RPMs and Fuel Burn

[ironj32]

Anyone have rule of thumb as to how much more fuel is burned per every increase of, say, 100 RPMs? I've got a few Acme props that I'm testing out, and am wondering if there is much difference in how much gas I'm burning.

I know I could go out and just drive them for 15 minutes each, under the same conditions (wouldn't want to do it while pulling a rider as there would be too many variables, with falling and all), but am wondering if anyone has already figured this out.

[GMLIII]

Anyone have rule of thumb as to how much more fuel is burned per every increase of, say, 100 RPMs? I've got a few Acme props that I'm testing out, and am wondering if there is much difference in how much gas I'm burning.

I know I could go out and just drive them for 15 minutes each, under the same conditions (wouldn't want to do it while pulling a rider as there would be too many variables, with falling and all), but am wondering if anyone has already figured this out.

Try this thread

• General Nautique Discussion

3081 or 3087 G23 with H6 stock ballast


Forum member, Greggmck is the expert on topic and has performed extensive research

[ironj32]

Thanks for steering me there, GMLII!

[Bevostein]

550 (which I believe you run in your boat) could be different than the results in that thread

[ironj32]

yea, i talked with Acme as well. it appears the only way to really find out is for me to go out and do some controlled testing. probably will drive each prop at wakeboard speed for about 8-10 minutes (no rider, so no falling or anything). will start each one with a completely full tank, and see how much each needs to refill.

[jbwake]

If you had an OBD2 reader you may be able to find out fuel burn rate from the ECM?

Can't say I have seen an OBD2 port but I imagine there must be one somewhere for engine diagnostics?

[Wake.BC]

I’m curious on this too. Won’t dropping my rpm by 400 is worth it in gas savings


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[Infinity]

yea, i talked with Acme as well. it appears the only way to really find out is for me to go out and do some controlled testing. probably will drive each prop at wakeboard speed for about 8-10 minutes (no rider, so no falling or anything). will start each one with a completely full tank, and see how much each needs to refill.

I do not see how you can get accurate results on 8-10mins only. "topping off tank" each time could yield slightly different results each time and with 8-10mins of run time, ounces will make a difference in outcome.
I am no expert in this field, but I do not see how running less rpm could not increase your efficiency...but also would think that its not as simple as a calculation between 100-200rpm as I assume that burn would also change depending on the load that is on engine. For instance....running empty at 3300rpm VS full ballasted surfing at 3300rpm? I could be wrong on the direct injection engines.
Certainly would like to hear what you find out! BTW, There is an diagnostic port in engine area (wire harness)....don't know exactly where it is, but have seen one hooked up to a G23 and know its there.

[ironj32]

from what i've been reading from other posts, it should burn the same amount fuel at 4000 of rpm with a "light" load as 4000 with a "heavy" load. also, it sounds like the engines are more efficient when running closer to the rpm that produces max hp - meaning if max hp is achieved at 4000 rpm, fuel efficiency should be better running 4100 rpm vs 3500 rpm (these are completely random numbers, by the way). that at least seems to be the case with the H6. there hasn't been much discussion on the XR7, so I'm going to try to see what's what with that engine.

10 minutes might not be enough to get super accurate info, but I'm pretty sure I'll burn around 4-5 gallons of fuel in 10 min, which would be enough for my analysis (15-20 minutes would definitely be more accurate, but driving for 20 minutes up/down the lake, with no one behind the boat would just feel awkward, lol)...so if there is a difference of 1/2 gallon, that would mean that there would be a 1 gallon difference per 20 minute set (if that was the case, i'd rather have a prop that gets me out of the hole quicker.

[Infinity]

I sure hope you are not gonna burn 4-5 gallons in 10mins...thats 25-30GPH! You may be right tho....guess I was thinking you would prob only burn like 2 gallons in 10mins and if you were only talking 100-200rpm difference between props, then ounces could count on total measurement, I could be totally off tho. My thot is to go with the prop that does everything you want it to do and also runs at optimal rpm for you since I don't think there will be a huge difference in fuel unless you are going from torque prop way up to barefoot prop. On rpm according to the owners manual...you are not supposed to run engine over 4000rpm for "extended periods of time", so you likely do not want a prop that puts you over that for the sport you do most (Wakeboard/Surf?). I could be wrong, but thot that max HP was made at WOT, which is your rpm limit, torque curve/peak however will be at a different rpm and each engine is different.
The G's are big, heavy boats plus obviously you have the XR550, I have a friend that had done some calculations (H6 engine) on his G3 last summer and was averaging 12-13GPH for the day (pretty sure he had the 17x17 prop).... that "average" included the idling, cruising to spot, fully ballasted surfing, etc. He filled boat every time, so was easy for him to calculate, I did the same thing on my boat all last year so I would know my GPH. Guessing your engine will burn a little more fuel than his since it will be making more power, let us know what you find out please. I have another friend that just bought a G25 and he wants to pick up a spare and asked me to find out what would be a good option, I know it will be slightly different, but doubt too much different between G23 vs G25.

For information purposes only....here is a video that BoardCo did, it was actually a "fuel comparison" between Centurion and Malibu, but they also threw in a G23 in this 2nd test and I think it has the XR7 engine in it. Obviously not apples to apples, but he does have a diacom system hooked up to it, may give you some useful info:

http://www.mmwatersports.com/boat-pe...consumption-2/

[jharris]

With all of these posts going on, I thought I would try to post something here that better explains fuel consumption at different speed and load conditions in an engine. In engine development, the attached graph I is referred to as BSFC map. (Brake Specific Fuel Consumption) map. This is a generic graph I found on line but is a good representation of what happenes in an engine. I tried to search for something for the GM Gen5 small black engine (H6) engine but these are not usually made public. To explain the map, rpm is on the x axis and engine load is on the y axis. The plot in the chart represents the fuel consumption for a given power at the engine load and rpm. As you can see the most efficient portion of the engine map is near full load. One primary effect of this is the throttling of the engine. When you throttle an engine you restrict the amount of air that flows into the cylinder. This makes the engine actually work harder to pull the air in to the cylinder. This effect is known as pumping losses.

This is what drives the idea of down speeding. For a given power, and engine will be more efficient at a higher engine load and lower rpm. So you can adjust a transmission ratio to have the engine at a lower rpm engine but a higher engine load.

Now relationg this to our boats. To push a boat through the water at a given speed it takes a given power. By changing the size and pitch of the prop we can adjust where the engine run on the map. This is easier in a car with an 8 speed transmission because we can continuously adjust the gear ratio to a more optimized range. In a boat the selection is more difficult because we have a single speed transmission and the inefficiency of the prop slippage. The efficiency of a prop and all of the subtle elements of the design could probably be a good thesis project but well outside my scope of knowledge.

Hope this helps people understand what is going on in these engines. Just remember on these graphs, the lower the number in the plot area, the more efficient the engine. Hope this helps.

[xrichard]

Thanks for that write up. I've read in the past that the most efficient area to run a heavy load tends to be between horsepower and torque peaks. I don't know if that is true.

Boating magazine reviews typically measure actual fuel consumption. Almost without fail, inboards tend to get the greatest MPG around 3500 rpm. (Back when magazines were popular, multiple magazines published fuel consumption reports and my recollection is 3300-3600 rpm typically gave the best mpg in other publications.) In any event, Boating has quite a few tests of wakeboard / v-drive boats that you can access on their website. They run the boats w/o ballast and it could be the optimum rpm will be different weighted. I'm inclined to think their tests are pretty close re: efficient operating range even with more weight in the boat.