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Thread: MT engine wear fear (concerned about high engine RPM at highway speeds)

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    Don't worry so much about the RPM - instead, consider how fast the pistons themselves are moving.

    At 3200 RPM, our Mirage engine is moving the pistons at ~1889 feet per minute, or 9.6 meters per second. Lets call it about 21 mph.

    That's easy to handle. The pistons themselves don't even move as fast as the car does.

    Lets play games with numbers, however. At 6500 RPM, the pistons are going 3838 feet per minute, or 19.5 meters per second, or 43 mph. Not so fast now, is it?

    Lets go firmly into the realm of the absurd - I have a Nitromethane powered RC truck. It has a single cylinder two-stroke glow engine that ticks over about 50,000 rpm. The piston speed is 4822 feet per minute - about 24.5 meters per second, or 54 mph.

    The Mirage engine will be fine.



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    Quote Originally Posted by Tinker1980 View Post
    Don't worry so much about the RPM - instead, consider how fast the pistons themselves are moving.

    At 3200 RPM, our Mirage engine is moving the pistons at ~1889 feet per minute, or 9.6 meters per second. Lets call it about 21 mph.

    That's easy to handle. The pistons themselves don't even move as fast as the car does.

    Lets play games with numbers, however. At 6500 RPM, the pistons are going 3838 feet per minute, or 19.5 meters per second, or 43 mph. Not so fast now, is it?

    Lets go firmly into the realm of the absurd - I have a Nitromethane powered RC truck. It has a single cylinder two-stroke glow engine that ticks over about 50,000 rpm. The piston speed is 4822 feet per minute - about 24.5 meters per second, or 54 mph.

    The Mirage engine will be fine.
    Not to argue, but there has to be something to be said about an engine piston moving as fast as ours vs a CVT model or another type engine altogether... why in the world would they not want the engine to turn over as slow as possible?

    I would recon a CVT engine will outlast a 5 speed engine due to just higher wear alone.

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    my 5MT also hovers around 3k RPM to maintain a cruising speed of 100km/h (around 62mph)

    not worried too much though I wish the 5th gear was taller. it's almost as close to 4th.

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    Quote Originally Posted by TFCfan2021 View Post
    Not to argue, but there has to be something to be said about an engine piston moving as fast as ours vs a CVT model or another type engine altogether... why in the world would they not want the engine to turn over as slow as possible?

    I would recon a CVT engine will outlast a 5 speed engine due to just higher wear alone.
    This is interesting subject I need to get through my old links. He is making a valid point RPMs do not necessarily translate into high piston speeds. As I recall one of the slower revving engines with redline <3k had one of the highest piston speeds. And motorcycle engines with 15k redline actually have piston speeds lower than slow revving diesels.

    To answer the question it is not that you don't want lower piston speeds, the way to get them is to increase bore/reduce stroke and this will reduce thermal efficiency. From what I read Mitsubishi made many improvements to lighten up pistons and make skirts slippery, so RPMs should not be an issue, and judging by the 4G63 track record they know thing or two about building bulletproof engines.
    Last edited by cyclopathic; 12-09-2015 at 08:59 AM.

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage DE 1.2 manual: 46.4 mpg (US) ... 19.7 km/L ... 5.1 L/100 km ... 55.7 mpg (Imp)


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    Quote Originally Posted by TFCfan2021 View Post
    Not to argue, but there has to be something to be said about an engine piston moving as fast as ours vs a CVT model or another type engine altogether... why in the world would they not want the engine to turn over as slow as possible?

    I would recon a CVT engine will outlast a 5 speed engine due to just higher wear alone.
    This is a question, as I really have no idea - would piston speed/rpm be the main contributor to wear, or would load also play a part. Put another way - which engine would last longer - the one in 3rd gear running 40km/h with light loading, or the one in 5th gear running 40km/h but chugging along under load? I think of it as doing ten reps of a light weight at the gym compared to doing 4 reps of a very heavy weight (say lifting 100lbs X10, or 250lbsX4). Like I said, I have no idea, but maybe the total amount of work done matters?

        __________________________________________

        click to view fuel log View my fuel log 2014 Mirage SE 1.2 manual: 45.0 mpg (US) ... 19.1 km/L ... 5.2 L/100 km ... 54.1 mpg (Imp)


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    Hi,
    Just got my new Mirage G4 MT, everything seems to be perfect as it is a new car.
    One thing I noticed and bothers me is that the squeaking sound from the engine after several kilometers drive.
    The engine sound itself I know is already loud compared to others, but the squeak is really annoying. But the moment I turn off the engine for a minute or two then start it again the squeaky sound is gone.
    Just want to know if this is normal...

    Thanks

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    Quote Originally Posted by Canoehead View Post
    This is a question, as I really have no idea - would piston speed/rpm be the main contributor to wear, or would load also play a part. Put another way - which engine would last longer - the one in 3rd gear running 40km/h with light loading, or the one in 5th gear running 40km/h but chugging along under load? I think of it as doing ten reps of a light weight at the gym compared to doing 4 reps of a very heavy weight (say lifting 100lbs X10, or 250lbsX4). Like I said, I have no idea, but maybe the total amount of work done matters?
    Excellent question. Let's see if I can come up with a decent answer.

    Engines have multiple parts, and of course they all wear. They do not wear evenly, either.

    Let's look at the cylinder walls, first. You might think that the walls wear evenly where the rings slide. But in reality, the cylinder walls get a bit of a taper in them as they wear. It's not something you can see with the naked eye, but you can feel it with your fingers - the end of the cylinder closest to the head will be slightly bigger around than the end closest to the crank. This makes sense - it's hotter on that end, and less easily lubricated. Running the car under heavy load, lugging it, will cause this area to get hotter, and wear faster.

    Next, let's look at the rotating parts - the crank and rods, and associated bearings. The crank and the connecting rods are run on bronze bearings, which are continually pumped full of oil. They never really touch. Faster everything spins, the faster the oil pump goes, more pressure is supplied. The slower everything spins, the slower the pump goes, the less pressure, no matter the throttle opening. So if you are asking your engine to pull hard, at low rpm, you're pushing the oil out of those bearings, potentially causing metal contact.

    What does it all mean? Spinning our little engines pulling our small aerodynamic cars isn't going to hurt them. If you tried to pull a camper, it would be different. I think the timing chain will let go long before anything in the engine wears out. I took the head off a Metro that had 250,000 miles. You could still see hone marks in the cylinders.

    So why does the cvt version spin so much slower than the 5 speed? I haven't driven a cvt mirage, but I'll bet the instant you breathe on the gas pedal, the little thing slams into low range and spools the engine up to 3,000+ rpm. The 5 speed can't do this. Mitsubishi was worried that if they geared the car too high, people in the US would be trying to run the car at 45-55 in 5th gear, and engines would die as a result. So they did what a lot of other manufacturers of small cars did - they geared it low. It still gets amazing mileage, and now it will run forever too. It just spins faster doing it.

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    The CVT is definitely eager to rev up when not using the cruise control. Using cruise it will seek maximum overdrive ratio almost immediately and attempt to hold it. Like at 60mph it takes some finesse to get it to drop down to the .55 ratio which is about 2000 rpm using the throttle. Click the cruise control and almost instant rpm drop to 2000rpm.

        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage ES 1.2 automatic: 40.4 mpg (US) ... 17.2 km/L ... 5.8 L/100 km ... 48.5 mpg (Imp)


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    Quote Originally Posted by Tinker1980 View Post
    So why does the cvt version spin so much slower than the 5 speed? I haven't driven a cvt mirage, but I'll bet the instant you breathe on the gas pedal, the little thing slams into low range and spools the engine up to 3,000+ rpm. The 5 speed can't do this. Mitsubishi was worried that if they geared the car too high, people in the US would be trying to run the car at 45-55 in 5th gear, and engines would die as a result. So they did what a lot of other manufacturers of small cars did - they geared it low. It still gets amazing mileage, and now it will run forever too. It just spins faster doing it.
    Pretty awesome explanation. Thanks!

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    Quote Originally Posted by Tinker1980 View Post
    So why does the cvt version spin so much slower than the 5 speed? I haven't driven a cvt mirage, but I'll bet the instant you breathe on the gas pedal, the little thing slams into low range and spools the engine up to 3,000+ rpm. The 5 speed can't do this. Mitsubishi was worried that if they geared the car too high, people in the US would be trying to run the car at 45-55 in 5th gear, and engines would die as a result. So they did what a lot of other manufacturers of small cars did - they geared it low. It still gets amazing mileage, and now it will run forever too. It just spins faster doing it.
    I think it is more about being not psychologically acceptable to downshift for passing or on 5% highway hills, and this is not Mitsu only, all cars offered with auto and manual have the same issue.

    To complement this good post on wear, there are a couple more dimensions to consider:

    1) 80-90% of the engine wear happens during cold start, when you have cold oil which doesn't wanna flow, time delay for oil pressure to go up and dry metal to metal contact. Using oversized filter and 0w20 oil helps, but still. To reduce this mfg going to electric oil pumps which pre-pump oil prior to start.

    2) many engine wear issues especially on OHV engines are related to valve gasket condition. After 5-6 years they harden and start letting oil pass on intake valves. That if left alone will destroy engine in 20-30k.


        __________________________________________

        click to view fuel log View my fuel log 2015 Mirage DE 1.2 manual: 46.4 mpg (US) ... 19.7 km/L ... 5.1 L/100 km ... 55.7 mpg (Imp)


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