Gen 6 540 or Blueprint 632 in Magnum 27, Reversion?
03-27-2026 | 04:23 PM
#21
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From: Murrayville Georgia
so if you went with the 632 would you trust it as is? odds are you need to pull it apart to lower compression and check clearances on bearings and valves plus what are the valves and will it need inconel swapped in. then factor in exhaust issues due to deck and exhaust port height. after all that is it worth it? you have the parts to build up a nice 540 that will fit in easier and you will know what you have. at the end of the day in that boat another 100hp is like 3 or 4 mph so is is worth it?
03-27-2026 | 05:57 PM
#22
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From: St. Pete Beach, FL
CMG223,
Not that we don't all love Roots blowers, but I'm betting you'll never get that blower under your lid, given it's got a carb and breather on top of it. The Whipple is going to be a more likely candidate, as long as you go EFI, as the intake is on the back of the blower, not on top. They also generally have a flatter torque curve than a Roots with a higher effective RPM range. the Roots are low end torque monsters, but they run out of steam at higher RPMs. they also typically use narrower rib belts as opposed to cog belts, which might be just the space you need behind your seat.
If you are ultimately limited to NA, my reading and listening would lead me to advise against the 632. Just too high a rod angle. There have been guys do it, but I think it's always been to less-than-stellar long-term results. A well-built 540, or 565 if you can squeeze it out of your block, will suit a marine application better. I know my dream engine would be a Dart BigM 565, 4-3/8" stoke and 4.530" bore, hydraulic roller build. But I'm stuck with an otherwise stock 496/8.1 GM block and heads, and a CFO that has already discovered the edge of the checkbook and doesn't feel she needs her knuckles any whiter.
Thanks. Brad.
Not that we don't all love Roots blowers, but I'm betting you'll never get that blower under your lid, given it's got a carb and breather on top of it. The Whipple is going to be a more likely candidate, as long as you go EFI, as the intake is on the back of the blower, not on top. They also generally have a flatter torque curve than a Roots with a higher effective RPM range. the Roots are low end torque monsters, but they run out of steam at higher RPMs. they also typically use narrower rib belts as opposed to cog belts, which might be just the space you need behind your seat.
If you are ultimately limited to NA, my reading and listening would lead me to advise against the 632. Just too high a rod angle. There have been guys do it, but I think it's always been to less-than-stellar long-term results. A well-built 540, or 565 if you can squeeze it out of your block, will suit a marine application better. I know my dream engine would be a Dart BigM 565, 4-3/8" stoke and 4.530" bore, hydraulic roller build. But I'm stuck with an otherwise stock 496/8.1 GM block and heads, and a CFO that has already discovered the edge of the checkbook and doesn't feel she needs her knuckles any whiter.
Thanks. Brad.
03-27-2026 | 06:06 PM
#23
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From: frankenmuth michigan
Oil control is a big deal with a 4.750 stroke, so is piston speed. The equivilant piston speed on a 4.750 at 6000 rpms compared to a 4.0 stroke 502 is 6000 vs 7200, so turning a 632 6k puts as much piston speed or rod, piston "stress" as turning a 502 7200. The cams "big", the cu inches will eat it up but still its gonna idle like a handful and have a much shorter valvtrain life then a smaller cam in a different engine would. IF you do do it, add a crank scraper and best pan money can buy, Smitty
03-28-2026 | 01:58 AM
#25
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From: BC
A 3/8" raised port raises:
Height: 0.26"
Width: 0.26" eaxh side...so 0.520" total.
03-28-2026 | 02:04 AM
#26
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From: BC
Lots of low rod angle examples that just push though and do well. Bore finish/ piston finish + coatings all play a part in preventing piston to wall thrust issues.
Marine generally doesn't do a lot of steep rpm.
Marine generally doesn't do a lot of steep rpm.
03-28-2026 | 02:08 AM
#27
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From: BC
Oil control is a big deal with a 4.750 stroke, so is piston speed. The equivilant piston speed on a 4.750 at 6000 rpms compared to a 4.0 stroke 502 is 6000 vs 7200, so turning a 632 6k puts as much piston speed or rod, piston "stress" as turning a 502 7200. The cams "big", the cu inches will eat it up but still its gonna idle like a handful and have a much shorter valvtrain life then a smaller cam in a different engine would. IF you do do it, add a crank scraper and best pan money can buy, Smitty
High rpm BBCs will have lip seals on the main bearing to contain the oil...and direct it down to the pan IOT prevent crank entrainment.
Today | 03:15 PM
#28
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From: West Bridgewater,MA
Thanks for all the insight. I think I'll just stick with my original plan and use up some stuff I already have. And then I'll know what I have in the end.
Anyone have a solid recipe for a merc gen 6 solid roller 540. Shooting for 750HP if reasonable.
Anyone have a solid recipe for a merc gen 6 solid roller 540. Shooting for 750HP if reasonable.
Today | 05:56 PM
#29
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From: St. Pete Beach, FL
Gonna be pretty aggressive, need 375cfm+ heads, 0.600+ lift, 250-260 duration. 6500rpm, full headers, dry exhaust.
Today | 10:35 PM
#30
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From: On A Dirt Floor
Basic 502 w/8-71 will do it.
454 w/8-71 will be close.
The offshore marine world was full of these for a reason. Why the Merc 700sci so popular too. Well, not an 8-71 but same idea, just newer school blower and efi.
454 w/8-71 will be close.
The offshore marine world was full of these for a reason. Why the Merc 700sci so popular too. Well, not an 8-71 but same idea, just newer school blower and efi.
