408ci LS Build for Marine

[Art_H]

Looking to power a 24ft performance hull with a single engine.

475-500hp at flywheel is the target.
Peak HP at 5500 to 5800.
Cruise RPM 3000-3200.

LQ4 would be the likely donor.

4.00" crank x 4.030" bore.
10.25:1 SCR area. Maybe push it to 10.75. Depends on cam.
317 heads ported (larger chamber keeps SCR in check)
Aftermarket 6.125" rods.
Pistons Forged D dish style.

Essentially, want a reliable and strong bottom end.

Keep FI system. Maybe go Megasquirt 3.

The hyd roller cam is now the big question. Enough lift to be effective but maintain reliability, and enough duration to put peak HP around the Max RPM of 5800, or a little before.

I have a TRS drive with a MercTrans II. They are sensitive to lugging, so running full power at a higher RPM will be easier on it, and less prop loading at lower RPM.

Most LS builds are focused on peak HP in the high 6000 rpm area.

As for exhaust, I'll fab up some tubular jacketed headers, and a chambered muffler.

Are coatings on the chamber worthwhile? Or will they just get burned off? Seems like coating an alum head chamber gives the best of both worlds. Efficiency and thermal stability.

Intake likely the stock truck. Want to maximize the 3200 RPM efficiency.

Thoughts?

[CGT]

I think if a their was dish readily available large enough, I would consider a 706/862 head....I think the chamber is a bit better in my opinion, and the port being slightly smaller may help you bring things down into that rpm range some...just a thought...

A PSI or PAC beehive running Comp Marine or HUC lobes .550 -.600 lift..would be good....small diameter duals if you have single spring phobia. I have no marine experience on all the various drive system idiosyncrasies but based on your info, on land I would probably be in the 220's@.050. Truck intake is a good idea.

I would skip the coatings, that kind of thing could cause more trouble than it could possibly be worth(if their was even any worth) when it goes south.

[Art_H]

CGT,

I'm a big believer in good chambers. I also like as much quench area as possible, and quick burn chamber characteristics.

I would like to keep costs down and not go to an aftermarket head solution.

3000-3200 cruise RPM does not have to be the 100% focus, but I would rather bias the efficiency in the Tq end of the equation vs going for Max HP at peak RPM. It may turn out that I get better MPG with the boat travelling a little faster as well, so maybe a 3500 RPM cruise. I'm putting together a 383 from some spare SBC parts I have around, as a test mule to get good hull data.

I'd like to get a healthy cruise with a below 0.400 BSFC if I could.

Most dyno data is based on WOT conditions, with a climbing RPM.

Boat application at a steady RPM does have advantages of tune-ability with rpm change. having an active O2 measurement will help, but reversion of wet exhaust can be an issue.

My experience with the LS is limited, but 220@ 0.050" is likely a good place to be to start.

With a Hyd Roller cam, what is the risk factor for a 0.550-0.600" lift for sustained WOT @ 5500-5800 RPM? Stainless quality valves.

Single Spring Phobia...lol. I'm not super concerned, but reliability is a large factor, I intend to cruise from the south west BC up to the Alaska'n coast. Single engine and open seas... Relatively low RPM operation should preclude a lot of valve issues, and not require really aggressive spring rates.

I recently saw Holley's fabricated LS intakes with the twin TB. Reminds me of the Ferrari intake system with the adjustable plenum crossover. Seems the Holley duel Plenum Sniper really picks up the TQ at all RPM.

[CGT]

I'm a big believer in good chambers. I also like as much quench area as possible, and quick burn chamber characteristics.

I would like to keep costs down and not go to an aftermarket head solution.

The 706/862 casting is the earlier 5.3/4.8 head. They have a much nicer looking chamber. I've always thought the 317 casting chamber was crappy and odd. But who knows, may not make much difference...they seem to run ok.

It was designed for a smaller bore so the chamber is small, definitely has more quench area, and gives you some flexibility to make some changes to it for your larger bore. They are in the 60ccish range to start, where your 317 is in the 70ish range.
The intake port is also roughly 15cc's smaller as well. It may be worth looking at a set to see what you think.

With a Hyd Roller cam, what is the risk factor for a 0.550-0.600" lift for sustained WOT @ 5500-5800 RPM? Stainless quality valves.

I'd say low risk factor if done correctly. If it was mine personally I would wanna use the lightest valves possible with high quality beehives and slow lobes. GM part number 12569427 hollow stem intake is very light and high quality,, and part number 12605525 exhaust also very nice and light. Both would need to be turned down to your desired diameter....a pretty common thing done with those valves in the LS world.

I'd like to get a healthy cruise with a below 0.400 BSFC if I could.

That seems doable

I recently saw Holley's fabricated LS intakes with the twin TB. Reminds me of the Ferrari intake system with the adjustable plenum crossover. Seems the Holley duel Plenum Sniper really picks up the TQ at all RPM.

I think the truck intake will be pretty hard to beat in that rpm and cruise range, the only problem with it is that its fugly as hell.