06-09-2007, 06:35 AM #1
- Join Date
- Apr 2007
What do you think a safe rpm range is for the tripple crank& rods?
I am just curious, I wonder what a safe spin rpm level would be for the tripple cranks. I know mine turns 6250, I have heard others saying theirs turn 7000. But what is a "real" Safe turn rpm? I happen to have a spare engine on the shelf with two cranks, I am seriously thinking of building a modified engine in my spare time (Not that I have much with five kids)
But Am going to port it, Possibly different rods, etc.. Any idea anyone?
And yes, I will post pictures every step of the way Port job,flow and all ..
06-09-2007, 06:50 AM #2
06-09-2007, 07:48 AM #3
Calculate this in terms of "piston speed" in feet/minute. (fpm)
Race engines that are gone through (inspected) in terms of minutes, not hours, will run somewhere near 4,000fpm.
As an example...
Stock Pro 785 @ 7,800rpm = 3,480 fpm
Pro 785 @ 8,000rpm = 3,569 fpm
Pro 785 @ 8,200rpm = 3,658 fpm
So as you can see, even though they really turn up, their piston speed isn't crazy high.
Rec riders and builders in the watercraft game will usually like to keep it somewhere around 3,500fpm for reliability.
Exceeding the safe limit can result in failures like broken crankshafts or connecting rods, separating pistons, breaking rings, disintegrating lower end bearings etc.
06-09-2007, 07:54 AM #4
Limits are for Girls!!!! It's all about the smile on you face!!!!
06-09-2007, 07:58 AM #5
06-09-2007, 08:12 AM #6
60 feet per second. Thats just rippin!!!!
06-09-2007, 08:30 AM #7
- Join Date
- Oct 2006
- Cleveland OH
1350 FPS. That does damage!!!!
06-09-2007, 08:51 AM #8
I wouldn't want to turn more than 73-7400, without having some sort of better crank in my engine. I know a racer whose wife went through FIVE $1500 cranks in one racing season racing a Pro785...the same skiis that run 8000rpm. You're ok no matter what you probably do to your engine, as long as you don't have too high compression and need higher octane, and you're using your stock pipe. If you do it on a pro785, or run race team pipes on a 1200 motor(they'll spin that high). The real high rpm's(7400+) scare me when it comes to cranks. Cuz if a crank goes, a whole lot can go too....I like your question. But that's my opinion, others that have the $$ and the knowledge to rebuild don't care, because a couple phone calls, a couple grand is no biggie and they know how to work on them and are back in the water come the next weekend. Not me.
06-09-2007, 10:58 AM #9
- Join Date
- Apr 2007
Well, I guess I should have been more clear. As usual
I want the engine to last. I just know I spin 6250, I would like about 7200, I understand you can do this with a 785 cdi, but dont you have to have something else? Or, is a cdi all you need?
06-09-2007, 01:02 PM #10
What you really want is to spin the engine where it's producing it's max. horsepower. Put on the steppest impeller possible to keep you at this rpm and get the most speed. Higher rpms doesn't neccessarily make it go the fastest...hint hint people. I think I've tought a few individuals here about this issue...I know sirhc can give ya some personal experience about it.
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