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Limitations of a rev limiter?

Discussion in 'Wheels, Wings, Mud, and Water' started by Donny, Jul 17, 2017 at 8:23 AM.

  1. Donny

    Donny Big Damn Hero

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    So a co-worker and I have an argument about rev limiters. I would think a rev limiter is to protect the engine from the occasional oops.
    He thinks it will protect the engine no mater what. Example a motorcross bike ran wide open down the street.
    Opinions.
     
  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    My understanding is that rev limiters will limit the engine to maximum allowable revs, as set by the limiter. But if a fault or action causing the high revs is not dealt with, the engine will 'bounce' on the rev limiter (in and out of limiting) and eventually shake itself to pieces. :(
     
  3. techguy0192

    techguy0192 AK Subscriber Subscriber

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    Have him try jamming it down a few gears, that will be clearly too low, to test that limiter. :naughty:
     
  4. whoaru99

    whoaru99 Epic Member

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    As has been mentioned, the rev limiter will keep the engine RPM limited to the setting. I would not, however, consider that a unconditional guarantee against failure.

    I'd look at it sorta like airbags. They greatly increase odds of survival in an accident, but still not a guarantee against death.
     
  5. arts

    arts Super Member

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    With very few exceptions, engines are not meant to be run continuously at their red line. A rev limiter is a great ''oops'' catcher,but if you set it to the engine's max rpm rating and have an ''oops'',the engine will usually overshoot the set limit. Many engines,especially those already running at (or beyond,as many manufacturers do) their design limit, won't survive the experience.
     
    John James likes this.
  6. usedto

    usedto Lunatic Member

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    Rev limiter story.

    Back in the 70s, a good engine builder friend was into 1/4 mile boat racing, and had a blown alcohol hydro with a 504 cu. in. Arias Chevrolet motor. They decided to go with a newer better engine, so the old one sat in the shop waiting to be sold.

    Tractor/4WD pulls were just becoming extremely popular, and he also built the engine for one of the top 4WD pickup pullers that was running a cast iron 454 in a Chevy pickup. They decided to "partner up" for an upcoming pull, and dropped the Arias under the hood, de-tuned it a bit, filled it with methanol, and took it to the fairgrounds. Wifey and I went along with them.

    When he first warmed it up in the pits, it caught everyone's eye. No one had used this type of motor yet, much less running on alcohol. He pulled it out on the track, backed up to the sled, and started the pull. In the beginning he just barely cracked the butterflies and the pull started. When the weight got about halfway to the front, it started to bog just a bit, and he rocked his foot into it. Unfortunately, he wasn't used to all that extra power. And, unbeknownst to the engine builder, he had connected the rev limiter. The butterflies opened, the RPMs went up, hit the limiter, it killed the motor, then lit it back up when the RPM dropped. Unfortunately, the whole time there was no spark, the 14-71 Mooneyham blower was pumping alcohol and air into the cylinders. Add spark and KA-BOOM!!!

    One spectator in the stands was hit by a piece of the aluminum block. Fortunately, not seriously hurt. When we got back to the pits, if you looked inside the front fenderwell, you could see people walking on the other side thru the block. Two rods had broke, and literally sawed their way out of the engine.

    That was over 40 years ago, and I'm sure rev limiters are more sophisticated now, but the use of that particular one proved costly.
     
    John James likes this.
  7. Bob Price

    Bob Price Active Member

    Messages:
    131
    Location:
    Pleasant Garden, NC.
    I deal with rev limiters every time I build a new engine, and every time my customers are tuning their systems for whatever use they have for the engines. There are countless applications.
    First, I think many people get the term "rev limiter" confused with the term "governor". A rev limiter is typically adjustable, and is used to control launch RPM off of the start line, and shift points... typically for drag racing and has a "limit" function built in. Although I have several customers use them in autocross as well. A rev limiter behaves slightly differently depending on the transmission in this application. For instance, my personal race car uses a 6 speed Tremec manual transmission through a multi-disc hydraulic clutch. My rev limiter's goal in my application is to do three things. It will not let the engine past 9000 rpm in case I miss a gear, and it controls launch RPM off the line and a shift light that comes on at a certain point in the RPM range depending on what ratio gear I am in at the time (yes, my limiter knows what gear I am in and knows where I make the most power in each gear). The goal in my case is to keep the engine in each gear as close to 7500 RPM as long as possible until it reaches within 100 rpm of its max HP and torque range. I build my engines to have as flat of a torque / HP ratio as possible. So my nitrous engines peak HP and Torque cross about 7500 rpm, so I set my rev limiter to 7400 rpm which occurs at different times depending on what gear I am in. By the time I see the light and shift, it will just be passing 7500. In this way, I ensure I am getting every bit of power I make to the ground before it starts to lose power. So, does it protect the engine? Yes, it is there to step in if I screw up. But primarily it is a power management system. Most modern automotive engines have discreet software rev limiters that will keep the rpm in check if the driver starts to abuse the engine, this is merely a ignition timing governor, it simply won't let the timing go any further which will hold the RPM at wherever they want. In fact, the cars I build have this system built in and it takes quite a bit of work to defeat it since the modern hemi is "drive by wire" like most new engines. So if you try to pass 6500 rpm on a unmodified version, it holds ignition timing and shuts the throttle plate. You can imagine what would happen if this occurred during a Nitrous run.

    A GOVERNOR on the other hand will not let the engine speed pass a certain point to ensure engine damage has less of a chance to occur. This is it's only purpose. Will it stop engine damage from occurring? Sometimes yes and sometimes no. I usually see governors on heavy duty commercial diesel engines where anything past 3000 rpm can destroy it. Most of these big engines require it because (if you did not already know), do not have a throttle plate and RPM is controlled by injecting more fuel, which increases engine speed and increases airflow, not throttle plate position and ignition timing. Especially the older engines like the 2 stroke Detroit 6 and 8v engines. They all had to be governed and would blow really quickly since they could rev really fast like two strokes typically can. Almost all diesel engines are turned off by shutting off the fuel so if engine RPM got too high, you could get a "runaway" and even shutting off the fuel would not stop it. It would merely start burning the crankcase oil as fuel and at that point you better run, or cut the air supply by putting a piece of plywood across the intake plenum!
    Another primitive and very reliable type of governor can be seen on a small briggs lawn mower engines. It uses an air vane that is moved by the air created by the cooling fan on the flywheel that will close the throttle once the air pressure reaches a certain pressure. I think everyone has heard one of those engines when the governor springs get wore out. That perpetual surging idle that is so annoying?
     
    Last edited: Jul 17, 2017 at 1:49 PM
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  8. usedto

    usedto Lunatic Member

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    It creates a huge vacuum in the wallet.
     
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  9. Bob Price

    Bob Price Active Member

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    I have had a few failures. The last one due to that problem not only blew the engine, but perforated the front fenders and quarter panels so badly we had to re-sheet metal the whole front clip of the car.
     
  10. MaxxVolume

    MaxxVolume Super Member

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    A rev limiter will only protect things while going UP through the gears....if you make an unintended downshift from say, 90 mph, and had intended to go to 4th, but accidentally engaged 2nd in error, (the dreaded "Money Shift"), you will force the engine beyond the rev limiter, usually resulting in valvetrain/piston damage.
     
  11. grillebilly

    grillebilly Empty Head Subscriber

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    The rev limiter in my cars kills cylinders individually, not all of them at once, to maintain the max setting. So when it reaches the setting, it "misfires" as opposed to shutting the spark off completely.
    Listen to the engine "stutter" on the rev limiter when the tree comes down. Cars are floored but "on the limiter"
     
    Last edited: Jul 17, 2017 at 7:35 PM
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  12. Donny

    Donny Big Damn Hero

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    This thread has really confirmed by beliefs on the rev limiter. I know suzuki made a utility quad that is limited to prevent wheeling. Seems I also remember the hayabusa top speed being limited and people getting around that by tricking the computer into thinking it was in a lower gear.
     
  13. turnitdown

    turnitdown AK Subscriber Subscriber

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    AIKI - my BMW rev limiter dumps the throttle at something around 5k. There is a not-so-subtle dump in power.
     
  14. MaxxVolume

    MaxxVolume Super Member

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    I think you are feeling the effect of the DSC cutting the power if it senses impending wheelspin....BMW rev limiters don`t come into play until just past redline....
     
  15. gadget73

    gadget73 junk junkie Subscriber

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    I find them rather annoying. One of the other neighbors occasionally likes to test his, usually late at night when I don't want to hear it. I wish it would stop working, at least once the rods shot out of the block of his truck the noise would stop.

    For purposes other than being a bad neighbor, I'm sure they're quite useful.
     

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