mercruiser 350 to 383 stroker, 2mph gain??? what gives.
05-30-2019, 08:01 PM
#21
Whoever is doing the carb. call them and give all specs of engine and boat to order,
Your compression is a little too high for my tastes. Kind of Okay for a LS motor maybe, but this is an old style 23* degree motor. Interesting. 2nd small block here this spring with that high compression.
Your compression is a little too high for my tastes. Kind of Okay for a LS motor maybe, but this is an old style 23* degree motor. Interesting. 2nd small block here this spring with that high compression.
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hogie roll (08-27-2022)
05-31-2019, 05:14 AM
#23
Registered
Fore reference....My 87 Formula F223LS has 22 degrees deadrise and a published weight of 4000 lbs. I think it's heavier.
When I first ran it 4 years ago it had a 97 350 MAG carb, which is a 260 or 300 HP small block. It could not even crack 45 MPH. So built my own 10:1 383. I have seen 59 gps. But that can be called out as BS. To get that speed I'd need to be getting 11% slip. Fat boat, not likely. So I could be full of "wanna be"
My drive is a 1.5:1 Alpha, which I baby and run with Amsoil lube.
Over the winter I bought an air fuel ratio set and I want to optimize my fuel this summer.
The factory Weber carb was fine, but since the engine was running out of breath around 5000 I bought a bigger carb to see if it helped. It's a 750 Quickfuel Marine. Looks and works great, Maybe I save $50 per year on fuel with it. But I must admit very little additional speed, maybe 100 rpm. I hope my carb tuning, a smaller prop pitch and correcting a recently found cam timing error will finish things up. My normal prop is a 21" Mirage. I have a 19" Mirage to try on it, see if I can spin it up. My high speed was on a cheaper stainless 21 Apollo from Michigan Wheel. That prop found a rock and isn't worth the cost to repair.
When I first ran it 4 years ago it had a 97 350 MAG carb, which is a 260 or 300 HP small block. It could not even crack 45 MPH. So built my own 10:1 383. I have seen 59 gps. But that can be called out as BS. To get that speed I'd need to be getting 11% slip. Fat boat, not likely. So I could be full of "wanna be"
My drive is a 1.5:1 Alpha, which I baby and run with Amsoil lube.
Over the winter I bought an air fuel ratio set and I want to optimize my fuel this summer.
The factory Weber carb was fine, but since the engine was running out of breath around 5000 I bought a bigger carb to see if it helped. It's a 750 Quickfuel Marine. Looks and works great, Maybe I save $50 per year on fuel with it. But I must admit very little additional speed, maybe 100 rpm. I hope my carb tuning, a smaller prop pitch and correcting a recently found cam timing error will finish things up. My normal prop is a 21" Mirage. I have a 19" Mirage to try on it, see if I can spin it up. My high speed was on a cheaper stainless 21 Apollo from Michigan Wheel. That prop found a rock and isn't worth the cost to repair.
05-31-2019, 05:44 AM
#24
05-31-2019, 06:32 AM
#25
Registered
A 2 mph increase could equal a 40 horsepower gain in your build. If the 350 was built right that could be the difference.
I went from a stock carbed 5.7 2 bbl to a 383 4bbl in a prior boat of mine. Alpha drive that was 1.65 and I changed to a 1.5 ratio which , is like about 4 inches of prop pitch. Ish. So I went from approx 250 horsepower to a little over 400 horsepower. I gained about 10 mph. The hull was a donzi 16 classic. I am not sure of your initial power nor what kind of increase in power your hull would need but it is something to keep in mind.
I went from a stock carbed 5.7 2 bbl to a 383 4bbl in a prior boat of mine. Alpha drive that was 1.65 and I changed to a 1.5 ratio which , is like about 4 inches of prop pitch. Ish. So I went from approx 250 horsepower to a little over 400 horsepower. I gained about 10 mph. The hull was a donzi 16 classic. I am not sure of your initial power nor what kind of increase in power your hull would need but it is something to keep in mind.
05-31-2019, 06:54 PM
#29
Registered
iTrader: (1)
What prop do you run NHGUY? I forget.
05-31-2019, 10:19 PM
#30
Depending how far your pistons are in the hole, I think your compression is higher than you think. Even with 91 octane you will see detonation. if the block is essentially zero decked to get you to .030 quench with your .028 gaskets, your at 11:1 those KB hypers won't last very long in that environment.
I have a 21' that I took out a 305 and put in (2) different 383 now. about 3500lb. boat and 15hp = 1mph My original build was 9.9:1 with 6 degrees less duration and anything less than a 50/50 mix of 91 premium and 100LL aviation fuel would detonate. I rebuilt at 9.4:1 and runs great on 91 premium. Depending on your timing curve it very well could be falling on its face due to detonation rather than lack of power. Pull 4 degrees of timing out and go up 2-3 sizes on main jet and see if you pick up rpm with no other changes. Other option put a tank of 100LL in and see what engine does on same timing.
I have a 21' that I took out a 305 and put in (2) different 383 now. about 3500lb. boat and 15hp = 1mph My original build was 9.9:1 with 6 degrees less duration and anything less than a 50/50 mix of 91 premium and 100LL aviation fuel would detonate. I rebuilt at 9.4:1 and runs great on 91 premium. Depending on your timing curve it very well could be falling on its face due to detonation rather than lack of power. Pull 4 degrees of timing out and go up 2-3 sizes on main jet and see if you pick up rpm with no other changes. Other option put a tank of 100LL in and see what engine does on same timing.
