Thread: Planning to build a Marine 383 Stroker

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Pat, I have the small flame arrestor, but removing it completley didn't make any notable difference.

Kodiak, I disagree, "no matter how many horses..." The "gear ratio" hasn't changed and it must have more HP with all the mods. It behaves like I have a rev limiter, or something hits the wall at 4200.

What about advancing the cam?, I have that option. (Now I really wish I bought that 2-pc timing cover!)

hmmmm well this will be my last post i will not try any more .if you make more tq than you want to use your tq if this hits the wall at 4200? then why not try to load the engine down with the bigger prop and lower rpm will make more mph you engine stops at 4200 ? try a bigger carb . and heads the points are bouncing? remember this is a boat not a car it has to get up on plane and the less in the water the fast it will go so have you tried moveing the trim setting ? moving the cam will not help to much it may get you to 4500 but at a loss at the bottom and may not help much on a boat like a car ?there is more drag with it .so if this boat lets say had a big block in it then it would have not the same prop ? if this small block make power like a big block then it should be prop out like one and remember to that the prop and the drive to plays in to this as it has a gear ratio

Well, the OMC Cobra is also a combination over hub/through hub exhaust. It's the same drive as the current generation of Volvo SX drives.

I've got a Volvo 280T outdrive. It's got over prop exhaust. All I know about this is that it's hard to find 4-blade stainless props for it. Do the 4-blades hold the water too well at low speed without the ventillation from the over hub?

The Prestolites are bad? I have a Prestolite. What kind of problems should I be looking for? I've got a Prestolite starter, too. It's much better built than the Delco starters but doesn't have the solenoid operated Bendix gear. This isn't a problem on my boat, but hard to start boats tend to kick the Bendix out of the flywheel just as the motor is about to start. Then it dies on you again.

Nauti, how long is the boat and what is the weight? To give you an example, my boat is 21 ft long and ~3500-4000 lbs with fuel, gear, and people. I'm running a 15x19 prop and have a 1.8ish gear ratio in the drive. I wouldn't run more prop, the boat should run more than 4200. The points are suspicious, especially single points if you have them like I do.

The boat is 26' LWL, 9.5' Beam, weighs 7000 Lbs (best guess with tender, 4 people, full tanks and lots of beer). Prop is the standard 3-blade 15.5' Dia. x 15" pitch. Drive gear ratio is 1.41:1, and using synthetic oil. I used one of the online prop calculators and at 3500 RPM/26 MPH i have .27 prop slip, and at 4200 RPM/35 MPH with .18 prop slip.

The carb is universally used between a 5.0 thru 7.4L size engines.

Points are new and dwell is steady on the mark.

The only 3 things that I can play with are cam advance, ignition, and carb tuning. Incidentally, the engine likes to run on the hotter side than before, my exhaust manifolds used to be tolerant to touch whereas now they can't. Running with no t-stat it holds at around 120F. T-stat in and it overheats in 3 minutes at cruise. This could be a good clue....

Originally Posted by 76GMC1500

Well, the OMC Cobra is also a combination over hub/through hub exhaust. It's the same drive as the current generation of Volvo SX drives.

I've got a Volvo 280T outdrive. It's got over prop exhaust. All I know about this is that it's hard to find 4-blade stainless props for it. Do the 4-blades hold the water too well at low speed without the ventillation from the over hub?

The Prestolites are bad? I have a Prestolite. What kind of problems should I be looking for? I've got a Prestolite starter, too. It's much better built than the Delco starters but doesn't have the solenoid operated Bendix gear. This isn't a problem on my boat, but hard to start boats tend to kick the Bendix out of the flywheel just as the motor is about to start. Then it dies on you again.

Nauti, how long is the boat and what is the weight? To give you an example, my boat is 21 ft long and ~3500-4000 lbs with fuel, gear, and people. I'm running a 15x19 prop and have a 1.8ish gear ratio in the drive. I wouldn't run more prop, the boat should run more than 4200. The points are suspicious, especially single points if you have them like I do.

prestolite dist are not that good for hi rpm did i say any thing about there starters ? or any thing else they make ??i am looking at something at the stand point of hi rpm i did work at a of the biggest boat yards and tune up many boats a year prestolite dist were not what i think were that good ? light point set up not as good as the mallory with the post and base to hold the points as one unit and the mallory did make heavy points.with the prestolite that is the way it is . i said they do work for low rpm but a msd or a thunder bolt is not any good? the thunder i used in two cars on the street work fine to 6500 .i would not of put the prestolite in a car for hi rpm use or boat

The 15" pitch prop is way too small. I am running a very simular boat 25 bayliner about 7k pounds with a tired 307 with only 200 hp, and a 17" pitch prop. top speed with the bottom clean is 32 mph. I am building a 383 for the boat and plan on running a 23 pitch. By my calculation I should be running 48 mph with the new motor and prop. Im sure your wondering where I will get a 23 pitch prop?? ya can buy a 21 and have it modded at any good prop shop.
There is definatly something limiting your rpm, but that really doesnt matter since you shoudnt be running in that range anyways. I've got half your hp with my 307, and a bigger prop, and am running a simular speed, that should convince you to get a bigger prop.
Lance

Originally Posted by inspectorlance

The 15" pitch prop is way too small. I am running a very simular boat 25 bayliner about 7k pounds with a tired 307 with only 200 hp, and a 17" pitch prop. top speed with the bottom clean is 32 mph. I am building a 383 for the boat and plan on running a 23 pitch. By my calculation I should be running 48 mph with the new motor and prop. Im sure your wondering where I will get a 23 pitch prop?? ya can buy a 21 and have it modded at any good prop shop.
There is definatly something limiting your rpm, but that really doesnt matter since you shoudnt be running in that range anyways. I've got half your hp with my 307, and a bigger prop, and am running a simular speed, that should convince you to get a bigger prop.
Lance

yes you are right .the 383 should get it up on plane so a bigger prop will not hurt we reprop a 22 sea-ray454 and did get so much more top end that the speed-o pickup stop working and started to chime walk bad .we pass a 39 stinger and not much of anything was in the water but the prop. when we went by them. that is what they told me

Originally Posted by Nauti-Escape

Well, I built this monster and man it can get the boat out of the hole like never before. The only expectation not met is that it won't rev any higher than the stock 350 ever did (4200). I would have thought that the extra oomph in size would have bought me a bit more top end (all I want 4600-4800 RPM WOT) So I ask my machinist and he says drop a Holley 750 on it. My Q-jet is likely already that size, even if it was a 650 probably would still breathe ok?. I'm talking this thing jumped from 70 to 90 Liters/hour on the fuel meter, so am I under-carbed or should I pay attention to my stock point-based ignition?

Anyone have some ideas where I should start looking or do ya think that's all I might get? I have the same prop as before too.

No matter if you reprop or not you still have a problem. Especially with the smaller prop, that thing should turn up too high.

1. It jumped from 70 to 90 on the fuel meter....Have you run a fuel pressure test at WOT to see if the pump can keep up with the engine's needs?

2. No matter what, you could use some better ignition. Won't hurt and just might help.

3. One of the things that separates boat engines from car engines is a car uses around 25% of it's rated power to cruise and a boat uses 50-75% of it's rated power to cruise. Therefore, your engine could maybe require a bigger carb to feed the bigger cubes.