Impacts of hitting rev limiter?
 

Caught an episode of Australian offshore powerboat racing over the holidays and it got me wondering. What are the problems that continually banging the rev limiter can produce? The Australian boats had sealed 525EFI's and we're all over the limiter the whole race. I know the 525's have the ECM555, and it cuts fuel vs spark. Also, it seems the 525's are tuned very conseveratively. My previous MSD setups on my carb motors cut spark vs fuel. I would think this is the safer approach. What are everyone's opinions?

I am no expert but I can tell you what my engine builder told me. Say your rev limiter is set to cut fuel at 6000 rpm. When your prop leaves the water and your engine climbs to 6000rpm the fuel is cut of course, but that does not stop your rpms from climbing. The engine can freewheel well past 6000 rpms before lack of fuel flow effects it. Same goes for cutting spark. Since your prop is out of the water when you hit the rev limiter there is very little load to stop the rpms from climbing the instant spark is cut. I cannot remember now how many hundred rpm he told me that he typically saw past the limiter.

I am hoping some of the experts will chime in here with more info

im not sure if merc cuts the fuel but i believe they do cut spark.msd box starts dropping cylinders 250 rpm before the set rpm limit and the engine will not go over set rpm limit.

for OEM stock marine engines - either way continuously bumping off the rev limiter is not good.

Dropping spark and not fuel - this will wash the cylinders down

dropping spark and fuel can causes lean conditions and or hot spots within the cylinders.

Also the Stock Merc HP 525's are PCM 555 and calibration

I'm by far an engine expert however I know of a triple engine black thunder that I scanned that had 525s and the center engine showed over 1800 bumps of the limiter. They had over 300 hours and still running. Finally blew up after adding a whipple and another 100 hours. The center engine was under propped for years and this man ran the boat WFO like it was going out of style. I could just imagine him and his buddies riding around with the center engine hard on the limiter. To say I was shocked is an understatement. It's now propped right lol

when msd drops cylinders it rotates through the firing order to stop fuel wash.i would think bouncing off the limiter for a few seconds won,t harm the engine.now if you were to do it continuisly that could create a problem.

post # 5 do you mean 1800 seconds total of bumping off the rev limiter ?

Again refering to Mercruiser stock apps and cals.

Overspeed will be the fault code for hitting / bumping off the rev limiter.

What this means and does is - recommended stock rpm range was exceeded - so if continued hitting the rev limiter / overspeed what happens is - first the horns sounds, second is the cylinders stop firing (drops spark) third is more cylinders stop firing then after that if still hitting the rev limiter the PCM / ECM will fault to overspeed 1 which could cause power limited meaning a % of guardian mode.

If Overspeed level 2 sets, the horn / alarm / fault is critical and only 5% of power is available and or engine shut down. If I remember correctly for PCM 555 this will be fault code # 135.

Its the same as pulling back the throttles, doesn't hurt at all.. If your not bumping it your not going for it

I have friends that THINK its a throttle stop..LOL It aint good. Motor harmonics are very important to longevity. Killing cylinders don't help that at all.

My 525 is propped to hit the limiter in perfect conditions. That usually keeps rpms down around 5,200 with passengers. I get the rev warning horn a lot when running alone and light and cool days. I have never noticed anything harsh after the horn starts. I thought the engine started pulling timing first. Can an engine guy verify for certain what happens when you hit the limiter on a 525 which I believe happens at 5,450 rpm.The power is at 5,400 so bumping the limiter is inevitable if you want get full hp.

If its an EFI motor and cuts fuel, it does just that, cuts OFF fuel. It's not lean, its just a dead 'jug' at that point pumping air. Its not lean under our normal definition.

If it cuts spark the fuel is not washing down the cylinders. Think about it, whether the motor sparks or not, the same amount of fuel is present. The only difference is that after TDC the unburned fuel vapor gets tossed out the exhaust chute. It's not sitting there cleaning out the cylinder bore any worse than a normal ignition sequence.

So unburned fuel continuously hitting a hot exhaust valve(s) is OK - would like to hear how that over time is fine.


A step further would be take the stock HP 525 out the mix completely but we have currently way more OEM stock marine apps for the past 7 years with O2 sensors - 2 incoming (pre) and 2 post plus a catalyst system / cat sub strake installed in the exhaust risers. Do you know what happens when raw fuel hits these areas because you think bumping off the rev limiter is ok ? When too much raw fuel hits the very hot cat sub strake it will flashover and blow a hole straight thru the cat. One cat sub strake is 1200 to 1800 bucks alone. Just saying. There will be a day when all marine apps really tighten the emissions and the owners of the boats. It has already started.

BUP, will the the fly by wire systems cut back on throttle angle to stop the continual 'bumping the rev limiter' scenarios. This would be far more optimal then continually dropping spark or fuel if someone leaves the sticks all the way forward.

SB it is here already - Mercruiser has released their new engines so far, the new 4.5 L V-6 and the new 6.2 L V -8 that has Adaptive Speed Control - aka ASC - I test driven the new Mercruiser 4.5 L in Oct 2014 and you could not get that to over rev no matter what you did.

Even at any speed / throttle setting with drive trimmed up and driven very hard corning - the jist of it was like a cruise / speed control would set in not allowing the rpms to increase no matter what even when the prop completely blew out, rpms stayed exact the same. Also worked at very low speeds to maintain planning speeds.

I ran the pee out that engine on the West coast of Florida. Anyways this link just talks about a bit.

http://www.boattest.com/engine-revie...5L-250-HP_2015

So with a system like that... you could drive like a maniac, launch the hell out of the boat, leave the throttle on the dash and never worry about throttling??

^^^ I hear you - its like a RPM maintainer for the boat or like I said cruise / speed control. Well I really did not want to go into more new systems nor the indepth details but you will like this as well.

Its becoming a boat does drive all by itself and the newest for 2016 is Active Trim Control plus already out to market is Joystick & Axuis & steering angle read outs and ASC plus other new tech brought forth to the marine market for the end user.

Watch the Video in the link scroll down. Talk about being lazy while driving a boat nowadays. jeez

Mercury will be offering a race version of this active trim for high performance boats that use surface drives as well.

https://www.boats.com/reviews/mercur...oating-easier/

For me and possibly most of us, a big part of the joy of driving a high performance boat is mastering the skill of throttling, tabs, trim etc. Making constant adjustments while driving at speed and reading the water. Like banging gears in a hot rod, or driving a dirt bike or quad. The more control you have and skill you develop the more fun it is. I can't let anyone else drive my boat. It took me years to master my little Donzi. Many people don't realize the skill it takes to drive it at speed or to carve that high banking slow turn. If we wanted boats that drive themselves we'd all have cruisers and kick back at the martini bar.

^^^^ agree and that holds true for anything - bikes, fast cars and fast women haha

Here is more for your enjoyment. Watch the Vid everything is taking it out of the hands of the end user. Anyways I have lots to say about all of this stuff going into boats.

http://www.tradeonlytoday.com/2016/0...sletter-010516

See, God does listen to your prayers. Ha @

Thanks for the replies. Just curious of everyone's opinions on this. With the 555 ECM cutting fuel when the revlimiter is reached, Whipple said " do not continually run on the rev limiter, continued abuse of the rev limiter can cause severe engine damage". Especially in my stock piston 496HO setup scenario.

^^^^ great for the reply so THX - and someone else that agrees with what I have said in the beginning of this post and if you search up here, I have said a bunch before.

And actually OEM merc and in all their OEM service manuals for ECM 555 and PCM 555 lists it drops spark hitting the REV limiter.

Yes. Its hot air with fuel vapor. Not liquid fuel. It's hitting the exhaust valve regardless and we are talking milliseconds not immersion of a glowing valve into water.

Whipple raises the rev limit with their programing, Merc's limit is totally fine. BANG ON

Bup, Thanks for the info. Good to get a spot on answer as to what actually happens when the 555 hits the rev limiter.