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joel mcqueen
02-13-2007, 07:27 PM
i know you guys are more interested in high speed performance but i am going to repower my 2 donzis and need info curently am running carb 454s 330 hp and would like to go with 5.7 L 325 hp small blocks question is fuel consumption less weight close to same hp will it burn less with newer fuel injected small blocks and get close to same performance ?????

p729lws
02-13-2007, 07:47 PM
my brain hurts now

joel mcqueen
02-13-2007, 07:51 PM
sorry it make my head hurt to!! and my wallet !! but someone out there knows the answers ???????

Rootsy
02-13-2007, 07:54 PM
doesn't matter if you have 350 CID or 3000 CID... fuel consumption is dependant upon developed HP and efficiency in an engine... mixture, internal friction, windage, inertia, etc.... as a general "rule of thumb" figure 1/2 lb of fuel / developed hp / hour of fuel consumption (BSFC - Brake Specific Fuel Consumption).. meaning that, as an example... (be aware that efficient racing and production engines, fuel injected and so on and so forth might be in the .4 lb / hp / hr range)

a 300 MAXIMUM HP engine that is in an 18 donzi that is say cruising at 4000 rpm.. maybe it requires 250 developed HP to maintain 4000 rpm under the conditions you are running... in that case... if you maintain that rpm and load you'll burn 125 lbs of gasoline / hr... figure somewhere around 6.2 lbs / gallon for fuel... therefore you'll consume about 20.2 gallons of gasoline an hour at that load which requires 250 hp, developed, to maintain...

btw, an engine only develops the amount of torque it needs to maintain a steady state at an rpm... and HP is a function of torque and rpm....

unlike a car... a boat's load curve is nearly linear from on plane to WOT... mainly a boats load curve is like a car trying to climb a mountain....

clear as mud? :bonk:

realbold
02-13-2007, 09:23 PM
Now my head hurts, but the fuel injected SBC's combined with less weight should definitely burn less fuel.

Cuda
02-13-2007, 09:43 PM
I doubt you will notice a significant difference. Torque is the name of the game, and it's about directly proportional to cubic inches. I think the big blocks won't have to work as hard. I know when I went up 55 horses on my fishboat, I got better fuel economy, because it always had to work it's ass off before.

BUIZILLA
02-13-2007, 09:50 PM
in box stock form, the Crusader FWC 6.2/375hp mouse is hard to beat...

Raylar is introducing a 450'' LS2, all aluminum, marinized package, at the boat show this week too... now that IS bad a$$

oldandtired
02-13-2007, 10:02 PM
At part throttle conditions, a smaller engine definitely uses less fuel than a bigger engine at the same power level and RPM. The reason is that the engine is also an air pump. Pumping horsepower is a function of flow rate and delta p. The smaller engine has less pumping displacement than the bigger one and therefore consumes less horsepower in its pumping stroke. A small block can also run a slightly higher compression ratio on the same fuel quality which helps some. Then there is the fact that a small block weighs a lot less and results in higher speeds at the same power level. I still prefer my big block, but small rules when it comes to part throttle fuel efficiency. At WOT, the smaller engine should do a bit better in efficiency because smaller diameter pistons mean that it takes less time for the flame to propagate through the cylinder, resulting in a cycle that is closer to ideal.

My head is starting to hurt too,

Rene

farmer tx
02-14-2007, 01:14 AM
At lake Powell Trueser running an 86' 22-C with a smallblock burned the same amount or a tad bit more fuel than I did with a 86' 22-C with a big block. Ran together all day so I thought it was a fair comparison. These are consecutive HIN# boats.

DonCig
02-14-2007, 01:41 AM
http://www.donzi.net/forums/showthread.php?t=24078&highlight=fuel+mileage

I went back and looked at some old posts from GEOO, myself and others.
GEOO was getting 16 gph at 4,000 rpm, 60 mph, with his 383MPI X-18.
DonCig was getting 15.5 gph at 3,800 rpm at 62 mph with his SBC 427 carburated engine.
Old Yeller is running 61 mph at 4,000 rpm in his boat.

So it appears that a SBC in a reasonable state of tune in a Classic 18 will yield a 16 gph boat that can cruise at 60 mph in the 3,800 to 4,000 rpm range.

joel mcqueen
02-14-2007, 03:32 PM
so it looks like there is not enough difference between old big block with a carb and new small block fuel injection in the same hp range as far as fuel economy goes

gcarter
02-14-2007, 06:34 PM
At lake Powell Trueser running an 86' 22-C with a smallblock burned the same amount or a tad bit more fuel than I did with a 86' 22-C with a big block. Ran together all day so I thought it was a fair comparison. These are consecutive HIN# boats.
But Mike runs a 21" prop, so he's not as efficient as he could be.
Larger engines do have more pumping losses and more internal friction....larger bearing area, ring area ditto.
Large engine and small engine with the same load, the larger will be producing more (or expending more) energy to have the same output at the flywheel.
P.S.;
Before it became a regulation, this is why Formula 1 engines were V-10 instead of V-12 (lower friction and pumping losses). Everything else being equal, the multi cylinder engine will produce more power because, the more cylinders, the more piston area.