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Thread: 1971 Donzi Classic 18 power....
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01-16-2017, 06:35 AM #15000 RPM
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1971 Donzi Classic 18 power....
I am new to this forum, although I have been watching from afar - there is a bunch of talent here!
I hope to get some stronger technical input to my issue. I removed the H/M 351/Volvo 250 powertrain and will replace with a 5.7L SBC Mercruiser Bravo. My target is about 450ish hp. My top speed target (assuming a top end prop) is 70-75mph with about 6500erpm.
So the question I have is what's the best SBC to build for this application. I do plan to run on open water at speed (sustained 70-75 mpg, est 4500erpm) so I need something that will stay together under those conditions. I also plan to bring in fresh air from the front hull to feed the engine. I currently have a Bravo outdrive that I plan to (cut the hole) and position about 2 inches higher than nominal (for speed considerations).
Currently thinking forged pistons, shaft-mounted rockers, 0.030, cast iron dart heads.....but not really sure. I also prefer to have premium recommended not required. Thinking Holley or Edelbrock fuel injection. I've heard supercharging, but not sure this is right for my application.
Any thought?......and thank you in advance!Last edited by Classic18; 01-16-2017 at 08:39 AM.
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01-16-2017, 03:20 PM #2The Historic Photo Master
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I personally have owned a few 18's. Its a hull you really don't want to push fast. Why you ask? They are dangerous over say...65 or so mph. What!!?? What do you mean? Thats B.S., right? No, it's not. Because of the round V-bottom & small lifting strakes, they like to barrel-roll at higher speeds. Speeds that the 16's & 18's never went, back in the early days & they never experienced, until the late 70's, and owners started pushing them faster. I have multiple friends, that have either died in a 18 that barrel-rolled, or are wearing a diaper their whole life, because they broke their back & can't control they're bowels. I'm originally from your neck of the woods & this is where these accidents happened as well. Just a FYI. Thanks for reading this.
Last edited by FUJIMO; 01-16-2017 at 07:01 PM.
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01-16-2017, 03:44 PM #3The Historic Photo Master
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Now. With that important classic Donzi performance warning over, lets proceed to answering your post. Nice choice going with the Bravo 1, over the Alpha. Its the same cutout with either drive, by the way. Limiting X-demension's installed height is always flame arrestor(carb) height, exhaust riser height, and of course, transom height. The standard Bravo 1 can go to 17 1/4", and still get gearcase intake water. No higher. Don't know if you can get anywhere near that, with the 18's transom. But thats good. It will keep the speeds down a little. And plane off much easier, with any load. The exhaust risers will need to go under a larger & wider raised hatch cover, intake/carb/flame arrestor too. You sound like you want to go old school type small-block with this project, but will you consider a newer MerCruiser EFI motor, which started coming out in 1996? They offer the ability to use a much lower engine hatch & run excellent & are very dependable. Horsepower upgrades are not a problem. Knowing what I know nowadays, having built hundreds of new MerCruiser powered performance boats, and enjoy the old 18's as much as you do, I would simply price out a brand-new MerCruiser/Bravo "package" and install it. It goes in, in less that a day, and is warranteed, and extended warranty-able, and it runs like a raped-ape, and it always starts, and the throttle response is incredable, and it uses less gas, and it idles like nothing else, and, and, and... But thats just me. lol
Last edited by FUJIMO; 01-16-2017 at 07:03 PM.
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01-16-2017, 03:59 PM #4The Historic Photo Master
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Regarding having enough fresh air supply to the engine, it should not be a problem using the factory Donzi double reversed gunnel top scoops that the boats came with. Depending on what engine hatch you end up using, you can always install an additional one on the front side of the raised engine hatch, for more fresh air. Try to stay away from putting one on the rear side of the hatch if you can, as backdraft spray gets into them at speed. With the front air intake grill/screen/intake, the only negative is, that the cockpit passengers get alot more engine noise from it. A lot more from a carburated engine, that an EFI engine though.
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01-16-2017, 05:12 PM #55000 RPM
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Fujimo, thank you for the great thought starters! I did add some photos in the album if it helps....
Regarding the hull, I did notice when I redid the hull there was little directional guidance for the boat towards the back where its rounded, which is the part left in the water at higher speeds (aside from the outdrive). Barrel rolls seem inevitable with the right combination of waves. Seems like I could add something like a 24" fin, or glass something in? Have you known anyone to do something like that? Seems pretty risky.
Regarding the position of the outdrive, I think I'll back it down to 17.25" as you suggest. I have it marked for 18" now. I have not cut the hole yet after glassing it in from the Volvo.
As much as I like the idea of a package, I can't afford it. I have a 1991 Mercruiser 5.7 from a donor boat now and a used Bravo outdrive. Somehow I'll bring this together. It's really a father and son project....So, what I need to do is determine what content would be best for the motor to achieve my targets. Thinking through the speed issue, I'm not sure I would ever run it at 70-75mph, its the mid range torque that I'm after. Also at those speeds I should be near redline and don't want to be running there either.
I do want fuel injection, if I can afford it. Seems better overall for startability and transient performance. You also made some good points about the front air induction noise.....still contemplating whether or not to do it. Maybe my eyes are bigger than my wallet!
You mentioned building many marine motors, what would you do to strengthen the 1991 donor motor I have? I'm still targeting ~450hp and 6500 erpm even if I don't use it all....it's one of those things! It also depends where the "knee" is in the cost curve if I ever meet my 450hp target.Last edited by Classic18; 01-16-2017 at 05:29 PM.
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01-16-2017, 05:32 PM #6The Historic Photo Master
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450 h.p from a '91 5.7...? Can't help ya much on that. I don't know, other than better heads, cam with low or no overlap, so that you don't get any reversion, which is exhaust suction through the exhaust tailpipes, that allow water mist/vapor to enter the engine. This happens from the downstroke of the pistons, with the wrong cam, that has too much overlap, at idle speeds, on a marine application. It can ruin a marine engine in short order. Reasearch better marine exhaust systems, because I'm pretty sure the stock MerCruiser exhaust manifolds/risers are not going to support 450 horsepower. Tailpipes or exhaust tips definately need flaps(internal and/or external) to help with backwash & reversion. I've gotten into the newer GM LS motor stuff in the last 15 years, and left the old generation engine stuff waaaaay behind.
Last edited by FUJIMO; 01-17-2017 at 12:30 PM.
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01-16-2017, 06:05 PM #7
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01-16-2017, 08:18 PM #85000 RPM
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01-16-2017, 08:30 PM #9
I've done a bit of business with Tyler in the past. Nice guy. That kind of reliable HP out of a small block isn't easy. Give him a call and tell him what you want to do. He may have some or know of some used dart or bowtie blocks to use. GM production blocks especially of that era are a poor choice for that kind of sustained power level in a marine application. 350-375 yeah they'll work. A 383 out of some nice parts will get you where you want to be without having to do it twice or more.
Another guys here is Saginaw builds some nice pieces too, in house dyno etc, and knows how to make it live in it's application.
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01-17-2017, 12:10 PM #10The Historic Photo Master
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Like the Captain said above, not a good choice of engines for 450 horsepower, in a "pleasure" boat anyway. If you are using a "standard" Bravo One drive, I would not go any higher than a 17'' X-dimension on your boat. The low water pickup Bravo's can, and do, go much higher...not yours. I'm not an engine builder, but have installed, tested, & raced hundreds of new MerCruiser/Mercury Race engine packages. Hundreds of new outboards as well. All big power, nothing small. If you are insistent on no less than 450 horsepower, I would consider a different engine base. Just my two cents.
Last edited by FUJIMO; 01-17-2017 at 12:27 PM.
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01-17-2017, 12:21 PM #115000 RPM
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01-17-2017, 12:37 PM #12Screaming And Flying!
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Put some GMPP "fast burn" heads on that 5.7 and you will get close to 450. But as said, the reversion will have to be dealt with....With small blocks, heads/compression are the numero uno thing to choose correctly.
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We gotta clean this liberal mess up, VOTE TRUMP TO MAGA!
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01-17-2017, 12:49 PM #13The Historic Photo Master
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Should be o.k. as long as your not talking about using the old original bronze "grilled" thru-hull pick-up that came on those old 18's, and as I remember, were located way forward of the transom. That will not work, for a couple different reasons. One, the slipstream is disturbed entirely too much, by that old type/location thru-hull, for a Bravo that is mounted as high as you want to go. It will disturb the flow of water too much, to the gearcase, and the propeller. Two, you need to use/install a sea-strainer. Three, the old factory Donzi location of the thru-hull will very possibly be sucking air, at higher speeds. Four, that old-style type of water pickup is creating drag. Transom mount pickup with a strainer. Or a transom mount pickup that has a course screened bottom. Many to choose from out there. Your pushing that old hull design faster & things change with regards to rigging concerns.
Last edited by FUJIMO; 01-17-2017 at 12:54 PM.
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01-17-2017, 01:00 PM #14The Historic Photo Master
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You think an old school high horsepower 2-bolt main small block will take jumping in & out of the water, the way those old Donzi 18's do? I don't know. Agree with the heads/compression thought. But he will have to spend some money. No matter how you choose to attain a "dependable" 450 h.p., you will be spending some money.
Last edited by FUJIMO; 01-17-2017 at 01:06 PM.
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01-17-2017, 01:17 PM #155000 RPM
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Thank you for the feedback....it is all very helpful. I'm feeling some "closing in", but more to go. I like the idea of the fast-burn heads. As I understand this SP383 has them......maybe a better option for me (if I can afford it)! Here's a link.
http://www.chevrolet.com/performance...p383.html#dyno
I currently don't have a raw water pickup in the hull, it was tied to the Volvo outdrive. I'll look for the transom-mounted style....
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