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View Full Version : # 13 Donzi MINX Build!!! Please Help



biggie
01-14-2013, 03:03 AM
Okay guys, I have hours and hours and hours of forum time and I seem to be going in circles now. I just bought my first boat (against my dads wishes..."son never buy a boat, make friends w people that own boats. Break out another thousand" ) It seems to be in pretty good shape but its an old boat so Im restoring everything. Right now Im stuck on the power plant. As you all know it has a stock 350 Mag in it w/ a base WOT speed of around 60ish. Well my desired results are 90z, Im building my own motor and have the time and money to do it. I don't wanna spend 20k on it but I want a "wow" motor that'll give me the results Im after. I was thinking a blown 383 stroker to save on weight and space but I dont really know which direction to go for the best "bang for my buck". Could I please get some specific build combos so I can weigh my options based on the best advise and start buying parts. Thanks in advance and Ill post timeline pics of my build as it progresses.

katanna
01-14-2013, 08:08 AM
How reliable do you want it to be? A small block pushing that boat to 90 wont last too long. To get there you will be using up your 20k in engine alone. Now you have to address the steering, thats another 3500. The drive wont hold up so you will be switching to bravo set up I assume, at least another 4.

Good luck-post some pics, I have an 88 Minx

dsparis
01-14-2013, 08:21 AM
Bravo won't hold up either.

VetteLT193
01-14-2013, 08:53 AM
If you really want 90's start by pre-planning everything and add it all up.

My gut says the Minx will run best setup like the Cig 20's. They tend to run flatter Vs. the 22 which really likes to run bow high.

A Big block will fit in the minx, there are a couple of them out there.

The short list (most already mentioned)
-New Drive, probably Bravo XR or better
-New Transom assembly
-Hydraulic Steering
-X dimension change? Going for a higher X to get more speed? fill in the transom and re-cut
-Beef up stringers (the later Minxes had a different setup from the factory that was beefed up, so at least that ++)
-Props
-Exhaust
-Optional... Extension box?

An educated guess puts the Minx in the 80's with a Merc racing 525 + XR drive + hydraulic steering + raised x or shorty or extension box with raised X. Substitute a blackhawk (with XR gears) + box + raised x instead of the XR drive and you might get upper 80's on a good day.

Consistent 90's you'll probably need 650 HP. In order to get there with a small block reliably you probably need a new block like one of the bigger inch motown blocks.

ralphvc
01-14-2013, 09:15 AM
I am 3 years into Donzi's and originally wanted to do same to an 18. The cost to get that fast is extreme plus the boats aren't built for it. How much time do you have in a 60 MPH boat???? That is fast. Suggest you pull the 350 go with a mild 383 stroker or if starting from scratch go with a 406. I think the Alpha will hold if you don't hammer on it plus not costly to replace.First crawl then walk then run if you must. Have fun and be SAFE. Again 60 with the wind in your face and a light chop is very fun. Good luck.

VetteLT193
01-14-2013, 09:28 AM
I posted a detailed reply but it got lost I guess.

Based on an educated guess: Bottom line 90's in a Minx you'll need 650-ish HP.

The Alpha won't last.

You are looking into at least a Bravo XR, plus transom assembly and either a shorty or extension box + X dim. increase. Props, hydraulic steering, beef up the stringers, etc. $$$$$$.

My gut says the Minx will run like a 20 cig with power (i.e. run pretty flat at speed) so take a look at them to see what it takes to make them move. Most run raised X, blackhawk drives with XR gearsets, and have big power Big Blocks.

With a Bravo XR, shorty lower, hydraulic steering, Merc 525 or similar, and props you should be in the 80's. Emphasis on 'should' because no one has really gone this route before... at least no one has and documented it for others to learn from.

olredalert
01-14-2013, 09:45 AM
----Saying you want to go 90 in a Minx is a bit like an obese old guy saying he wants to diet back down to high school weight. You would better serve yourself if you go in steps. If the boat goes a true 60 now think 70-75. Thats realistic, but still wont be cheap to do right. You can think about higher speeds after you have hit the first plateau. Remember that steering comment above. I have two 383s built for my Spitfire more for durability that dynoed at 416hp, but the really important figure is the torque and both these motors pulled 460 ft/lb! Torque is what really moves your boat, not HP.
----As well, you would be over-stressing the Minx hull even at 75 without some extra strengthening of the hull in the right places. Any Donzi you see mentioned on this sight that actually really goes 90 has a bunch of extra fiberglass work done, unless the owner has a death wish!!!.......Bill S

VetteLT193
01-14-2013, 09:54 AM
----Saying you want to go 90 in a Minx is a bit like an obese old guy saying he wants to diet back down to high school weight. You would better serve yourself if you go in steps. If the boat goes a true 60 now think 70-75. Thats realistic, but still wont be cheap to do right. You can think about higher speeds after you have hit the first plateau. Remember that steering comment above. I have two 383s built for my Spitfire more for durability that dynoed at 416hp, but the really important figure is the torque and both these motors pulled 460 ft/lb! Torque is what really moves your boat, not HP.
----As well, you would be over-stressing the Minx hull even at 75 without some extra strengthening of the hull in the right places. Any Donzi you see mentioned on this sight that actually really goes 90 has a bunch of extra fiberglass work done, unless the owner has a death wish!!!.......Bill S

I agree and disagree at the same time. From a perspective of seat time, etc. your thoughts are well understood. However, from a value perspective it's a whole lot cheaper and easier to pick a goal and go after it one time Vs. going after it chunk by chunk. I'd wager that every guy running 90 that did it in steps could have saved 10 grand by doing it all at once in the beginning.

BUIZILLA
01-14-2013, 10:03 AM
while I always love someone's admiration to undertake a project, and Lord knows i've done a few of those :cartman:

nobody has ever gone 90 in a Minx... :eek:

or even 75 for that matter

much less 80ish

you'll spend 40k + attempting on doing it, and wish you bought the ebay 22 that does run a legit 90 on a good day...

it's free to dream it, but it's purty darn expensive to do it :smile:

gcarter
01-14-2013, 11:00 AM
Yep!
I had $25K+ in my Minx that'd do 65 on a good day, not that going faster was a goal.

I can think of 15 or 20 reasons why a Minx would be the wrong boat to start with.
Not the least of which is that the hull layup is WAY too light.

ralphcrocker
01-14-2013, 12:19 PM
Hello olredalert. Although ok at drag car motors I am embarking on building my first marine motor. I would like to build a 383 and your outcome sounds dead on. if you would be willing to share specifics on your 383 please call Ralph at 248-214-9285 or email ralph@nutechgraphics.com. Thanks in advance.

HallJ
01-14-2013, 05:43 PM
Pull the deck and replace the stringers first. The layup and workmanship is poor at best.

Jeff

biggie
01-14-2013, 05:59 PM
Thanks for the advise, Im aware that the Alpha wont hold up and the steering needs to be upgraded and I am addressing those issues as well Im just specifically stuck on my power plant issues. Okay maybe 90's is a dream but 80 for sure isnt. I don't want to build in stages bc then Ill have to spend more money after Im bored w that stage. I have a 2004 Kawasaki zx10r on nitrous dino'ing @ 220 mph but I don't ride that fast daily. I want to build a "good" RELIABLE engine that'll give me the results for when Im ready for them. So a 383 wont last, what does everyone suggest? I saw 22 Shelby w a v10 that ran around there. Im just wondering what motor I should build to achieve that and what the suggested way about doing it is. If a built bbc is gonna give me that then thats what Ill do but if I can do a reliable sbc....this is my issue lol someone save me!

mattyboy
01-14-2013, 06:05 PM
it should be noted the only 2 similarities to the Minx and the 20 cig is the LOA and the beam . the cig is 800 + lbs heavier, the bilge is designed for a Big Block and the newer ones have a pad. even with that said the general rules of thumb that apply to making the 22c faster hold true for the minx. make it stronger both hull and motor, and raise the X

joseph m. hahnl
01-14-2013, 07:16 PM
Thanks for the advise, Im aware that the Alpha wont hold up and the steering needs to be upgraded and I am addressing those issues as well Im just specifically stuck on my power plant issues. Okay maybe 90's is a dream but 80 for sure isnt. I don't want to build in stages bc then Ill have to spend more money after Im bored w that stage. I have a 2004 Kawasaki zx10r on nitrous dino'ing @ 220 mph but I don't ride that fast daily. I want to build a "good" RELIABLE engine that'll give me the results for when Im ready for them. So a 383 wont last, what does everyone suggest? I saw 22 Shelby w a v10 that ran around there. Im just wondering what motor I should build to achieve that and what the suggested way about doing it is. If a built bbc is gonna give me that then thats what Ill do but if I can do a reliable sbc....this is my issue lol someone save me!

http://ts1.mm.bing.net/th?id=H.4605094845547700&pid=1.7&w=244&h=154&c=7&rs=1http://ts4.mm.bing.net/th?id=H.5011729458465447&pid=1.7&w=203&h=154&c=7&rs=1

Pat McPherson
01-14-2013, 07:56 PM
My 87 20' Minx was not built nearly as strong as my 75 18' Classic. Good looking boat but not the strongest stringers so start with strengthing those. You'll want an offshore front mount. I added one to mine after repairing/strengthening cracked stringers and didn't break the boat again.
I ran a 350/350 in my boat and she hit 70. I think 450 would get you close to 80 and 600 to 90 with the right setup. If your going more than 450HP, I'd go Big Block. The balance will be thrown off a bit but you can use alum heads and add weight up front.
Swap the drive for a Bravo I style with a short lower and add twin ram full hydraulic steering.
Plan on spending 30K in parts alone for your project and keep in mind you'll never recover even close to what you invest when the time comes to sell.

gcarter
01-14-2013, 09:18 PM
The other item no one has mentioned is that the engine compartment on a Minx is somewhere around 6"-10" SHORTER than a 22C. The hatch size is similar to an 18C. A SBC has to come out sideways. So, while a BBC will fit (and I even saw one brochure that listed a 454 as an alternative engine) it's a tight fit.
Anyway, really long engines would probably require removing part of the back seat.

PS, Like Matty said, the 20 Cig (at least the later ones) were designed for a BBC w/a lot of muscle, while a Minx is a very nice performance cruiser that's very comfortable and dry.

smidgen too
01-14-2013, 09:52 PM
[Pull the deck and replace the stringers first. The layup and workmanship is poor at best.

Jeff ]

I would take what Jeff said to heart, he rebuilt his Minx last year. What he found out about the way there built will scare you. Jeff's has a nice small block with a Arneson drive, speed ?? :beer:

biggie
01-14-2013, 11:37 PM
I've had a serious reality check today so thanks for all seasoned help. I think w/ everything being said Ill pull the 350, strengthen it up a bit and drop in a decently built 383 and be happy w the results. Ill make it pretty and trade up later for a faster boat if 75 isnt fast enough. One more question, if I am staying under 500 hp is it really necessary to pull the deck and redo everything? That sound costly?

smidgen too
01-15-2013, 01:03 AM
After I saw how wet the wood was in Jeff's and the way the stringers were set in place I would . Jeff does not run his hard but when he & I were on Lake St Clair this past summer it was nice to know this Donzi would make it back.

Pat McPherson
01-15-2013, 10:11 AM
I think you can get some good direction from this sight on how to pull the deck off and strengthen the stringers. Remember you're working with a 25+ year old boat. If you're going to add big power, you need to fix those stringers first.
A 400+HP stroker smallblock, good used B1 drive, and new B1 gimble is the route I'd recommend. I think you can easily do the glass work and repower for the 20K number you spoke of earlier.

MOP
01-15-2013, 12:01 PM
Everyone always recommends a switch to a Bravo, which I did! It will cost you about 3mph, the gear case has a larger profile it also take more HP to twist a Bravo. The cost of the swap will cost you about the same as buying 3-4 SEI drives that come with a 3 year warranty. You will only get a 1 year on a brand new Bravo package and nothing on a used setup. Think that one over real careful I consider it one of the most costly dumb mistakes I made when I did my 22 over, cost me near 8K. By the way my old Alpha has survived 5 years so far on my kids 27 Searay which gets a ton of hours fishing every season.

VetteLT193
01-15-2013, 02:17 PM
Everyone always recommends a switch to a Bravo, which I did! It will cost you about 3mph, the gear case has a larger profile it also take more HP to twist a Bravo. The cost of the swap will cost you about the same as buying 3-4 SEI drives that come with a 3 year warranty. You will only get a 1 year on a brand new Bravo package and nothing on a used setup. Think that one over real careful I consider it one of the most costly dumb mistakes I made when I did my 22 over, cost me near 8K. By the way my old Alpha has survived 5 years so far on my kids 27 Searay which gets a ton of hours fishing every season.

I agree if 75 is the goal. over that you'll have to carry a spare. :banghead: Plus, if the real goal is 80+ and you'll end up with a Bravo... steering, etc is all easier and cheaper to do once.

MOP
01-15-2013, 03:54 PM
THe beauty is blow your Alpha slap an SEI on and then you have three years for about $1,300 beats the heck out of the cost of the Bravo conversion and loss of speed.

MOP
01-15-2013, 04:19 PM
I've had a serious reality check today so thanks for all seasoned help. I think w/ everything being said Ill pull the 350, strengthen it up a bit and drop in a decently built 383 and be happy w the results. Ill make it pretty and trade up later for a faster boat if 75 isnt fast enough. One more question, if I am staying under 500 hp is it really necessary to pull the deck and redo everything? That sound costly?

75 is a stretch! You would need to build a very hefty 383 to get too or near that number, I have a 383 in my 22 it has a full Lunati Pro Mod kit pushing 400+ it does 65.5. the previous engine was a 300hp Tempest which ran 61+ spending a tad over 5k got me 4+mph, don't set your hopes to high.

biggie
01-15-2013, 05:19 PM
THe beauty is blow your Alpha slap an SEI on and then you have three years for about $1,300 beats the heck out of the cost of the Bravo conversion and loss of speed.


I agree, whats the difference from that and an alpha 1? and what gear ratio do u recommend? After having my bubble busted several times over yesterday I think Im going to buy a SEI and build a tough blown 383 and be happy w my results and later down the road buy a faster boat. I appreciate all u guys chipping in w the info. I think its safe to say that the Donzi community or boating community in general is closer than my military family or even my skydiving family. Im glad to be a part of it.

Ghost
01-15-2013, 07:39 PM
That's good for 3 years if it's under warranty. A blown 383--that'll be way above the SEI-warranted 300hp, no? Sounds like that might risk just losing the $1500. I'd bite the bullet and buy a drive actually rated for that load. FWIW. And sell the Alpha now, before grenading it. ;)

Pat McPherson
01-15-2013, 08:27 PM
I've heard lots of stories of Alphas running for years hooked to 400HP smallblocks but I'd make the switch if your going to build a high torque motor. A good used B1 drive can be had for $1500 and new leftovers are about $3200. The gimble housing will run you $1500 for a reman. The transome cutout is the same you just drill 2 more holes. I believe the trim pump and power steering are the same.
A Stock B1 with last with 500HP and 500ftlbs for years.

osur866
01-15-2013, 09:30 PM
My .02, make the boat safe for added HP, that would include the stringers, and steering, I'd think 450-500 HP would be ok In front of a Alpha if driven with respect, I'd think a mild 383 with low boost in the 3-4 psi range would get ya in the mid to high 400 HP range and possibly into the mid to high 70's. Or like Buzz said buy the 22 that's done and runs 90. If your true goal in 90 it's gonna cost way more than your 20k budget, ask me how I know, and will take a great deal of time to not only to dial it in but also learn how to drive it to your 90 goal. Make the boat safe then add power

biggie
01-15-2013, 10:48 PM
I saw a youtube video of a guy with something like a 625ci in front of an alpha that he claims has lasted for years. From what I understand its the torque that kills those things and if I "take care of the alpha" while I save for a bravo it'll last. Not to mention Ill retain like 2-4 mph by keeping the alpha. Is there a way of beefing up the alpha? What gear ratio does everyone recommend for a 383? Ill build my 383 and run it alone for a bit while I save for the 5k whipple or even a vortec. This is a staged engine build and I wanna do it in stages to where I can utilize the boat while I do the project. I think after the engine is built ( without the supercharger) Im going to take the old 350 Mag out and at that time inspect and strengthen the stringers and the hull...is that a decent idea?

younger
01-15-2013, 11:06 PM
If your goal is to make the Minx fly by all means go for it. You will need big hp and a drive train to support that. What you would spend on a blown or forced induction sb you could build a monster of a stroked sbc in the 415-434 cubes. That will produce some serious tq and have good dock manners. I'm all for boats that where never intended to run fast get pushed past the treshold. First and foremost make it safe!

younger
01-15-2013, 11:25 PM
If your goal is to make the Minx fly by all means go for it. You will need big hp and a drive train to support that. What you would spend on a blown or forced induction sb you could build a monster of a stroked sbc in the 415-434 cubes. That will produce some serious tq and have good dock manners. I'm all for boats that where never intended to run fast get pushed past the treshold. First and foremost make it safe!

biggiefl
01-18-2013, 12:05 PM
1) I had a stock Alpha(1987 w/1:32 gears) bolted to a 400+ BBC and it lasted for almost 20 years never being apart on a boat weighing much more than a minx. The 1:50 on yours probably won't last as long. I would keep the alpha, just watch mashing it out of the hole and make sure you are off the gas when you catch air or she will(and engine) grenade quicker than you can blink.

2) First boat and you want to go 90 but will settle for 75? Better spend $15k on the rebuild and $5k on a term life insurance plan so you don't dissappoint your father by making him pay for the funeral. In time you will realize, if you live that long, that he is right. A stock Minx in my 1986 Boating mag ran mid 50's with a 260. A 383 might get you upper 60's which is plenty as a newbie....you'll see.

3) Boats are not bikes or cars. 90 in a minx would be like doing 220 on a bike...with waves.

BUIZILLA
01-18-2013, 12:13 PM
1) Boats are not bikes or cars. 90 in a minx would be like doing 220 on a bike...with waves. word..

RBT
05-19-2013, 09:19 PM
I have a "restored" minx that I found on here a year ago. It needs to be restored properly, I may do it this winter.

That aside, it has a bravo drive and a 350 mpi motor in it. Ran it today and the best I could manage was 57.1 with two aboard and 3/4 tank of fuel. It is a major work in progress. 23 merc cleaver at 4500 rpm.
Too much prop.

years ago, there was a minx that was restored by hot knots up here and the put a bravo drive and 454 mag in it. (365 hp) this was before GPS, but I went for a ride and recall it turned a 27 mirage to 5000, assuming it was 1.50 gears that is around 75ish mph.

I am considering removing my 350 and adding a 502 mpi that a friend has, and a shorty drive.

Just Say N20
05-19-2013, 10:03 PM
It seems to me that a strong SBC, like a 383 would be a better solution as the Minx likes a lighter back end.

You could get a 400 - 450 horse small block that is very dependable. My engine is a 400 SBC; 430 hp @ 5300 and 490 ft lbs @ 4300. At 3000, it already has 458 ft lbs of torque. That would be more than enough to get a Minx moving.

RBT
05-20-2013, 09:40 AM
I understand your thinking. But I really want efi, I hate carbs.
There won't be a huge weight difference with aluminum exhaust over a stock 350.
I have been looking on this sight and can't seen to find much info on minx's

i do not think the bravo helped this boat, as what I can find is mine is slower, yet more powerful than stock.

gcarter
05-20-2013, 10:15 AM
Here's a thread about my Minx rebuild. Just a note, the stringers are a problem, and while I made some corrections, I never intended to install 400 or more HP, so the reinforcements I made aren't as heavy as the work I did on the TR project.
Also if you do a thread search on my screen name, you can find several more Minx threads.

http://www.donzi.net/forums/showthread.php?36703-Break-s-over!

RBT
05-20-2013, 03:45 PM
I just ran it again, 59.2 by myself 1/4 tank of fuel.
the bottom does have a hook in it. I have it on my lift in the boathouse and lying on the dock at the bow and looking aft if is noticeable, more so on the passenger side.
I am curious as to how fast a stock from the factory minx went, and what have some of the other small block minx's have and or do run.

thanks

rt

gcarter
05-20-2013, 08:19 PM
60-65 MPH is about average.

RBT
05-23-2013, 04:10 PM
60-65 MPH is about average.

Yike, I am way short. It is 4 mph faster with this cleaver than it is with the 24" turbo it came with.

gcarter
05-23-2013, 06:42 PM
W/a 260 HP SBC, or a 320 HP more contemporary SBC, the range will be 55-65 MPH.
An Alpha might be a bit faster as it has a smaller profile.

joseph m. hahnl
05-23-2013, 08:07 PM
I just ran it again, 59.2 by myself 1/4 tank of fuel.
the bottom does have a hook in it. I have it on my lift in the boathouse and lying on the dock at the bow and looking aft if is noticeable, more so on the passenger side.
I am curious as to how fast a stock from the factory minx went, and what have some of the other small block minx's have and or do run.

thanks

rt
I'm still breaking the 400 in .But I have brought it up a tad , in short burst and hit 65 with the 21p Mirage. My Former Mag 350 @270Hp rating, averaged 58mph in the heat and 60in the cold. with the 400 it only takes a 1/4 throttle to get on plane where as the Mag was pretty much 100% throttle.

CHACHI
05-24-2013, 07:01 AM
Stock 350 MAG, 270 HP, Alpha w/21 cleaver. Best ever 61 MPH. My 3 seat Sea Doo would slowly walk it.

Even with a S/S lower, no difference in speed.

Ken

RBT
05-24-2013, 07:20 PM
Great info guys, keep it coming. I need to try and find some props to try.