BBC Build Opinions

boatstogo · 09-26-2009, 06:18 PM

I am building a motor for a 1986 22' Velocity project I acquired. Boat had a BBC Mercruiser serial number F810042 which I believe is a 7.4 Bravo Gen VI motor vintage '96 or '97 that was cracked. I have a Mercruiser block serial number K189211 that is missing the main joirnals. Questions to you parts guys. Will the main journals from the cracked block work on the other motor if it is line bored? Both are 4 bolt mains. Also, I can use the heads off the cracked block or a set off of a 1990 502/465 HP Motor. The cracked block heads have casing numbers M125 10114156 and the other are C080 14036188. The latter heads also have HI-PERF cast in them and are larger valves with rectangular intake ports. I have an Edelbrock tall intake manifold and a Holly 780 carb. Will the stock Mercruiser exhaust manifolds, risers and 4" hose work OK? I do have a set of Gil Big Boy manifolds and Stainless Marine tailpipes off a 572 blower motor but would have to rework the tailpipes and cut larger holes in the transom for the 4 1/2" pipes to work. What kind of hydralic cam should I use with the roller lifters? Lastly, what piston and compression ratio do I want if I only have access to 89 octane? Any advice or counsel would be much appreciated.

Rookie · 09-26-2009, 08:40 PM

sorry wrong thread

OutlawMan · 09-28-2009, 10:38 PM

If you are speaking of the main caps, they should work but dont quote me on that. Alot of BBC heads say hi-perf on them which usually isnt anything to get excited over. The fact that they are rectangular port is good since they should flow better than oval port in the higher rpms if that is what you are after. Get them checked out though. Your tall intake if it is a single plane should compliment the heads nicely, as will the carb(save up for an 850 though). Your gonna want a decently lopey cam to bring this all together though, as matching parts is what dishes out HP. Which brings us to compression ratio. This setup is begging for a decent amount of compression, but as you stated you are stuck with 89. So you going to want to stick with around 8.5-1 so you can put a decent amount of timing in, and in the future if you can get your hands on some cash and decent fuel you have the perfect blower motor.

Assuming of course you use forged internals. IMO

boatstogo · 10-03-2009, 04:07 PM

Thanks Outlaw man. The HI PERF heads came off a 33 Fountain with 465HP Mercruisers so they should flow well. I can't use too much cam as I will probably stick with the stock exhaust manifolds and risors for budget and space reasons. I could use a set of big Gil manifolds and 4 1/2" tailpipes but it would take some work getting them to fit. This boat was a hand full with 350 hp at top end of mid 70's, so not sure if it is worth building more than another 100 hp or so and keeping it right side up. Thanks for your response.

fireboatpilot · 10-09-2009, 11:02 PM

Save yourself the time and expence of building an engine and buy one of mine. I have a matched pair of 468's. One is newly freshened with "0" hours and the other has about 40 hours on it. fresh water. BBC, Merlin heads, H beam rods, 8.5:1 pistons, solid roller cam. Selling as long blocks. These motors made 580 HP on the dyno with single plane intake and 900cfm carb at 5800rpm. I would rather keep them together but I need the cash for one right now. Fresh one, $6K, 40hr one, $5K.