Re: 383 dyno success!
No idea about lube differences. I would hope today's lubricants would be head and shoulders above the old stuff. For the price I paid for the Quicksilver synthetic, it ought to cure a couple diseases while it lubricates my drive.
The heat thing is a good point. This is where I'm glad I can choose where my exhaust exits. I know thru hull performance gains (or lack thereof) can be a hot topic, but I do think that keeping the extra heat out of the leg at WOT might help the lube do its job.
I think Tim postulated this on the thru hull discussion a few months ago.
Regarding props and gears, for now I am leaning toward just buying a 24 or 25 Mirage Plus. I have read many good things about these props. Any rough ideas on what speed I should shoot for? It ran 60 mph with the 300hp Mag (330 at the crank) so should I assume 430hp at the crank would get me 10 more mph.
Lot of good info in the posts above. will be fun to hear how it goes once you get out and about in it.
The lubricants of today are dramatically better then 20 years ago and yes, I run high quality synthetic lube in my drives. I also keep a spare alpha drive (I think I have enough good uppers and lowers to put three together rght now).
I agree with the point above that the breeakdown in lube from any source is a huge source of failure issues. Another with alpha drives with respect to bottom end failures from what I understand is chipped teeth from shifting at too high an rpm. becomes a source of weakness and gernades under high load / high rpm conditions.
I did have a temperature probe (made a fitting and installed a type T thermocouple in the upper drive through the upper plug, bent down so was in the fluid). Was mainly interested in upper temps assuming that with lower drive submerged cooling's not an issue there, and because the higher gear ratio transfers much of the torque load to the upper gears. Only ran it a few times and temps were well within acceptiable levels, I would have ot find my notes to remember exactly. Got over 200, but not up to decomp temps for synthetic oils. I've never found burnt looking lube and I change it often. Will know more for sure once I get back on the water this year. I've never seen the chalky deposits on mine that I hear people talk about. My exhaust exits above the rub rail a couple feet outboard of the exhaust manifolds and nothing goes through the drive. I'm considering adding a cross pipe to reduce backpressure and a smaller tube from the cross pipe to the exit in the drive (wher ethe y-pipe used to be) mainly to let water run out the bottom rather than acumulate in front of the mufflers. Just am too tight to pay to have the stainless pipes custum fabricated right now.
Heat from exhaust being an issue came from the guy who built my current boat engine. He said it was a very large and often neglected aspect of keeping an outdrivedrive running in the right temp range behind a little more power. He also said they nearly always take drive showers off of boats when they redo them for customers and never saw much benefit from using them. Said an alpha drive ran great hot so the oil could circulate well, but needed to be set up right and not neglected. (note that circulating lube is what carries the heat away from the gear surfaces and internal bearings - get lube viscosity too high and you actually decrease cooling even if temps are lower). They build a lot of high end marine engines for offshore applications and circle track engines, and also always seem to have complete (hot rod) boat rebuilds just repowers for customers that don't have to ask what the bill is going to be. I've dealt with a lot of machine shops and they do some of the best work I've ever seen.
In my opinion, through hull exhaust gains are usually not seen by most people because once you depart much from stock engine builds, the exhaust manifolds are the limiting factor. WIthout a significant upgrade in manifolds / headers, what happens after that doesn't make a big impact. Note that this problem gets worse as you increase rpms... so if I'm spinning my 383(5) to 5500 or more instead of the 4800 for a 350, have compression of about 10:1 (with cast heads), i'm putting a lot more exhaust gas out at wot than a stock 350. So the exhaust is a huge piece of the puzzle overall. Good manifolds are worth the money for a number of reasons.
Mirage is a decent prop and there are a lot of them around in those pitch ranges for good prices - that's why I run one for daily use. A hydromotive quad 4 or a bravo (especially the merc labbed bravos from what I've been told) are probably both a good bit faster, but pricey. I've got a 27" 4 blade spinelli on mine right now to try once I get it back together.
I would assume that 8-10 mph increase is a good start maybe even a little less (note that is with the same rpms... if you do that you're actually accounting for more like 12 or 14 mph once you account for the increase in max rpms!) and then see what you get and go from there. Have you got any shops in the area that keep a large supply of used props? I borrowed a half a dozen or so (with a deposit or them knowing I have and would buy one or more of them, etc..).
I like wot rpms set where I would push for max speed - more like 5500 rpms, but then daily playing I dson't spin the engine over 5000. I don't accidentally go the last 500 or so rpms... in my boat there is a significant difference in pucker factor once you're going over 60 or 65... you've got plenty of midrange power to pulll whatever you need in the middle if you're propped for max speed at wot - especially with the higher gear... and will get better cruising speed fuel economy with the higher pitch range props, so I bet you'll wind up in the 25 or 27" range in the end, but you'll have to play and see what works best.
also, remember that "accidentally" spinning a carefuly balanced and blueprinted engine with a high quality, forged crank, H-beam rods, forged srp pistons, All ARP bolts, solid valve train, etc... to 6000 rpms is a whole different thing that pushing a stock motor much past redline.
As mentioned above your best top end will be propped with wot right at max horsepower. note that your dyno numbers look great, but you'll give up a good bit especially on the upper end to the marine exhaust (I'm assuming that was with dyno headers and not your exhaust).
Not a professional - just a backyard hack procrastonating about going out to grind and do glass work this morning...
