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View Full Version : anyone have any info on 18' donzi big block boats



throttlemanro
05-22-2014, 09:43 PM
starting on a 18' donzi project / restore , was a volvo boat (1985), looking to install aluminum head big block with bravo xr , any thoughts on the hull speed ? how do they run over 70 mph ? would hate to get her in the water and have handling problems . opinions are welcome also .

mattyboy
05-22-2014, 09:48 PM
they did make a few BB factory boats

I think the formula for a real fast 18 is a high HP SB and a shorty or surface drive due to weight and room


https://www.youtube.com/watch?v=1jKhIXJ0fME

osur866
05-22-2014, 10:39 PM
My experience with my 18 was the more weight I took out of the back of the boat the better it handeled, I'm running right at 600 HP, Bravo 1 with -2 shorty. If you have plans to run big HP invest in a good steering system full hyd.

Steve

Tidbart
05-23-2014, 09:37 AM
There is not enough room in the engine compartment of an 18 to work on a small block.:wrench::mad: Why anyone would want to put a big block in one, I don't know.

Lose the weight, as osur866 said. I think, in the long run, you will be happier.

B

Greg Guimond
05-23-2014, 10:10 AM
"Just Say No" ........... to fat bottom big blocks in an 18 :wrench:

osur866
05-23-2014, 11:45 AM
There is not enough room in the engine compartment of an 18 to work on a small block.:wrench::mad: Why anyone would want to put a big block in one, I don't know.

Lose the weight, as osur866 said. I think, in the long run, you will be happier.

B

Bob I heard that!! Having this conversation later today with another board member on just doing simple things like impellers and plugs and greezing the coupler can be challenging when it's a SB I couldn't imagine what fun I'd be with a BB.

mattyboy
05-23-2014, 12:00 PM
what's in a name

the BB 18 I am thinking of was called "genuine trouble" or "genuine risk " If I recall correctly

gcarter
05-23-2014, 03:36 PM
I understand that "Genuine Risk, or, Trouble" had a habit of swamping the rear seat passengers when it came off plane.
Probably not a good decision.

I wish I could have afforded a 454 LSX when I was in the engine stage of the 22 TR. I still believe that's the way to go for our boats.

wwahl
05-23-2014, 04:51 PM
These are some comments from the person I bought my 18' Donzi from:

", I decided to change the engine to a 454 and installed a Mercruiser 420 (454 CID, 420 HP) which was the largest at the time. This is a high-performance Mercruiser engine that came with a 4-bolt main block, roller rockers, aluminum intake, and a Holley 850 CFM carburetor. I did head work and installed a slightly larger camshaft. I also installed an Alpha SS lower unit, Kiekhaeffer trim tabs, Gil exhaust, and Latham steering with a full hydraulic helm. All the work was done professionally with the use of correct Mercruiser parts for the engine conversion. The engine with the Gil exhaust, head work, and cam change is conservatively 450 HP now. With the engine change the boat runs in the mid 80s with a 28" pitch prop at approximately 5600 RPM. You can cruise at 60 MPH at 4000 RPM, then hit the throttle and the acceleration to 80 MPH pins you back in the seat. It pushed the Nordskog speedometer to 88 MPH on a brisk fall morning with a low tank of fuel."

I personally have observed 84 MPH on my GPS before backing off.

Lenny
05-27-2014, 02:17 AM
... nice ... great reply and inspiring

Thanks

RedDog
06-01-2014, 08:59 PM
I had the 18 George refers to. 454 King Cobra drive with approx 420 HP. Yes it set low in the rear and if you chopped the throttle water coming over the stern was a concern but it never happened. The acceleration was incredible. Top speed was about 73 gps. It wasn't factory. Blackie on the board has it now