Why Do Some Engines "Bounce" Off The Rev Limiter?

"Wobbles the Mind" (wobblesthemind) 09/21/2016 at 16:50 • Filed to: Questions

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Hnnng, no-limits EVO!!!

Just curious, why do some engines bounce off the rev limiter instead of just cutting fuel delivery and holding that revolution? Basically, why come the engine goes, “Brapp-pappappappapp!!” while others just go, “Braaahhhhhhh...”

For those here just for the gif:

MonkeePuzzle > Wobbles the Mind 09/21/2016 at 16:54

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settings within the ECU, most likely aftermarket settings within the ECU

that GIF is gold!

Blondude > Wobbles the Mind 09/21/2016 at 16:54

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Dude what the fuck, I was literally just thinking this.

Wobbles the Mind > Blondude 09/21/2016 at 16:57

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I know you were so I figured I’d ask. By the way, your fly is down.

Nah, I’m just messing around with you! Hahahaha...you’re out of milk by the way.

RamblinRover Luxury-Yacht > Wobbles the Mind 09/21/2016 at 16:59

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It’s hard to cut off fuel to the exact level that will be a “safe” previous RPM - it’s way easier to cut completely. That way, the engine is just an air pump instead of an engine attempting to run on too little fuel (and detonate). Depending on the extent to which the throttle is drive-by-wire, though, it could anticipate you hitting the rev limit and “catch” you - starting to cut fuel before you actually hit, and slowing rate of increase as you approach, increasing EGR, etc. etc.. Much harder to pull that off with a mechanical throttle + EFI.

Also to some degree a function of how hard the engine can physically hit the rev limit - if it’s a very free breather but they want you to keep off of valve float or something, the engine may be happy to pump much faster than the rev limit, rather than slowing its ascent and straining to get there.

Bottom line - mechanical throttle and free breathing with a rev limiter: bounce very likely. Highly managed throttle with a lot of intake hardware: probably way less so.

HammerheadFistpunch > Wobbles the Mind 09/21/2016 at 16:59

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Holding the revs is basically what rev limiters are trying to do by cutting the fuel.

Overrev - Cut fuel

Reves drop enough to be back in compliance - Fuel back on

Overrev- Cut fuel

Over and over.

Bouncing of the limiter is just a function of where the “fuel on” upper limit is set and the interia of the engine and flywheel.

i.e. some rev limiters (coupled with very low interia engines) are tuned to respond to overrev in a very smooth way, others are more conservative.

e36Jeff now drives a ZHP > Wobbles the Mind 09/21/2016 at 16:59

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The ones that bounce are cutting fuel entirely and the engine is just windmilling down to a lower RPM until the fuel it turned back on and develops enough pressure to spin the engine up again.

The ones that just hit redline and stay there are only cutting enough fuel to stop it from gaining any more revs.

TheTurbochargedSquirrel > Wobbles the Mind 09/21/2016 at 16:59

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I believe it has to do with how when you cut fuel or ignition to the engine the revs are going to drop until you resume the supply. Some are tuned either so that the rev limiter is actually the highest the engine can rev or so that the supplied amount will only allow it to rev so high.

MonkeePuzzle > Wobbles the Mind 09/21/2016 at 17:04

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having watched the video, I am now suspicious. It never goes to 0 rpm, so I feel like the needle is just positioned incorrectly.

RamblinRover Luxury-Yacht > e36Jeff now drives a ZHP 09/21/2016 at 17:04

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Yep - and only cutting *enough* fuel is impossibru without being also able to control ignition and/or throttle position to a high degree so that the engine isn’t leaning out with an open throttle and not enough fuel.

The Stig's former college room mate > Wobbles the Mind 09/21/2016 at 17:06

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because brapp-pappappappapp sounds cooler than braaahhhhh.

but seriously,it is probably more likely that some vehicles cut fuel flow while others reduce spark and injector pulse.

the PCM can command the injectors to stop firing, which will cause the rpms to drop. once the rpms are below redline, the injectors come back on and engine revs again,.

In other situtations, the PCM can control the rate at which the spark plugs and injectors fire and ignore throttle position, maintaining a constant RPM.

My Charger would seem to have both- in park or neutral RPM is limited to 4000 and it will sit there at a constant 4000 with the throttle floored. but in drive you can get it to “bounce” off the limiter. If you punch it while cruising at 30 mph, it will downshift to first, break the back tires loose and rev quicker than the transmission can upshift, causing it to “bounce off the limiter” for a half a second. the sound is incredible. makes you want to yell “fuck yeah! America!” when it does it, and it’s been known to scare small children and Prius drivers.

BvdV - The Dutch Engineer > Wobbles the Mind 09/21/2016 at 17:07

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It’s probably some ECU tuning parameter, because there are cars that just slightly lower the amount of fuel, while others cut harder, and thus “Bounce”. Though I think most OEM ECUs are calibrated to be soft limits.

I once drove a Fiat 500 Twinair 0.9 turbo(2 cylinder, so it sounds a bit like a cross between a 2-stroke and a Harley), it had such a soft rev-limiter that I didn’t notice it the first few times. I was literally thinking:

“why does it slow down, I can’t be near the rev-limiter, I’m not going that fast yet”

*looks at revcounter*

“Ah, I should just shift....”

bhtooefr > Wobbles the Mind 09/21/2016 at 17:08

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You’ll notice that the engines that softly rest on their rev limiter tend to slow down their rate of increase of revolutions as they hit it. That’s because the software (or hardware in some cases - old diesels with mechanical pumps use a sprung governor, and spring tension cuts the fueling off as it approaches the rev limiter) effectively gradually reduces fueling in the last couple hundred RPM.

The ones that bounce off of the rev limiter don’t do this, so they’re more responsive when close to the limiter. And, at that point, you can’t gradually hit the right point, you need to cut all fuel/spark/whatever.

Scott > HammerheadFistpunch 09/21/2016 at 17:10

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He makes a good point though. Most rev limiters I am aware of cut the fuel entirely. However it is completely possible to simply reduce the injector cycle to not allow any higher RPM than that. It requires more of a PID system, which is a little more complex, but on higher end cars it would certainly be worth it, the vast majority of cars it would be a little extra development labor for something almost nobody would car about. You would probably still want a fuel cut off at a higher RPM in case the PID system did not respond quick enough. It's certainly possible that some cars already do this, my experience with high end cars at the rev limit is purely from Gran Tourismo games.

JawzX2, Boost Addict. 1.6t, 2.7tt, 4.2t > RamblinRover Luxury-Yacht 09/21/2016 at 17:17

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also, going back to motorcycles (and rarely, older cars) the lack of direct fuel control presented by a carburettor (as opposed to FI that is fully managed by the ECU) means that older bikes rev-limit by cutting spark. This causes that distinctive fast “brabpbrabrabppp” as the fuel is still being sucked through the engine by pumping action and is immediately there when spark is resumed. (often resulting in exhaust fire-balls)

Scorpio GTX1 > Wobbles the Mind 09/21/2016 at 17:22

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BTW, a lot of cars will do both. My car came with a “soft limit” which gently pulled back engine speed for about 500 rpm before fuel cutoff. It’s a less rude way to alert you to approaching redline, but it sucks for performance because you lose out on a lot of your rev range. When I had my car tuned the soft limit was removed and a higher hard limit was implemented.

DarrenMR > Wobbles the Mind 09/21/2016 at 17:25

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My car wont bounce off unless the wheels are spinning free.

crowmolly > Wobbles the Mind 09/21/2016 at 17:29

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Hard vs soft cut style

Daily Drives a Dragon - One Last Lap > Wobbles the Mind 09/21/2016 at 17:30

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I love how that Evo on the street is all wheelspin.

Also, Toyota engines sound amazing up on the limiter

they-will-know-my-velocity > Wobbles the Mind 09/21/2016 at 17:34

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Well, you mentioned the sound. To clarify terms here

Two step/Launch control

These systems can be integrated into the engine management or existing anti-lag system, or can be fitted as a standalone unit. The basic method of operation is to artificially lower the engine rev limiter to hold the engine at a speed where the turbo can produce usable boost, by altering the ignition. Because the ignition is alternately cut or retarded, there is similar noise and misfires associated with other anti-lag systems. Systems for two-step launch designed to be fitted in addition to the existing engine management work by interrupting the crank position sensor signal, so that the engine develops a controlled misfire at a pre-determined RPM. The basic premise of the launch control system is to build positive boost pressure from a static engine, releasing full or increased power to the wheels when the car starts to move off.

Two step/anti lag systems will also make that popping-crackle that can be confused with bouncing off the rev-limiter. Rear how it works and you can see why it would make a similar noise.

Rev limiters have changed over the years. They achieve the same purpose but depending on application they change. Most common are the ones that cut fuel. Reach rev limite, cut fuel injection. This can cause the popping noise you were talking about. These are more common because of the mechanical simplicity and less wear on mechanicals.

Cutting spark is the other option. This isn’t used as often since it cycles unburnt fuel though the exhaust, which causes quite a bit of backfiring and wastes fuel/smokes/etc.

In reference to “bouncing” off the limiter, that too is a choice by manufacturers/aftermarket. It is what is called a “hard-cut” limiter. As the RPM reaches the set speed, it cuts either fuel or spark and the RPM drops. If the engine is at WOT, it revs back up and bounces off the limiter again. Hence the “bounce.”

“Soft-cut” limiters retard timing and fuel/spark before you reach the set RPM limit and do not let you exceed it. It will slow the acceleration of the engine until it reaches the limit and then maintain it. Performance vehicles commonly use the hard-cut limiters.

Jordan and the Slowrunner, Boomer Intensifies > Wobbles the Mind 09/21/2016 at 17:38

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I wish the FiST had a rev bounce like that. It gets violent when it cuts out as is.

Tekamul > Scott 09/21/2016 at 17:45

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Why would it be worth it? Cut offs are almost universally past peak power, and only as high as they are to allow for overrun. There is no good reason to sit at the cutoff

Scott > Tekamul 09/21/2016 at 18:38

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For the most part you are right, and probably why for most cars a cut out is perfectly good. However if you are on track with a car, (not an all out race car where gear ratios would be changed to suite the track) you may find yourself running against the rev limiter on a long straight. Personally I experienced that far more often on the Autobahn.

Since this really costs nothing more than a simple coding function in the software it really is almost no cost to add it. While almost nobody would notice it, those that drive on the Autobahn, or the few other unrestricted roads around the world where running flat out would likely notice that the manufacturer has payed attention to that tiny detail. While most tracks don't have long enough straights for a car to reach its top end, it does happen, and I see that as where people might most want it.

I'll fully acknowledge it's really just a nice attention to detail that few will ever realize is there. But I can recall many trips on the autobahn where I was going flat out bouncing of the rev limiter and I found it annoying to try and keep at the top end, but under the limiter. On track I could see it as being far more annoying, but granted far less likely to occur.

.

SidewaysOnDirt still misses Bowie > Wobbles the Mind 09/22/2016 at 08:24

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Low intertia engines with a hard cutoff sound best. FC20s and 4AGEs sound glorious bouncing off the limiter.