351W Optimum Max RPM?
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=2870
Printed Date: November-23-2024 at 7:28am
Topic: 351W Optimum Max RPM?
Posted By: Morto
Subject: 351W Optimum Max RPM?
Date Posted: January-12-2006 at 2:34am
Hi everyone, just wanting to get some learned opinions on what maximum RPM for the stock PCM351W should be. I know the book says 4400. Why then do I ask? Well, my local marine engineer claims a few things:
1. The American V8's are strong enough to run into the high 5000's.
2. Many marine engines (he uses Volvo as an example) are propped too big for a particular boat to artificially limit the maximum RPM and so avoid warranty claims. The downside to this, he claims, is sometimes not adequate cooling. To this end we have junked the standard raw water pump and are making a bracket to mount a Mercruiser pump on the PCM. He says this pump is crap too (!!) but flows more than the PCM. He is planning on sea-trialling once we are finished and trimming the prop to achieve 5500 rpm max, with a pulley that will turn the pump at 8000 when the engine is doing 5000. Eventually we may replace it with the brass Jabco which he likes. Remember that we are in Brunei, and the water is not ever cold (I'm guessing minimum 17 or 18 deg C all year - there are no wetsuits on skiers here!).
Any thoughts before we start cutting?
Cheers all,
Shaughn.
|
Replies:
Posted By: nuttyskier2002
Date Posted: January-12-2006 at 3:45am
Morto, what is your main objective here? Is it to pull more skiers at the same time and get out of the hole faster or just to watch the needle on your tach climb higher? You need to state this first! Yes the 351W will turn 5500 RPM safely, but how often are you going to be doing this. Because, you are going to reduce the longevity of this engine by twisting it that tight all the time. You have to remember that a marine engine operates under a constant load all of the time (unlike a car). It's like always climbing a steep hill and never reaching the top. For this reason, marine engines wear much faster than automotive engines to start with.
Also there are other things to consider. For one - if your transmission output is 1:1 you are going to be turning the shaft at this same speed. The stuffing box and strut are not designed to handle much more than 4000 RPM shaft speed. Also, you're talking about turning a pump with a rubber impeller at 8000 RPM. That's not going to last long either.
Ask your engineer if he has made these mods before and if he can show you a working example that is still holding up.
-------------
95 Malibu Echelon w/Mercruiser 350 Magnum Skier
Former boats:
88 Ski Centurion Tru Trac II
59 Chris Craft Capri (woody)
|
Posted By: GottaSki
Date Posted: January-12-2006 at 4:57am
Well said, nutty
Sure it will hold together but the stock 351 heads don't have the lungs to run above 4500+ krpm unless the boat is light and small as well as the loads.
If you use a mercruiser pump you have to upgrade all the fittings and hoses from 1" to 1.25", else its for not. I think a 2x pully is too much.
Propped such, ski speeds will require the secondaries to be open some otherwise it just can't fill the engine enough with only the primaries, but a vacuum seconday carb is not very precise at those settings. Its best to be in the upper half of the primaries for best atomization and throttle responce.
To sum up, you need the modify the engine to make it perform well at those rpms. I regret to say, a chevy would respond better to what you desire than the basic ford due to larger valves.
-------------
"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
River Rat to Mole
|
Posted By: 64 Skier
Date Posted: January-12-2006 at 7:26am
Speed or Ski? That's the question you have to ask yourself. Why does a ski boat need 550 rpm's?
I live in the South (similar or hotter than Brunei waters....surface temps of shallow ski lakes are easily greater than 18C) and we pull slalom at 3200 to 3600 rpm in Tournaments (an old 289 and a new 350) so the engine has a heavy load at very warm water temperatures and have never had a heating problem...although if you pay close attention to the thermometer you can see slight swings as the thermostat adjusts to hot surface water.
If you want to see your pump in action...hook it up to a garden hose and run the rpm's to about 3000....that pump will outrun supply and collapse the hose! Maybe the eccentric was worn out on the pump your mechanic has experience with???
-------------
64 Skier
66" HO VTX and 67" HO Triumph
http://www.correctcraftfan.com/diaries/details.asp?ID=1071&sort=&pagenum=3&yrstart=1971&yrend=1975 - 71CC
|
Posted By: 79nautique
Date Posted: January-12-2006 at 8:57am
Go ahead and let him blow up your motor if you like. Most late model V8's run 5000 rpm's tops with a gear reduction tranny for short periods of time. Don't agree with his assement of the cooling system and over proping herts it's cooling ablity but if you like pissing in the wind, enjoy and have a line on a new motor to drop when this one gives out. Now if you want to rebuild and beef up the motor and have it balanced then you can turn 5500-6000 all day long but not with a stock motor.
-------------
http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
|
Posted By: jbear
Date Posted: January-12-2006 at 9:55am
79; As always, straight to the point. Can't believe that there is alot of replacement motors in Brunei. Where in H is that anyhow?...jbear
-------------
"Loud pipes save lives"
AdamT sez "I'm Canadian and a beaver lover myself"...
|
Posted By: 79nautique
Date Posted: January-12-2006 at 10:16am
middle east???
-------------
http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
|
Posted By: JoeinNY
Date Posted: January-12-2006 at 12:28pm
These guys are on the right track on all accounts. Even if the motor could turn 5500 occasionally without issues it is not going to be making its max HP at that point unless you change the cam, heads, ignition, etc. Therefore you would be cutting down your top speed, and decreasing your fuel economy (yeah I know the stuff comes right out of the ground over there but it still has some value). The only reason I would see for doing this with a stock engine ever was if you were pulling massive loads and wanted to develop your max power at a lower boat speed, but I seriously doubt your loads are that extreme and would justify the high end performance loss. If extreme towing loads were the issue you would not want to be cutting down your propeller as you would need all the face area you could get, the proper way would be to lower the pitch.
If I was going to do that I would oversize the pump pulley to slow down the rpm in the pump, not speed it up as your "marine engineer" is suggesting. Speeding it up too fast will result in cavitation and decreased water flow (especially if you do not increase the size of the hoses and pickup), and more importantly decrease pressure at the circulation pump, meaning the cooling water will take more time to enter the system when the thermostat opens. This combined with the extra heat generated by turning 5500 rpm (engine and transmission heat both being dissipated by the same flow of water) is most definitely going to leave you looking for a new engine long before you should be.
My main reason for kicking in my two cents to the above correct replies is that I have worked with Volvo Application Engineers designing while boats for a couple of manufacturers during my contract engineering stage, and not only are your source’s comments wrong but they aren't even logical. Volvo does not choose the outdrive/prop combination for a boat they just approve it. If a boat is over propped it is because the boat maker wanted to be able to publish better fuel economy or top speed numbers, and is willing to sacrifice low end power. Depending on the engine Volvo will typically approve a lightly loaded top speed RPM within a 1000 RPM range leaving the boat builder with a lot of leeway to pick how they want the boat to perform. If Volvo were trying to limit RPM to keep down warranty issues and limiting RPM causes inadequate cooling, then Volvo is not too bright. They aren't my favorite marine engine manufacturer but they haven't gotten their market share by accident.
If you were really just worried about the extra heat and longevity, run one of the better full synthetics in the engine and the tranny. If it helped you sleep at night you could buy a higher rated pump and run it with a pulley that matches its highest rated flow at your top rpm, increase the line size, add a second pickup, and make sure you have a good high flow thermostat. Get your money back from that “marine engineer” and use it for gas, beer, and water toys.
-Joe.
-------------
http://www.correctcraftfan.com/diaries/details.asp?ID=1477 - 1983 Ski Nautique 2001
1967 Mustang 302 "Decoy"
http://www.youtube.com/watch?v=cO5MkcBXBBs - Holeshot Video
|
Posted By: 79nautique
Date Posted: January-12-2006 at 1:29pm
doesn't 18C equal 50F??? freezing is 0 c & 32 F??? seems like the water isn't realy that warm at all. And it sound like a marine engineer is drumming up extra charges that don't need to be made.
I have an engine that can and does turn 5000-5500 rpm's on a regular bases and for pretty long runs at times (Gota love smooth water)and all I ever see is an extra 5-10 degrees on the gauge using a stock RWP. Now keep in mind my engine is far from stock ans is balanced as well.
I really don't see a need for you to change RWP or try and getting 5500 rpm out of your stock motor, if you do re-pitch the prop to get the RPM you want, you are going to lose top end speed, fuel milage (granted you are paying pennys for it) and the stress on the motor is going to be way beyond the design limits.
-------------
http://www.correctcraftfan.com/diaries/details.asp?ID=756&sort=&pagenum=1&yrstart=1979&yrend=1979 - 79 nautique
|
Posted By: AWhite70
Date Posted: January-12-2006 at 2:37pm
Just to clarify 18C is 64.4F. 1 deg C is equal to 1.8 deg F. The conversion formula is: F=C*(9/5)+32.
The engine in my '79 is far from stock and I have never had a cooling problem with the stock raw water pump.
Additionally with the design of SN hulls you're not going to get really fast top speeds no matter how high your rpms or hp are. 50mph is about it. There is a lot of the hull in the water and drag increases with speed exponentially, along with the power needed to overcome that drag.
-------------
AWhite70
http://www.correctcraftfan.com/diaries/details.asp?ID=562&sort=revyear&pagenum=5&yrstart=1976&yrend=1980" rel="nofollow - '79 Ski Nautique
|
Posted By: 91nautique
Date Posted: January-12-2006 at 3:34pm
Morto
Make sure you keep all the old bits off your motor cause when it self distructs (and it will) and you have to rebuild/replace it you will hopefully see that this so called marine engineer hasnt got a clue as to what he is talking about.
Why would you want to modify a well desighned and proven setup?
Why would you want to make an engine run colder? So it can wear out faster?
Johnson/Jabsco/Sherwood impellor pumps are not rated to turn over any faster than 1800-2000 revs,they will not flow any extra water -the impellor blades just fold over, and the pump is likely to self destruct and put an window in your engine cover!You could turn the pump at 10000 revs and it wouldnt pump any extra water.
Basically his setup is going to flow bucket loads of water at idle,then nothing at high speed and if the fan belt manages to stay on the pump it might slow down the pulleys transit through your engine cover when the pump detonates.
Is he planning on upgrading all the fittings and hoses to feed this pump?
How is he planning on getting the water out? You going to upgrade the trans cooler/manifolds and risers to releive this pump of the pressure it creats or just blow the seal out of the pump?
The only cutting I would do is cutting a path down to this fools workshop(if he has one) and rescueing your boat before it is trash.
What is he planning to trim the prop with a nine inch grinder?
|
Posted By: Morto
Date Posted: January-13-2006 at 12:24am
Thanks for your replies everyone. That's why I ask before I commit. Correction for you:
Brunei is on the island of Borneo in south-east asia, not Dubai which is where I'm going as soon as I finish typing this! (don't feel bad - half my family still thinks I live in the Middle East.) Fuel happens to be a give away here too, as it's all just off the beach.
What rpm's should the Mercruiser pump turn then? And the boat is at my place!! The guy is an exceptional engineer, no question, but I guess this wasn't one of his finer ideas!
Cheers everyone.
|
Posted By: 91nautique
Date Posted: January-13-2006 at 5:15am
Shaughn
Sorry if my post was a little harsh,but recon the sherwood pump and check out your risers -they may be blocked,thermostat ect
Its a small world,when in Dubai, check out
http://www.dwsa.net/facilities.shtml
I spent 2 months there, sweet water ,they will hook you up,they run nauts 7 days a week and have a couple that have run over 5000 hrs and the water temp is HOT 24/7
|
Posted By: 81nautique
Date Posted: January-13-2006 at 6:47am
Hey Guys, that DWSA joint looks pretty cool. I had no idea there were those kind of facilities in the middle east. Guess I have to get out of the midwest a little more often. FYI,By my calculations gas is $1.36 per gallon in Dubai.
-------------
You can’t change the wind but you can adjust your sails
|
Posted By: Morto
Date Posted: January-15-2006 at 2:43am
|
Yeah, and it's $0.53/litre (that's Brunei/Singapore dollars.....diesel's $0.31!) here in Brunei. Hey, no problem 91Nautique, like I said - that's why I ask. To be honest, there's no reason to suspect any problems as we took on the boat part way through a major restoration, according to everyone who knows the boat she was just a little tired. We are simply trying to make sure we do the best by her while we've got the chance. The old pump, by the way, basically fell off when disassembled, it is way beyond repair. The mercruiser was here ready to go. It's taken us so flamin' long we could of had a new one here but what the hell.
|
Posted By: jbear
Date Posted: January-15-2006 at 10:42am
Man I love this place. A couple weeks ago we were talkin' to a guy in Isreal about his platform and now we got Brunei on the line. WOW. And I used to think it was cool to talk to BKH in CA....john
ps to BKH; it's still cool to talk to CA also.
-------------
"Loud pipes save lives"
AdamT sez "I'm Canadian and a beaver lover myself"...
|
|