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Originally Posted by Rookie
(Post 3211625)
502ss, do not prop to peak torque. You will torque load your engine and wear it out real fast. We tore down a pair of stroker 496's with only 50 hrs on them and they looked like they had 500hrs on them. The rings were worn out, cylinders bad, and the mains looked horrible. The owner threw some 30's on it and ran at 4600RPM, cause he liked the cruise speed at 4600. You want to prop just a little below peak HP.
I am spinning 25 mirages (tons of cupping added) 5800RPM almost 78mph. I ran some 27 4 blade hydro's but they kept blowing the STBD prop out at 4000RPM. I didn't try any speed runs but eventually pulled 5200 with throttle left. The boat ran with a lot of hull in the water. I am now trying to get some 26/28 bravo1's to try. The 3 blades carried the bow much better and a smoother ride, that's why I want to try B1's cause the hydro's lift the stern. You want to prop for peak HP, not peak torque. Peak HP is probably at least 500-1000rpms above peak torque. |
Originally Posted by Griff
(Post 3211720)
X2
You want to prop for peak HP, not peak torque. Peak HP is probably at least 500-1000rpms above peak torque. I would have to disagree with you on this. Excessive RPM, Increased rod angle, Increased piston velocity, etc can all effect wear on an engine. I have never heard of an engine running at peak torque causing premature failure or excessive wear. Rememebr torque is what an engine is capable of producing for power not what it's internal components can handle. Again these internal stresses are based on things like piston velocity, rod angle, valve train capability, etc. A good example is your stroker application where in order to "Stroke" an engine some of the side effects are increased rod angle and piston velocity which have been know to shorten the life of "stroker" engines. I am not saying this was the problem with these strokers but just making a general reference. Marine application has a lot more in common with prop planes then with automobiles. Airplane engines are designed to output maximum torque within the cruising and max RPM range because much like a boat the engine is run at a sustained RPM for long periods of time. One additional benefit of propping a boat for peak torque yet lower RPM is better fuel mileage (not like it matters much) and definetly less wear and tear as sustained running at higher RPM's is known to cause increased wear. I know several professional engine builders and will ask the question but I am pretty sure that running an engine at peak torque will not cause any more wear then an engine running at sustained higher RPM's and in actuality it should cause less. ...Jim |
Propping for peak torque is hard on the outdrives, and engine. Not only is the outdrive gears loaded, but the crankshaft also. Sort of like having a manual 6spd tranny in a vette. Cruising at 60mph and mashing your foot to the floor and holding it in 6th gear without downshifting. The engine is dying to make power, but the resistance of the trans, rear end, and vehicle weight are all tied to that crankshaft like a boat anchor holding it back. Merc always has a recommended WOT range on their engines, and that is never at peak tq rpm. Peak HP is where you should turn your max rpm.
Turning 5500-6000 rpm on a bbc isnt a problem IF your motor was built properly using the correct components. A friend of mine had a 30FT scarab with healthy 540CI naturally aspirated motors. He spun his motors to 6k rpm. He had TRS drives, and never a drive issue with it, and he runs it HARD. He had 26p props running 90+mph with that boat. Sure, he could have put some 32pitch props on and ran it at 5000RPM, but I will bet his drives wouldnt have lasted very long. Overpropping is never a good thing, and is a invitation to detonation. |
Spinning a BBC engine to 5500rpms is nothing with forged cranks and decent build. I did it with a mildly hopped up 454mag with a stock bottom end with 500hrs on it.
I spun my 525SC with upgraded cam and 8# of boost to 5300rpms and it had almost 500hrs on the bottom end. The only way you are going to make the 565hp you claim is to spin the engine at the rpm it made that HP. I doubt it made that at 4800rpms and if it did then its 650hp engine at the correct rpm. Propping an engine to run at peak torque lugs the engine hard and can cause detonation. Just because you are spinning a bigger prop at lower rpm does not mean you get better mileage. You still have to push the sticks just as far forward and are using the same or even more fuel. |
Hard to tell what the props are doing when you have a tranny problem ! I would say the tranny is slipping more with the bigger prop's.
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Originally Posted by Jeff P31
(Post 3214960)
Hard to tell what the props are doing when you have a tranny problem ! I would say the tranny is slipping more with the bigger prop's.
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Originally Posted by 86scarabIII
(Post 3215159)
I completely agree. I'm going to send them to BAM this winter then in the spring I'll really be able to run it with no worries.
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Thanks for the recommendation. I obviously knew BAM and the reputation they have but I would also much rather use local guys when possible.
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I know a great guy in Medina Ohio
www.fantasticfinishmarine.com |
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