View Full Version : Fuel economy

Pat McElmeel
01-15-2003, 04:44 PM
" Have a 27' phoenix express fisherman with
a pair of Rochester 4 barrel carbureted
350 marine powers . These motors are 97s
and run sweet.I average 3/4 of a mile per gal.
and wonder if I'm being delusional to hope for
a tad more. Normal cruising with the wife
and kids is not a big deal. But on a run to
Bimini, 65 miles angling into a 4 kt current
I can drain 80 gal or more. I re proped per
Michigan wheel from 17X20 to17X17 and my RPM'S
are 4300 WOT and 30 MPH empty in flat conditions.
16 MPH at 3200 RPMS. Top speed and RPM's are per manual. A local shop says I'm under powered
and a new pair of Merc 300HP fuel injected
motors would be right spiffy. What is your
opinion? I love the boat and would consider
anything short of diesels "

01-15-2003, 06:49 PM
" Gee Pat, I had almost forgotten engines ever had carberators. You will notice a sugnificient improvement in fuel economy with a pair of new MPI engines. If you can obtain 30 mph, I have to say you are not underpowered. Keep in mind, if you increase your HP too much (8.1L) you will probably have to change your gears as well. Your dealer is right, 300 hp small blocks would be a good choice. You can probably put your 17X17s back on, still get the rpms you need and the performance you want. I would price Crusader as well as Mercruiser. Crusader is using GM's MEFI4 fuel injection while Mercruiser went to their own system which is more complicated and fewer people have the software to work on it. Crusader will most likely be less money as well. Good Luck. "

01-15-2003, 08:10 PM
" If you're committed to new engines, I'll add my vote for Crusaders. But if you're not and are at all concerned with costs, the $$$$$ spent repowering would probably fill your gas tanks for years to come.

FI engines do offer improved fuel economy and performance, but in a marine application the difference can be marginal. Also, FI is more expensive to buy and maintain. Your break-even point could be many years away if you reach it all.

I'm showing my bias here, but FI can't match the carb when it comes to cheap, simple and reliable operation. I agree with Rick that you're not underpowered. Your Rochester Q-jets' small primaries give excellent fuel economy if you can stay out of the secondaries when cruising. Also, if your engines haven't had a tune-up lately, that would help, too.

Why not stay with those "sweet runnning" engines until they justify replacement? "

Pat McElmeel
01-16-2003, 12:41 PM
" Rick and cruzerto
Thanks for the input guys.I guess I'll wait
till something breaks to repower. My mechanical
secondaries start to open at around 1800 RPM'S
and the speed is a little better then a sail boat
maybe 12mph , at 3200 my usual cruise I'm at 15 or
16, at about 35 to 3600 rpms is where it makes the sound like a car when you floor it. Is there an optimum or majic rpm range. Is Floscan worth the bucks ? thanks Pat in Pompano Beach "

01-16-2003, 07:30 PM
" Aha! Secondaries opening (beyond just cracked open) at 1800 is way too soon. My Q-jet's big flaps don't begin to open until above 3,000. The difference? Vaccuum secondaries. It might be worthwhile putting vaccuum Q-jets on your engines - I'll bet you would see considerable improvement in fuel consumption. Improved throttle response at low speed, too. Not sure if mechanicals can be converted, but the oldest mechanic in your marina would probably know.

I've never had FloScan meters, but everyone seems to rave about them, especially for extended cruising. "

01-16-2003, 09:02 PM
" Pat,
If I were going to replace my carbs, it would not be with new carbs. The best use for a carberator in 2003, is as a doorstop. Holley Marine (www.holley.com) makes a bolt on fuel injection system to fit your engines. Available in both TBI and MPI, they are not cheap, but compared to new motors are a real bargain. As far as Flo-Scan goes. A Flow-Scan is merely going to tell you that you are getting 3/4 MPG at the speed you like to cruise. And if you wand good economy, you must run at, as you would say, "sailboat" speed. Good luck. "

Randy Magnuson
04-21-2003, 09:14 PM
" For comparison, I have a pair of 454's (Marine Power) with Q-jets that also are smooth and push a Luhr's T320 and average .8 mpg at cruise. The secondaries begin opening at about 3200 rpm. It took some fiddling to get there, and the secret was becoming adept at rebuilding Q-jets. It took a couple of tries: despite popular opinion and great myth, this not a difficult task.

I was intimidated at first, but a Q-jet book by Doug Roe (available at Amazon among other places) is pretty easy to follow, with a couple of tricks to shore up some common factory problems with fuel well leaks and floats set up wrong.

After a time or two through a Q-jet, you can do one in less time than it usually takes to get to it and remove it. All you need is a couple of common tools, some epoxy glue and a generic Q-jet kit. They are available at auto parts stores (Walker is the brand name I believe) for about $15, and you can do at least a pair. Usually the problem becomes obvious once you get into it. Personally I prefer knowing that $7 and an hour can fix fuel problems wherever you are. EFI gets gets a few drops of water (always a problem on a boat), and sends them into major protest, fixable only by mechanics with magic boxes and "name that price." A fellow boater has the same boat and engines with EFI and flows 20% more fuel.

I differ opinion as to the value of Flowscan, as it is the best indicator of engine and carburetor health along with being able to "see" the secondaries during operation. With a little experience, one can also "see" stuck primaries (common Q-jet problem), bad float adjustments (correct needle valve installation goes against common sense and supplied configuration) and so forth, often during which the engine runs just fine. A little correlation and you'll become an ace at optimizing your engine(s) performance and keeping it that way. "

04-22-2003, 06:40 PM
" Randy, I have a couple of 23 year old Q-jet's that as far as I know have never been touched -- at least since '96 when I bought the boat. I've noticed that they seem to dry out when the boat sits for more than a few hours -- requires about 10 sec. of cranking before they fire off, pumping the throttles doesn't help. Would it be worth the time and $30 to do a rebuild on them?

I have no strong indication that anything is wrong and I'm inclined to not touch anything that isn't broken. Fuel consumption was about dead even before I replaced the port short block. Now the port uses a little more fuel than the starboard.

I don't notice a marked difference in fuel consumption between my normal cruise at 3000 rpm (secondaries should be closed) and high cruse at 3400 rpm (they should be open).

Could I expect a fuel consumption improvement and/or quicker lightoff with a careful rebuild? "

Dale Gardner
03-27-2004, 09:22 PM
I had a 87 Trojan 8.6 meter with 5.7 260 h.p.
with I/O. I would cruise at 3000 rpm at 28 mph
I would get 1 mpg with Rochester Q-jet. It would
top out at 43 mph. I used 16x16 props.

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