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PaulO
08-22-2000, 04:50 PM
The replacement 383 is in and running!!
Sounds good and runs pretty good. At least it has stayed together so far. I need to try some props and re-jet the carb to see where we are in regards to performance. Not getting anywhere near the RPMs I had anticipated and am hoping that is due to the carb problems and the fact that the borrowed prop is questionable. May get a chance to re-jet tonight. Will keep anyone who is intersted posted.
PaulO

Hotboat
08-22-2000, 06:49 PM
Please keep us posted. We are all interested.

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Hotboat

GEOO
08-22-2000, 08:36 PM
PaulO, If you have the prop you ran with the old engine try that and you can compare the power from engine to engine. Then add pitch as necessary. I'm sorry I repitched my prop this winter. It's better to make only one change to the boat at a time then note the change. GEOO

Riley
08-22-2000, 10:20 PM
Hey PaulO, glad to hear your new mill is up and running. What rpm's are you turning right now? I have access to quite a few props, let me know if I can help. Please keep us informed of your progress....Chris Riley.

seano
08-23-2000, 06:19 AM
PaulO,

Sounds great man...I've got a set of 24" Bravo I's although you may need something BIGGER! I'll be in Baltimore this weekend, so you're welcome to try the prop if needed, let me know.

Jamesbon
08-23-2000, 08:44 AM
Paulo,
What's you timing set at?

Nate

PaulO
08-23-2000, 09:45 AM
Re-jetted ( re-rodded really) the secondaries on my Q-Jet last night. Only had time for one change and it made a fairly big difference. Went to CK rods and lost most of the bog when I opened up the secondaries and gained 3-400 RPMS. Still much lower than I expected so, I am still looking. The timing is set a 33 degrees total and I am only hitting 4700 RPMS tops. I need to get it to 5200 or so to get the HP out of this thing. I originally had a Mirage 21 which I figured would be too little pitch so I temporarily exchanged it for a 23 Mirage re-pitched (supposedly) to a 24. I really expected this motor to spin a 25 to 5200. The prop is old and looks a little funky although it has the stamp of a reputable prop shop on it. Changed tachs to be certain and will try another secondary rod size next and then would like to try some other props.
Seano,
Dont't know much about the Bravo1 props but, if it is a contender, I would like to try it. Have an offer to try a 22 cleaver also and wouldn't mind seeing what that does although common consensus is that the Mirage is the prop for these setups.
Someone on this board recently posted that they were running a 25 Mirage on their late model 18 with a 350 Mag (300HP) and spinning it to 4700.
Would have expected this motor to do considerably better but, I guess I am jumping ahead. Need to cover the basics first.
PaulO

RickSE
08-23-2000, 10:30 AM
PaulO,
Do you remember what RPM's and speeds you had before with your original engine and the 21 Mirage. I'm trying to figure out what prop to try next, either a Laser-II or a Mirage+. I'm assuming you had a stock 350 Mag before?

PaulO
08-23-2000, 11:29 AM
RickS,
I had a 260 Mercruiser Alpha with a Performer RPM and a mild cam. The engine was probably a little tired though. I was hitting 4700 RPMs with the 21 Mirage plus and am not sure about the speed. My damn handheld GPS was too slow to respond and too much of a handfull at high speed. I did confirm that my speedo numbers were about 4 MPH shy of my actual speed in the 45-50 MPH range and I could pin my 60 MPH speedo at WOT. I am thinking low 60s.
PaulO

GeneD
08-23-2000, 10:38 PM
PaulO,
Once again, I am here to tell all that it just never ends!
It's cool though. We'll get that bad boy Donzi moving out in no time.
1. Most Q-Jets are 450 CFM. Not only will you have to change out the rods, but the mains need to be changed to.
Think about this, how about a Q-Jet from a big block? The engine is going to take what it needs from the BB carb and leave the rest. With the SB carb, it just isn't breathing right.
BTW, I was pulling 4700-4800 RPM's with my regular 350 Merc using the 25 Mirage, pegging the speedo @ 60 MPH+. But I am geared special. Using the stock Alpha, with the 1.50 ratio, I would think that you want a 23 Mirage. Stay away from the cleaver. Or at least don't spend any money on one, borrow it and get the experience out of it. Old technology.
A chopper is nice, maybe a 22?
Good luck, remember, you are our newest guinea pig for this type of thing.
Be an example or us all!
Because...
We're all counting on you!

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GeneD
007
Melbourne, Florida

RickR
08-24-2000, 08:47 AM
Sorry Gene I have never heard of a 450 CFM QJet? Typo?
They come in a 750 and a 800
Primary venturies for the 800 are 1 7/32"
750 are 1 3/32"

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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

Ranman
08-24-2000, 09:13 AM
PaulO, Good memory. I'm running a 97 18' with the MAG 350 MPI (300HP) with a 1.65 Bravo and a 25p Mirage Plus. The boat turns 4900 rpm at 67.8 mph.

PaulO
08-24-2000, 11:48 AM
Ranman,
Ah Hah! it is the 1.65 ratio that makes the difference! That mystery is solved.
PaulO

PaulO
08-24-2000, 11:55 AM
Ranman,
Trying to calculate slip. Is that speed GPS, Radar, or speedo?
PaulO

seano
08-24-2000, 11:58 AM
I didn't know they made a Bravo with 1.65 gears...what is your cruise at @3200 rpm?

RickR
08-24-2000, 12:01 PM
1.65 is Bravo's "High Altitude" option
9% Slip

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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>

[This message has been edited by RickR (edited 08-24-2000).]

Ranman
08-24-2000, 01:50 PM
The speed was taken from a "max speed" run using my Garmin GPS II. This morning was the first time I actually looked at the drive for the ratio #. It lookes like it is stamped in one of the grey MerCruiser stripes on the port side of the drive. Also there is a sticker on the cavitation plate that indicates the ratio. I have a couple of questions now that we know what it is.

1) RickR, How did you come up with a slip of 9%? I came up with 2.8% which I instantly knew was wrong. 9% sounds more like it and is a very good # according to Hot Boat.

2) I believe the boat was initially purchased in AZ and used on Lake Havasau (sp). Then it was sold to a fella in TX which is who I bought it from. Is AZ a high altitude state? Does that explain the optional ratio?

3) Now that the boat is in MI should I have any concerns about air/fuel ratio or performance, or will the MAG 350 computer automatically adjust using a mass air sensor etc...

4) I assume you can compensate for any ratio through propping, but is this ratio "stronger" than a 1.5 due to torque multiplication (ex. 3.08 gears are easier to break with 500hp than 4.10 gears)

The gear and prop seems to be fine, however,
all thoughts on this setup are appreciated. http://www.donzi.net/ubb/smile.gif

RickR
08-24-2000, 09:06 PM
9% seems very low for a 18 Classic
Has the prop been reworked? Has cup been added? Does it have a lab finish?
Some prop finishers will guarantee 2MPH with a lab finish but the prop is so thin it won't last.


I use a handy dandy Merc slide rule for slip caculations.
25+1(cup)=26"
5280x12=63360inches per mile
4900RPM/1.65=2969.69propshaft speed
2969x60x26=inches per hour
67.8/([2969x60x26]/63360)=.927496
All you engineers please excuse my equation.

The Bravo should be bullet proof with the SBC if you don't let your rpms go up when the prop comes out of the water. Don't be shy about jerking the throttle back. Your cone clutch is threaded so it will slip when reverse torque is applied during reentry.


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RICKR
mailto:riggerb@aol.comriggerb@aol.com</A>



[This message has been edited by RickR (edited 08-24-2000).]

GeneD
08-24-2000, 11:46 PM
I'll stick to my original statement.
Most SB Q-Jets were built for 450 CFM carburation.
Most SB chevy's can not produce more than 450 CFM WOT. And I repeat MOST.
Check your numbers.
It might be different for Marine. In this respect, I am some what of an automotive expert, or perhaps let's just say I have a lot of experience.
You can give me venturie diameters all day, they still came with varied jet sizes per motor HP.
Yes, I know venturie diameter is very important in determining CFM. I was trying to make a distinction between BB and SB. There is a difference.
The 450 CFM is a number I received back in 1982. It made sense back then, and it makes sense now.
I'll bet there are over 100 types of Q-Jets out there.
PaulO needs a carb that can breath, and put the fuel in.
Gear ratio is another thing. I have a standard 1.50 in the top gear, and a 1.65 in the bottom. That is what makes my boat breath fire. In addition the 25 pitch Mirage.
Sorry to be so terse.
But carburation is a big pain in my ass with high HP motors. I am still running the stock Q-Jet in my Corvette and Donzi and both run great. It takes a real knack to tune a high HP motor and use the stock carb with no jetting changes. Of course, they are both 350 cubic inch engines. Not 383. Let us remember, one of the first chevy BB motors was the 396. Only 7 cubes bigger than the stroker 383. Most of us don't bother changing the jets when we rebore, (2-7 cu in.) and no one thinks about this when re-stroking. It's still a SB right?
A 383 cubic inch motor is going to go through some CFM's. I for one am ver interetested in what happens with PaulO's motor, since I plan on building a 400 cu in. motor (408 actually).
I would run the BB carb and not worry about much else.
(unless of course it didn't run right, then of course, I'd buy a Marine 750 Holley, Q-Jets are a lot of trouble.)

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GeneD
007
Melbourne, Florida

[This message has been edited by GeneD (edited 08-24-2000).]

Ranman
08-25-2000, 08:43 AM
RickR, The prop looks like your run of the mill bone stock Mercury Mirage Plus. I'm fairly confident that no work has been done to it. I have practiced your suggestion of pulling back the throttle in the past, however, for me the prop doesn't leave the water too often (if ever). That advise has led me into my next post though.

RickSE
08-25-2000, 11:26 AM
Ranman,
Arizona lakes range from 400-ft (Havasu) to 7000-ft (Lake Mary). Most lakes around Phoenix are between 1500 to 2000-ft elevation. Your boat may have been on the dealers lot with the high altitude gear and re-propped to run at Havasu or sea level. The advantage of your gear is that it allows you to use a more efficient prop. My understanding is that higher pitch = less slip. The disadvantage of higher gears like the 1.8? ratio is that they wear out lower shafts because of the large thrust forces associated with large pitch props.

Also, the fuel curve on your MAG is programmed into the ECU. Unlike automotive ECU's, marine ECU's do not have the ability to change the fuel mixture irt altitude, there is no O2 sensor to close the loop.

Ranman
08-28-2000, 08:43 AM
RickS, If that is the case, how do you adjust the air / fuel ratio for altitude?