Re: I hate to ask, but a prop question...
If your motor is in good shape, you're not going to hurt it by running it at 5800 for a WOT test. You're only going there to see what the max rpm is.<br /><br />The whole point of prop selection is to make sure you're running in the correct band for the motor's power characteristics. The mfr's guidlines are typically very accurate, but in this case there's a baseline of over 30 year's experience in running the Big Inlines and some changes had to be made to make them last. <br /><br />If you 'lug' a motor by having too much pitch on the prop, this shows up even at speeds less than WOT. For any given boat speed, inline 6's like to be running on the faster, not slower side. <br /><br />Lugging a motor increases piston/rod/bearing loads and also increases heat in the combustion chamber. These motors are very sensitive to this because of the bore size, and the fact that Merc ran very thin rings which are easily heat-damaged.<br /><br />Bottom line is, don't lug an inline 6. If you want it to live, prop it for WOT 5600-5800. Doesn't mean you have to run it at that speed all the time, it will mean that the engine will be spinning a bit faster for any given boat speed. The engine loading will be less, and the engine will 'work' easier. This will make it last longer.<br /><br />Back in the mid-80's I built a 1350 for my 16' Sidewinder. It was a screamer on that boat and would run right up to 6000 rpm. That motor found its way onto my next boat, a 17' Seaswirl Spyder. I had to prop down because the boat was heavier.<br /><br />The motor saw many hours of skiing/playing and never missed a beat. I sold it in '97 (to much regret later on) and last year it happened to come up for sale again. The motor had never been touched, ran as good as ever, and the boat sold before I could even look at it!<br /><br />Had an old Merc 1000 on a 14' Fiberform that likely saw 6500 many times, burnt up from a bad impeller and rebuilt, ran for many many years after that without a hitch.<br /><br />The 6's like to run, they're not like the newer Merc 3's and 4's that don't tolerate rev's. <br /><br />Inlines have a small bore and stroke, and you're not even close to dangerous piston speeds at 6000 rpm. The 1500 XS Merc was essentially stock internals and revved much higher than this.<br /><br />At any rate, IMHO, check with the other Inline Mavens on this board and I expect they will confirm what I'm saying.<br /><br />BTW the 90HP six is probably an exception, they have small ports and don't breathe real well at higher speeds. But other wise they have the same internals and are a very durable, low-stressed, and economical motor. They still have a good hole shot, just not killer on the top end.<br /><br />HTH...........ed
<br /><br />Chris....................