Upgrading mercruiser 350 v8 260 hp

[knightowl]

Got a 1979 350 260 hp engine on my 1979 completely restored and remolded boat. The engine is about 2 years old now with only about 15-25 hours on it. However, my boat is about 27 feet long but its also VERY tall boat (as you can see and compare to other boats on the lake).

When i first got the boat it was just a pile of JUNK. Few friends of mine couldn't believe it only had 1 engine and alone a small 350. Many felt it would be underpowered. But since it was my first remolding boat project, i decided to drop the same engine in. Besides it had an Alpha one setup and a bigger engine i figured might of destroyed it quick.

Ive read on these forums before that alpha drive can only take up to about 300hp. I have one of those Florida SE 106 replacement drives installed instead of the merc version. So far so good.

With 3 people including me, the fastest the boat can go is about 25 to possibly 30 mph with about 4 rpm. Usually at around 3.3 rpm i get around 22mph to 25 with that many people.. Had about 10 people in the boat the another day and it would only go about 15 mph.

So i wanted to know if its a good idea adding the extra 40 horsepower to my boat and if it is, what are the best/safest upgrades i can do? Also how much speed do i expect from the upgrade?? I was thinking a intake manifold upgrade would put me at 10-15 hp? cost around $150 at auto store (at least what the kid said at the store).

Here some boat picks so you can get a clue of the scale of the boat.

 

[mkast]

Re: Upgrading mercruiser 350 v8 260 hp

You might even get 1 mile per hour with the intake manifold, maybe.
You already know the story. You can spend $5,000-6,000 for a stroked small block. Ten horsepower will usually net one mile per hour. How much do you want to spend? How fast do you want the boat$$$

 

[gadget73]

Re: Upgrading mercruiser 350 v8 260 hp

If its only spinning to 4000 rpm WOT, I think you may get more gains with changing the prop than messing with the engine. That boat is never going to be fast, but you should be able to turn 4600-4800 at WOT with the right prop. Max RPM should be specified on the engine, but thats a typical value for that motor.

 

[knightowl]

Re: Upgrading mercruiser 350 v8 260 hp

So the best way to find out my rpm is to go WOT while by myself and i should reach 4.8 rpm? I noticed that after or around 4 rpm, my tach gets all jumpy and screwy by going to 7 to 4 to 6 to 4 to 5 rpm very jumpy and so forth.. Why???

 

[knightowl]

Re: Upgrading mercruiser 350 v8 260 hp

You might even get 1 mile per hour with the intake manifold, maybe.
You already know the story. You can spend $5,000-6,000 for a stroked small block. Ten horsepower will usually net one mile per hour. How much do you want to spend? How fast do you want the boat$$$

I don't want it to be a speed boat like i mentioned before. I think the SE106 replacement drive and mercruiser outdrive is only rated at like 300 max hp. Em i correct? If so, it only leaves me at 40 hp max that i can upgrade.

Its got Eldebrock 1409 auto choke carb on it.

 

[Bondo]

Re: Upgrading mercruiser 350 v8 260 hp

I think the SE106 replacement drive and mercruiser outdrive is only rated at like 300 max hp. Em i correct? If so, it only leaves me at 40 hp max that i can upgrade.

Ayuh,... That's Correct,... And, 40hp will barely be noticable in that boat,....

It looks like you've done a Beautiful job on it,...
Enjoy it as it is for a few seasons,+ save your coins for a BBC/ Bravo upgrade...
Find a Junked/ Salvage boat with the right driveline,+ Swap it into your hull....

 

[knightowl]

Re: Upgrading mercruiser 350 v8 260 hp

so no luck for me i guess right?

 

[Showtime22]

Re: Upgrading mercruiser 350 v8 260 hp

I have the same engine but on a 21 Ft boat. And I think its weak on my wreck....I've seen blowers available for the 350 Mag engines but not for our set up.

Buy 2 props...a speed prop and a stump puller. Change them depending on your days agenda.

 

[180shabah]

Re: Upgrading mercruiser 350 v8 260 hp

Remember that the 260hp listed was at the CRANK so you are only puting about 230 to the prop. ALL current engine and drive ratings are specified at the prop.

Your 350 with vortec heads, a good intake manifold with 4bbl, center rise exhaust manifolds with 4" elbows will net you a gain of at least 70hp at the prop. It will be dead reliable and very noticable...

BUT were talking about $2k in parts.

That boat is the perfect candidate for a 383/bravoIII setup, but now we're talking some serious money.

 

[knightowl]

Re: Upgrading mercruiser 350 v8 260 hp

Crap! well that blows. How much will intake manifold or fuel injection system or even turbo improve?

 

[180shabah]

Re: Upgrading mercruiser 350 v8 260 hp

As noted above an intake manifold by itself will net a few (maybe 10) hp for a noticble gain of approximately zero.

Fuel injection does not MAKE power, that comes from the heads, intake, cam and exhaust choices.

Turbo, opens a big can of worms. you will need to have a custom waterjacked exhaust built and use a water cooled turbo. Otherwise things in the engine compartment tend to spontainiously combust.

 

[knightowl]

Re: Upgrading mercruiser 350 v8 260 hp

By the way what prop would you recommend for my boat? Why is my tach gets screwy after 4 rpm? Also, if i had to do it all over again which engine should i have bought for this boat and what type of out drive?

Check out this lucky son of gun!

http://www.youtube.com/watch?v=T79b3hXrHXw

 

[ringmaster72]

Re: Upgrading mercruiser 350 v8 260 hp

You will be fine adding some power. I have over 350 HP and have over 40 hours on an Alpha with no problems so far. Your drive has a 3 year warranty correct?

If it comes down to it, why not get/build the powerplant you want and then run it on the alpha until it craps out then get the bravo? At least that is my plan anyway.....

 

[180shabah]

Re: Upgrading mercruiser 350 v8 260 hp

Check out this lucky son of gun

GM 350 with distributor in FRONT????? and it needs twin turbos to get "over 300hp" he thinks... I'd stick with vortecs on a 383 - solid power and dead reliable.


What prop? needs to be asked in the prop forum. But first you need to get your tach fixed. you don't have any useful number from your current setup yet.

Generic suggestions - Hustler/Legacy if you want a three blade aluminum - great all around prop. four blades may serve you better out of the hole, if you have the power to spin it, might have to play with pitch. And of course there is the end all, be all of props - the shifting propeller.

 

[knightowl]

Re: Upgrading mercruiser 350 v8 260 hp

wait a minute, i thought putting bravo in would be a huge process?

 

[180shabah]

Re: Upgrading mercruiser 350 v8 260 hp

Yes, converting to Bravo is involved, but not "difficult". You will need to remove your entire drive assy and the engine. Then drill two additional mounting holes in the transom, install the Bravo inner and outer transom, gimbal, etc. reinstall the engine, with engine mounted seawater pump, align the engine and install the bravo drive. There are going to be some small parts along the way that will also need replaced, but the above covers the big part.

 

[Cptkid570]

Re: Upgrading mercruiser 350 v8 260 hp

What size prop do you have? When I had my 26' Sea Ray with only 260hp, the recommended prop was 15x15... and I was still underpowered..

Bravo would take too much work and money.. you'd have to change the entire transom assembly and I belive add an impellor onto the engine.

You did a great job with your remodel..but you could always keep an eye out for a smaller/lighter boat that needs an engine and outdrive and drop your engine backage into it.

 

[Bondo]

Re: Upgrading mercruiser 350 v8 260 hp

Also, if i had to do it all over again which engine should i have bought for this boat and what type of out drive?

Ayuh,... I answered that in post #6,....

Enjoy it as it is for a few seasons,+ save your coins for a BBC/ Bravo upgrade...

I have over 350 HP and have over 40 hours on an Alpha with no problems so far.

Ayuh,.. So Far,..?? So Far,..?? You've barely got the break-in behind you,...
I build boats for a 4000hr lifetime,... You've made it to 40hrs,... It'll Never see 400hrs...

 

[gadget73]

Re: Upgrading mercruiser 350 v8 260 hp

By the way what prop would you recommend for my boat? Why is my tach gets screwy after 4 rpm? Also, if i had to do it all over again which engine should i have bought for this boat and what type of out drive?

Check out this lucky son of gun!

http://www.youtube.com/watch?v=T79b3hXrHXw

What prop is on it now? Might want to contact the manufacturer and see what they originally equipped it with. I've seen some larger boats with small motors that used a fairly small wheel in order to move properly. With too much prop, the engine won't spin up and you suck a whole lot of fuel doing it.

Could be a loose wire on the tach, loose connection at the coil, corroded connections in that big black plug on the motor, a bad tach, or a coil with a cracked tower that arcs over to the terminal. I've had all of those things occur on my boat, and every time it made the tach go wonky. Check for connection problems first since thats most common and cheapest to fix.


As for power in front of an alpha, more than once have I seen big blocks bolted to them. I used to work on a mid 80s 28 foot Cruisers with twin 454/Alpha setups. And yes, he blew those drives several times, thats with a 340 hp 454. I strongly suspect that the size of the boat and how you run it plays more into how much you can get away with. A big block on a small boat probably stresses that drive a whole lot less than it would on a big boat. At 28 feet on a single engine, I wouldn't try it.

If you were to buy a similar boat new, and it came with a single engine I'd bet you'd be looking at a big block with a Bravo III or Volvo DP, or maybe a 6.2 with the same drive. Base power might be as small as a 350 Mag, but you're still talking 330 hp and twin screws vs what you have now.

 

[knightowl]

Re: Upgrading mercruiser 350 v8 260 hp

Sportsmans II Iron Heads from world products are saying its an improvement of 30-70 HP. I found some on craiglist for $300 for both. 1 Needs a small crack welded though. But prices on Summit raceing is anywhere between $400 and $700. Will this work? Good or bad idea?

http://www.worldcastings.com/products/sportsman-ii-iron-heads.html

http://www.summitracing.com/search/?keyword=SPORTSMAN II Iron Heads&dds=1