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Discussion Starter · #1 ·
Hello. I have done a lot of searching and haven't found exactly what I'm looking for regarding converting an older 350 block to a vortec via heads and intake.

The block is a 1987 350 (merc, non-mag, 4200-4600 rpms, 260 hp)...was remanufactured 2 yrs ago (before I owned the boat). Boat is 1987 Baja Sunsport, 196, 23P Mirage SS.

My questions are this:

Will just changing the HEADS and INTAKE actually yield a 40hp increase??? I have heard claims that a cam change is needed as well....answers?

What else needs to be changed? Do I need an electric fuel pump? Can I still use my quadrajet carb?

Does anyone sell a complete kit for this? I would need cast iron head and intake (boat in brackish water), so no aluminum.

I have also heard/read that because of power increase there will be 300-400 rpm without a prop change (currently revving around 4500-4600, thinking this would take it to 4800-5000 range)....anyone with experience with this??

Lastly, I've heard with Thunderbolt V ignition and vortec engines there is a knock sensor...if I do this changeover, would like to make sure thats included and upgrade to thunderbolt V as well (currently have tbolt IV which has no knock sensor or rev limiter).

All other thoughts and opinions welcomed. Seems like some easy HP for a relatively easy job. Not looking to do major engine mods or changes, this just seems like a relatively simple switch for not too much $$.
 

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40 Is very possible with these heads. I am doing the same swap as you , but throwing in a cam as wel. You dont need an elec fuel pump. You can still use your quadrajet carb. You will need new Self aligning rocker arms. Even though you have centerbolt heads now, the 217 heads that are on there have pushrod guides built into the head. Nobody sells a complete kit. As for intakes you are very limited. For cast intakes you can "sometimes" find an old Volva penta one. Thats it for cast unless you want one of mercruisers 2bbl vortec intakes. Merc never made a 4bbl intake. It was 2bbl or the MPFI option. Your best bet would be to run an Edelbrock Aluminum one and flush it. Your RPM's will rise. You just need to reprop. With the Vortecs, you want the 906 heads for marine use vs the 062. You hear alot about hot the 906 heads flow a little less, and thats because of the Iconel exhaust seat in the heads. That helps us out alot with a boat motor as its always under stress. You can sometimes find the wiring and sensors on ebay for the V unit. Heres a few pics of my heads that got done and intake.
 

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I made this same upgrade last year. I used the stock igintion without a rev limiter or knock sensor with no issues. You will notice a performance gain without changing cams but I would go ahead and change the cam also. I used the 906 heads with an Edelbrock Performer intake.
 

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Discussion Starter · #9 ·
Not looking at a cam change (at least not at this point, dont want to pull motor at least until season is completely over)

Will I see the gains without a cam change?? Will they be noticeable, minimal?

Thanks for the posts so far...keep em comin.....
 

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tcgravy said:
Not looking at a cam change (at least not at this point, dont want to pull motor at least until season is completely over)

Will I see the gains without a cam change?? Will they be noticeable, minimal?

Thanks for the posts so far...keep em comin.....

If I was going to pull the heads, and adjust rockers in hull, why not just pull the motor and do a cam as well, you could have it out in 2 hours easy. cam, heads, intake-rockers and maybe a carb, now you are making some power, if you don't want to pull it now I would wait and do it all in the off season, just my two cents
 

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Discussion Starter · #13 ·
Yea, on my sunsport there is a back bench and a sundeck engine hatch....no dog house to remove, although I can certainly see it being an easy change in that style without removing the engine.
 

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Change the cam and get an intake. The power increase would be worth the effort. You can easily achive 400hp with those heads a cam an Air Gap and better carb. The Quad would work but the primarys would need to be fattened out some.

Builders use those heads all the way up to at least 600hp. Hot Rod had a issue out about 4 years with the specs for a 400hp setup I'll see if I can dig it up for the cam specs.
 

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OPtimus Prime said:
Change the cam and get an intake. The power increase would be worth the effort. You can easily achive 400hp with those heads a cam an Air Gap and better carb. The Quad would work but the primarys would need to be fattened out some.

Builders use those heads all the way up to at least 600hp. Hot Rod had a issue out about 4 years with the specs for a 400hp setup I'll see if I can dig it up for the cam specs.
They used a LT4 Hotcam. I have the article bookmarked. I'm going for the same combo!
 

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The stock 5.7Lcam sucks ! :laugher:

Stock 5.7 260hp Mag:
Stock flat tappet cam:
.004 " 270, 284
.006" 258, 270
.050" 200, 212
.400", .410" with 1.5
110 LSA

The stock Merc 5.7 Vortec cam isn't all that much better - just maybe a tiny bit.

Stock hydr roller
GM#14097395/ Merc#431-811658
.004" 258, 272
.006" 249, 264
.050" 197, 207
.430", .450” with 1.5 rockers
109 LSA

-------------------------------------------------

Just adding better heads will give you little more power.
You need to open the valves a little longer and higher to let the better heads live up to more of their potential.
To make it all worth it I'd add a cam at the same time !

In fact, if I had to choose either the heads or a cam, I'd do the cam ! But, of course I would do both.

Oh, keep the T-Bolt IV distributor

IMHO.
 

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Discussion Starter · #19 ·
why would you stick with the TB IV ignition/distributor????

I just like the idea of the TB V because it has timing advance and a knock sensor to keep the engine from pinging, as well as the rev limiter to keep from overrevving engine and drive when the prop comes out of the water.


Sounds like a cam change may be in the cards as well, but if that happens the swap wont happen anytime soon.

Thanks for the responses guys, keep em comin.
 
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