fuel efficiency changing from small blocks to big blocks?

[Frosty]

I don't think altitude compensation really affects boats.

It a long time since I drove it up a hill.

[Boston]

well said but you never know what carb some fool put on that thing so its worth checking
I think they have specific jets just for marine applications and its entirely possible thats not what is on those engines
Ill throw you a few points on humor value though
I got a good laugh out of that Mr Frosty

rrrrrg
wont let me points you mate

[Fanie]

Hi Tim,

All the chevy spares are available to make that big block a state of the art modern as you can get motor, fuel injection the works.

As Frosty said, big machines suck fuel. No matter how nice you make those big blocks they will be heavy on fuel. You could maybe tow a tanker with it it is that strong, but heavy on fuel.

Small blocks are on the verge of being heavy on fuel, depending what the motor is made out of and how efficient it is. Marine motors use a bit different setup from cars. Any of the chevy component sellers will happily assist you as to what is needed to get them a bit more lively.

Imo it would benefit you more to give the small blocks a bit of a facelift.

A nice 5.7L chevy motor should kick at average around 400hp. Don't just swap the old broomstick with a hot cam, it won't work. There is a combination of spares that characterise a motor. If you do the right things you will get better fuel efficiency as well as better performance.

Of course an outboard still is the best power to weight ratio and fuel efficiency...

[CDK]

""CDK... you keep refering to GM product as dinosaur engines with no economy... what's your choice of engine??? ""

I use the word dinosaur for grey cast engines, developed shortly after 1945, pushrod operated valves, specific power 50 HP/liter or less, raw water cooled so they never reach optimal temp. Now facelifted with EFI, but for decades sold with carbs only.

My boat engines are old-fashioned VW 1,9 turbocharged diesels, liquid cooled, my cars also have diesels, but the CRDI type, except the vintage Porsche in my garage, which has a light alloy V-8 gasoline engine with Bosch mechanical injection. Not quite a dinosaur yet, but hi-tech from the early 80's.

[TeddyDiver]

Originally Posted by pkoken

Higher compression gasoline engines do not have diesel efficiency.*

Nobody said so.. They just have better effiency compared to lower compression gasoline engines

[mydauphin]

Originally Posted by Tim.M

A 6,000 lb boat currently has twin small block GM 350 cubic inch 5.7 liter engines / 260 hp each. It cruises at 3,500 rpm around 35 knots.

If this boat were repowered with twin GM 454 cubic inch 7.4 liter engines / 330 hp each what would you expect to happen to the fuel economy cruising at the same 35 knots?

I had 1979 27' Glastron Carlson CVX27 6000lb empty... Originally had 260hp Mercruiser as above, replaced with 330 hp bigblocks. Boat went from doing 60 knots to 75knots... Fuel economy was so bad, I ran out of fuel almost every time I went out. It ate 120 gallons in 30 minutes... It was fast in between tows .... Sank - water got into engine compartment docked behind my house - it was too low in the stern...

Never want a boat like that again.

[mudman]

If you want to move anything any ammount of distance you will need a certain ammount of power. Power = force X distance / Time. A 350 chevy should burn the same ammount of fuel as a 454 big block for a given weight and speed.

A gallon of gasoline has 1.3X10 to the 8th Joules of energy. You can not change that. You will use a certain ammount of energy to go a certain speed on any given boat. You can have an inefficient engine, poor carb, so on. Simply put, if you want to go faster, you will use more power.

This is true in cars trucks etc. too. Go look at the stats for a full size truck with different engines. A full size F150 with a V6 gets the same fuel economy as the ones with the V8's. The V8 has more power, towing capacity and better accelleration. Same MPG on the highway though.

Many other factors come into play on a repower including hull design and prop selection. This changes the Force part of the equation. With 454's you will probably run less rpm to get to 35 knots, but you will burn the same ammount of fuel in theory. If you want to go over 35 knots, I don't know. Depends on the hull.

I know that I upgraded from 115 hp to 150 hp on a Sabre hull and saw very little change in speed, but drastic change in fuel consumption at WOT. Seems that the hull that I had was designed to run at 40 mph. When I tried to get over that, I gained about 3 mph. The boat did 40 mph wide open with a 115. It also did 40 at about 4800 rpm with the 150. Push the 150 to 5500 and gain 3 mph. Not worth it for that hull. The faster you want to push through the water the harder it gets.

[mydauphin]

Sorry MUD but your wrong and here is why.
The 350's had 600 cfm carbs, the 454 had 780 cfm. So at idle the 454 consume more directly proportional to additional air. volume and fuel. Also 454 were many hundreds of pounds heavier. So that added to fuel consumption. Then then lets rev then to 3500 rpm .. 350 ... 40 gals of fuel per hour per engine. 454 ..60 gallons of fuel per hour per engine.

With fuel injection and lean burn technologies, these numbers go down... But most people that have these boat want to run at fuel throttle and there the numbers are really amazing....

[Boston]

as I said
I made this conversion and it ended up costing me a fortune in both swap costs and in fuel
I'd have to say Mr Douphin is got it down to an art as to why
I did however get the additional power I needed out of the deal

[Jimbo1490]

Originally Posted by speedboats

A correctly tuned carb will offer the same burn rate as that of its injected brother. That is because to make 260hp you need to burn 260hp worth of fuel. Any difference will come in the way of transfer of the energy to the crank, so unless you can find an engine that wastes less energy making heat and noise when combusting fuel then they'll all be much of a muchness.

There's a lot of truth in this statement, but the complete truth is actually a bit more complex.

The chief advantage to EFI is not really that it meters the fuel better. As you've pointed out, a carburetor that is set up properly will meter the fuel just as well.

But this does not address the very important subject of intake manifold design. Intake manifolds intended to mount a carburetor look a certain way (the '180 degree' type is the most common) because of design limitations due to the location in the air stream where the fuel is introduced, namely at the beginning of the intake tract. The manifold has to be designed to provide just the right amount of turbulence to keep the fuel suspended in the air charge at low and moderate engine speeds (part throttle). This need ultimately limits not only peak power, but adversly affects cylinder filling (volumetric efficiency) at ALL engine speeds and power settings.

Take a look at this typical EFI intake (for a 'dinosaur' V8) for comparison to the 180 degree style in the first link. Now what do you think would happen to the fuel/air mixture if you mounted a side draft carburetor to the EFI style manifold in the picture? Would the engine run? Would it run well? The answer is "NO!" it would not run well, if you could manage to start it, at any throttle setting except wide open.

A properly designed EFI manifold is actually a lot like an old-school tunnel ram type intake which is used mostly for drag racing. These intakes could make the most peak power (using a carburetor) but had dismal part throttle performance and were VERY hard starting. Without turbulence, the fuel will simply precipitate out of the air stream and puddle on the floor of the manifold. Not Good

The EFI manifold can have long, straight, tuned length pipes of large cross-section, rather than smaller cross-section pipes with deliberate bends. And all the pipes can feed from a significant plenum volume to greatly reduce inter-cylinder negative interactions. Carburetors are incompatible with large plenums (the plenum in a carburetor manifold is minuscule by comparison) because large plenums reduce 'signal'(velocity) at the carburetor, another big no-no in carburetor intake manifold design.

All this is possible with EFI simply because the fuel is squirted right into the cylinders at the end if the manifold rather than and the beginning of the manifold.

And we have not even touched on the cam design changes that this permits!

Jimbo

[TollyWally]

Quite interesting Jimbo, please go on.

[CDK]

Originally Posted by mydauphin

I had 1979 27' Glastron Carlson CVX27 6000lb empty... Originally had 260hp Mercruiser as above, replaced with 330 hp bigblocks. Boat went from doing 60 knots to 75knots... Fuel economy was so bad, I ran out of fuel almost every time I went out. It ate 120 gallons in 30 minutes... It was fast in between tows .... Sank - water got into engine compartment docked behind my house - it was too low in the stern...

Never want a boat like that again.

Good to know I wasn't the only one. My Glastron Carlson came with 260hp, the engine exploded after one year and was also replaced by a big block. It was not just the financial side that bothered me: circling around a crowded fuel station and the tedious process of filling a large tank that 'burped' twice a minute made me hate this thing. Sold it after a few years to someone who crashed it midships into another boat.

[Boston]

ah you plumbed your tank without a feed back hose on the fill tube
thats why it burped gas at you

[speedboats]

Jimbo... While your argument is concise and compelling, and for the most part I agree with you, however we are talking fuel efficiency rather than volumetric efficiency. That is gallons of fuel burnt to produce a given horsepower rather than horsepower per cubic inch of displacement.

Herein lies my statement, the far majority of engines will burn about the same amount of fuel to produce the same amount of hp.

Everything else aside...
what is the burn rate on the carb sbc @ 260hp?
What is the burn rate in the inj bbc @ 260hp?

I'd hazzard a guess +/- the same?

[Frosty]

120 gallons in 30 minutes is 240 gall per hour!! is that imperial or Us gallon? it does'nt matter.

1 ton per hour. Sigh--- makes a Lancaster bomber look economical.