cmoore7

Hi,

I am a new over of a Donzi 35ZR (twin 496 h.o) and was wondering what size props i should be running. Right now i'm running a 4 blade 28p and turning ~5200+RPM @ 80 mph (fresh water). The current props have some small dings and nicks so i'm considering getting them reconditioned or buying new ones of a different size. I've been told that the rev limiter is at 5250. Has any one ran a 30p on this set up? Any input would be helpful.

Thanks,
Chad

BUP

iTrader: (9)

rev limit is at 5150 / 5200 for the 496 HO unless your PCM 555 has been recaled by whomever.

Bouncing off the rev limit overtime is not a good thing. Just saying

dereknkathy

so, if you are regularly hitting the limiter, an additional 2 inches of pitch should drop revs about 7% down to about 4850. unless you have enough power to push right back to the limiter with more prop.

class6

Why is hitting the rev limiter a problem?

BUP

iTrader: (9)

Are you Asking ?

If not some info about PCM 555 - off the top of my head - Merc does not rev limited with timing - so that means they limited (drop ) spark and then continuous Overspeed will keep droping spark in other cylinders... Dropping spark but still adding fuel in cylinders can cause wash down of the cylinder walls and when the overspeed is reduced with rpms, the spark will pick back up. This creates ALOT of pressure in the cylinders for a short jist of this info. So how can this be good for an engine. Well its not if one does this a lot and or continuously..

If one owns a PCM 555 Merc fault for rev limit is Overspeed fault 19. This is the first setting a warning horn that will sound, secondly is the cylinders stop firing, third is more cylinders stop firing -- continuously dropping spark - upon that situation past overspeed warning on to Overspeed 1 in which is fault code 134 --- once level 1 overspeed is set, it rolls into exceeded from overspeed 1 to overspeed 2 in a matter of a seconds or tad more - Fault code 135. .This causes engine guardian limiting engine power, .as a critical FAULT with ONLY 5 percent of available power.

Also in which many do not know this period. - watch what number 4 cause is in the video link below. Listen carefully to number 4 reason as I will let this guy in the video hurt your feelings if one thinks about bumping off the rev limiter a lot or even continuously is not a bad thing. Great info in the number 4 reason.







. .

class6

So you should not use the 555 in any race Applications. Rev limiter's act in milliseconds. I don't believe I'm going to ruin my motor with milliseconds on the limiter. And I truly believe it's better to hit the limiter once in a while then over prop it and strain the motor. At that rpm I doubt cylinder washdown would be a problem. The fuel would vaporize and get pushed out of the cylinder Way before it would wash. Just think how many times that piston goes up and down at 5400 RPM's
as for the video. He's talking about people that just stand on the throttle in neutral to make noise. Or drag racers that stand on the line for five seconds or more holding it on the limiter. I don't think he is talking bumping off the limiter now and then. Watch it again

dereknkathy

It sounds to me like the OP is regularly running his boat at 5200. Also sounds like if you know your limiter setting, you should either pull throttle or re-prop to stay just under the limit. You hear these big blocks can spin 6000 to 7500 rpm. That is fine and dandy for 10.3 seconds or so, as long as engine is built for that kind of use. When GM puts a motor together, they have ONE primary goal. Make that engine last long enough that they aren't gonna have to replace it for free. Light valve springs and low valve lifts make valve trains last, but can't control valves much above 5k.

neva satisfied

I think a 30P is going to be too much prop for that boat. I think you are right where you should be. You don’t mention how much fuel or gear you have at that rpm?

dereknkathy

If rev limit is 5150 or 5200, what is rated WOT rom? 28 to 30 pitch is 7% diff. If he is right at 5200, he will be down around 4850. BUT, for all we know, boat would run to 5300 or more if rev lim wasn't kicking in. I think he should look into brett's prop trial deal. If 30 turns out to be too much, since these props need work, mebbe have brett add a half inch or an inch.

dereknkathy

Double post.