305 Block

[fabrimacator21]

Re: 305 Block

If you drive both boats identically meaning that you accelerate at the same rate, drive a constant speed, they are going to have similar fuel consuption. The only difference is that with the 350, you will have some reserve that you can use if you need it.

I agree with that... but I'm talking 3/4-WOT. The 350 will burn more fuel.

A 350 will inevitably burn more fuel at 3/4-WOT. That is assuming that the 305 is running a 2bbl.


Like I said whats the point in going with a 350 if you never get past 3/4 throttle? Might as well stay with the 305 if thats the plan.

 

[jtybt]

Re: 305 Block

Having spent the las dozen or so years trying to improve my mileage, I have found more power has always increased my mileage. I have a '78 searay 240 weekender that ostensibly weighed about 4500 dry from the factory. When I got it without engine/drive it weighed 4000 lbs. My first build was pretty much stock and the average mileage was about 1.55 MPG. After adding a cabin, full SS railing/radar arch etc. the weight went to 6800 lbs and I built my first 406. Mileage increased to an average to 1.7 MPG. That was good enuff for the first 7 years.

Next I decided to swap the MR/Alpha drive for a B II but that required a transom replacement which ended up removing ALL the wood from the transom to the cuddy doors. I built the cockpit deck to cover the engine to increase the open fishing area and added two large fish boxes.

Weight now came to 7660 lbs with full main tank(100 gal) but I increased power with another 406 build so I now have somewhere above 325 HP @ 4600 RPM and 450 ft/lbs of torque @ 2500 RPM.

Presently doing break-in of this engine but mileage is now showing 2 MPG.

That's a 30% increase in mileage from the stock engine and 2000 lbs lighter set-up.

I fish exclusively and drive exactly like Futz.

 

[Bondo]

Re: 305 Block

I agree with that... but I'm talking 3/4-WOT. The 350 will burn more fuel.

A 350 will inevitably burn more fuel at 3/4-WOT. That is assuming that the 305 is running a 2bbl.

Ayuh,... Now you're hedging on your Bet.....

As already explained,.... It takes X# of horsepower to move a hull....

Bigger motors working at lower percentages of available power, burn Less fuel....
And,... Assuming the 305 2bbl. motor,+ the 350 4bbl. motor are merely set to 3/4 throttle,....
You'll be going Much Faster with the 350, which would Explain the higher fuel usage at 3/4 Throttle....

 

[futz]

Re: 305 Block

So as Charlie has stated, and i think Bond-o agrees,2 boats weighing the same, same hull one with a 305, one with a 350 going side bye side 20 mph, both using the same carb. the 350 will use less fuel, correct?

 

[jtybt]

Re: 305 Block

NO!

With more power you can change prop pitch. With more power I went from 1.5:1 gear ratio with 16X16 prop to 2:1 gear ratio to 17 3/4 X 21 prop.

NOT doing all that, you can run at a slightly lesser throttle opening and probably get the same mileage... simply because of the increased torque. My engine displacement is about the same over a 350 as the 350 is over the 305 though my torque is around 100 Ft/lbs more.

Torque is the name of the game. all things being equal, the more displacement...the more torque.

 

[dr_bowtie]

Re: 305 Block

yes torque is the name of the game....

Torque = how much weight you can pull....

HP = how fast you can pull it....

 

[n2ostroker]

Re: 305 Block

Even running a 4-barrel if the guy is using that much more fuel to regret the change then something isn't set up right. My friends 454mag in a 24' Checkmate uses just a hair more on an average weekend than my 18' with a 350. He runs WOT everywhere he goes though, doesn't know what cruising is... I run mixed driving with a mile long wot blast once and a while.

 

[fabrimacator21]

Re: 305 Block

I agree with that... but I'm talking 3/4-WOT. The 350 will burn more fuel.

A 350 will inevitably burn more fuel at 3/4-WOT. That is assuming that the 305 is running a 2bbl.

Ayuh,... Now you're hedging on your Bet.....

As already explained,.... It takes X# of horsepower to move a hull....

Bigger motors working at lower percentages of available power, burn Less fuel....
And,... Assuming the 305 2bbl. motor,+ the 350 4bbl. motor are merely set to 3/4 throttle,....
You'll be going Much Faster with the 350, which would Explain the higher fuel usage at 3/4 Throttle....

Hedging on my bet?

My first post in this thread:

Beware of the 350 swap.... I know a guy that went from a 200hp 305 to a 260hp 350 and he's kicking himself for it because of fuel consumption. If you have a big enough tank I wouldn't worry about it but the 350's do suck more gas.

With all your posts I'm sure you know that a 200hp 305 is a 2bbl and a 260hp 350 is a 4bbl.

We could argue this forever.... because we are both right.... down low the 350 can use it's torque and a higher pitch prop to get better mileage.

But once you open the carb enough on the 350 enough it will suck more gas then the 305.

That guy could have very well had his carb settings screwed up(probably does with the way he whines about gas mileage)...... I wouldn't think it would be as bad of mileage as he described it.

Either that or he does alot of high speed runs and is suprised how quick the gauge drops.

 

[jtybt]

Re: 305 Block

Also, 2 barrel carbs are not as efficient as 4 barrel carbs simply because 4 barrel carbs can spread it's flow rating between 4 smaller barrels while 2 barrel carbs have comparatively larger bores.

Example a 450 CFM 2 barrel is flowing 225 CFM thru two barrels while a 600 CFM 4 barrel is flowing 150 CFM thru each barrel.

It's possible to set up a 4 barrel carb to be just as efficient as a 2 barrel with more power if you need it.

 

[fabrimacator21]

Re: 305 Block

Also, 2 barrel carbs are not as efficient as 4 barrel carbs simply because 4 barrel carbs can spread it's flow rating between 4 smaller barrels while 2 barrel carbs have comparatively larger bores.

Example a 450 CFM 2 barrel is flowing 225 CFM thru two barrels while a 600 CFM 4 barrel is flowing 150 CFM thru each barrel.

It's possible to set up a 4 barrel carb to be just as efficient as a 2 barrel with more power if you need it.

effeciency and fuel consumption are two different things.

If a carb flows 650 cfm then it needs enough fuel to match that amount of airflow. There's no way that a 650 cfm 4bbl will use the same amount of gas as a 450cfm 2bbl at 3/4-WOT..... if it does then it's lean. It's a matter of fuel mixture. If a carb flows a certain amount of air then it needs a certain amount of fuel. That doesn't change when going from a 2bbl to a 4bbl carb.

 

[Honest Don]

Re: 305 Block

getting back to the original question here, if you have the motor ready to go minus the crank issue, you can adapters on ebay for $20. they sell them to go in both directions, and the only block differences between the years in the rear main seal, and dipstick tube placement. Be sure to use a marine oilpan, or add a windage tray to an automotive one. this will keep the oil in the pan and off the bottom of the motor.

 

[bkhdmh]

Re: 305 Block

Thanks for all the info. I found a flywheel that worked at a local marina for $200. Came off a 350 with a 2 piece rear main seal. Supposedly its the same as a standard Murcruser 350 flywheel and I just bolted my old coupler to the new flywheel. Engine is on the boat and running. A little rough but I thihnk that is due to the year or so old fuel. Going to clean the carb today or tomorrow, get new plug wires and check the fuel to make sure there is no water in it. I have some lucas gas treatment I am going to dump in also.

 

[Silvertip]

Re: 305 Block

And then there is the combustion chamber and valve size differences between 305 and 350's. Compression ratio and breathing are issues with 305 head on a 350 block.

 

[180shabah]

Re: 305 Block

effeciency and fuel consumption are two different things.

If a carb flows 650 cfm then it needs enough fuel to match that amount of airflow. There's no way that a 650 cfm 4bbl will use the same amount of gas as a 450cfm 2bbl at 3/4-WOT..... if it does then it's lean. It's a matter of fuel mixture. If a carb flows a certain amount of air then it needs a certain amount of fuel. That doesn't change when going from a 2bbl to a 4bbl carb.

You forgot something - a 350 spinning at 5000RPM cant draw 650 CFMs. At 100% VE, it would only take in a hair over 500CFM. Real world would be 400 and change. If setup properly, the 4bbl introduces less resistance than the 2bbl in the example above, thus should be more efficient(but only barely, probably not enough to notice outside of a lab).

BUT

in most cases when more power is available - it gets used. Harder acceleration, faster cruising speed, both mean more fuel used overall. It's not a design issue, it's an opperator issue.

 

[Bondo]

Re: 305 Block

BUT

in most cases when more power is available - it gets used. Harder acceleration, faster cruising speed, both mean more fuel used overall. It's not a design issue, it's an operator issue.

Ayuh,... That's what I said up in post #12....

 

[jtybt]

Re: 305 Block

It's absolutely how the driver likes to run the boat.


BUT...I have my carb set up so I can run at 3/4 throttle on the small spreadbore primaries(holley 4170). If I wanted to, I could wire the secondaries completely closed. and run full throttle on just the primaries. The fuel air ratio is set by your jets and float level.

All I do is fish and trips run between 50 miles to over 150 miles and more than 50 miles off shore. That's not a place to be hot rodding around. I try to run at the lowest throttle opening to get the job done.

 

[dr_bowtie]

Re: 305 Block

effeciency and fuel consumption are two different things.

If a carb flows 650 cfm then it needs enough fuel to match that amount of airflow. There's no way that a 650 cfm 4bbl will use the same amount of gas as a 450cfm 2bbl at 3/4-WOT..... if it does then it's lean. It's a matter of fuel mixture. If a carb flows a certain amount of air then it needs a certain amount of fuel. That doesn't change when going from a 2bbl to a 4bbl carb.

Wrong....just plain wrong on so many levels...

first of all size DOES NOT matter in carb...a carb is a carb is a carb...the differential is the engine...the carb MUST be set to the engine...

the carb does NOT know what engine it is on nor the size...

A carb is only a "controlled fuel leak" no more no less...its main job is to "leak fuel to match the air flowing thru the venturi(s)....

if you have an engine running a 3/4 throttle...we'll call this 3400rpm...

and that engine is a 350cu engine...the CFM rating at that RPM goes like this...

350x3400rpm/3456=cfm

350x3400=1190000/3456= 344cfm

if you have a 400cfm 2-barrel carb or a 600cfm 4 barrel carb it wont matter they will both provide the SAME amount of fuel....

or even a 850cfm or...what ever...there is NO such thing as over carbing an engine...

the engine will ONLY take what it needs..you cannot force more into the engine under natural aspiration ( excluding forced induction)

if the carb doesnt work it is either jetted wrong or the vacuum pulses the engine is signaling arent reaching the boosters...

if the engine is efficient it will be fuel effiecient as well this goes with the BSFC (Brake Specific Fuel Consumption) this goes directly with Volumetric Efficiency at given RPMs....

a larger carb gives you a higher RPM ceiling...but carbs are the same as heads...if you go too large you have the potential to lose bottom end throttle responce IF (only IF) you lose Venturi Velocity...(air speed past the Venturis)

a 305 and 350 are very close in BSFC...but a 350 in poor tune can eat gas like no tomorrow...

 

[jtybt]

Re: 305 Block

'zactly!

The only addendum I might add is a small displacement engine just can't 'pull' the same amount of air as a larger displacement engine. In this case, a smaller CFM carb with smaller throttle bores will flow air 'faster' and pull(leak) fuel more efficiently and atomize the fuel better.

 

[dr_bowtie]

Re: 305 Block

Depends...

I ran a 850 CFM carb on my 283 cu in engine for years and I ran 12:30s with it regularly...I also ran it 8500rpm too... a smaller carb made the car slower as well...

But then again no one had the guts to mod a Q-Jet to do the things I do with them...

at that point we are right back at the second part about the vacuum signaling...this is the KEY to making any carb work...once you know the signaling you can make any carb suit any engine....

 

[Bondo]

Re: 305 Block

there is NO such thing as over carbing an engine...

Ayuh,... I Agree,... Excellent post Dr....
I with you up to that line,+ you explain My reasoning with This line,...

or the vacuum pulses the engine is signaling arent reaching the boosters...

Seems to Me, the vacuum signal gets Lost when Overcarbing....
I think a 600/ 650cfm is the Idea size for Most SBCs....