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mercruiser 350 to 383 stroker, 2mph gain??? what gives.

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mercruiser 350 to 383 stroker, 2mph gain??? what gives.

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Old 05-30-2019, 04:54 PM
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Default mercruiser 350 to 383 stroker, 2mph gain??? what gives.

i have a thundercraft diablo 230 23ft boat. It had a modified mercruiser engine 350 - vortec heads and eldelbrock intake RPM. Then the intake gasket took a dive and let water in the oil and spun the bottom end. That being said. it was time for a 383 Stroker build. From what i can remember it was a 53mph On GPS boat with a 19p Mirage Prop at 4400 RPM WOT.

Now over the past year I built this stroker.
It has a sbc 400 crank in a 350 short block bored over .30.
Flat top two cut relief KB pistons.
.028" head gaskets.
Aluminum heads 64cc , 205CC intake runner.
1.60/2.02Valve sizes.
Comp cams 295-124188
Retro fit hydraulic roller lifters
1.6 ratio roller billet rockers.
push rod length is perfectly set up.
Eldelbrock Performer dual plane intake
STOCK quadraJet 800cfm carb i believe.
fuel pressure gauge 3-6 psi
holley marine intake filter
Stock mercruiser ignition system at 32 degree advance.
.030 quench deck to piston height.
Stainless marine headers

If im missing something please ask.

So I took it out for its first ride and was not impressed at all. Out of the hole the boat is impressive. but the top speed was only 54-55mph GPS reving at 5200 rpm with the 19p mirage prop. That prop is only 14 1/4" diameter.

I ordered a 23P x 14- 1/4" prop on ebay. Took it out. Out of the whole it sounded like i had a spun prop hub. It was slipping like a clutch. till the boat got up on plain then caught water and took off. i got 52mph at 4500-4600RPM. I am thinking prop diameter is not large enough for that pitch and weight of boat that its slipping.
The hubs in the props are plastic. no rubber.

Why havent i gained speed? do i need to retune my Carb? bigger jets for the stroker engine? cause thats the only thing i can think of. Also i am thinking of trying a Turning point Mach 3 prop that is 15.6" diameter 21P. hopefully prop slip will then be minimized.

ANY advice greatly appreciated as im about to buy a jet stage 2 quadra jet soon.

Last edited by guimond47; 05-30-2019 at 07:01 PM.
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Old 05-30-2019, 05:42 PM
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You must retune the carb for the new engine. You are probably lean. Hope you didn't cause any damage. Did you pull the plugs to check tune ?
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Old 05-30-2019, 05:45 PM
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I did look at the plugs last night . The tips are white/grey ish. The ring ove the plug is mildly black. Not excessive tho... im thinking i dont have enough fuel up top. It does run smooth tho with no bog
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Old 05-30-2019, 06:09 PM
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Agree, motor needs tuning
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Old 05-30-2019, 06:23 PM
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What cam and what fuel psi at WOT at top rpm ?
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Old 05-30-2019, 06:53 PM
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Comp cams part number in first thread. Plus the 1.6 ratio rockers makes it about a .520 lift. Wot 5psi fuel pressure
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Old 05-30-2019, 06:58 PM
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I've read the 1st post 10 times. I don't see it. So please flatter ,my old age. Thanks.

Originally Posted by guimond47 View Post
If im missing something please ask.
I tried.
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Old 05-30-2019, 07:01 PM
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Originally Posted by SB View Post
I've read it 10 times. I don't see it. So please flatter ,my old age. Thanks.



I tried.
here is the cam part number 295-124188. it is comp cams.
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Old 05-30-2019, 07:11 PM
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what gear ratio is your drive? based on a given ratio and the numbers you posted your slip went up 10%+. no amount of tuning is going to fix the slip numbers. if you have the ratio I can give you better before and after slip numbers.
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Old 05-30-2019, 07:12 PM
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Originally Posted by compedgemarine View Post
what gear ratio is your drive? based on a given ratio and the numbers you posted your slip went up 10%+. no amount of tuning is going to fix the slip numbers. if you have the ratio I can give you better before and after slip numbers.
my ratio is 1.50... i've tried to use the mercury calculator but theres no prop diameter input...
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