Blueabyss

Anyone ever thought of trying a 383 with a 2bl carb. I know you are not going to get 400 hp but maybe 325 and better fuel economy that a 350 with 4bl carb.

Chris

SB

iTrader: (1)

You are mistaken.

Having a larger air restriction will have lower volumetric effeciency because of larger pumping losses. Larger pumping losses can/will cause higher BSFC (amount of fuel used per hp)

Say if for some reason both motors have same vol eff / bsfc, then if you use the same hp, you use the same fuel.

Which brings me to a more important fuel effeciency topic with boats...

Off the engine topic......your boat's efficiency (ie: how much HP it takes to drive at X speed) and how fast you drive it (ie: X) has way more to do with fuel efficiency then the engine is responsible for.

Exyreme but real Example: Friends ski boat with stock 260hp 5.7 takes over 4 times the fuel to get to a certain place we go than I with an approx 375hp 5.7. Both are 20ft boats.

Psst: I get there faster too. Way faster. LOL.

Why is that ?

innerrage

iTrader: (1)

because you have full race peanut butter race ports

Wes Burmark

From my experience the 5.7 260 h.p. was not in a very efficient state of tune. When they came out with the Magnum version they rated it at only 10 additional h.p.
I saw dyno results showing 12 h.p. alone switching from the 3" rises to the 4" risers. Then there was the high rise intake which made a huge difference plus the
less restrictive flame arrestor. I'm sure, with your skills SB, that you put together a package that is super coordinated and powerful. Ever wonder what you could do with a 427 cu. in.
smallblock and a large budget? I'm guessing 450+ on pump regular.

SB

iTrader: (1)

Creamy, not Chunky. Shhh. That's a speed secret.

On the stock 260hp Mag, a few more degrees of timing, the Highrise Bowtie intake as you mentioned, along with Felpro 1204 intake gaskets (blocks the exhaust heat crossover) and getting a dialed in prop really wakes up small single Alfalfa driven boats.

So, yes, Alfalfa can compliment peanut butter. LOL.

450hp 427 ? Serious ? A 383 can make 450hp no sweat. Medium budget. For a little more a 421cid or so SBC can go well north of 500HP on pump. Time to rethink your Alfalfa though.

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PS: Cheaper way to make your 4bbl into a 2bbl is to use less throttle. Same thing. In this instance swapping back to a 4bbl is as easy as pushing the throttle farther foward.

SB

iTrader: (1)

BTW: it's not my mad scientist tuning skills, it's because the ski/wakeboard boats plow thru the water. It doesn't ride on top of the water with little hull touching the water.

Again, my point is that what you use for horsepower to go a certain speed is what you are going to get for fuel mileage.

A little 20ft boat dancing on it's butt (ie: only say 3ft or so really on the water) will use far less than a 20ft boat with 20ft of it plowing thru the water. At 45mph his ski boat is at or near WOT - ie: 260hp. I'm at 1/4 throttle or so and a close estimate might be 100-150hp.

At 260hp I'm probably around 52-55mph. As that's what the boat did stock. So, should be a close guess.

FIXX

iTrader: (2)

i got a 2 bbl that out flows a hollt 600 4v..came off a old 70's school bus....hey they said stock gm carburetor in the rule book...

Blueabyss

Ya I have seen those Fix.. So back to the question.. Has anyone ever tried or do they think a 2 bbl would work on a 383 marine. I know mopar use to do it in cars. And like fixx is referring to I have seen it on an asphalt stock car race motor. I understand the flow restriction but on a big heavy boat, trying to keep it from sucking gas with the secondaries open while just on plane is also important.

mike tkach

iTrader: (3)

most engines with that displacement will need a 750 or bigger carb.it takes power to push a big heavy boat and power is reduced when you restrict the air flow.the sucking of gas is just going to happen if you want power.

SB

iTrader: (1)

Mike get's it.
Air pulls fuel thru the carburetor.
More power requires more air and fuel.
Reduce the power by using a smaller carburetor , you will use less fuel.
Reduce the power by not using as much throttle opening on a 4bbl and you will use less fuel.
When you use less throttle, you are effectively making the carburetor smaller.

The power requirement of the boat and the driver is what causes your fuel economy #'s.

As an FYI: A bigger engine usues more fuel at idle/very low speed than a smaller motor......frictional losses, reciprocating weight differences, etc,etc amongst other reasons.