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Old 15-04-2014, 07:50   #1
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Crusader 425hp 8.1ho Fuel curve

HI all
Im hoping that someone here on CF has a fuel curve for the above mentioned engine.

And in addition Id be keen to hear of any experience of these engines in relations to longevity and economy.

Cheers in advance!
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Old 15-04-2014, 08:48   #2
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Re: Crusader 425hp 8.1ho Fuel curve

None of the gasser suppliers offer a hp/torque curve, much less a fuel curve. The following is a generic GMC Marine Vortec V8 hp/torque curve- the one used by Crusader, etc but for a 5.7 liter V8.. Create your own fuel curve by multiplying each hp point by 425/292 (the horsepower ratio) and dividing by 12 (12 hp per gph for most FI gassers) to get gph at each rpm.

This will be a ballpark curve but probably within 15%.

But it won't be very useful. To know how many gph your engines are burning at x rpm you need the propeller fuel curve. This takes a little more math. If you want to do it, reply and I will walk you through it.

David
Attached Files
File Type: pdf GM 5.7 Vortec data sheet and hp and torque curve.pdf (361.6 KB, 60 views)
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Old 16-04-2014, 04:26   #3
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Re: Crusader 425hp 8.1ho Fuel curve

Many thanks David. Ive worked out some ballpark figures now thanks to your info. Id be interested to go through the extra math with you to see what we come out with. As of today we have booked a time with the broker of the boat running these engines and will be taking it for a run next Wednesday.
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Old 16-04-2014, 08:05   #4
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Re: Crusader 425hp 8.1ho Fuel curve

OK, you asked for it ;-):

Theoretically a propeller absorbs horsepower in an exponential relationship to its rpm. That exponent varies from 2.5 to 3.0 for various propellers and boats. So lets use 2.7.

The formula is hp = f *rpm^2.7 where f is a factor based on a lot of stuff- transmission ratio, diameter, pitch, blade area, etc. But since we know the hp of the engine is 425 at an assumed rpm of 5,000 and all of that power is absorbed by the prop (if propped properly) we can back calculate the f factor.

So for your engine the factor is .0000000438. Plugging that into the formula for various rpms gives the following and dividing the hp by 12 hp per gph (a good rule of thumb for FI gassers) gives the gph burned at each rpm:

RPM------HP--GPH

1000----5.5---0.5
2000----36----3.0
3000----107---9
4000----233---19
5000----425---35

Now all of this is theoretical and based on the 12 hp per gph rule of thumb which is no good at low rpms. It takes a certain amount of hp just to turn over the engine machinery irrespective of the load on its crankshaft. Also the thermodynamics of a gasser engine get poorer at low rpms and low power loadings. So don't pay any attention to the 1 and 2,000 rpm values.

But the table definitely shows the benefits of slowing down on fuel consumption!!!! Your best fuel consumption will probably be the point where the boat is just up on a comfortable plane. Any more just burns more fuel.

David
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Old 16-04-2014, 09:38   #5
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Re: Crusader 425hp 8.1ho Fuel curve

Are these big block Chevy's, 454's? If so I have run them on a boat for considerable time. From a lifeing perspective anything above about 3,000 RPM starts to hurt, boat I use to run wouldn't plane until about 3,400 though if I remember correctly. 36' Pescomore (excuse the spelling please)
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Old 18-04-2014, 08:49   #6
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Re: Crusader 425hp 8.1ho Fuel curve

Thanks David.

Your information has really helped and painted a clearer picture than that supplied by the broker!!
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Old 18-04-2014, 09:14   #7
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Re: Crusader 425hp 8.1ho Fuel curve

Have a 501 in a 25 ft Carolina Classic. About a 9000 lb rig. I plane around 3000 rpm's and have obtained about 2.5 MPG over a measured run inland water. Trolling around a 1000 rpm 5 or so knts less than a gallon an hour. Usually 25 or so gallons per trip.All open ocean trolling. Hope this helps some
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