Twin turbos on a Mercruiser 330 hp BBC 454 in a boat. Which turbos
 

Here is my current project. I have a pair of 454 BBC in a 30' offshore boat. They are 330 hp engines with the larger oval port heads, Q-jet carb with thru hull exhaust, basically stock engines. I want to put twin turbos on the engines. Before everyone starts saying "just do a single" remember this is a boat and all the exhaust is water cooled. I have a pair of Gil center riser manifolds I am going to bolt the turbos too. This is going to be a low boost blow thru system. I want it to make 5 lbs because I don't want to have to plumb in a intercooler. max RPM will be 5000. Im shooting for about 500 hp. Yes I know heads and cam can get me there, but it doesnt have the same effect at the dock as twin snails when the hatches are open. I have looked at HX35 because I want an internal wastegate but not sure if they are small enough. I want to use ebay turbos too since I need to buy 4 of them and make water cooled heat shields, so no S400's for me, 4 of them is not in the budget. does anyone have a better solution than the HX35's?

HX35s are probably small enough - I'd almost worry about them being too small, but they'll probably be OK for an engine in such a mild state of tune. With this being a boat, why not add an air to water intercooler?

What about using HX40's Which blade count would be better for the HX40's?
I though about an intercooler but that may be for the future. To intercool it I would be looking at $200 for an intercooler, $100 for piping, $100 brackets and misc, $100 for the water pickup, $100 for a water strainer, $50 hoses. total $650 per engine.

A worthy project indeed.

I am along the same lines but with only a 21' boat and single 8.1L mercruiser (non-HO) with jet. I picked up an old school Banks TT 454 draw thru setup that came with a boost referenced carb (modified 850DP I think) and water cooled I/C, and watercooled logs n elbows etc. As my motor is MPFI I am going to reverse the intake manifold and feed the TB from the I/C (behind the engine). It is the only way to fit it down inside the doghouse. I have a holley hp ecu to allow for mis-tuning.

Not to be your mother, but how do you alter the carbs to accept blow through and keep them CG compliant? If you use a pressurized carb box - be sure your floats are not hollow, as they might collapse.

https://cimg9.ibsrv.net/gimg/www.hom...6c9a73c300.jpg
not my actual boat, this example has a Gen iv 454 but you get the idea. The Mercruiser has a big HE across the front.across the front forcing the TB up and top wont close.

It uses old Rajay 301E10 turbos, old but tough and have carbon seals to allow air/fuel to be drawn thru. They say you gain 12% air flow by putting taking the fuel out of the intercooler. Good luck with your tricks.

Hey I am very interested in your project as I have wanted to do this for a long time. Where are you in the build process?

Installing a NA small block so the boat pulls the child on a tube this summer! - I need to drive the boat at least.

BBC - collecting parts. working with the parts I have. Im in Vancouver - Canada and my new parts are held up in a receiving warehouse just across the "closed" border.

I sold my Mercruiser long block and Ecu for better money and bought a PSI 8.8l aftermarket Gen7 block (has piston coolers\squirters), forged stroker crank, 6.7 H rods, Dart iron heads and a Holley EFI to run it. (should have got a dominator to allow 2ea O2 sensors) I have Inconel valves, decent beehive springs, and necessary hardware to build the heads, not sure if I will touch the new CI guides with bronze right off the hop or not. I have everything but time to build the heads but my overseas trip is cancelled so I will have some soon.

I have no pistons yet, the block is in a busy machine shop to be sonic tested to determine how big I can bore it and leave reasonable walls with a rebuild or 2 left, without hardblock. New block bore is 4.350, I want un-shrouded valves and round bore head gaskets so 4.4-4.45 to start with. I am fabricating a simple fake aluminum piston to install and evaluate deck height/warp, stroke clearance etc. before I spec and order custom 20-30cc forged d-dished pistons, I need to finish heads so I have a real CC # to calculate with, wanting 8-8.5 cr. I have the oem intake which sucks, I was going to try and cut/weld out the known internal restriction and port it and re-orient it to allow the TB to be on the back BUT...I recently bought a Dart dual plane manifold. This allows the Banks Turbo inter-cooler hat to just bolt-on as designed and allow the TB to go where Banks had the single 4bbl, across the rear of the engine between the turbos. So much more of a bolt on. The intake has pads to drill out for injectors, and will use standard billet rails etc. I don't think the oem GM FI spider (5/16" inlet) is big enough when you go well past oem power levels. The intake system will now be dry unlike the OEM Banks so a 10-12% flow increase should result and the carbon sealed turbos could be upgraded, but I dont plan to. Did I mention my intake is stuck in the US, grrrr.

The little OE Banks turbos are not supported but are good, so I will run them at this time, Im not looking for more than 20psi, ever. I can still get rebuild parts (quality ??) but I have gone through them and will just change the gaskets $20ea. I have closed up the OE bypass holes in the turbine housings and have added 44mm external WG for each. I am re-machining the turbine exits to allow WG output to re-enter the water cooled exhaust stream, and add O2 sensors on the outlets before cooling water is re-introduced. I am not sure how long the O2 sensors will last near water (6" away) but I have little choice in an enclosed engine setup, it always sucks when the boat burns to the waterline.

I don't know what else
Hull reinforcement - 4/6 point engine/pump mounting perhaps when power level is up?

I have 2 big block 502’s that were built for superchargers. However I want to go with turbos. I eventually want to run efi but want to start with carbureted. I have o2 sensors in the water cooled dry exhaust To the tips and then water dumps under side of exhaust at the transom. So far no problems.
I run in Saltish water not the ocean but Chesapeake bay. So I am trying to figure out what turbo would be good to use Being fresh water cooled.

Hullofjustis:
Sorry to say, my project is hung-up with Covid restrictions...I can't run across the border and pickup more reasonable parts in the USA.
I am not sure how I would feel about running saltish water thru the Alum. water jackets, no more than I have to. I have a brackish river that I would be in (similar to you) and I would probably be rinsing the raw water-circuits when I trailer the boat home. If I upgrade to 4" HE from 3" I would probably run engine coolant to the exhaust logs and Turbo shields instead. But being aluminium the cathodic anti-corrosion system would likely deal with it already.

For others, I was wondering about what to do about the heat loss in the Banks aluminum exhaust logs. As a boat with a closed engine compartment I have no choice but to run water cooled exhaust manifolds and turbo shields. The Turbos themselves are ceramic coated and wrap around water cooled shield to keep things from burning to the water line. The turbines will still turn red under use, The logs will run warm (I can throttle the water so they run warmer but a screw-up here and they will meltdown I think, adjustment at full throttle would be key. I am considering glass beading the exhaust passages and getting them ceramic coated internally to retain as much heat as I can and thus prevent it from even reaching the cooling system. I have some aftermarket risers (Dana Marine - better than logs) but that will re-orient the physical layout of the turbo system and it might no longer fit in the doghouse, we will see)

My junk has the original little Rajays, they are in good shape so I will run them unless it ends up a slow turd.