Ryan00TJ

iTrader: (8)

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?

hadleycat

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

mike tkach

iTrader: (3)

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.

BUP

iTrader: (9)

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

mickeymcclgn

iTrader: (1)

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

mike tkach

iTrader: (3)

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.

BUP

iTrader: (9)

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

BUP

iTrader: (9)

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.

class6

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

Bawana

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.