why is my gas mileage so bad

My 1993 7.4L B2 would burn about 15 - 16 gph at 3500 rpm's my cruise rpm was around 3100 burning about 12 - 13 gph. This was my fuel flow meter reading. a little over 3500 rpm the 4 barrel would open the the fuel flow would jump to over 21 gph
 
Four Suns said:
For those posting fuel burn numbers, how are you calculating this? Do you have fuel flow meters?... or are you using the BS approach of dividing the hours on the meter into the gallons you took in at a fill up?
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Floscan, is within 5 gallons on 200 gal reading.
 
I'm using a fuel flow meter as well. Just seemed that my rpm's are running high for the speed I'm going.
 
I think you should look at the props first. Call a qualified big boat prop shop and tell them what you have, maybe they can recommend a change.

Boat should be able to do over 30 mph? Probably have a great hole shot as it sits now I bet.
 
Whitenights said:
I think you should look at the props first. Call a qualified big boat prop shop and tell them what you have, maybe they can recommend a change.

Boat should be able to do over 30 mph? Probably have a great hole shot as it sits now I bet.
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:thumbsup::thumbsup::thumbsup:
 
Everybody's correct in "whats your WOT" rpm?
This is how to see if you are propped correctly.
If too much rpm WOT you need more prop.
If not rated WOT rpm..you need less prop.

IMHO you should not cruise over 3500rpm with an older tech 454.
Boat will be best at 3000-3400 with a little tab.
My friend had your boat, years ago and its a great riding boat and a BIG 34.
Not a speed boat tho......
 
I am not sure of the spec. for your engine.
Mine (1995 454 gen V, bigger boat) is 4400-4600 so, if the same you are good.
Maybe a fresh bottom job will bring up the rpm a little..

We really need a member with the SAME boat to compare performance.
Keep in mind your boat is a totally different boat than the newer ones.
They dont build them like they used to............
:)
 
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itshamortime said:
WOT is 4400 rpm and speed is 30 MPH per GPS.
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That is about where I am, but I am V-drives, yours are straight?

If so they should give you a little better performance since the boat is balanced better and uses less tabs. I pretty much have to run with tabs 80-90% down and am still nose high.

When I bought a set of props for mine, from Lauderdale Props, they added a "Sea Ray" cup, no idea what that was for but boat runs better with this set.
 
The Sea Ray cup is for all us "boobs" that worry about .2mpg, like we're really going somewhere......
Ocean crossers rarely have a wave emblem on them
 
If you search for "props" under Sprink56 there is a thread with burn rates and RPM for these era boat.
Mine generally runs 14-16 GHP on FloScans at 3100 RPM and 19-23 Knots. Mine is a Straight Drive express Cruiser which carries 50ish gal of fresh water and usually 75% fuel load.
Your numbers seem to be so far off, but sounds to me like you are running her wide open. What are your numbers at 3000 RPM?
I do know that above the cruise speed the speed doesnt pick up parallel to RPM and GPH. GPH goes way up over 3100 RPM.
 
Since you are powered by 454 cid engines you have four barrel carbuerators. The primary barrels are used to idle and run the engines up to a certain RPM level. Probably around 3100 RPM max depending on your camshaft and timing. Running these "Big Block" engines above this RPM does not give you efficient performance. You are essentially running twice as much carburation or even more, once you start running the carburators on the "Primary and Secondary Barrel" combined, when you run the engines over this RPM level, you more than double the amount of fuel sprayed into your intake manifold.
There is an efficient RPM Range in which you should run those engines. My guess is between 2900 and 3200 RPM. Any higher operation level will lead to excessive fuel consumption, wear and tear on the valve train and piston/rod assembly and deteriorization of the engine lubrication properties.
Hover, the reason all that extra power is there is for getting the boat on plane in heavy seas or making a run to shelter from heavy seas. The 454's were not engineered to run at 4000 RPM continuously.
 
Just to point out, this is a 13 month old post. I sure hope he found the problem, but he never posted it if he did!
Welcome to the Club!
 
I found that right about 3100 rpms is great for me. I am running in fresh water versus salt water. Conditions can make big difference on what you will get. I never to to WOT or even close 4000 rpm. I can watch my gas gauge (now that I put new sending units in) drop as I move.
 
relentless1996 said:
I have a 370 sundancer with 454's We cruise to Catalina island off LA Cal 1.5 hours each way at 3400 rpm I burn 18 GPH your RPM is whats killing you.
pump the brakes !!!
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I do that trip all the time as well and cruise around 3450 RPM's and burn between 31 and 32 GPH with 8.1's.
 
I have had a dual Floscan meter on my boat for the past five years and a Navman fuel management system before that. The sweet spot seems to be around 3200 to 3300 rpm depending on how much fuel and people are on board. Also, the condition of the bottom (clean versus growth) seems to have a profound difference on fuel economy.

At best, the fuel economy is .7 mpg and at worst it is .6 at 3300 rpm. You cannot compare fuel consumption from an 18' or 24' foot boat to a 37' boat....the physics are completely different.

While it is a staggering number....burning 15 gallons an hour per engine means you are burning 30 gallons per hour ($90 an hour at $3 a gallon). A .7 mpg gives you 21 mph which can vary based on sea conditions.

However, given that I find myself on the Potomac river....most of the fuel burn is at 1200 rpm where the fuel economy gets closer to 1.0.

Sure, I am jealous of the diesel guys. Who wouldn't want to burn 50% less fuel and have the same speed. But .....I like gas engines because they are cheaper to maintain and replace.

Whenever I read that someone is cruising a 37' gas Sea Ray at 3500 rpm and only burning a total of 15 gallons an hour.......it just isn't credible.
 
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