if the PLCs in these engines do a sweep of the sensors on start-up. Maybe Seahorse will read this and give us some more background on this subject.