Diesel vs gas horsepower/torque comparisons

Toolate

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So, as some of you may know, I have been looking for a 30 to 35 foot down Easter. The search inevitably leads to looking at boats that would be better with new power and the question of gas versus diesel comes up.

My question is, for a 300 to 350 hp diesel engine, where does it make them majority of its torque? At what RPM? i assume that it is around 2400 RPMs and not at the top of the power band Which is why diesel engines are generally used for boats and also why they are more efficient than gasoline engines cruising speeds. I think I remanufactured Cummins diesel would be the only engine I could afford to install in about and barely at that So if anyone knows where I can see horsepower/torque curves for these motors, I would appreciate some direction.

I have owned several gasoline engines and understand the technology much better (gas engines usually make their highest work and horsepower ratings at the top of their RPM range). What I am looking to do is take make a comparison between the horsepower and torque ratings of a Cummins diesel to a gasoline V-8 that I can buy for much less And see if it makes sense for this boat.

The boat I am currently looking at is a 30 Sisu which is, in my opinion, at the limit of where gas engines can be used With any degree of efficiency and longevity. I would love to discover that I could put a 383 stroker or a 454 in this boat and match it to the right transmission and prop and achieve 14 to 16 not cruising speeds with fuel consumption under 10 gallons per hour.

I think for a larger, heavier boat, that this analysis would probably not be worthwhile because gas engines simply don't make the torque they need to to push a bigger boat But for this book, it may prove a reasonable experiment.

Anyone have any experience with this? Also need a source for V8 gas power/Marine transmission combinations in Connecticut area.
 

Toolate

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Forgot to add that, if possible, I would like to do a small block General Motors Marine conversion that has been remanufactured. Wondering where I could get a good deal on a good motor. Not looking to cheat myself in the long run by installing junk But I don't need a brand-new engine with the best everything. Same goes for transmission. Any suggestions on sources for combination of both what the problem.
 

Toolate

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No one? Starting to think this subject has been beaten to death and no one wants to even start. How about some gas engine aNd trans sources in addition what has been posted ? Thoughts on 30' sisu performance with various gas engines? Help!
 

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:.
 

Toolate

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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).
 

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
 

Toolate

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THis is getting good:

DJ- look at this- http://www.centerstateengine.com/_CTR_6.2L_Flyer_PDF.pdf

Same site but its a 383 (6.2) and the torque curve is almost flat across the rpm range at 400 lbft the whole way. Hp at my intended cruise is over 200 so its not far from the diesel we looked at earlier.

(That 330 Cummins you posted was a torque monster developing 800+ (is that right?) and no gas engine could match it)

In order to maintain prop rpms I will need a higher reduction through my tans. I will look into these sites and hunt around to see what i can find for reman diesels and trans combinations.

Would either of you (or anyone else on the interweb) disagree that this boat could be pushed by a gas engine though? Seems like there is no question it could be done but at what rate of fuel consumption. Seems like the diesel curves are just under 10 gph so the gas will be more but how much is the question.

This takes this conversation beck to the age old question. Hours use/run time/fuel costs etc. and how to add all that up to sort out which way to go. I am a 100-150 hour/year boater so 5 gallons an hour (if the gasser ran 15gph) is maybe 5-700 gallons more gas which is about $2000-$3000. If the diesel costs $20g's and the gas is $10g's then it might just make sense to go to the diesel. One step at a time...
 

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
 

Toolate

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Thanks for confirming my suspicions DJ. Still lots to sort out though obviously..

I am looking at a boat that has a gas engine and that is the reason for this thread. Frankly, as has been discussed a million times, a diesel isnt needed for smaller boats. I think its a modern fad to expect them in a 28' boat and almost more representative of how little knowledge the average boater has about this kind of stuff.

I am 38 and grew a up around outboards and have owned 2 inboard gas boats which is why I am struggling with the idea that a 10000 lb DE boat "needs" to have a diesel. Plus I secretly want a gas engine because I know the parts on a GM based V8 intuitively and diesels are strangers. I started out wanting a 35 Nauset but have now (I think, wife has to be consulted too...) sort of talked myself into a smaller boat so I can have gas.

For the record, the boat I am looking at has a Chrysler 318 v8, of which I have owned 3, and I want to change it to go faster than it will push the boat, not because I dont like the engine or think it suits the boat. At 8 knots, what the hell is the difference except noise
 
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