fuel efficiency changing from small blocks to big blocks?

[mudman]

According to Mercruisers website a Crownline 260 LS with a 350 MAG will run at 45.2 mph and burn 20.2 mpg (5000 rpm). The same boat with a 496 MAG and of course proped different will run at 45.5 mph and burn 19 mpg (4000 rpm). Top speed of this boat is 48 mph with a 350 (22.6 gph) or 53.2 mph with the 496 (31 mpg). More speed = more fuel consumption. I agree that at idle you will burn more fuel with a larger engine, but for normal running, I still think that If you want to go 30 mph, you will burn 30 mph worth of fuel.

RPM __mph 350__gph 350__mph 496__gph 496
1000__5.4 ______1.4 ______6.5______2.2
2000__9.1 ______4.1 ______13.4_____6.8
3000__23.7______8.0______31.0_____9.8
4000__35.5______12.8_____45.5 _____19
4700____________________53.2______31
5000__45.2 _____20.2
5250__48.0 _____22.6

I am not sure about the reduction on the drive units on these 2 boats, but hey, GPH is GPH and HP is HP. Prop selections and reduction on the drive units will have more impact on fuel consumption if you want to go 40 knots, more so than the engine displacement IMO.

Carbs are a primitive form of fuel delivery, and I think that fuel injection will offer better fuel/air mixtures. Especially when you start getting into HPDI. I still like a Holley though. Mainly because I can adjust and fix them if need be.

[Jimbo1490]

Originally Posted by 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?

While making 260 Hp (the peak power at or near WOT) they will be about the same, assuming they are running the same cam. If these are OEM setups, they probably are running the same cams.

This is one reason that racer's are not all starry eyed over EFI; they know that it may not help them much, if at all, since they are usually at WOT all the time. And when an EFI screws up, you have to figure out what's wrong, then fix it. When a carb screws up, you just remove it and install a spare. Takes 5 minutes.

But with both engines running part throttle, the situation changes. The EFI, due to it's superior manifold design, will have a decided advantage due to reduced pumping losses and better cylinder filling.

The very nearly 'stratified' charge of the EFI engine introduces another wrinkle, namely cam design. Because the fuel and air are not mixed until the last possible moment, and the fuel injection even can be timed to an exact moment in the valve cycle (this is sequential EFI, another thin we have not mentioned yet) a cam profile can be selected that would be a disaster if a typical carb and intake were installed.

So in your example, both engines are running at WOT, and presumably running the same cam. In such a case, the EFI just looks like an overly complex way to get the job done, kinda like a computerized can opener.

You'd probably be amazed that many OEM's were very lazy about changing cams when the EFI revolution hit. They often kept using the same 'holdover' cams from the carb era. (inititally they even used the same intake manifolds; that is why there's such a thing as 'TBI)

But if you take advantage of what EFI permits you to do with the intake and cam, there are power and efficiency advantages over a carburetor.

Then there's a whole other conversation about combustion chamber design for EFI. Finally in the 1990's all the big three US auto makers redesigned their cylinder heads to take advantage of EFI's capabilities. As with cams, they had lazily re-used the old heads which are for homogenized charge, not stratified. The shape and size of the combustion chamber is radically different.

This is the stuff that made these engines 'dinosaurs', NOT their large displacement or even that they 'only' have 2 valve heads and pushrods. But if you need 300HP, and your old design cams, intake and combustion chambers will only let you make ~50 HP per liter, then 6 liters makes sense. When you add EFI with all that comes with it, and now you can quite easily make 80 Hp per liter with reliability and fuel economy (ask any owner of a 5.0 Mustang hot rod) then 6 liters makes no sense at all, as you only need 300 Hp, not 500. And why carry all that extra weight for power you don't need?

Jimbo

[CDK]

Originally Posted by Boston

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

Nope, I didn't plumb anything, the boat was new.The filler hose was very long with a couple of short curves. The ventilating hose and non-return valve were toy size. Glastron Carlson was always more concerned with good looks....

[Boston]

thats bad
someone can get seriously hurt by splashing fuel
not to mention the fire hazard
and ya You need a hose about half the size of the filler tube so froth doesn't get in the way of return air

sucks mate Ild have been pissed off if my new boat spit gas all the time and I had to go fix it myself

[mydauphin]

CDK, I never understood it, but come to think about it my Baby spit gas also. One time at marina, I almost got ticket for it.

[speedboats]

Jimbo... again, I agree with you and can't fault your statement.

However, you are talking about volumteric efficiency. How much hp / litre of engine displacement.

My statement is about how much litre of consumption / hp.

Changing a cam, or the way the fuel/ air mix gets atop of the piston will not change how much fuel is required to make the said 260hp.

Changing cam, intake runners, head design, compression, fuel distribution affects how much air you can get into the chamber. More air means MORE FUEL can be added, resulting in more bang per cycle and MORE POWER MADE. I agree that the changes mentioned above mean you can easily get more power out of the same displacement engine, but 260hp will +/- always consume the same amount of fuel no matter the type of engine or configuration used.

[Fanie]

The fastest cheapest way to improve fuel economy and performance is to increase the compression ratio. The chevy motors used to run at 7.8:1 in the old days. If you up this to to 10.4:1 there is a difference already. Now you can replace the cam with one that has a bit more cam in it, but not much. You can buy std pop up pistons the original flat ones gets replaced with.

Smaller motors give better fuel per kw performance. Unfortunately there is not a cheap as in cheap solution, and no happy ending in sight either.

In the meantime think about sails and the trade winds

Eh, and what about a nice kite. If you can overcome the current and wind drag on the boat you can actually go somewhere.

[Jimbo1490]

All the newer EFI heads allow higher C.R.'s for sure. We ran 10.25-10.5 real measured CR all the time in our street Mustangs with Dart and Edelbrock heads with little or no detonation. Those new 'heart shaped' chambers made a huge difference over the old Ford heads. We had the same experience with the newer Vortec heads for Chevy after '96. They were simply superior to any head we had run, regardless of how much money we had poured into them at the machine shop. More CR with less detonation.

Jimbo

[Boston]

in a diesel the higher compression is what makes it so efficient
or at least one reason

but in gas engines you get into preignition problems with the fuel being a joke of mismanagement
the petroleum distillers are even allowed to put up to 100,000 tons of toxic waste into the fuel annually each, annually.
Thanks Reorge for the great legacy ( and nice job with the economy as well ).

The octane rating of fuel is highly dubious as its not checked all that often and when it is, its always lower than whats reported
Friend of mine was a formula one driver and he was always going off on passenger fuel and its lack of quality control. as apposed to all the controls the industry puts into the fuel he uses, same stuff but extremely clean

higher compression engines gas means you need higher octane levels and more expensive gas
trade offs no mater which direction you turn

[speedboats]

Again... up-ing the compression ratio doesn't mean that you get more energy (power) out of the same amount of fuel. As posted above, it forces the chemical reaction to go faster (ask any high school chemistry student).

A faster reaction means you can put MORE FUEL on top of the piston, creating a bigger bang means MORE POWER...

[Fanie]

Again... up-ing the compression ratio doesn't mean

It does too... You get better fuel consumption as well as more power.

The higher compression gives a bigger explosion than the low compression. You are limited in petrol engines by the compression ratio tho. A higher than 10 point something comp ration can result in spontanious combustion, which means if your engine doesn't stop running when you kill the ignition you may have to get out and shoot the darn thing Yes, arnour piercing works best

What chemical reaction ??? The gas ignites by the spark and explodes. The smaller chaimber and higher compression burns more complete and faster than a low compression ratio. You can check all the new cars have higher comp ratio's.

That said, the higher the octane fuel the lower the compression ratio, the lower the octane the higher the comp ratio can be.

[Boston]

I think he should get a steam engine

[Fanie]

Neuclear driven steam engines. VERY efficient, and you can go day and night non stop,,,

[Jimbo1490]

Originally Posted by Boston

in a diesel the higher compression is what makes it so efficient
or at least one reason

but in gas engines you get into preignition problems with the fuel being a joke of mismanagement
the petroleum distillers are even allowed to put up to 100,000 tons of toxic waste into the fuel annually each, annually.
Thanks Reorge for the great legacy ( and nice job with the economy as well ).

The octane rating of fuel is highly dubious as its not checked all that often and when it is, its always lower than whats reported
Friend of mine was a formula one driver and he was always going off on passenger fuel and its lack of quality control. as apposed to all the controls the industry puts into the fuel he uses, same stuff but extremely clean

higher compression engines gas means you need higher octane levels and more expensive gas
trade offs no mater which direction you turn

Diesels are NOT more efficient because of their higher compression ratio, rather they are more efficient despite their higher compression ratio. See if you can dig up some info on the Continental Motors AVDS1100 to see what a variable compression ratio diesel can do.

The higher compression ratio is only needed for cold starting and is absolutely worthless at all other times, robbing the engine of power and efficiency by uselessly over-compressing the air charge. The optimum compression ratio for best power in a diesel engine is surprisingly similar to that of a spark ignited engine, about 12.5:1. Trouble is, the engine will not start at this CR.

Various schemes have been worked out over the years to address this including dual fuel engines and variable compression ratio, but most are deemed too costly or complex for widespread use.

This problem declines with increasing engine size due to the greater heat loss per unit displacement and some other factors like engine design speed which all work against the smaller engines in this regard.

Many large diesels have surprisingly low CR's and still start just fine. The GM EMD engines use 14.5:1 without any starting aids like glow plugs but then they are 567, 645 and 710 cubic inches (9.3, 10.6 and 11.6 liters) per cylinder, so these are big engines

Jimbo

[mydauphin]

Back in the old days.... About 20+ years ago. When my friends and I would buy old muscle cars with 10 to 1 compression for $500. We would port the heads, manifolds, shave heads, install dome pistons . We made sure combustion chambers were smooth no sharp edges. We raised compression to 10.5 or even 11. We used avgas mixtures and water injection. Sometimes we retarded ignition timing or even cams. Anyway, we would get an extra 20% for all the trouble. Nitrous would also help cool intake and reduce detonation. This was all without EFI, or anything fancy just timing work and trial and error. Imagine if you did all this work on a big diesel or efi modern engine.