Engine Cooling Fan Speeds, CFM's, and Efficiency
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Thread: Engine Cooling Fan Speeds, CFM's, and Efficiency

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    Just Some Dude in Jersey My location The Scifi Guy's Avatar
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    Engine Cooling Fan Speeds, CFM's, and Efficiency

    It's been my observation over the years in the mechanical world and somewhat based upon things I've heard, that running a fan at high speeds versus low speeds does not equate to more cooling at a 1:1 ratio. Window air conditioners are a good example. Running them on high doesn't really enhance their cooling effect. I've found that they work just as well, if not better, at low speed and you don't have to listen to the sound of a turbo-prop mounted in your window. Years ago, RallyBob sold me a set of pulleys for my GT that slowed down the speed of my water pump. It worked great and provided a 15*-30* reduction in thermostat temp. The theory being that the slower moving water has more time to absorb heat from the engine. I have this combo on my car right now and it is one cool cat. I also have a cheapie, plastic, electric radiator fan on a thermostat to supplement the 7 blade engine fan on my 2.4. My car's thermostat never goes past 180* even in backed up traffic on a high humidity 100* day. I also have the 3-core heavy duty OGTS radiator. Highly recommended for ALL GTers.

    There's a bow wave or cavitation effect(please supply the correct terminology) that happens as the speed of water or air increases as they flow over surfaces. This "effect" causes a static barrier of air or water to form at the surface which impedes the heat transfer to the air/water passing by. Having air/water linger a bit longer in the vicinity of a heat source greatly improves the heat transfer to the air/water.

    When I was shopping for an electric radiator fan I saw all sorts of speed and CFM claims from various manufacturers, with commensurate higher and higher prices for said fans. Based upon my experience, I contend that expensive, super high CFM, fans are a waste of money on a street driven, non-raced, Opel GT. I contend that ANY cheap average CFM fan will get the job done just as good as expensive, cyclone-inducing, noisy, fans. I also observe that common cars with electric fans don't have fans that spin at light speed. In fact, they seem to spin at a fairly tame speed.

    What do you guys think?

    General discussion on this topic.


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    Member Timbo's Avatar
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    What, nothing to do today?

    I think there may be two things going on with the fan. One is cooling the engine coolant and the other is moving air (cooling) through the engine compartment. The first has to do with maximum contact with the heat exchanger, lower air flow, and the second would be maximum air flow to move as much air through the system.

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    Quote Originally Posted by The Scifi Guy View Post
    It's been my observation over the years in the mechanical world and somewhat based upon things I've heard, that running a fan at high speeds versus low speeds does not equate to more cooling at a 1:1 ratio. Window air conditioners are a good example. Running them on high doesn't really enhance their cooling effect. I've found that they work just as well, if not better, at low speed and you don't have to listen to the sound of a turbo-prop mounted in your window. Years ago, RallyBob sold me a set of pulleys for my GT that slowed down the speed of my water pump. It worked great and provided a 15*-30* reduction in thermostat temp. The theory being that the slower moving water has more time to absorb heat from the engine. I have this combo on my car right now and it is one cool cat. I also have a cheapie, plastic, electric radiator fan on a thermostat to supplement the 7 blade engine fan on my 2.4. My car's thermostat never goes past 180* even in backed up traffic on a high humidity 100* day. I also have the 3-core heavy duty OGTS radiator. Highly recommended for ALL GTers.

    There's a bow wave <SNIP>

    What do you guys think?

    General discussion on this topic.

    Actually Gordo, from what I have read, I believe that you are "upside down". If the "slower moving water" ABSORBS more heat, the water temp goes up. The "slower moving water" going through the "heat exchanger" (AKA radiator although it does not function by radiation) DOES have more time to dissipate the heat to the air the fan is moving and the coolant temp is lowered. Of course, I have been known to be "upside down" on occasion also (truthfully I don't remember the last time, but I don't remember a lot right now!) so don't trust my memory on this one. -- HTH, Doug

    PS - Doesn't "upside down" sound so much better than "WRONG"?
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    1000 Post Club kwschumm's Avatar
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    I use Water Wetter in my coolant. It's supposed to reduce bubbles that can form on a hot surfaces, like water passages surrounding cylinders, and help with cooling. Seems to work well.
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    It was explained to me like this...

    The coolant in the engine gets hot. The thermostat opens. The coolant moves from the engine to the radiator. The thermostat closes (as cooler coolant entered the engine). The coolant in the radiator cools. The cycle repeats.

    Without lag time in the radiator, the coolant doesn't have time to cool properly and the temperature in the engine can rise as a result.

    I have seen this in action on my current Ford Windstar van. The PO removed the thermostat. The temp gauge goes from low (idling) to medium high when going up a hill. During normal winter driving the heater barely gets warm.

    I will be installing a thermostat soon.
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    I'm going to split this into two topics. Water flow and CFM for fans...

    The slower your water flow, the more it will saturate with heat. This can be good or bad. Good because it can really help pull heat out, but if you can't cool it back down again, it becomes bad. This is where the radiator comes into play to prevent heat saturation of the cooling system, thus overheating the engine.

    The fan CFM is rather important because a heat exchanger is very dependent upon airflow. You actually want a high CFM fan on a commuter car and don't need it on a race car. Why? Race cars don't spend a lot of time in stop and go traffic, where airflow from vehicle movement is minimal. F1 doesn't have them at all.

    So for a commuter car, I'd go with a moderate speed water pump and a high CFM electric fan that had variable speed control. If the fan is manually driven by the pump, then focus on coolant flow and not fan speed.
    Last edited by Autoholic; 04-08-2017 at 06:15 PM.

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    This thread can be broken to 3 questions:
    1. How is flow of coolant affecting cooling
    2. Benefits (if any) of using mechanical and electrical fans simultaneously
    3. Required capacity of various electrical fans (cfm)


    Let me tackle the question related to the flow of coolant.

    There is a long standing legend that slowing the flow of coolant through the radiator helps the cooling “as it allows more time for coolant through the radiator”. That legend is probably influenced by the racers which are indeed reducing the pulleys ratio in order to slow down the pump (and the alternator), however they are doing it to prevent physical damage to those components as they are typically running very high rpm during the race.
    There must be a balance between how much the heat coolant can absorb while flowing through the engine vs. amount of heat that will be removed while flowing through the radiator. In simple words, slowing down the flow of coolant will make it flow slower in the whole circuit, spending more time in the engine and absorbing more heat. Also in the opposite case, faster coolant flow will absorb less heat from the engine (as it will spend less time in the engine).
    If the cooling system does not have enough capacity, slowing down the flow of coolant will not help.


    Regarding having mechanical and electrical fan at the same time:

    Mechanical fan main disadvantages are:
    - It robs the engine of some power
    - It runs even when it is not required (during start up when the engine is still cold) or during the drive on the highway when the natural flow of air does not require any radiator fan.
    - It often does not have enough capacity in stop and go conditions as the engine and the fan are running at low rpm.

    This is the reason why basically all modern cars are using thermostatically controlled electrical fan.
    Running mechanical and electrical fan simultaneously has little sense. Adding electrical fan is good but keeping mechanical fan did not remove its disadvantages. So why? In the case of the GT there could be possible explanation of maintaining constant flow of air under the hood (electrical fan goes on and off).


    Which brings us to the subject of electrical fan capacity.

    Electrical fan is popular add-on for Opels (and many other classic cars) which raises the question of choosing the fan with the right capacity.
    I ditched my mechanical fan long time ago, at the time that I was still running more or less standard engine. I used 12” fan, 80W, declared to have 1500 cfm. It worked fine for me and with Honda radiator I never experienced overheating.
    When I installed AC, the situation changed. With AC running, there was no problem on the highway but under stop and go condition in traffic, the needle on the thermometer was creeping to the right, so I had to turn off AC. Next step was to replace the fan with 14” model, 90W, declared to give 2000 cfm. It was better but there were situations that I still had to turn off the AC.
    I located on the Ebay 14” fan 230W declared to develop 3000 cfm. That worked great but it put a lot of stress on the electrical system. Yes, I upgraded my alternator but it is still 45 years old electrical system, so I have finally settled on 14”, 130W fan 2200 cfm which seems to does the job.

    Based on above experience, I would recommend 14”, 90W fan for any Opel without AC. There are many on the Ebay. Adding AC opens the whole new can of worms.

    I also wish there is enough space between the engine and the radiator to install the sucker fan. Sucker fan is more efficient than blower fan and also allows installation of the shroud. Keith had notched radiator crossbar and moved the radiator forward to create space. I might go that way.

    It is beautiful day outside and I am going to the garage now. Damn you Gordon.
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    1000 Post Club kwschumm's Avatar
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    There will be an optimum speed of water flow through the engine to maximize cooling that varies with engine temperature.

    There will also be an optimum speed of water flow through the radiator to maximize heat exchange, which also varies with air and water temperature.

    Those two optimum flow rates are likely different.

    Since the mechanical pump varies flow by engine speed, and not flow requirements, we end up with a not-at-all-optimum complex system that is difficult to control. A better system would have a controllable variable speed water pump with sensors in key places, along with a "smart" controller, to implement closed loop control logic.

    Has any car made has ever had such a thing?
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    I believe that the theory is slightly different to limited cooling in GT, confined engine compartment.
    Exhaust manifold heat dissipation needs to be addressed, as well as, the liquid cooling issues in a GT.

    Maybe the new OMC management will allow an upload of this older OMC document?
    http://www.opelgt.com/forums/6b-cool...tml#post241649
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    Lindsay, would you please send me the document you are referring to? I'd appreciate it.

    Lots of very good points, as we all try to explain the various aspects to this. As far as I am aware, there has never been a car produced that has a variable speed water pump and a variable speed electric fan. Variable speed electric fans are pretty common today though, and they tend to be pretty high CFM fans. Listen to a brand new car that is really hot from driving, and sitting parked with the engine running and the AC on. They kick on those fans to max, and get pretty loud.

    Is it possible to have a variable speed water pump on a vehicle? Yes, electric water pumps are out there but rarely are these stock equipment on any vehicle. This is more of an aftermarket part. If someone really wanted to do this, they could. Is it needed to have a really good cooling system? No.

    One thing that we haven't mentioned yet, that is also an aspect of this conversation, is the coolant itself. How many of you are running Evans for coolant? Already mixed 50/50 regular coolant? 100% Water? The exact coolant you use will play a role in how effective your cooling system is.

    I liked that the problem of heat from the exhaust manifold was mentioned. This is something I have said a few times in related threads, that the exhaust needs to be ceramic coated. The hotter your IAT's, the hotter the engine will run. Also, lots of radiated heat from the exhaust will give the engine bay a rather hot ambient temp. That will saturate everything with heat, and place a higher demand on your cooling system. I don't even own a GT yet and I can spot this flaw with the car. You have a major heat source, literally attached to your intake manifold. After a long drive, has anyone measured the temp of the passenger and driver sides of the engine bay? I bet there is a significant difference. This is going to contribute to vapor lock after a long drive, and it doesn't help that the stock fuel line routing has the fuel line hugging the engine. It amazes me that this (fuel line routing) often goes unaddressed, especially on a mechanical fuel pump car. I don't believe a GT has to have an electric fuel pump in order to avoid vapor lock. Is it the easiest and cheapest way to solve vapor lock? Yes. But the ambient temp of the engine bay, fuel line routing and lack of a fuel pressure regulator also are issues. Going back to the initial point in this paragraph though, ceramic coating doesn't have to be expensive. Many people have had decent results with VHT and similar DIY products.
    Last edited by Autoholic; 04-08-2017 at 07:13 PM.
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    7,000 Post Club My location wrench459's Avatar
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    Quote Originally Posted by kwschumm View Post
    There will be an optimum speed of water flow through the engine to maximize cooling that varies with engine temperature.

    There will also be an optimum speed of water flow through the radiator to maximize heat exchange, which also varies with air and water temperature.

    Those two optimum flow rates are likely different.

    Since the mechanical pump varies flow by engine speed, and not flow requirements, we end up with a not-at-all-optimum complex system that is difficult to control. A better system would have a controllable variable speed water pump with sensors in key places, along with a "smart" controller, to implement closed loop control logic.

    Has any car made has ever had such a thing?
    Yes there is a few late model cars that use a pcm controlled electric water pump.
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    I think most BMWs since ~10 years ago have been using electric water pumps. Its likely a similar story with most other 'luxury' brands. Mechanical water pumps can be relatively large parasitic power draws on the system when in a low power, mid-rev situation like highway cruising. Mechanical pumps typically use something like 2-5% of the engine's peak power at peak RPM. That draw scales with RPM, not power or ground speed.

    So, say the pump uses 1% of the peak power from the engine when down at 2500 rpm and your engine makes 100HP. Thats 1HP of parasitic draw on the engine. And say while cruising on the highway, your engine is making something like 15HP. 1 out of the 15 HP produced is being used directly to pump water around. The thermostat can provide a restriction on the amount of water being pumped, which also increases the backpressure. Typical centrifugal pumps are good at mass flow and bad at back pressure, so a restriction in this way effectively reduces the amount of parasitic power the pump is using. For arguments sake, lets say it drops the draw by half and now the engine is leaving 0.5 of 14.5HP produced to the water pump. Thats ~3.5% of the total power produced going towards pumping water around.

    An electric pump lets you choose when to doll out those parasitic losses by decoupling the pumping of water from engine RPM. It also means that you don't need to make your pump more inefficient with a thermostat but can instead just choose when to run it.

    kwschumm summed up the logistics problem pretty well, but I'll provide another example to help others think about this problem in another sense.

    Say you have a closed circuit of water. In your circuit are two cylinders connected in a loop by hoses. In between one of these connections is a pump. One of these cylinders sits in a room with a draft. The other sits atop a bunsen burner. The speed at which the pump runs is independent of the amount of heat the busen burner produces and it is independent of the strength of the draft in the room.

    All 3 of these variables can be altered without directly changing the others. Can you see how this system can quickly become unbalanced towards either a frigid fluid or a boiled fluid?

    The idea of a fluid 'moving too fast' to absorb energy is a little weird IMO. Turbulent flow enhances heat transfer in fluids since it is introducing new fluid molecules to the heat boundary more often than a laminar flow. And faster flows are generally more turbulent. Q = cmdT. Keeping Q constant, if you drop m you are going to make a larger dT.

    The best guess I have right now behind a slower mechanical water pump dropping engine temps is that the original pump speed was too fast and was cavitating. This essentially is locally boiling the water in the engine which will make it transfer heat in a much worse way. And in a similar way to how you see steam stick to the bottom of your spaghetti pot, those bubbles of steam will adhere to the cylinder walls and prevent heat transfer from the walls to the water.
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    Quote Originally Posted by Swiftus View Post
    So, say the pump uses 1% of the peak power from the engine when down at 2500 rpm and your engine makes 100HP. Thats 1HP of parasitic draw on the engine. And say while cruising on the highway, your engine is making something like 15HP. 1 out of the 15 HP produced is being used directly to pump water around. The thermostat can provide a restriction on the amount of water being pumped, which also increases the backpressure. Typical centrifugal pumps are good at mass flow and bad at back pressure, so a restriction in this way effectively reduces the amount of parasitic power the pump is using. For arguments sake, lets say it drops the draw by half and now the engine is leaving 0.5 of 14.5HP produced to the water pump. Thats ~3.5% of the total power produced going towards pumping water around.

    I think your math got a little messed up in this example. I think we can both agree that the parasitic losses incurred by the water pump will remain somewhat constant, as a function of RPM and power output. It will fluctuate a little, but nothing crazy so for the sake of the discussion, constant. Lets say that is 2% of the power produced at a given RPM for a specific water pump. At peak power, we'll say the engine makes 100 HP. So that means 2 HP went to moving coolant. At say a 2,250 RPM for cruising, lets say the engine makes 25 HP. 2% of 25 HP would be 0.5 HP. The power required to operate the pump will be proportional to the RPM, not constant. This is also seen in superchargers. The faster you pump something at a given pressure, the more power a pump will need. It gets a little more complicated when we change flow and pressure. 1% of 15 HP would be 0.15 HP to move the coolant while cruising on the highway. To reach 1 HP, the parasitic losses would have to reach 6.7% for just the water pump, when at peak power it was 1%, resulting in a rather drastic change of 670% in parasitic losses at a lower operating speed, not higher. Pumps don't fluctuate that much, especially when slowing down.


    Also, Hot Rod did dyno testing and found that open loop vs closed loop for the cooling system, to create back pressure, resulted in no change in power output. They did find underdrive pulleys to recover most of the power that was lost when comparing electric and mechanical water pumps. This is desired by racers because they are typically at higher RPM's normally, so flow isn't a problem. The focus becomes a parasitic loss reduction, assuming cooling is still sufficient. The slower the coolant moves, the move heat it absorbs, so more time it needs to spend in the radiator. Without doing actual testing, it's hard to say what speeds work best for coolant in a CIH. Every heat exchanger problem will have a spot where it works the best and frankly, you have too much time on your hands if you're doing this testing and you don't work for a factory F1 or LMP1 race team. Just like sonic tuning of an exhaust system, anything you improve will only work in a narrow band of the power curve.

    Baseline Testing - Do Water Pumps Suck Power? - Hot Rod Network
    Last edited by Autoholic; 04-10-2017 at 03:36 AM.
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    I did not read every word in this thread but I don't see anything mentioned about radiator cooling capacity which plays a huge role in the equation.
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    Quote Originally Posted by Autoholic View Post
    I think your math got a little messed up in this example. I think we can both agree that the parasitic losses incurred by the water pump will remain somewhat constant, as a function of RPM and power output.
    I think my explanation might have been poorly worded, but you repeated the math I did in your correction, so I'm going forward saying that we agree on that

    However, I don't think a mechanical water pump is power dependant for the parasitic losses it contributes. It is purely an RPM associated loss.

    That is how it is unlike a supercharger. Superchargers are connected to the air pump that is the engine both mechanically through a belt / chain / whatever and via the fluid connection of air pumping in to the engine. Because of that air connection, a supercharger will sap more power as more power / airflow is required by the engine, even if you generally ignore RPMs.

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    Right. By power output, I meant that the parasitic loss would be computed as a function of HP, which is what both of us did. We didn't say at 5,000 RPM's, the pump requires 10 HP. We said at peak power, the pump would consume 1% of the power output. That's what I was referring to.
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    Just curious how much of a drag does a fully energized alternator pull?

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    Quote Originally Posted by Swiftus View Post
    say the pump uses 1% of the peak power from the engine when down at 2500 rpm and your engine makes 100HP. Thats 1HP of parasitic draw on the engine.
    I abbreviated my previous statement. Its right Joe.

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    Quote Originally Posted by Swiftus View Post
    I abbreviated my previous statement. Its right Joe.
    That part was fine, and not what I was referring to as a mistake. The possible mistake was assuming at cruising RPM, the water pump would still require 1 HP of the 15 HP produced. We already established that a pump's power requirement will be dependent upon the RPM of the pump. Peak power very, very rarely happens at 2,500 RPM in a petrol engine. Unless you're trying to talk about the precise conditions under WOT that an engine produced the most power at a specific RPM, which is not what I believed you were mentioning. If you're saying at 2500 RPM's, the pump required 1 HP, and cruising was at 2500 RPM, then ya it would still be 1 HP. However, we're rarely concerned with parasitic losses while cruising in sports cars.
    Last edited by Autoholic; 04-11-2017 at 02:25 AM.
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  22. #20
    Opel Intern Swiftus's Avatar
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    So water pumps are sized based on some function that the manufacturer has devised, and part of that equation incorporates the maximum power the engine can develop and the duty cycle at which the manufacturer thinks this engine's max power will be used. The reason max power is important in this equation is because that also accompanies the maximum impulse of heat into the cooling system. Remember, ~1/3 of the energy in an engine goes out towards the wheels, ~1/3 goes out the exhaust and ~1/3 goes into the cooling. So if you are making 100HP in power at the crank, you are also rejecting 100HP of heat into the cooling system.

    If you, as a manufacturer, decide that the use case for the engine will be typically a very small portion of peak and only very brief periods of peak power, say to overtake on the highway or when merging, you will likely select a relatively small water pump. Maybe one that at its maximum draw (which is at max RPM) uses ~2% of the engine's peak power (here you need to remain relative which is why a % of power is used). In choosing this small pump you are ensuring that the parasitic losses during the majority of the use cases in the engine's life will be minimized - even if technically the cooling system cannot cope with a peak load situation indefinitely.

    If, on the other hand, you decide as a designer that the use case will have predominately high load situations, say in a circuit race car, then you will choose a pump which has more flow capacity but will also use more power from the engine - likely in the 4-5% of peak power at max RPM range. There will be much more parasitic loss in low-load situations, but who has time for that in racing? WOT is life in racing.

    I can understand some confusion in this description, but let me try to explain further. Saying % of peak power at max RPM does not mean that peak power exists at the max RPM, it is merely relating the maximum heat input to the maximum mass flow for the pump. A mechanical pump's mass flow is directly related to RPM and largely only RPM. The heat input to the cooling system is directly related the current power production of the engine and largely only the power production of the engine.

    In saying 1% of peak power at 2500 RPM when I had said a pump was sized for 2% of power at max RPM, I was trying to relate that the power draw of the pump was only related to RPM and not the current power being produced by the engine. It was, that in fact, the pump would be using 1 HP at 2500 rpm no matter what the position of the go pedal. The fiddly interpretation of 670% etc. was purely a misunderstanding of my intent, although in re-reading my original post I am still confused as to how it was interpreted so poorly.

    I do think that understanding parasitic losses and being concerned with parasitic losses is important in just about every car. Its just that the goals of why we want to reduce those losses are different.

    In a commuter car, we are concerned with parasitic losses mostly due to fuel economy. If 1 out 15 HP being produced is going towards pumping water around an engine and it currently doesn't need that amount of water pumping around it, then you are wasting something under 1/15th of the energy you are consuming. In a 30MPG car that's 2 MPG being lost due to pumping water.

    In a sports car, we are concerned with parasitic losses because those things reduce the available amount of power which can go towards accelerating the car. 2% power is HUGE in the racing world. 2% of a 60 second lap (I know power does not equal drops in laps times) is 1.2 seconds! That can be the difference between first and last in many racing disciplines.

    On that note, I am very open to understanding how I might not have made that initial statement in a clear manner so that I can improve my explanations in the future.
    Last edited by Swiftus; 04-11-2017 at 02:49 AM.

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