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5.7 engine build 6.2 ?

Old 10-24-2018, 08:59 PM
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Default 5.7 engine build 6.2 ?

Hello I have and 20ft Bryant runabout the motor is tired it's 5.7 260 HP has alpha 1 gen 2


I'm looking to build a solid good running reliable engine don't want to give up lot on the low end or hole shot so going mild on the cam we ski and tube a lot I'm thinking either go back 5.7 or stroke it to 6.2 but don't want drive problems looking to build 300HP ? and be able to turn it 4800-5000 max to keep the drive happy with a shower and synthetic lube
Also want to be able to run 87 octane gas


here is the 2 I was thinking of building any advise ?


5.7 build option
Wiseco PTS508A3 pistons
eagle or scat 5.7 rods (I beam rod)
Gm ramjet 350/HT383 cam 14097395
New Vortec heads (stock GM ?)
Dual plane Intake


6.2
Wiseco PTS533A3pistons
eagle or scat 5.7 rods (I beam rod)
eagle or scat crank (cast don't think I need forged )
Gm ramjet 350/HT383 cam 14097395
New Vortec heads (stock GM ?)
Dual plane Intake


Both motors would be decked to 9.000" and use .041 head gasket to get .041 quench


5.7 would have 9.3:1 ratio
6.2 would have 9.5:1 ratio
I'd think either would live on pump gas ?


Is the cam enough with the vortec heads to get the 300HP ?


what do you degree the cam in at looks like it already has some advance built in


On bearings its been a few years since I've built motors I know they took the lead out of the bearings and that was a problem on big motors are the clevite 77 bearings good enough or do I need to go to King or Childs Albert bearings ?
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Old 10-25-2018, 12:23 AM
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Hello, can´t comment a massive amount on your engine build specs, a couple of things do come to mind though, 300hp is really the limit for that drive, even with shower, yes I know a long time ago they were paired with big blocks, the newer motors, really the 350 MAG 300hp was about the max and was not paired in all ratios, we had a couple of clients with that setup and they had some problems with the top ends, choose cam carefully, there are people who are more expert on those than I but Mercruiser have a came with more overlap (??) to stop engine sucking water back in when you stop it, I hope someone else can give you more advice on it, I am also not sure if the new Vortec heads have the exact same bolt pattern if you were to use your original block, Merc does have a 383 stroker but I think it would destroy your drive, hope this helps.
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Old 10-25-2018, 06:55 AM
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Raw water or fresh water cooled?
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Old 10-25-2018, 07:34 AM
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Unless you are not happy with the 260 power, I would stick with the 5.7. Pretty tough to find a more reliable package.
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Old 10-25-2018, 07:40 AM
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Look at Chevy crate engines, 383 HT is a truck engine already built for you, mine was $4800 out the door
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Old 10-25-2018, 07:47 AM
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You will have a seriously restrictive exhaust issue if you use the thru-leg exhaust and high rpm.
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Old 10-25-2018, 08:23 AM
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Originally Posted by 805gregg View Post
Look at Chevy crate engines, 383 HT is a truck engine already built for you, mine was $4800 out the door
That amount of power will destroy the Alpha drive, will probably need a different cam (would need proper expert advice on this), replacement core plugs, different head gaskets etc. Price is very good though, would have to check that exhaust manifolds would marry up to that block also.
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Old 10-25-2018, 08:27 AM
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I dropped one of these in my Albemarle, it has been flawless...
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Old 10-25-2018, 10:35 AM
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Thanks for the replies
I don't want to buy a crate engine ! I've seen lot of the aftermarket crates a cheap junk they build them with now factory are alright but the clearances are all over the place deck is to high and not square I want to build my own I've built lot of engines in the past if you told me you wanted 1250hp BB naturally aspirated I can tell you exactly how to get that I'm just trying to get a little more than stock balanced blueprinted nice smooth engine without over powering the drive ... BTW I have a problem of crossing the limits in the past !!!! ... but hey you never know where the line in the sand is till you cross it !


Raw water cooled but wont see any salt water
If it eats up and intake every year oh well
Boating season is short around here anyway

it will run through the leg
stock runs through the leg and they get 300hp ?
I really don't want loud annoying boat with through hull exhaust just getting old
so I give up 10-15 hp /1 mph to keep it quiet
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Old 10-25-2018, 10:45 AM
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you'll be rebuilding that drive every year or two.
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Old 10-25-2018, 11:24 AM
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I’d find a junked boat boat with a bravo drive and take all the necessary bravo hardware and use it to convert your boat to a bravo drive. An alpha drive won’t hold up long with a 383. Yes, merc did pair big blocks with alpha drives... for a few years, and then they promptly stopped. Probably because it was a recipe for warranty claims.
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Old 10-25-2018, 11:42 AM
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I don't have to build the 6.2/383 Im just looking to to get around 300 hp and make the drive live trying and not to cross that line to unreliable


I'd like to put a bravo on it but cant find them around here or they want 80-90% what I can buy a new one for and I think* the big block low end torque is what killed the alphas but could be wrong and wasn't that the gen 1's ?
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Old 10-25-2018, 11:55 AM
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a stock Mercruiser 5.7 Vortec 4 bolt with a AM single plane intake with a 650 holly vac secondary will get you 300hp..
no rocket science there !.
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Old 10-25-2018, 12:33 PM
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Originally Posted by in812 View Post
I don't have to build the 6.2/383 Im just looking to to get around 300 hp and make the drive live trying and not to cross that line to unreliable


I'd like to put a bravo on it but cant find them around here or they want 80-90% what I can buy a new one for and I think* the big block low end torque is what killed the alphas but could be wrong and wasn't that the gen 1's ?
If you have any high performance marine shops around you, I would check with them if you're interested in pursuing a Bravo. They usually have a few complete take outs kicking around for a decent price. The guys wanting to hit 100mph pull them before dropping in their big power packages.
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Old 10-25-2018, 01:12 PM
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From above
a stock Mercruiser 5.7 Vortec 4 bolt with a AM single plane intake with a 650 holly vac secondary will get you 300hp..
no rocket science there !.
rocket since Hum ? so it has to be 4 bolt main ? and a single plane intake to kill low end torque ? and 650 holley ?
4bolt main is a joke the other bolts are drilled into the webbing making it weaker now if you take a 2 bolt block and ad splayed caps that works
Single plane intake really ? you want a dual plane to make off idle power
hum 355 @ 100% VE only need 500cfm @ 4800 over sized carbs hurt throttle response
sorry you sound like race car guy that doesn't have clue what it takes to build torque and HP what are you going to tell me next to put a stall converter in it next to make up for it ? LOL


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Old 10-25-2018, 02:10 PM
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0.041 head gasket is not a stainless gasket and may rust out. Quench only works with a flat topped piston in the quench area AND the deck height is flush with the piston top.
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Old 10-25-2018, 04:08 PM
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That's why I was looking at the reverse dome wiseco pistons they should fit the vortec head nice (not like the soup bowls in crate engines ) and yes I under stand quench





BTW for those that dont know if you have 70cc trapped and 56cc of that in the combustion aera and 14 cc end gasses with .080 squish and go to .040 squish theory you move 7 cc of the end gasses into the combustion chamber you just made 10% more fuel available to be ignited making in theory 10% more power (free power as long as the squish velocity doesn't go to high or compression ratio) squish has another advantage to a point as the piston in moving up the trapped end gasses create velocity across the piston you don't want to much velocity think it this way if you pour some gas out light it burns blow on it and it gets violent blow hard enough you blow the flame front just have to know how fast your fuel burns meters/second keep the Squish velocity under that gives you more power less emission ... now that's Rocket Science !
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Old 10-25-2018, 04:25 PM
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If it were mine, (and I have the same plans when my 5.0 MPI B3 gets tired) a fresh 350 block with a stroker kit, factory 5.7 Vortec heads (they flow like the old Fuelies), Mercruiser 6.2 camshaft. Being you're carbed, dual plane intake for sure. Keep it simple, with this setup, you'll easily be in the conservative 320hp range.
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Old 10-25-2018, 04:49 PM
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Your no stranger to chevys by what you wish to do .But for your own well being and wallet . Check marine cams , Mercury had problems with to much cam and suffered cooling water from the exhaust intrusion into combustion chamber with rapid slow down / reverse . I also like the idea of reverse dome piston engines Good luck.
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Old 10-25-2018, 05:05 PM
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Did you figure out the dynamic compression ratio? 9.3:1 with iron heads doesn't sound too 87 octane friendly.
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