Cummins Marine Diesel Repower Specialists Forums General Discussion Stack EGT drops at higher RPM?

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  • #68795

    Joe Monaco
    Participant
    Vessel Name: Tunacious
    Engines: 3126 TA 420 hp
    Location: San Diego
    Country: USA

    Looking at the below chart I noticed the exhaust manifold temps increase steadily with engine RPM but exhaust stack temperatures decreases from 2400 rpm to 2600 rpm in the 420 And 385 HP version. All 3 HP ratings have this drop.

    On other boards folks comment they “discovered” with EGT gauges that their motors are “working less” at higher RPMs due to a drop in the stack EGT. They increase RPMs at cruise to get this EGT drop thinking this is better for engine life.

    I think the exhaust valves/manifold are still getting hotter so more RPM isn’t helpful for cruising speed engine life. What do you guys think? Does anyone know what is causing this drop in the stack temps? Is it more boost air cooling coming in at a higher rate?

    Obviously too much free time this week!

    Sent from my iPad

Viewing 9 replies - 1 through 9 (of 9 total)
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  • #69375

    Larry Backman
    Participant
    Vessel Name: Skipjack
    Engines: QSM 670
    Location: Cape Cod, MA
    Country: US

    4-6’s off the stern quarter for 6 hrs

    The sea conditions I was in were honest 4’ seas with the odd 6 footer mixed in coming from about 8 o’clock. I could not run my normal 20 knot plane without burying the bow so I dropped down to the speed where the boat was barely in plane at about 15-16 knots to stay behind the seas. I was really going uphill on th backs of waves the entire way home.

    This put me in the max. Torque and heat range for the QSM for those 6 hours which combined with an overprop situation likely had my dry exhaust manifold red hot.

    I have no issue going that speed or those RPMs for a half hour jaunt to the Vineyard but for long offshore runs I try and stay out of that danger corner.

    #69365

    Joe Monaco
    Participant
    Vessel Name: Tunacious
    Engines: 3126 TA 420 hp
    Location: San Diego
    Country: USA

    Thanks Steve,

    If the conditions are such that I won’t be able to stay on plane I’m staying at the dock :-)! Ive become a fair weather only boater. Off shore SoCal has its bad weather days for sure. Pleasure boating is supposed to be fun, not scary. A misguided trip to Catalina during what was suppose to be a “mild” Santa Ana condition popped into my mind. If the weather comes up quickly or a supposed “weather window” closes, you’re correct, hull speed is all that’s left. Been there, done that. Never again…

    When the Ocean is angry cocktails and sea stories tied securely at the slip with ice cold EGTs sounds about right for me.

    I’ll check out the QSM exhaust manifold info. Thank again.

    #69364

    Joe Monaco
    Participant
    Vessel Name: Tunacious
    Engines: 3126 TA 420 hp
    Location: San Diego
    Country: USA

    Thanks Steve,

    If the conditions are such that I wouldn’t be able to stay on plane I’m staying at the dock :-)! Ive become a fair to great weather only boater. Off shore SoCal has its bad weather days for sure. It’s supposed to be fun, not scary! If the weather comes up quickly or a “weather window” closes, you’re correct, hull speed is all that’s left. Been there, done that! Hopefully never again…

    I’ll check out the QSM exhaust manifold info. Thank again.

    #69279

    Steve Lewis
    Participant
    Vessel Name: Just Us
    Engines: Cummins 480CE
    Location: Marblehead, OH
    Country: USA

    Shades of Grey

    Joe I think you finally have it. Sea Conditions are not constant. They vary. I boat on Lake Erie. Our predominant conditions are 3foot seas or less. That all depends on wind strength and direction. The resulting state of the waves determines how fast I want to go based on how my boat handles those conditions. I can safely and comfortably maintain my preferred cruise speed, 22mph or so, in waves approaching 4 feet. We have sharp chop not nice long rolling waves.

    That means as wave height increases beyond that point, I slow down. At some point I slow down enough that it makes no sense to stay on plane. Drop off plane to stay safe and comfortable.

    That grey area of decision is influenced by how much pitch is on the prop as that, along with engine rpm, will generate speed. So I will aim to setup my boat for MOST but not all conditions. It’s a balancing act.

    Do I want to get a little more speed for the prevailing conditions or do I want a prop that works for even the most extreme edge cases of operation. That is an individual choice. How much do you want to squeeze out of your setup while up and on plane.

    Larry, or anyone with a QSM, has a certain set of well known operating parameters. Do some reading on the QSM and the exhaust manifold. Use the search feature of this site and read read read.

    Most people will optimize for the predominant conditions, not every single possible condition. At some point the sea state is too much to stay on plane. Propeller pitch will load the engine and the seas will, at some point, dictate dropping off plane. The point at which that happens is different for every boat.

    The voodoo is finding out and setting things up for your specific boat, engine and predominant conditions.

    Happy boating

    #69256

    Joe Monaco
    Participant
    Vessel Name: Tunacious
    Engines: 3126 TA 420 hp
    Location: San Diego
    Country: USA

    Thanks Steve,

    So wouldn’t sea state loads be transitory? The motor going above and below the fuel burn curve with big seas is whats avoided by dropping to displacement speeds?

    Under these circumstances you can’t keep the QSM11 on plane without damaging it? That seems odd to me. Why put a motor/boat/prop combo together that will not be able to operate in the open ocean? Unless it’s just an extremely rare sea state that this problem occurs.

    What is it about a QSM11 operational limitations that’s different from CAT 3126? Thanks for the education. Very interesting.

    #69236

    Steve Lewis
    Participant
    Vessel Name: Just Us
    Engines: Cummins 480CE
    Location: Marblehead, OH
    Country: USA

    Sea Conditions

    Joe,

    You asked “what am I missing?” Sea Conditions. Each boat has its own set of operating conditions that will vary based on all of the relevant external forces that end up presenting load to the engines through the propellers. Larry is telling you that even though he is propped at or under the curve for most operating conditions, there are times when conditions do not allow him to raise rpm because that would be operating his boat too fast for the prevailing conditions. So prudence dictates that he slows down and his set if circumstances means operating at displacement speed.

    Load is not constant. You must accept that load will vary almost infinitely. When I am cruising I can use my electronics to watch my fuel burn. Waves, wind, fuel level, bottom conditions all contribute to the load presented to the engines. At 3500 rpm my fuel burn can be as low as 35gph and as high as 42 depending on what is going on. Am I pushing the boat up the back of a big wave or am I surfing down the front side of a big wave. Do I have a head wind or tail wind. We have lots of canvas so that makes a difference. Are we full of fuel and food and liquids at the beginning of a trip or are we light in load because we are just going out for the day to swing on the hook and swim.

    You prop your boat as prudently as possible but there will be circumstances that require you to alter the operation of your boat as not to push things too far. Larry has a QSM11 in his boat which means he has a very specific set of operating circumstances he needs to stay within. Those are different than your CAT 3126’s.

    What we are all trying to do is take the concepts that Tony presents here and adapt those to our own set of circumstances. They are all different by definition. So Larry has to slow down sometimes so he does not cook his exhaust manifold. Even though he is propped correctly he will still find himself in sea conditions that do not allow him to run his QSM Within ideal spec. He must back her down and still the sea conditions end up with the load presented to the engine as too much. So prudence dictates slowing down even more.

    Know your own unique boat. Be a prudent master of your vessel which means knowing when you can advance the throttles and knowing when to back down and save your equipment for a long enjoyable future.

    2 users thanked author for this post.
    #69229

    Joe Monaco
    Participant
    Vessel Name: Tunacious
    Engines: 3126 TA 420 hp
    Location: San Diego
    Country: USA

    I don’t understand why you would need to drop to displacement speeds. You should be able to cruise your boat, if propped correctly, at planning speed. What am I missing?

    Looks like EGT readings after the turbo can be dropped by increasing RPM but it’s not always making it easier on the motors. My chart shows the temp in the exhaust manifold goes steadily up with increased RPM.

    I’m thinking a boat motors actual fuel flow vs manufacturers stated RPM fuel flow may be the only way to know if your asking for more HP than the manufacturer says is “safe” at the various RPMs. If changing RPM gets you on or below that fuel flow you should not be overloading the motors.

    #69046

    Larry Backman
    Participant
    Vessel Name: Skipjack
    Engines: QSM 670
    Location: Cape Cod, MA
    Country: US

    See the same with my QSM

    I’ve observed the with my QSM11/670 for many years. EGT is 50 degrees higher before plane in the 1500-1700 RPM range, drops at plane in the 1800 RPM range and drops 25 additional degrees at very high Cruise, 1950 RPM.

    These numbers are against 2350 WOT on a single engine downeast.

    More importantly I have years of observations of what Tony is saying, no matter what I do with my props, there is some corner of my planing cruise speed where I can find myself overloaded, fuel burn a gallon or so too high, EGT up 50 degrees. It can be due to weight, sea conditions or tide. When I see that situation I now adjust my speed to get out of that corner, more often than not slowing down to displacement speed.

    I cooked a QSM exhaust manifold 7 years ago before I understood this performance corner by dropping to 1600 RPM just below plane for a 6 hour run home. I didn’t have an EGT gauge back then and had neither an understanding or a measurement of the damage I was doing.

    Since that point I run my boat by GPH and EGT!

    #68974

    Tony Athens
    Moderator
    Vessel Name: Local Banks
    Engines: QSB 6.7 550 HP
    Location: Oxnard, CA
    Country: USA

    The chart you posted is made in a certified test cell under very strict loading conditions.. This really is just a guide and is not truly applicable to what goes on in a vessel.. Also, the stack temp is AFTER the turbo and the turbo is absorbing more heat at some ranges per HP produced.

    Just about all planning hull vessels are at some level of over loaded condition in cruise RPM ranges, yet at WOT they may make the minimum RPM spec…….. That’s why I try and explain that on engines 50-60 HP per liter range, prop to the fuel burn chart where you like to cruise as that is where you spend your time, not at WOT..

    Cummins engines do the same– when over propped in the cruise RPM ranges, most of the time the EGT drops at as you get closer WOT as the boost now meets the “test cell spec” at that RPM.. More air per the HP produced usually drops the EGT…… When a planning hull is at a range to where itn is in-between hull speeds and over the hump, EGTs can get very dangerous……………..If you have accurate pyro’s & a GPH display that is accurate, plot what you have and compare to test cell specs.. They rarely match.

    Tony

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