Safe OEM power Head RPM

[bacaruda]

Tried to get an answer from Mercury, but they wouldn't budge from the book. 5000-5600 operating range.
'91 175 XRi. Currently running 75MPH @ 6200, with 26P Predator 4Blade.
It's been suggested on here I try a 24P ChopperII. But with less pitch, and
more speed(?), my revs could be approaching 6800-7000. Other upgrades
are from 5" to 7" setback, BMS cone, transom hieght.
What are you other guys turning your stock OEM short blocks too?

[HStream1]

Well IMO sence you say STOCK. I can assume you still have the STOCK oil injection system. With that being assumed. I would never turn that motor over 6200. Even then you're asking for a crank shaft oil pump drive gear failure.

You may get other openions.

[BagMan]

I regularly turned my '91 200 XRI w/23 Hi-5 (skiing/tubing) to 6400. This was maxed RPM and had no issues. Never went down and tried pushing further however.

[Raceman]

My response to this question is always the same............... WHY do you want to turn it harder. That engine doesn't make the power at 6800 that it makes at 5800. You will lose speed. durability AND fuel mileage. There's ONE REASON ONLY to turn an engine such as this harder................... acceleration. If you want MORE SPEED, you need to be thinking in the opposite direction. Pitch it UP, get the RPM's DOWN from where they are now and you'll gain everywhere EXCEPT acceleration.

You may very well pick up some top end numbers by going from a 4 blade back to a 3, so changing from the Predator to the Chopper might give you an increase in itself, but first thing I'd try is a chopper in the same pitch you're running now as a starting point for comparison to see which design blade your boat likes best, and THEN add 2" of pitch and see what happens, OR you could just try a 28 chopper right off the bat.

[bacaruda]

Raceman, I understand and agree. I was asking because a few on here
recomended the 24P Chopper over the 26P Predator. I assumed they had
all the considerations in mind. Or, concerned with holeshot, as I ski alot.
But the hole shot is now more than enough to get up with both feet in ski,
I can't hold it. So losing some bottom end may help that.

[Capt.Insane-o]

6200 is perfect.

[Raceman]

I'm thinkin' if you're usin' a 4 blade through hub for skiing that you're probably not going to like a chopper all that much for two feet in starts on a slalom, but it could vary from boat to boat. My ole' Sidewinder that I usta' ski with years ago with a 200 Merc wouldn't pull a skier up with a chopper.......... it'd just fan and not get on top.

[bacaruda]

This predator is over/through hub. Smaller than gearcase. Stalls to
5000 out of the hole. I've heard differing about the chopper, some have said it wouldn't spin up like this prop. But you seem to say it'll spin like a race torque-convertor. Not good.
I'll try to find a 26P II, and see what it does. Should be able to resell for what I paid for it.

[bacaruda]

Where this all started,,

http://forums.screamandfly.com/forums/showthread.php?t=200896

[JR IN JAX]

I do not turn my ProMax over 7K due to still using the stock oil injection system. I got over 1000 hours on my first powerhead [that was still running]. Turning them tighter just "lightens" your wallet.

[bacaruda]

It would be really nice to get a HSP/Torque curve chart from a real factory dyno
run. Then you could nail a stock engine prop on the head. I don't know if I'm under propped and running out of air long time ago, or not. If I am, bigger wheel is definietly the way to go.
Oh, and the stock oil injection coment really got my attention. I looked at exploded view in my FSM, and is the oil pump gear metal or plastic? Like a car nylon cam gear.

[Capt.Insane-o]

The driver gear on the crank is nylon, that is the one that fails. The driven gear is steel.

[bacaruda]

At least they could have reversed that so could upgrade to a steel oil pump gear instead of blowing motor!

[Capt.Insane-o]

I'm not sure, but it's possible Yamaha's patents might of had something to do with it all. They ran a bronze/steel gear set up. I've worked on quite a few Yammers and have'nt seen a failure in that area.

[BoatR]

I am having the same issue. I too phoned merc. they would say go to a higher pitch. I have a 28 chopper II on my 2000 175 hp EFI(oil injection in tact). the merc guy said over 5600 rpm you dont make any more power in fact you loose power, he said the rev limiter dumps more fuel into cyl # 4 then #1 cyl and so on until rpm comes down. I thought it would have been electronic not fuel! and more gas would make it go faster. when warm weather comes Im going to try a 30 " pitch but I cant imagine it swinging it!
right now im running pretty close to 6500 rpm @ 73 mph (gps) hydraulic jack plate on a Charger 20 'STV. I also question how accurate the merc tachometer is , the merc speedo say 80 mph when garmin says 73 mph.

[sschefer]

Having been inside only two 2.0l now and soon a 2.5 I can see why Merc holds the line at 5800 RPM. These are production engines and although they're pretty darn good they still end up with pretty wide tolerances compared to a blue printed and balanced race engine.

All those inconsistencies that end up in a production motor equate to one thing. Harmonic Vibration. Merc is telling you that your safe up to 5800 rpm but after that the harmonic vibrations begin to start taking their toll on bearings, seals and yes, the infamous oiler gear.

So why can some guy's turn 7000 rpms on a stocker and others can't.. It's because of the inconsistencies that some engines are better, some are normal and some are flat out piston burning lemons.

[bacaruda]

Wow, assuming you're running 1.87 gears like me, our numbers don't add up. Currenty my TF tach. or voltage reg. is kaput, because I have no rpm. I just quote RPM from a couple of years ago because I've made no changes. Also, I don't know speed for sure. I calibrated my Airguide to a
200' foot course, but not the most scietific method. Also gauge reading near full scale. Not any intruments most accurate range. I figured out how
to display GPS data on my Cobra, AFTER the season was over. So I need
to get some accurate data before I make any prop changes.
I see prop slip for me around 9% (little tight), and yours around 20% ! ?.
WAY LOOSE! We both need to get some good base line to work from. After that I might like to give your 28P Chopper II a shot, if you go another way.
Thanks, jim. :cheers:

[bacaruda]

Also BoatR, isn't that basically a ModVP hull? That I would think should run 85 - 90+.

[bacaruda]

Oh yeah thanks Steve. I'm going to reprop when I get an accurate baseline. Oil gear scares me the most. I don't want to go back to mixing oil just to dump the oil injection for safety reasons.

[T-REX]

SAFE RPM=lower rpm.........UnSAFE RPM=higher rpm........Safe and unsafe rpm ain't got nutt'in to do wit tha peek power rpm ov tha mota...

Find tha peek torque ov tha mota, prop it for that rpm......don't git no better.....

But, one word ov warning...LUGG'IN an OB iz worse than turn'in a few extra rpm!!!...Juss my unprofessional RPM!!:thumbsup:

[bacaruda]

Gotcha T-Rex. DYNO doesn't measure HSP, just Torque. I'd like to find someone close with a Land and Sea Propshaft Dyno-Mite. Probably 10% past Torque peek would be the sweet spot.