Diesel vs gas horsepower/torque comparisons

Toolate

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BillD

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Take a peek @ the performance curves of the Cummins 6BT 220.

http://boatdiesel.com/Viewers/index.cfm?ESF=7506JA5A4E767E5458B49A964C76AE76A4A89448B04040441&PU=3

You can buy this engine for well under 20K.

Take a peek @ the Crusader Vortec 5.7 MPI 330 hp.

http://www.crusaderengines.com/NewFiles/5057specs.pdf

@ 2400 rpm 220 hp 6BT is producing 205 hp, 450 ft.lbs of torque and burning 9.2 gallons per hour.

Crusader is pretty stingy with "performance curves", I can't find them on the web. Maybe somebody here can.

I'll guess @ 4000 rpm the 5.7 330 is producing well under 300 hp and "maybe" 375 ft lbs of torque and the fuel consumption "would be substantial" over the smaller diesel.

OK, what can you swing for a prop and what gear ration ?

Let's take a 22X22 prop in an under 28 foot DE.

I figure the little 6BT 220 could swing the 22X22 prop no problem with a 2:1 gear and the engine would make rated rpm (2600) no problem. Go to a 21 inch prop you could maybe go to 1.75:1 or 1.5:1

I figure the 5.7 330 MPI would need a 2.9:1 gear to swing a 22X22 square prop.

Again, I'm no prop guy but I look @ torque to move boats. Keep the rpms down, gear correctly and burn less fuel.

A 6BT 220 will spin all day every day @ 2400 spinning the 22X22 prop while burning 9 gph.

The gasser?? do the math, you'll be up around 3600 rpm to get the same speed as the 6BT @ 2400 and burning twice as much fuel maybe ???

Again, no expert.
 

steveinak

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As much as i don't care for volvo's there are some D4 & D6 used engines showing up on the market, i'd think one of those would be nice for a smaller boat. A friend has a D6(300hp) in his 30 aluminum bowpicker it pushes the boat nicely at 20-22 knot cruise and he hasn't had any engine problems in aprox 3000/hrs now the outdrive is another story :rolleyes:.
 

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djmarchand

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HP and Torque Curves- gas vs diesel

Here is a hp/torque curve from a generic GMC 5.7 gasser. As others have noted Crusader, Mercruiser and others never publish their curves. Go to http://www.centerstateengine.com/_CTR_5.7L_Vortec_Flyer_PDF.pdf

Here is a hp/torque curve for a nominally equivalent diesel, the Cummins QSB 305 hp- http://www.sbmar.com/Engines/PDF/QSB%20Curves/QSB%20305%20Curve%20May%202010%20M-91369.pdf

There are two significant differences:

1. The gasser peaks at 4,800 rpm whereas the diesel peakes at 2,600 rpm

2. The gasser curve is much steeper. The diesel curve reaches near its peak at 2,000 rpm and holds it near their up to and beyond 2,600.

So there are several things to consider when installing one or the other of these in a boat:

1. The gasser will need a numerically higher transmission ratio so it can turn the prop more or less at the same rpm as the diesel.

2. The diesel installation will sometimes need a beefier prop shaft due to its higher torque which can break the shaft while accelerating over the hump near peak torque.

3. The diesel will usually be easier to get up on plane because of its higher torque.

David
 
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BillD

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Great response Bill- thanks for taking the time to write that all out. Just the kind of analysis I need to do.

I couldnt get the boatdiesel link to work but if the 6bt make that kind of power then the only gas equivalent is the big block (8.1 or 454) which, from every performance curve I have seen, should be making about 200 hp and just over 400 lbft of torque at around 3000-3200. I dont want to have to cruise a gasser any higher than that- just too much IMO. Its surprising how flat the torque curves are on modern gas v8's (see here- not marine specific but its a big block at least- Graph of 8.1L Factory Dyno - Diesel Place : Chevrolet and GMC Diesel Truck Forums ).

SO:

1) Can you fix that link? I tried and cant find it.

2) where can I get that 6bt for under 20? I am not above a reman or used engine but would prefer a reman obviously..

3) transmission seems like it needs to be decided on after the engine is found (while being considered all along of course) but the goal would be to turn the biggest prop as slow as possible. I have no idea what prop will fit on a Sisu but I think 22 is within an inch or so.


I really would like to stay away from a Volvo as you stated. I got myself all excited over a number of boats with them but I have the opportunity to start fresh here and would really like to stay away from the volvo parts cost/delays if possible. My mother had a volvo car and the radiator took 3 weeks and cost almost $2k...add the word marine to the order and it doubles in my experience (like the work "wedding" in case any of you have recently married).

Try this link:

http://www.sbmar.com/Engines/PDF/6BT/6BT 220 Curve -- Aug 04.pdf

And while you are on the Seaboard Marine website do some reading.

Transmission ?? I'd be using a ZF63A gear, $1900-$2000 depending, many gear ratios available. Rugged robust gear.

And remember, there is not much RECON about Cummins. The engines are essentially 100% NEW factory fresh.

Any Cummins marine dealer has access to the line of RECON engines. I bet if you "shop around" you could get a 6BT210/220 ZF63A package for $20K or under.

Keep us posted
 

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djmarchand

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Yes the Cummins makes a lot of torque. It peaks at 783 ft lbs at 1,800 rpm. But that 6.2 liter gasser peaks at a little over 400 ft lbs at 3,600 rpm. But they are about equal at the prop.

If the diesel needs a 1.5:1 transmission ratio then the gasser will need almost twice as high a ratio and the numerically higher ratio means about the same torque as the diesel at the prop shaft.

A 6.2 liter or so gasser in a low 30s foot downeaster should push it fine. If you get up to 35' then it is going to take 250 or so hp to push it to 15 kts. That is a bit much for a gasser.

Modern diesels produce 18-20 hp per gph. Modern fuel injected gassers make 11-13 hp per gph.. Do the math at 250 hp and see how many hours it takes to breakeven.

A remmaned Cummins will cost about $20K. A Crusader 6.0 will cost about $12K. Less if you buy one of the lesser known brands.

But there is another point. I have a well maintained Cape Dory 28 for sale with a strong Chrysler gasser. No body wants it. I could have sold it ten times over if it had a Yanmar 240 hp diesel and for much more money of course. People are fine with gassers in express cruisers but not a downeaster.

David
 

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