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Extension boxes and blackhawks
I am doing basically a CT525 conversion on a Scarab P29. I have already pulled a bunch of weight out of the boat last year (with the original 5.7’s) and the boat ran too flat. Going to the aluminum aluminum engines it will be worse. I am considering Imco extension boxes. My question is what height should I go. I am thinking +3. My current x dimension is 16.5”. Thinking +1 for the 12” setback and going a little higher. I think the drive is a little low and forcing the bow down. |
I can not recall Blackhawks on extension boxes and I am even currently working on boats that were set up with blackhawks.
On the GM crate dirt car engine CT 525 will need some internal parts changed out for the marine set up. If I am correct the same CT 525 GM engine here as you posted |
Basically ct 525. I got 2 19329865 crate long blocks, Howard’s ASA cams, with the correct springs, new valves, refitted the pistons and ring gap, sheet metal intakes, and a few other things. Not worried about marinizing the engines I know what to do there. Just not real clear on drive height and my thinking on it. I know the blackhawks work good with a rocker in the bottom (my boat was already built for that) and world good with notches (the drive extension boxes take care of that). |
FYI, when drives are deeper, they give more leverage and will lift the bow easier. A drive being deep does not force the bow down.
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Was this boat originally equipped with blackhawks? |
Normally I would agree, but blackhawks ride on top of the water. Just experience with outboards that propshadts even to 1” above the bottom can cause the lower unit body to push the bow down. Like a trim tab.
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It’s a scarab p29. All p29’s came with blackhawks. |
The P29 bottom and balance was designed for the blackhawks. They all were mounted WITHOUT extension boxes from the factory build. Larry Smith redesigned the P 29 bottoms for the blackhawks . I have worked on 2 different P29s in the past and another one 1 last year.
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Originally Posted by BUP
(Post 4659791)
The P29 bottom and balance was designed for the blackhawks. They all were mounted WITHOUT extension boxes from the factory build. Larry Smith redesigned the P 29 bottoms for the blackhawks . I have worked on 2 different P29s in the past and another one 1 last year.
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If you don’t go +3, you’ll probably get the stern lift from the gearcases being too deep as you described earlier. Marc @ BAM did a ITS with a zieger extension box( to bring it back a foot) on a 20 Cig, maybe he’ll chime in |
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