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Two Bolt Main or 4 Bolt Main ?

8

88-checkmate

New member
Hi guys , I have an 88 Convinsor with a 454 330hp Motor with what I think a two bolt main. It is not a magnum motor. I would like to move up in horse power something in the 500hp range. I have a couple of questions I would like to ask before I start this project. (1) Should I get a 4 bolt main motor or is the block I have good enough for this hp and rebuild? (2) I have a stock original bravo drive do I have to upgrade it or get a diffrent drive? (3) When I do rebuild the block what parts do I have to buy to make 500hp ?(pistons,cam,heads,valves ect) (4) Is it better and cheaper to buy a creat long block or rebuild mine? I would really like to hear what you guys think. Thanks:sssh:
 
I was told since a boat motor does not have the "shock from shifting gears" that a 2 bolt will hold together fine. I believe most marine motors are 2 bolts. However I used 4 bolt mains on my motors. Even the junkyard motor that I am using till my stroker is done is a 4 bolt. Another thing you can do is use "splayed main caps" turning your 2 bolt into a 4 bolt. Some say it is a stronger setup than a stock 4 bolt block. If you trust a local shop, it will save you some cash to build a motor vs buying a turn key. There is alot of performance marine parts for the BBC, way more than the SBC. I would start with bolt on performance parts. Adding performance intake and exhaust manifolds and a set of aluminum heads is a easy bolt on that will yield you some power, and reduce weight without removing the motor. If thats not enough, you could pick up another block and build a good and solid short block and use your top end parts to complete the setup.

There is quite a few more people with a lot more knowledge on here than I have but that's just a couple ideals. Ask 10 people what to do and you will get 10 answers! Those are my ideals so good luck with whatever you chose.
 
A 4 bolt is better & can hold more power, however a 2 will hold 500hp just fine. The 4 bolt cuts down on the mains shifting under load, but I wouldnt worry about it too much unless you were looking for a good bit more than 500hp. 425-525hp should be ok on your block. It has been done many times before.

What would most likely be done to get you in the 500hp range is:
Bore & stroke the engine. (in essence go to about 496 cid ).
Then a set of higher compression heads with better runners, larger valves & blended bowls.
Top it off with a good high rise dual plane intake for low end & mid range power, or a moderate sinlge plane intake.
then top the intake with something like an 850 mechanical secondary carb like a Holley..
Also dont forget the right cam. You will probably end up with a Hyd roller cam and running a set of roller rockers in the heads to let enough mix in and exhaust out..
You will also need a good free flowing exhaust system. Like a good set of HP manifolds or a set of headers.

We also offer performance rebuilds & new power upgrades.. ;)

Hope this helps.. :) Jamie / Lakeside
 
I have done mild race engines up to that H/P with now issues...but if you want I have a cherry 454 std bore 4 bolt main bare marine block for $600 if you want..Rob
 
To awnser #2: imo, your bravo will last at that hp level, as long as you don't abuse it. The one other thing you might have to possibly upgrade is steering. What size convincor is this going in?
 
As mentioned a 2 bolt will hold up I ran some 2 bolt mains in a few boats years ago. I had one it was a .040 over 427 c.i. 2 bolt in Hondo runner bottom v-drive I owned, Never new the HP but that thing twisted 6800 rpm. I had a Mechanical flat tappet cam, ported heads 10:1, forged rotating assemble. I would guess it was around 400 to 450 hp. but it was a light boat compared to a Convincor. It ran great the two seasons I put on it, I sold it and the new owner ran it for 3 more years I know of then I lost track of the boat.

If you are going to invest into a custom high performance engine I would recommend using a 4-bolt main. Even though you are not shifting gears if you ever air your boat out there is alot of shock transmitted at re-entry
You will be eliminating A weak link in you high performance engine.


MKHammer
 
I agree a 2 bolt will hold up. Your not banging gears or smoking tires in a boat, that's where all the strain is in a car. I would however go with a good balance job.
 
I think the real key question here is how much do you want to spend.

A 2 bolt will stay together with 500 HP but I would rather have a 4 bolt.

The bravo drive can handle the 500HP if you do not hammer on it everytime out of the whole and wait till you get on plane.

The hyd steering is the best way to go but 500 hp in a 25 convinsor will not be putting you in any speed danger with regular steering.

I would spend my money on a good roller cam set up and some good aluminum heads and exhaust.

I would say 10 grand would be a good ballpark figure to acheive the 500hp mark useing good parts
 
If you don't already own a block, just find a 4 bolt block and you will have 1 less thing to worry about. The price difference is really small when you consider all the other parts it will take to get that project finished.
 
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