I found my Bravo drive was making an unhealthy sound when running the motor on the hose:

That’s not right. Removing the top cap and having a look at the bearings, it all becomes clear:

Infortunately, the water damage also included the bearings in the gears:

So there was nothing for it other than a full strip down and rebuild. This is the pinion gear – every bearing showed some damage, and the big bearings are only available as part of a full kit of gears:

A fairly expensive round of ordering and the rebuild could start:

Carefully put it all back together using all the necessary special tools:

Major bits back in:

Checking pinion gear backlash:

And finally pressure testing it all:

Background

The engine started life as a Mercruiser Bulldog 540, originally part of a triple engine setup on a sunseeker.

Apparently the sunseeker ended up sinking when the engines were virtually new and they were removed and written off by the insurance. The engine then sat in my friend, Tony Davis’ garage for a few years.

When I got the engine, I knew I wanted to improve it a bit to take full advantage of the bulldog “tall deck” block and increased capacity, but without going supercharged or turbo. I always loved the “stack” intake manifolds you see on retro & vintage racing cars and I liked what kinsler had done on some of the mercruiser engines, so I thought this might be an interesting path. I found a small bore hillbore manifold that looked like I could convert it to EFI, along with a set of Eickert exhaust manifolds and mocked it all up:

This looked like it would all fit together, so I worked with Bob Madara (rmbuilder on OSO) to put a spec together. This is the specification we ended up with. It meant I would need to run exhaust dry to the tips, but should also give a nice idle, strong torque all the way up, with peak horsepower at about 5500rpm.

Specifications

Bore	4.5″
Stroke	4.375″
Capacity	556.7 cu (9.1 litres)
Compression Ratio	9.56
Exhausts	Keith Eickert Cast Headers
Cyinder Heads	Airflow Research 315cc with Inconel Valves & Anodising
Intake Manifold	Hilborn 2 7/16 Mechanical Injection
Fuel & Ignition	Megasquirt 2 V3 running Bank Fired injectors and Wasted Spark ignition
Camshaft	Custom Marine Kinetics Grind (see below)
Crankshaft	Lunati Signature Series

The camshaft was designed by Bob Madara with the following specs:

Modelling this all in desktop dyno, it looks good. I know the numbers aren’t doing to be “real world” accurate, DD is inevitably a bit enthusiatic on the numbers and is crank, not propshaft, hp. But anyway, this should produce an engine with similar power to the mercruiser 600 supercharged motors and should “just” be OK on a Bravo XR drive. The engine will never be dynoed to verify, but it’s nevertheless good to see some nice high volumetric efficiency numbers (we have all the head flow numbers entered correctly) on a big block chevrolet motor.

This set the recipe, so a few wise purchases later, along with having the block bored out slightly and a crank journal machined, I was ready to start assembling it all.

The Build

The block was bored from 4.44 to 4.5″ bore, leaving enough meat still in it for a rebore in future if needed (that would then make it a 572 as well). After cleaning all the block very carefully, new cam shells were pressed in to the block:

And the bottom end assembled:

so far so good

Next the cam was installed and timed:

And the heads, along with the scorpion marine duty roller rockers:

Including using test springs to work out that stock length pushrods were OK:

And the intake manifold:

It started to look decent:

And eventually it all came together pretty well:

Lessons

I learnt quite a few things duiring the build, here’s 3:

Avoid the single piece steel core sump gaskets – of it you do use them, use them carefully. I had a leak where one was a bit too thick and didn’t compress down properly – which meant engine out of the boat again tp put a traditional 4-piece sump gasket in.

The 140 amp single V-pulley alternators from DB Electrical are excellent value for money

Always use the bypass hose on the circulating coolant pump. I originally rigged my engine without this, making the block “full flow” – which also works for the full flow heat exchanger I have. Bad move. Without the little bypass hose on the reciculating pump, it generates enough back pressure to blow the core plugs of the engine. I wouldn’t have believed it if I hadn’t seen it. One popped out at fast idle on the hose – I assumed I had not put it in properly. One then popped out when I was out on the water – so I pulled the engine out to investigate properly and found that a 3rd would have come out, except it had wedged itself against the engine mount. Since re-installing the bypass hose, all is fine again.

Exhaust Upgrades

After the last run, it was clear I need to put some effort to quietening things down a bit around marinas and so forth. My initial plan was to just downrate the camshaft slightly – sacrifice a few mph top end and be able to run a wet exhaust – however rmbuilder was reluctant to spec me an alternative camshaft and instead recommended mufflers. So a fair bit of hunting round, eventually I found that Gil mufflers can run dry, so I set about making some new tailpipes that would fit the gil mufflers, plus also let me adjust how much water goes in to them.

At the same time, also wanted to reduce reversion. Original plan was to have a large crossover pipe between the banks – since the major point of reversion is due to the uneven firing cycle between banks. Originally planned to have a full 3.5″ inner bore pipe mounted immediately after the collectors, but eventually I realised this was impractical.

Also wanted to have a flange between the collector/manifold and the tailpipes – to give me a better mount point for the O2 sensors and also to have a EGR check valve in – this is another idea, to have an EGR type check valve in so that at idle, rather than suck exhaust (water) back in, fresh air is pulled in via the check valve. Don’t need a huge amount of flow here, just enough so that at idle there’s a little less reversion.

The flange idea also means that as I try different packaging/ideas out – I just need to make a new flange, not a whole new tailpipe each time!

Exhaust Flange

Part 1 was to make the flanges and see if this plan was viable. I needed an aluminium flange, 50mm thick,

Unfortunately, nobody wanted to make me one of these, so had to make it myself.

Started off with a couple of bits of 50mm thick aluminium:

And then slowly using the biggest holesaw I could get to work, cut a hole in the middle:

That was enough to then get it into the lathe with a boring bar:

I think they came out OK:

Tailpipes

Next it was time to build new tailpipes to fit both the flanges and the mufflers. I had new stainless 316 flanges laser cut from fractory, so the first steps were easy to mock up the inner tubes:

However, if you’eve ever looked at a dry tail pipe boat, often the exhausts look asymettrical coming out the back of the boat through the transom – because the exhaust manifolds are offset slight (one bank of cylinders is about 3/4″ forwards than the other, which messes up the angles). This meant that while I could have one bank with a straight tailpipe, the other bank would need to be adjusted to make sure the exhausts are symmetrical on the transom). This took a lot of backwards and forwards for what appeared like a small cut. in the pipe You can see in the below pic that I’ve already welded the inner & outer to the flange, along with the water dump pipes fabricated and welded on, along with half the outer.

It was also necessary to have the flanges bolted to something pretty sturdy to stop them distorting when they were being welded:

And I’m still a beginner at tig welding, but I think it came out OK:

Next it was time to weld on the last bit of the outers (which then means I also need to remove the manifolds to install & remove the tailpipes – aggro!)

Then needed to make some clamp water dumps for out the back. The “idea” is that the mufflers get cooled externally from the outside to avoid burning them up too much – pics below will show:

Final install. I think they came out OK. Hope it works! this pic is with the flaps closed.

Oil Leak

Also noticed a small oil leak from the sump last time I ran it. A bit of googling tells me that the “modern” single piece oil pan gaskets are notorious for leaking as they’ve a bit thick & rigid, so yet again, engine out:

And then when I drained the oil, I found a couple of drops (literally) in the oil. That’s not right! So I pulled the intake manifold off as the most likely culprit. I hope I found it:

So a bunch more work redoing that:

29th July Update

Finally got a chance to test all the work out. Everything seemed to work well, oil leak is fixed and with the mufflers closed, it’s quite a lot quieter. No high-speed footage unfortunately, but you can hear the idle is now tolerable, similar to a wet silenced engine. Not quiet, but acceptable. It currently has a 3mm water hole into the mufflers, primarly to let the tips drain down, but it also helps muffling. Once everything settles down and all the efi map is tweaked, I’ll add another one that’ll help silencing (water is the best silencer) and then review it from there.