SBC to BBC Conversion Questions
 

If you guys have ever read my Formula 206LS build thread, you will see that I had a 383 stroker built up for my boat. It was shortly after stolen and stripped. To get it back in the water, I had an extra 305 sitting around so I threw that in. As you can imagine, NO FUN AT ALL. It runs a max of 30ish MPH and uses full throttle all the time and alot of gas to get there. I am ready to build another engine for my boat and want to go to a 454 this time. My question is, what all do I need to change for this? My boat has the newer Vortec style engine with 1pc rear main setup in it now. Also, I have a above the swim platform thru hull exhaust(pics below)...do you know if the exhaust for a BBC will line up from the SBC? Any advice would be great and if anyone has a solid performing and reliable build up on a budget for a 454 let me know. Now even though I did say budget, I don't mind spending money to make it right and dependable.

Thanks alot!

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BBC and SBC share the same bolt pattern for motor mounts and bell housing, and the distance from the bell housing flange to the motor mounts is the same, so it will drop down in there, provided you have enough room in front of the motor in the bilge, because the BBC is longer. You will need to obtain the correct 454 flywheel, because they are externally balanced. Keep in mind that the Mk IV (early) 454 flywheel is different from the Gen V/VI late model 454 flywheel. You will need to find the correct alternator, PS and seawater pump brackets for the big block, along with the correct crank pulley. I think MerCruiserParts.com might be a good source of schematic diagrams with part numbers. Likely that you could find that stuff in the Swap Shop, or just find a complete marine take-out motor with all the brackets, acc, exhaust, etc.

Oh yeah, regarding exhaust, the SBC exhaust is closer together, so you may have alignment issues. Depends on how much distance you have between the exhaust tips in the transom and the risers - if 10 inches or so, you can probably make up the bend with some of the high temp silicone exhaust hose, as it is much more flexible and works well for alignment issues.

Good luck - been there, done that, and have the T-shirt and much lighter wallet to show for it.

Also, just noticed this is a 206 Formula. For a boat that size, I would consider staying small block. Much easier, and you could build a stout 406 small block that would make that boat scream. On a boat that small, I would be worried that the extra weight of a big block right over the stern would throw off the center of gravity and make it ass-heavy.

Just went through the flywheel issue on another thread you should read. http://www.offshoreonly.com/forums/d...heel-help.html

I do have a BB merc bell housing and engine mounts if you need them...Rob

427 Small block might be fun

One thing not yet mentioned although important is short of the rare person who has never had an issue an alpha will not live behind a big block.

If you have an alpha stick with a small block.

I hear this all the time about the back end being heavy with a big block. The weight difference is like have one extra person in the boat. Do not let that be a deciding factor. If stuck with an Alpha drive, I'd likely use a SBC. If you are going to install a Bravo 1, go with the biggest SBC (Dart SHP 427) or the standard big block. Have fun and report back.

A standard 330 hp 454 would push a 20' boat mid 60's and you can find takeouts complete for around 2500, with all acssories alt steering pump etc.

If you have the technlogy an ls based sbc would be sweet in that and 450 hp is supposed to be really easy to get and dependable with an ls.

the big inch SB get very pricey to do....

Why not another 383 to replace the one that was stolen? How did it run with that combo in it?

What drive?

Dart SHP short block is $5k. That is right in line with other builds and all the existing small block stuff will bolt on.

And to think there was a well built stroker with a brand new Dart block that sat in the classifieds for months before it sold this summer.

Why ? the crank kits are about the same price

Hell build a 454 SBC....now we're talking!!

I'm talking about well over 400ci aftermarket block etc...

Thanks all for the replies.

It is an alpha boat. I would like to eventually do the bravo swap, but right now with space being a limiting factor...it's not in the cards.

I think the main reason I was wanting to go BBC is that from what I understand about them, it would be easier, more reliable, and maybe a little cheaper to do a mild BBC and have close to the same performance my SBC stroker had.

Am I wrong in thinking I could do a BBC with upgraded heads, cam, intake, and carb and be close to the same performance my SBC had? I never had my stroker on the dyno, but me and my builder estimated 450-475 hp.

Overall at this point in the boats life it's used as mainly a family boat vs speed boat meaning lots of water sports and such. So I am looking for a reliable solid performing 60's range. When I can do the bravo swap I may look into more power at that point.

Thanks again for all the insight!

Maybe just put it back to stock, sell it and buy a bigger boat with the Big Block, it may be safer.

The BBC will weigh in at around 200+ lbs more than your SBC. That's figuring both with iron heads.

Check this out......

http://www.gomog.com/allmorgan/engineweights2.html