Going from 4.3 to 350 in my 18 foot, 2300 pound wet boat. (hull has a pad and tight power steering) with alpha 1 outdrive. runs 52-60 with 4.3 depending on prop and loading. I'ts been 20 years since I built a small block chevy from the ground up, but I'm generally pretty handy. I do worry about selecting a good combination of carb, cam, intake and heads though.
I picked up a complete mercruiser 350 with older style heads, roller cam ready block, oem manifolds, 4 barrel, etc... it came with everything from the transom forwarx including electrical, ignition, distributor, water pump, starter, etc... all oem mercruiser parts.
I've done a lot of reading and here's what I'm currently considering:
Looking for fairly flat torque curve with usable power from off idle with peak hp at 5200-5500 rpms.
Components:
Cast dart or world products heads with 2.02 / 1.60 valve heads and 165-170 cc intake runners (If I pick up a little extra work in the next few wweks, will consider going closed cooling and aluminum heads).
Comp cams hydraulic roller grind Xm270HR (duration at 0.5 lift: 218 224, lift: .495 .503; LSA: 112?) with appropriate springs, pushrods, magnum 1.6 rockers.
good cast lower end components, carefully balance and blueprint everything,file fit piston ring end gap, new rod bolts at a minimum, have rotating assembly balanced. Ideally will buy new 383 rotating assembly once I talk to machinst and see what cost will be for block work.
Edelbrock 750 marine carb (have been really happy with the 600 on my 4.3).
Exhaust will be at a minimum aluminum glm, most likely second hand headers or stainless steel manifold / riser combo.
and whatever else makes sense...
Oh, and the alpha drive gets a different upper half with 1.5:1 gear ratio, a drive shower, and good synthetic lube changed often. maybe a temp transducer. Then I've got enough parts to piece together a second outdrive as a back-up if I fry the first. I may get bored / ambitious and buy the tools, do homework and try to rebuild the second drive with heavy duty gears, cryo / heat treat everything myself and whatever else I can learn to help strengthen that link.
Perhaps if I try and build for lots of increase on midle and upper end of hp band with mild increases increase in low end torque (corrected for gear ratio) to what I have now, it'll spare the drive some on take-offs (it has plenty out of the hole now...).
Thanks in advance for any thoughts or experience - especially with respect to the general combination and approach... I know it probably seems simple / obvious to a lot of you, thanks for bearing with me. Oh and thanks to a few of you who have posted so much information in the past and those who have been patient with my questions.
I picked up a complete mercruiser 350 with older style heads, roller cam ready block, oem manifolds, 4 barrel, etc... it came with everything from the transom forwarx including electrical, ignition, distributor, water pump, starter, etc... all oem mercruiser parts.
I've done a lot of reading and here's what I'm currently considering:
Looking for fairly flat torque curve with usable power from off idle with peak hp at 5200-5500 rpms.
Components:
Cast dart or world products heads with 2.02 / 1.60 valve heads and 165-170 cc intake runners (If I pick up a little extra work in the next few wweks, will consider going closed cooling and aluminum heads).
Comp cams hydraulic roller grind Xm270HR (duration at 0.5 lift: 218 224, lift: .495 .503; LSA: 112?) with appropriate springs, pushrods, magnum 1.6 rockers.
good cast lower end components, carefully balance and blueprint everything,file fit piston ring end gap, new rod bolts at a minimum, have rotating assembly balanced. Ideally will buy new 383 rotating assembly once I talk to machinst and see what cost will be for block work.
Edelbrock 750 marine carb (have been really happy with the 600 on my 4.3).
Exhaust will be at a minimum aluminum glm, most likely second hand headers or stainless steel manifold / riser combo.
and whatever else makes sense...
Oh, and the alpha drive gets a different upper half with 1.5:1 gear ratio, a drive shower, and good synthetic lube changed often. maybe a temp transducer. Then I've got enough parts to piece together a second outdrive as a back-up if I fry the first. I may get bored / ambitious and buy the tools, do homework and try to rebuild the second drive with heavy duty gears, cryo / heat treat everything myself and whatever else I can learn to help strengthen that link.
Perhaps if I try and build for lots of increase on midle and upper end of hp band with mild increases increase in low end torque (corrected for gear ratio) to what I have now, it'll spare the drive some on take-offs (it has plenty out of the hole now...).
Thanks in advance for any thoughts or experience - especially with respect to the general combination and approach... I know it probably seems simple / obvious to a lot of you, thanks for bearing with me. Oh and thanks to a few of you who have posted so much information in the past and those who have been patient with my questions.
Last edited:
