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IMCO Lower Decisions

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Old 09-01-2017, 12:00 PM
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Talking IMCO Lower Decisions

Good morning. Back to drive decisions. Looking for advice on what lower to choose. The boat is a 2004 American Offshore 2600 NSX. The power is an Eddie Young 850SC. 547ci Merlin Block, 871 Roots, B&M intercooler, Brodix Aluminum 345cc heads, running 6lbs. of boost with 2 750cfm carbs. This engine makes every bit of that power and is turn key so downtime due to engine problems is minimal unless something breaks (which is usually drive related). We put approximately 50 hours of run time on the boat per year. We accumulated almost two seasons out of an IMCO SCX upper/rebuilt SC lower. The SC lower developed a stress fracture in the lower unit this month and is leaking gear lube. We purchased the boat 4 years ago with an IMCO raised 3" box, Bravo upper and SC lower. The boat has been dialed in with the IMCO 3" raised box, SCX upper/SC Std. length lower, and a 1.5" spacer. The drive has a 1.5 gear ratio and we're spinning a non-labbed 34 pitch B1 prop. The boat with this setup runs low 100's and pulls skiers etc. meaning this prop stays on the boat during all activities which is nice. Now's the time to think about going to the SCX lower and potentially a cleaver style prop on a number 6 prop shaft. Again the boat is used for generally boating, water sports etc and we'd like to keep the 100mph performance. Should we gamble on another SC lower for performance and chance reliability or gamble on the SCX lower for reliability and chance performance?
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Old 09-01-2017, 05:20 PM
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Drive hasn't treated you that badly. Doesn't sound terminal. I assume you have looked into replacing the lower case and repairing the internals? Repairing the case would probably be a bad idea as at least one dimension has changed when it cracked.
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Old 09-01-2017, 09:04 PM
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Your gonna loose speed with the SCX Lower , Why not a new SC Lower and atleast 4 more years?
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Old 09-01-2017, 10:04 PM
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How deep is your drive?
How far below the hull is your prop shaft?
Is it possible that your prop is too deep and your slip numbers are too low.
This is causing to much stress on the drive.
A little bit of slip is good for the drive.
If you go to an SCX you will want to raise the prop shaft height and run a larger diameter prop.
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Old 09-02-2017, 08:07 AM
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Quote:
Originally Posted by dereknkathy View Post
Drive hasn't treated you that badly. Doesn't sound terminal. I assume you have looked into replacing the lower case and repairing the internals? Repairing the case would probably be a bad idea as at least one dimension has changed when it cracked.
Yes would definitely replace the case if we go back to SC lower.
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Old 09-02-2017, 08:11 AM
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Quote:
Originally Posted by dunnitagain View Post
Your gonna loose speed with the SCX Lower , Why not a new SC Lower and atleast 4 more years?
I agree doesn't sound like a terrible idea. IMCO recommended tearing down and inspecting the SC lower after every season with this setup. That doesn't sound great.
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Old 09-02-2017, 08:22 AM
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Quote:
Originally Posted by Tinkerer View Post
How deep is your drive?
How far below the hull is your prop shaft?
Is it possible that your prop is too deep and your slip numbers are too low.
This is causing to much stress on the drive.
A little bit of slip is good for the drive.
If you go to an SCX you will want to raise the prop shaft height and run a larger diameter prop.
The prop shaft is even with the bottom of the center pod. The center pod on this hull is slightly deeper than the sponsons at the transom (tunnel hull).

Slip is in the double digits. We tried 3 different spacers and multiple props to dial in the X-dimension to where it currently sits with SC lower.

If we go to SCX we would remove the 1.5" spacer for starters (I think) and run a larger diameter prop like you suggest. The problem is the expense of the cleaver style props and availability. I'm a boater on a budget and don't want to try and reinvent the wheel here with this hull design. The hull was setup as a Bravo style boat from the builder. I'm not sure how a cleaver style prop will effect handling and performance.
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Old 09-02-2017, 08:42 AM
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Another idea is going back to a new SC lower and installing a Drive Guardian. Maybe that's the ultimate compromise.
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Old 10-25-2017, 11:44 AM
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With how well your setup performs currently, triple digits plus being able to pull tubes and skiers, inspecting the lower after every season sounds like a fair trade off to me.

If you do decide to go with one of the scx lowers, most likely the 1400, I'd get it up as high as possible, which would be 3.5" above the bottom of the center pod if I read your first post correctly? I haven't read of many or any scx lowers retaining the same speeds of an sc lower if they aren't raised above the bottom of the hull. Imco also makes a #6 propshaft adapter that could be used for testing #6 props as well if you couldn't get a bravo style prop to work out.

I'm going to have to try an sc lower on my outlaw next season because 800hp for lower 80s with the 1300scx lower just isn't cutting it if you ask me. My lower is 2.25" below which I believe leaves too much of the massive lower unit to drag.
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Old 10-25-2017, 01:16 PM
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Not sure how much a drive guardian will help in this situation as this is not a boat that sees air time. Where are your torque spikes going to come from that would cause the guardian to slip?
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