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Discussion Starter #1
Ok...how many times has this been asked? I see the answers for big inch motors...but not little ones. I'm also told that jet boats "need" more CFM than a car (which it was all the calculators are for). Right now I have an 850 Mighty Demon; and besides the obvious flaw of being a Demon, I figured it might be too big.


Motor specs:
383 ci @ 6000 rpm
Right now makes a verified 500 hp @ 5700 - but *has* made 500 rwhp at 6200 in an EFI application.
Current intake is a dual plane (Performer RPM)...side note: would it be beneficial to switch to a Victor Jr.?
Large-ish cam.
 

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i've been running 2 of the exact same flawed crappy carbs for the last 9 years. so if you decide to toss it in the lake, lemme know, i'll take it....:)
 

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i've been running 2 of the exact same flawed crappy carbs for the last 9 years. so if you decide to toss it in the lake, lemme know, i'll take it....:)
Ha ha...I know some work great and some don't. Mine falls somewhere in the middle.


I do hate to buy a new carb when I can just go back to EFI...
 

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I'd switch to a vic jr or super vic. If you don't like it, I'm sure you could easily unload it on LS1tech.
 

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Ha ha...I know some work great and some don't. Mine falls somewhere in the middle.


I do hate to buy a new carb when I can just go back to EFI...
mine didn't work great, all the time, for 2 years. i was about ready to strangle somebody. after reading everything i could find, trying numerous different things, i finally got connected to the guy that was head tech at bg at the time. 20 minutes later, everything made sense. made a few changes, and they've been great ever since.
if you don't want it, just lemme know. i need one for something else...:)
 

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A 750-780 would be what I would run. Your dual plane will work ok but you might get a bit more power at high rpm with the single plane. Depends on how much your boat weighs on the manifold I would use.
 

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Discussion Starter #7
A 750-780 would be what I would run. Your dual plane will work ok but you might get a bit more power at high rpm with the single plane. Depends on how much your boat weighs on the manifold I would use.
It's a 21' Liberator...so for the jet boat world I suppose it's a heavy boat.
 

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Hp for carb vs EFI shouldn't be much at all. So, there is carb tuning issues or some mismatched parts somewhere.

What are the following:
Compression ?
Cam specs ?
Cylinder heads ?
Exhaust ?
Flam arrestor type and sizing ?
 

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Discussion Starter #9
Hp for carb vs EFI shouldn't be much at all. So, there is carb tuning issues or some mismatched parts somewhere.

What are the following:
Compression ? 11.5:1
Cam specs ? Hyd Roller, 238/244 @ .050, .603/.609, 112+4 LSA (I think)
Cylinder heads ? Ported stock 241's - .600 lift flow - I=315cfm E=260cfm
Exhaust ? Rewarder through transom headers
Flam arrestor type and sizing ? I don't know on this - it's what fits inside the open front scoops?
The biggest change is from a Fast 90 type intake to the Performer RPM.

Anyway, the answers are above in bold.
 

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Looks like a stout combo.

Hard to guess on how much power a Vic Jr would be over the RPM on this combo. I really wouldn't think much.....but....have not done the comparison on your type build.

That CFM carb should be fine on both the dual plane and single plane. Can't answer if it's fuel metering is correct though.

As far as flame arrestor, I've seen a few make a good motor lethargic on the dyno.

On the water, as an example, I have even seen an approx 350hp 5.7L Chevy (older style) gain 1.5mph from removing the 9x4 (2.75" element height) K&N arrestor from a BG650 with choke tower (no choke blade) still present. This set=up did not make the motor lethargic but eye opening since you would think it was a big enough arrestor.

Something to think about.
 

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Discussion Starter #12
Formula ZR, are you looking for the difference in HP? Thats easy. Its in the dyno. Your comparing apples and oranges.
An enertia chassis dyno to a water brake dyno?
Bad enough to compare HPs between dyno brands, or even correction factors(specially if incorrectly calculated), but between a rear wheel chassis dyno and a water brake. Not hardly.
Your chasing a ghost.

I'm not looking for horsepower, I'm just trying to check my carb size to make sure I'm not leaving "free" horsepower out on the table. I only stated my horsepower so everyone knew what we were working with.

What I'm getting at is this: a simple calc of my 383 even at it's EFI peak of 6200 says I need a 687 cfm. I could see going to a 750; but an 850 seems huge. BUT - I keep reading that a jet boat needs more carb than car. I don't quite grasp that concept...but I am trying to see if I'm over carbed. A 502 spinning 5000-5200 rpm with an 850 sounds reasonable - but again, wasn't sure about my "little" motor.
 

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I could see going to a 750; but an 850 seems huge. BUT - I keep reading that a jet boat needs more carb than car. I don't quite grasp that concept...but I am trying to see if I'm over carbed.
Don't compare total cfm of carb(s) or throttle bodies on a tunnel ram vs single vs dual plane vs IR. Type of manifold dictates how much throttle bore/blade/venturi area each cylinder will see when demanding air.

To see if your carb is too restrictive check intake vacuum at top rpm and load.

Same with your flame arrestor ! Check inside arrestor for vacuum readings.
 

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I'm not looking for horsepower, I'm just trying to check my carb size to make sure I'm not leaving "free" horsepower out on the table. I only stated my horsepower so everyone knew what we were working with.

What I'm getting at is this: a simple calc of my 383 even at it's EFI peak of 6200 says I need a 687 cfm. I could see going to a 750; but an 850 seems huge. BUT - I keep reading that a jet boat needs more carb than car. I don't quite grasp that concept...but I am trying to see if I'm over carbed. A 502 spinning 5000-5200 rpm with an 850 sounds reasonable - but again, wasn't sure about my "little" motor.
You are on the right track, a R 4779 holley 750 would be perfect for that combo. Jetted accordingly and proper squirters and trust me---it's on!!! Mark
 
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