Purpose of High rpm Low Torque Engines

Discussion in 'Gas Engines' started by baboonslayer, Dec 21, 2010.

  1. baboonslayer
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    baboonslayer Junior Member

    I honestly don't see the point in modern 4000+ rpm gas engines that are complicated and have a lot of electronics. The high rpm wears down the cylinder sleeve way quicker than any low rpm high torque heavy flywheel engine does. They are too complicated for the average person to work on. Also, they suck down fuel like a jumbo jet. You could compare their torque output to that of an electric toothbrush motor. So, I ask again, why are we using these motors? Theoretically speaking, they are manufactured to break, right? What happened to the old gas engines, which were simple, reliable, and fuel efficient?
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  2. P Flados
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    P Flados Senior Member

    I am not an expert on power boat stuff, but I have had this discussion with a pretty knowledgeable former marine mechanic.

    Modern engine technology is adapting to a more "disposable" mindset. The cost of labor for major maintenance combined with inventory costs for needed parts is making engines more expensive to "maintain" and cheaper to replace. Diagnosis of hard to find problems across the spectrum of all of the motors out there can be real tough for a service dept.

    When they wear out or get unreliable, it can be cheaper and better business (for them) to just replace the entire engine.

    This mindset pushes thinking toward a primary objective of Horsepower vs. manufacturing cost with low weight as a plus. This gives us high rpm engines with lots of small parts but with minimum big heavy expensive material parts. Modern mass production and automation can push a lot of these out the door without a lot of skilled worker man-hours invested.

    The fact that these wear out sooner than older designs (after any warranty is over of course) is actually a plus for the supply side of the equation (manufacturers, distributor & service providers). There are probably still better options available, but since they have a smaller slice of the pie they cost more. Also, modern consumers do not push for products that last. This just feeds the situation.

    Elements of the above discussion are actually applicable to most consumer and industrial products. Everything is just too disposable and products are constantly changing.

    At the power plant where I work, we buy a "standard" pattern valve, but 5 years later the "manufacturer" can not sell us parts without an exact serial number and a long wait because his sub-suppliers that actually made the valve changed the details of the design twice since then.

    When we buy industrial electronic devices, the part is frequently obsolete before we can install it. When it fails in 3 to 5 years, the replacement device requires entire redesign of our cabinet and circuits. We do our best to make it work and then 1 to 6 months into service, some hidden "feature" of the new design causes all sorts of problems. The devices are made with components known to need replacement every 5 to 8 years (electrolytic capacitors), but the components are buried in the bowels of the device with no provision for replacement and nothing in the vendor literature tells you they designed the device for failure.
  3. cthippo
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    cthippo Senior Member

    Two words...

    Emissions standards.

    The need to meet the new emission standards has driven engine design for the last decade or so. It is apparently easier to get cleaner combustion in a hotter, faster engine, and much of the electronics on a modern engine are emissions related. That's also why it's so hard to import the older 1 and 2 cylinder engines because they don't meet the new standards and are therefore not allowed into the country.

    On the plus side, I would take a look at whether they are really less fuel efficient than the old engines. Great strides have been taken in this area and that should have crept into marine engines as well.
  4. Stumble
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    Stumble Senior Member


    I think you hit the crux of the issue. Emission standards are almost universally the driving issue in modern engine design. Since most marine engines are really borrowed from car engines, the same issues that drive those engines drive ours as well.

    That being said I have doubts about the concern over reliability in modern engines. Most engines today are light years ahead of the engines produced 40 years ago in terms of reliability. The real problem is that modern engines cannot really be worked on by a home mechanic. If something goes wrong it must go to the shop.

    Heck I remember when every weekend of boating also required a half day of engine maintenance. Admitedly it was simple stuff like checking the spark plugs, and cleaning out the Carb, things which today you wouldn't touch outside of a service shop. But these days the maintenance is just too complicated and requires specialized equipment.

    Also the reality is that we ask more of engines today than we did years ago. Thanks to modern design and construction techniques the industry is capable of turning out reasonable quality boats that are affordable that also can operate at better than 80 miles/hour. Years ago that would have set speed records and couldn't be imagined in a production boat. But these days even speed boats are expected to top at over 40kn. This extra demand is pushing the engines further and faster than ever before at the same time the maintenance is becoming impossible for the home boater. This is what leads to the mistaken impression that the engines are less reliable (at least in my mind).
  5. powerabout
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    powerabout Senior Member

    I would say modern engines use less and polute less both cars boats and outboards
  6. Joakim
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    Joakim Senior Member

    On what kind of boats would you like to use low rpm gas engines? If weight and size are not a problem, why don't you use a diesel instead?

    Which modern engine are less fuel efficient than old low rpm ones? Which engines have wear problems before rust and other problems in typical use?
  7. DennisRB
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    DennisRB Senior Member

    Another way of describing a low torque high RPM engine is a high power engine. These engines will be much smaller and lighter than a low revving engine of the same power.
  8. Karl2
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    Karl2 Junior Member

    The marine gas engines are completely reliant on what the automotive industry does. So, emissions, fuel economy, etc. etc. are the drivers.

    It is not the rotational speed of the crank that causes cylinder wear. It is piston speed. I.E a function of engine speed and stroke. I submit that piston speed has changed very little in the last 40 years while metallurgy technology, lubricant technology, etc. etc. has advanced tremendously. Today’s engines wear out at a slower rate.

    Simple - Yes.

    Reliable – Disagree completely. I owned my first car in the sixties (Car made in the late fifties) and has experienced these motors through the decades. My experience only: Until we got to the nineties I was happy to get 100,000 miles from an engine without major repairs. At 100K i filled oil regularly and spent time every weekend under the hood. The last three vehicles I owned (MY 2000, 2003 and 2005) have all clocked +180K without a wrench being put on the engines apart from maintenance. I open the hood so infrequent I tend to forget were the latch is. Currently I own a European made vehicle with 199K on the odo. Turbocharged, small displacement, 6,000 rpm. Not a problem what so ever, never filled a drop of oil, no fluid leaks....I would like to have my 1966 Mustang with the 289 back but no thank you, I will not trade the reliability of that 289 for what I have today.

    Fuel efficient - You are kidding, right ?

    IMO: Although the applications are totally different the marine applications has enjoyed the same advances in fuel efficiency and reliability as in cars/trucks.

  9. ondarvr
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    ondarvr Senior Member


    I think you should list the engines you're referring too, as I don't think many people want to go back to what was. Although being able to work on them more easily would be nice, I wouldn't trade it for the longevity and reliability of a modern design.
  10. rxcomposite
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    rxcomposite Senior Member

    Because it has specific use. Almost all high rpm engine are lightweight and designed for short service interval. If you are a pleasure yachts user, you will demand maximum power about 20% of the time but if you are operating a trawl, you would demand 80 t0 100% power from the engine.

    Manufacturers have three to five different service ratings for their engines. If you compare a 100 Hp pleasure rating to a 100 Hp continous duty rating, the continous duty would almost always be twice as heavy.
  11. gonzo
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    gonzo Senior Member

    High RPM engines are more economical. I don't know where you get the data to support that they use more fuel per HP than an atiquated engine. That is just "good ol' time" BS.
  12. baboonslayer
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    baboonslayer Junior Member

    Lots of opinions on the topic.
    Thanks for all the input on this. I should have known better than to open this ******* can of worms...

    We could couple today's technology with old technology and manufacture an engine designed for continuous duty using modern alloys, machining, and modern lubricants, and since weight doesn't really matter it would have water cooling. It would be low rpm and have a piston stroke longer than the bore of the cylinder to give it more torque. It would be simple so anyone could maintenance it.

    gonzo, you are right about the older engines producing less horsepower with more fuel. The main flaw in what you are suggesting is the horsepower part. all horsepower means is that you have x amount of torque at y amount of rpm, so if you put smaller flywheels on the old engine and increased the rpm from 750 to 2000 rpm then it would have more horsepower, but less torque.
  13. cthippo
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    cthippo Senior Member

    Hey, if we all agreed this would be a pretty boring place :D

    It could doubtless be done, but you could only sell it in the third world. That's why places like India and China are building old Listeroid designs. They're cheap, reliable, and made with more modern materials and techniques, but good luck ever importing one due to emissions regulations.
  14. Yellowjacket
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    Yellowjacket Senior Member

    The driving force behind smaller more fuel efficient engines is the CAFE standard. That pushes auto makers towards smaller, lighter, and unfortunately lower powered cars. In order to keep performance up, these smaller engines spin at higher RPM's to get the necessary power. Unfortunately most of these engines aren't very good for boat propulsion.

    The higher efficiency comes from better fuel control and lower internal friction, and those are good things, but if you need bigger power you need more displacement and those engines aren't as plentiful as they were when more than 75% of the cars on the road had V8 engines in them. Some of the 4 stroke big hp outboards are based on their auto engine brothers.

    Truck engines still have the power and torque for marine use and that's where you can still find engines for a reasonable price that can be converted to marine applications. There are just fewer of them in production and they are more expensvie than before.

  15. rxcomposite
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    rxcomposite Senior Member

    Short stroke engine have less piston sweft area than a long stroke, therefore less friction per rpm. It is not the piston itself that is in contact with the cylinder wall. it is the piston rings.

    Because it has to rev at much higher speed, it's mass has to be kept low. The block is also low and does not have to be as thick as the long stroke thus making it lighter.

    Short stroke suffers from not getting enough air in a normally aspirated engine so a turbocharger is added. Pound for pound, it produces more Hp.
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