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Xcomunic8d 03-06-2021 10:16 PM

LS Swap Seafari 25

I wasnít sure if I should put this under my restore thread but I ultimately decided this way might be easier for others searching. My apologies if this bothers you, dear reader.

Iíd like to thank and acknowledge all of you guys that have helped me LOTS. Strick, Blue Heron, Terry, Don V, and K Moose. Thank you and thanks to all of you that have encouraged and given input. Especially for the CG estimator! You guys are the best almost as good as our boats.

My seafari 25 was converted from twin I/Oís somewhere in its life to an Outboard. The deck space is just amazing. But the Outboard went DOA due to a flywheel. New Outboards are expensive and even heavier. When you do the math you will get better hp and handling and retain a purity that was the original boat.

So you will lose some deck space no 2 ways about it. Iím consoling myself with plans of a teak swim platform. My plans for my boat is serious offshore fishing for ono (wahoo), ahi (yellowfin tuna), and marlin.

Step 1 you need to locate a LS. I have a LMG 5.3 out of a Silverado 1500. Iíve known my friend for years so I know itís history. I also got the engine for free.

After you locate your donor vehicle you will need to plan out your build. I HIGHLY recommend a closed cooling system in salt water environments. Also consider what outdrive you will use. Iíve heard of guys using 400hp on alpha drives. Iím finding awesome deals on Bravos. Thatís a better option, IMO.

Iím looking for max hp on these but I canít find one. In 73 these had up to 500+hp through twin mercruisers based on 351W block and as low as 330hp.

My goal is to build a 5.3 as close to 500 as I can on a reasonable budget. There are plenty of how toís on pulling a Ls, so I wonít be labor that. I do want to suggest pulling the heads and undoing the bell housing bolts that way instead of removing driveshaft and unbolting the tranny. Saved me hours.

Hereís the oil leak I found. Look you will see the o-ring protruding from fill tube.

The engine is ready to pull today.

I will be deleting the VVT, Putting a new camshaft in, and a tuner (Holley terminator x?).

Iíll make a parts list and sources as I go. Wish me luck all.

Xcomunic8d 03-07-2021 04:26 AM

Here’s a beginner list from others. All input appreciated I’m making a plan as I tear down and build the motor. I believe all big hurdles are addressed so far.

I do plan on running direct port injection (injectors) vs a carb setup. This means you will need a map sensor. Many harnesses do not offer a MAP sensor, so be forewarned. I did really consider building a 383 stroker with the mercruiser elbows and risers. I did consider changing out the heads for merc style old 350 heads. The intake will not work and my engine has a TBSS on it and I’m not sure about 350 heads on a LS I know reverse can be done change my mind if you can. I got this engine free and it helps a friend. I believe it will cost less, produce the max power a bravo can handle, and be more reliable over the long run. Ps it has 243 heads on it too. I’d rather have 706’s but 243’s are quite good.

Here is a list of mechanical parts I have compiled so far:

1. Mercruiser Stock bell housing cover works just fine, early Alpha 1 large back opening or cut the hole bigger.

2. Step Dowel Pins may be needed to align the flywheel cover to the engine. Have some step dowels machined 0.62" on one end and 0.704" on the other. About 1" to 1.25" long splitting the length between the two diameters.

3. Considering a Hardin Marine aluminum flywheel.

4. Drive coupler is from a 7.4L 454 Mercruiser with the round mounting plate with six bolt holes in it. I believe this will be better than a 5.7 for hp ratings etc.

5. Engine Mounts ICT billet LLC part number 551628. $$35 eBay and some mercruiser motor mounts to dial in height angle and vibration. eBay $30

6. MARINE engine depot. Has crank mounted water pump. Johnson f6b-9 pump and adapters
Part # 512601- crank balancer adapter $69
Part# 527018- bushing $4
Part #302391- pump support bracket $69
Ordered the pump on Amazon- $135

7. Will reuse the factory serpentine belt setup for water pump, alternator, and power steering.

8 Edit: NOT NEEDED! May need to put a bracket in place of factory a/c compressor. This will allow a mercruiser impeller water pump for the closed cooling system. I’m hearing different things on the bravos having water impellers.

9. Closed cooling system for heat exchanger of antifreeze and secondary one for power steering

10. Hardin marine LS exhaust elbows and riser kit.

Note: this is just about how to marinize a LS. I have an entire other list of things to do to the engine first. We will go through that too as we go. And we will need to cover the outdrive portion too.

FLexpat 03-07-2021 09:46 AM

Bravo is a very good drive and can easily handle 500HP but the real rating is in torque (most folks can't/don't do the conversion). If I recall correctly a Bravo 1 can handle about 550 ft-lbs and a Bravo 1X can handle about 650. Alpha starts falling apart 350-400. Bravo and Alpha use the same hole but the Bravo has 2 additional bolt holes. You will need a water pump with a Bravo since they do not have an internal one. Here is some info on the evolution of the drives:

Make sure you use a marine alternator since the automotive ones are NOT ignition protected - ignition in the bilge= bad day.

Definitely go closed cooling and delete the VVT. Make sure you have a cam that gives low end torque (truck style) so you get better economy.

Like the Chevy small blocks, all the LS motors have the same block externally and just change bore/stroke. I see you have a free 5.3 but think about a 6.0 - maybe a L96 or LY6 if you can find it (even though those have iron blocks - it only adds about 75 lbs) or an aluminum 6.2.

Make sure you put on a remote oil filter. I would use aluminum exhaust manifolds with replaceable anodes. I have a source for those manifolds somewhere in my computer if you want me to run it down.

Xcomunic8d 03-07-2021 05:30 PM

Yes I agree completely on the alpha’s. I was going to go that direction with about 400hp but a bravo is near me and I just need to get it.

Hardin makes aluminum elbows and risers with dry joints.


Of course I’ll be doing the closed system. VVt is already gone but I need to tap and die and install set screws.

I agree on a big truck cam. Need torque down low, decent mid range, and high end power for when all out. So many options on cams is frustrating. Didn’t have this when I was a kid. If you can recommend one that’d be awesome.

Looking at compcams and Texas speed too.

I agree a ly6 would have been great but Trust me on island prices and availability. I was originally told it was a 6.0 but no such luck. I looked into boring it out to 6.0 but can’t be done. You could overbore it but not sure it’s worth the hassle given what a Ls cam, heads and tune will do. I’d have built a .383 stroker but this fell in my lap. I figure it will make 500+, should be plenty.

FishStretcher 03-07-2021 07:13 PM

As someone who has a 350 MAG MPI, I was curious. The specs for the LMG that I could find are not high performance- cast pistons, powder metal rods and cast crankshaft. That said, I don't know anything about LS engines.

What sort of revs are you thinking?

Xcomunic8d 03-07-2021 08:34 PM


Originally Posted by FishStretcher (Post 271020)
As someone who has a 350 MAG MPI, I was curious. The specs for the LMG that I could find are not high performance- cast pistons, powder metal rods and cast crankshaft. That said, I don't know anything about LS engines.

What sort of revs are you thinking?

Idle to 6500 I doubt Iíll get it out to 7k. The 243 heads are pretty good, Iíll port them, shave them, and put new springs and guides in. Maybe new valves if polishing seating etc is not enough. Cam will change the timing. It has a TBSS (same as ls3 intake). Itís a garden variety gen IV. But can be quite powerful and a good beginning point. Iím going to be trolling not much else. But itíd be nice to get where I want when Iím in a hurry.

The big difference besides distributor and direct port injection is the top end. Most 350ís required lots of work on bottom end to make power. LS blocks are (mostly) all same except for boring. That means throw a cam, intake, set of heads on you have instant power. Getting stuff here is the problem. I canít tell you how much stuff never makes it here. So we have to figure it out, if weíre not rich.

Check out 3:36. Doubt thatís my cam. But thatís a pretty close setup to my goal.

FishStretcher 03-07-2021 10:19 PM

You said you're getting a Bravo 1? I have a Bravo 3 with 2.20:1 gearcase. And with the 5000 RPM 350 MAG MPI.

But the Bravo 1 maxes out at 1.65:1 which may make prop selection a challenge if you want to make power by going to 6500? Also, a cam that works at 6000 RPM might not want to troll very well? In this case, VVT would be nice if it allowed the high end power you want.

I was pulling numbers from here:

Like I said, I could be wrong about this.

Xcomunic8d 03-09-2021 03:27 AM

Iím not sure which bravo Iím getting. Iíve got a lead on a bravo with dual props he claims itís a 2, but I thought only 3 had duals?

As far as vvt, compcams recommended removing it when you cam swap and tune. So Iíll be listening to them.

Been a long day pulling the motor. Separating the engine and tranny was a bear. But allís well that ends well.

Nonstop rain since Friday. Supposed to rain all week.

flyingfrizzle 03-09-2021 08:45 AM

This will be a sweet swap, I am about to swap a LSR block into my Camaro (one day if I ever get any free time again) and going to run MPI with Holley EFI dominator set up with the Holley dash. I have seen a few go fast boats with the holley EFI set ups and you could do the terminator x for reasonable money.

Xcomunic8d 03-09-2021 01:06 PM

Yeah that terminator x looks to be best way to go. I haven’t figured out the gauges yet. I wanted twin lowrance screens tied together on the outboard. But I’m leaning towards just analogs now. I just want this done.

Sounds like you have a great project too. Don’t forget about that throttle body... I’m going cable drive. And a 102mm vs 92mm adds no hp on dyno according to Richard holdener.

I’d get one of those Fast Lsxr intakes if I could afford one. Sharps custom intakes look great too. But I have the TBSS which is identical power to a LS3 intake and the best OEM you can get. I just can’t justify $1k for 30hp.

PS check out that F35 cam from Texas speed for your swap. It won’t work for me but oh I wish it would.

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