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Mercruiser HP420 Build - Recipe Input

Old 06-12-2022, 05:05 PM
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Default Mercruiser HP420 Build - Recipe Input

Sunday Funday OSO Family….

As my patience for chasing gremlins around my low hour 1990 Mercruiser 420’s has been depleted, I have decided to pull the twins out for rebuild and upgrade.

I am curious if anyone has a reliable recipe for 500-520 HP with stump pulling torque to push my 357SR1 with TRS drives.

Understanding the limits of the boat and the drives, I would like some input from our community on how I can get close to that power with the following considerations…

1. Use Stock HP420 Carbs - Holley 800DP
2. Use Stock HP420 Intakes
3. Use Stock HP420 Gil Manifold with my modified tails (dry to the transom)
4. Use Stock TBIV Ignition
5. Use Stock Fuel Pumps and Accessories
6. Use Stock HP420 heads
7. Use Stock HP420 Crane Rockers

I am thinking that bumping up the CR with some domes, cleaning up the heads, and changing the Cam would get me close? Trying to stay N/A for the time being but I also typically only fill the boat on the trailer with 93.

Look forward to hearing these responses!
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Old 06-12-2022, 08:04 PM
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I just rebuilt 2 Merc 420's. Suggestions, if you can up the compression ratio, bowl port heads, the intake is not good, (I'm might change mine out) I used 500 EFI roller cams. Good reliable inexpensive builds. They are not going to set the world on fire and you might not net much extra speed. But, you will be boating every weekend. Mine push my TRS Panther 70 mph.
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Old 06-12-2022, 08:13 PM
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Originally Posted by Rookie View Post
I just rebuilt 2 Merc 420's. Suggestions, if you can up the compression ratio, bowl port heads, the intake is not good, (I'm might change mine out) I used 500 EFI roller cams. Good reliable inexpensive builds. They are not going to set the world on fire and you might not net much extra speed. But, you will be boating every weekend. Mine push my TRS Panther 70 mph.
.
Very nice setup Rookie! Looking sweet! Reliability is what I am looking for here, my old 357SR1 is too heavy to light anything on fire except the gas bill. My 420's are a bit newer than the ones in your picture I believe, I have the dual plane intakes and the Gill Manifolds. Nonetheless, definitely some good information. What speeds were you seeing before the upgrades? I have the best of low 60's on her currently, would like to see mid-high 60's but I know that very difficult with my setup, a bump in cruise speed would be nice.
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Old 06-12-2022, 08:31 PM
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With the original 420's it would run 73mph. They were stock flat tappet cam, ported 188 heads, ported Wieand Stealth intakes and a little more compression. (cut heads) I then turned these into my ~600HP 454's and it best ran 82mph. I then transferred those to my AT and found some stock 420's out of a Formula 357 and rebuilt them. (above mentioned engines) These have the rectangle port heads that have the bolt hole in between the ports and don't flow like my originals did. I also sold my worked props for my Panther and might be the reason I'm down on speed.
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Old 06-12-2022, 10:21 PM
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If you run in freshwater, put on a set of Edelbrock E-street aluminum heads, an RPM AirGap intake.
if you run salt, then buy Dart Iron Eagle 308 heads, pay for a good port blend/cleanup. You can still run the RPM Airgap even though you'll need to epoxy coat the inside of the water crossover.
Whichever head you run (and you shouldnt run the GM iron rectangke port head) get a good 3 angle valvejob, and backcut the intake valves.
With your 454 sized bore you will DEFINITELY want to notch your bores to unshroud the valves - free 20hp.
You probably will end up with flat top pistons, but zero deck the block (to where the piston quench height us even with the top of the block. Then run the .039 gaskets.
With flattops, and 119cc heads you'll be lower than you want to be on compression. You can cut the heads to 108cc and end up at 9.0:1
or
You can run 119cc heads and use pistons with 11cc domes and be at the same place.
or
You can run a 4.25 stroke crank and have 9.0:1 with flat tops and 116cc heads.
But you'll need a bit of dome otherwise.
Keep quench clearance between .036 and .046" and your motor will be pretty detonation resistant and responsive at the same time.
Flat-top pistons, small well-shaped combustion chambers, high port velocities, tight quench clearance all make a huge difference in how happy your motor will be.
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Old 06-14-2022, 08:41 AM
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Originally Posted by Rookie View Post
I just rebuilt 2 Merc 420's. Suggestions, if you can up the compression ratio, bowl port heads, the intake is not good, (I'm might change mine out) I used 500 EFI roller cams. Good reliable inexpensive builds. They are not going to set the world on fire and you might not net much extra speed. But, you will be boating every weekend. Mine push my TRS Panther 70 mph.
.
Great minds think alike!

Josh, this was basically what I mentioned to do when we were talking about doing your 420s a while back. Only real difference here vs Rookie's is that your intakes are GM dual planes so you'll want to move to something a better there. Unsure if a dual plane would be a better choice versus a Victor Jr. or Merlin; with your heads the RPM AirGap would build better low/midrange torque, but my thinking is that fuel distribution upstairs is better with the single plane hence why Merc used them.

This is the most cost effective route though you will need to upgrade the lifters and pushrods (you know how I feel about flat tappet cams).

And no worries to be had with those drives... they're beasts!
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Old 06-14-2022, 08:44 AM
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Originally Posted by mcollinstn View Post
If you run in freshwater, put on a set of Edelbrock E-street aluminum heads, an RPM AirGap intake.
if you run salt, then buy Dart Iron Eagle 308 heads, pay for a good port blend/cleanup. You can still run the RPM Airgap even though you'll need to epoxy coat the inside of the water crossover.
Whichever head you run (and you shouldnt run the GM iron rectangke port head) get a good 3 angle valvejob, and backcut the intake valves.
With your 454 sized bore you will DEFINITELY want to notch your bores to unshroud the valves - free 20hp.
You probably will end up with flat top pistons, but zero deck the block (to where the piston quench height us even with the top of the block. Then run the .039 gaskets.
With flattops, and 119cc heads you'll be lower than you want to be on compression. You can cut the heads to 108cc and end up at 9.0:1
or
You can run 119cc heads and use pistons with 11cc domes and be at the same place.
or
You can run a 4.25 stroke crank and have 9.0:1 with flat tops and 116cc heads.
But you'll need a bit of dome otherwise.
Keep quench clearance between .036 and .046" and your motor will be pretty detonation resistant and responsive at the same time.
Flat-top pistons, small well-shaped combustion chambers, high port velocities, tight quench clearance all make a huge difference in how happy your motor will be.
Definitely some good advice here, too, though you'll run into significantly more cost going this route.
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Old 06-14-2022, 08:52 AM
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Lastly, might as well spring for new rods. A good set of H-beams will be money well spent vs rebuilding your current 420 rods. Cost to resize and fit new wrist pins plus new bolts will make new rods the way to go.
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