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1987 Donzi Classic 22 repower & rebuild

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1987 Donzi Classic 22 repower & rebuild

Old 09-19-2018, 01:07 PM
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Default 1987 Donzi Classic 22 repower & rebuild


This boat currently has a Mercruiser 7.4L engine with an alpha drive (not a typo). I have a two-stage repower plan and am interested in thoughts as well as improving the boat structure, larger fuel tank and more "car-like" interior. New gel and paint including a carbon-fiber hood.

Stage 1 - current - I'll pull the engine, complete stock rebuild minus a non-prod cam & intake. Update the Alpha to Bravo. Run that while I build the Real Deal...

Stage 2 - Real Deal - thinking I should go with an GM 8.1 truck engine. Is this smart, or should I stay with the 7.4 Merc? Either way, I'd like to line bore, cylinder bore, splay the mains, add lightweight pistons, high vol pan and pump and forged lower rotating components with a pair of aluminum dart heads, carb (dual quads?), cam, closed cooling. I think I can get to 600ish... and possibly a supercharger......I know I will need at least a Bravo XR, maybe more....

My goal is high-speed reliability. I like to run hard (4000-6000 erpm) more often that not and am thinking a blower might sub optimize the engines durability. Any tribal knowledge anyone can share?

Last edited by sandslot; 09-19-2018 at 01:09 PM.
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Old 09-19-2018, 02:04 PM
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Why not fuel injection?
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Old 09-19-2018, 02:19 PM
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Originally Posted by Gweedo View Post
Why not fuel injection?
HI Gweedo - its simple, robustness & reliability. I know automotive systems are reliable, but they have tons of calibrators and controls engineers working and testing them daily.....I don't see marine FI as being there. I know many have done it...and I have a ton of respect for them, but I am one guy and my son.....I just see it as an expensive nice to have!
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Old 09-19-2018, 02:53 PM
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Originally Posted by sandslot View Post
HI Gweedo - its simple, robustness & reliability. I know automotive systems are reliable, but they have tons of calibrators and controls engineers working and testing them daily.....I don't see marine FI as being there. I know many have done it...and I have a ton of respect for them, but I am one guy and my son.....I just see it as an expensive nice to have!
I dont think there is a carbed manifold available for the vortec 8.1.....have to go cu$tom.
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Old 09-19-2018, 03:52 PM
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Originally Posted by SS496 View Post
I dont think there is a carbed manifold available for the vortec 8.1.....have to go cu$tom.
Great point! That alone might exclude me from using an 8.1! I suppose I can get close with a 7.4L - 0.060"....
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Old 09-19-2018, 07:12 PM
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Originally Posted by SS496 View Post
I dont think there is a carbed manifold available for the vortec 8.1.....have to go cu$tom.
Dart has made one for a long time now. But,I have to ask why a Vortec 8,1L if modding anyway ?
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Old 09-19-2018, 09:10 PM
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My motivation for the 8.1 is I understand I can get more displacement from an 8.1.... could be wrong. I want to do a full roller valve train and not sure if I can do that on a 1987 7.4l? I may be misinformed
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Old 09-24-2018, 02:29 PM
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Its no problem to do a full hyd. valve train in the 7.4 just have to use tie bar lifters. there are a lot more performance parts for your 7.4 than the 8.1 by far. Regards Dan
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Old 09-24-2018, 06:13 PM
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502 based engine, XR upper, Imco -2" shorty.

See you on Maceday Lake.
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Old 12-18-2018, 08:43 PM
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Originally Posted by Gweedo View Post
Why not fuel injection?
ive thought about fuel injection but I think it will be expensive. Also not sure how many aftermarket systems are out there
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