cheech

The OP is long gone so this thread is sort of irrelevant as far as he's concerned and your case with the surface drive and your boat is obviously a one off situation more or less.

BUT to say in general an NA LS lacks enough torque, or doesn't work with "Z drives", is inaccurate info that'll be spread by internet BS.
It's case dependent.
Podunk is proof.
Do you realize every anemic SB 350 CI 260HP with an Alpha is in front of a 1.47/1.50:1 drive.
And every 300 HP 350MPI in front of 1.65:1 or 1.50:1 Bravos.
Even your LS376/480 produces 400 lb-ft at 2500 RPM according to the CPP catalog.

Are you saying a Big Block Chevy is big? Or archaic with the Mastodon statement?
If archaic you mean cam in block, pushrod, 2 valve per cylinder just like an LS?
(The LS with better OEM ports perhaps, but the aftermarket has the BBC taken care of.)
If so and also, you're arguing for torque but at the same time disparaging the king of torque. AKA, the big block Chevy.


So you slowed your prop speed down with the new trans then stepped up in pitch to recuperate and you gained 3 MPH?

plavutka

1.) I reduced the propeller shaft rpm from 4,000 to 3,100 and gained much over 3 mph. I also explained why. Again briefly:

The propeller takes power approximately with:
- diameter to the fourth potency.
- rotations to the third potency.
- pitch to the second potency
- linear with the difference of the front surface.
This applies to all propellers that rotate in liquids, including e.g. for airlines. I work professionally with bigg propellers for air flows and I know what I'm talking about, although the language barrier and simplification may make it look different.

Since there is one whole potency at revolutions and steps, it makes sense to lower the propeller revolutions as long as this is technically feasible. For speedboats, the lower limit is probably somewhere between 2,600 and 3,200 rpm, I did not search what is the best and I used 3,100 rpm becouse of lot of good propellers for this rpm and Re numbers still alow relative small diameter of prop.
If you keep gear ratio and change prop pitch you do not make a lot. With to high rpm on prop shaft you always have conditions like if you drive your car in 6 gear on steep slope, even if you change pitch. (Diference on the potency for rpm and pitch.)

If you use an LS or LT with a 1.5 or 1.6 drive, you get at least 4,000 rpm on the propeller, and these are conditions that are suitable for very fast and light catamarans, not at all for a V speedboat. There is to much rpm and to less torque on the prop.
Put a Mercury 400 or 500 outboard on a speedboat over 4,000 lbs and you'll run into the same problem. It will be less succesfuly than 8,2-HO.
But why did Fountaine give 1.6:1 and not 1.5:1?

Volvo Penta gives the LS engines a gear ratio of around 2:1 and realistically achieves better results than the Mercruiser 8.2 HO. If you put a 1.5:1 drive on the same engine, it won't go anywhere because there will be too many rpm and not enough torque. I know, they do not race, but this solution alove LS to push bigg boats succesfuly like BB and this is the point.

2.) Technically speaking, all these engines are very outdated, be it SB, BB, LS, LT. LS and LT still have some advantages over the other two.

Tartilla

LS engines are truly an amazing accomplishment. GM had brass ones to keep pushrod cam engines in their flagship cars and got amazing fuel mileage with performance. Compare that to the BMW V8s that have DOHC and are massive...with unlimited common issues.

SBC/BBC will be the dominant marine engines until there is something better enough...and it's not here yet.

TQ and HP are essentially the same thing...representing the net energy produced by the engine. Peak HP has more energy than peak tq. (Typically) Less TQ is produced at peak HP rpm, but there is more speed...so more energy. Of course TQ is the rotational energy, and HP is the power, and we generally focus on TQ at lower rpm, and HP at the higher rpm.
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plavutka

OK;

All Z drives and propellers are designed for arround 5.000 engine rpm. SBC and BBC are in this range.
If you take such drive for LS or LT engine, you have no chance to use all benefits of this engine. It turn over 6.000 rpm and prop rpm it is to high, that mean prop shaft torque it is to low.
For speeds arround 70 mph it is the best gear nr. round 2:1. In this case you have a lot of propelers with pitch betwen 26 and 34 inch.
For speds over 80 mph should be 1.6:1 ok, but only like twin on light boat. Or single on really light and small boat who go wery fast. Tuff did similar thing with single 450 hp outboard Tuff 28.

I will never say that SB, BB, LS and LT engines are bad, but they have old technology inside. Since there are no more suitable ones, I myself have one LS in a speedboat, but I would never install a BB!
Any explanations why did Mercury go away from SB and BB and start with good, modern outboard engines?

Wally

Plain market demand. The newer generations of boaters (as i see it) didnt have the exposure to the visceral experience of loud boats like we did growing up and having a show like Miami Vice going every Sunday afternoon.....or have neighbors that are gearheads tuning up a loud muscle car. These kids now a days are all about fast and furious and fart-can mufflers and/or saving the earth from burning down!
The guys like me that grew up in those times are getting older now and moving on to easier things. Hanging upside down while the blood is rushing to your head and your eyeballs feel like popping out of your skull while trying to change a starter motor on an inboard engine is just getting old. Dont get me wrong...im still doing it as im only 52 right now but the bones are starting to hurt when i wake up and working on an outboard is looking more and more appealing Wont be the same as any inboard open exhaust engine but that's life sometimes...