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Larger engine vs fuel economy

T

tuandabrainz

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"Gentlemen,
I am trying

"Gentlemen,
I am trying to decide if I should change to a larger engine. Right now, I have an in line 6 mercruiser 165 that pushes the boat along, but I am wondering if I go with a larger engine, say a GM 305, if the extra power would actually save fuel because I would be getting on plane faster and could cruise at a pretty good clip with a lower throttle setting. Its a pretty big boat, a 20 ft Tiara cuddy cruiser that weighs about 3900 lbs. The I6 pushes her along pretty well with two or 3 people on board and light gear, but with a heavy load she struggeles to get on plane, and the fact is it is harder to find goodies and upgrades for the current engine. However, if the 305 is going to gobble a lot more gas, I would just as soon stay with what I have now. Anybody got any suggestions or thoughts?"
 
"Even if it did save fuel, unl

"Even if it did save fuel, unless you boat several hundred hours each season, the cost of repowering would never be recovered in fuel savings in the life of the boat.
e.g. If you save $200/year in fuel, it would take 20 years to save $4000 and even if buying used stuff, $4G's is the lowest I could see budgeting.


If you were to repower for more oomph, I would suggest looking for a deal on a 292 inline 6.
A more straightforward swap that would let you reuse most of your marine specific components like exhaust/intake manifold, engine mounts etc...plus you keep the simple inline 6 design and you know it'll fit in the same place as the current 250 cu. in. you have

Just look for a 292 that has the front mounting on the block. Some did and some didn't. Merc rated the 292 at 200HP the 1 year they used it.
292:
http://i50.photobucket.com/albums/f349/luckystrikemike/100_0324.jpg"
"
 
"I'm not trying to recover

"I'm not trying to recover the cost of the engine in fuel, but I am trying to avoid spending 100$ on a tank of fuel every time I go out. I have a 5.7 liter engine in my truck, and it burns through gas alarmingly, which is why I don't want to go with a 350. I could go with a 292, but the reason I asked about a 305 is that they are a lot more common than the I6 engines,easier to find aftermarket parts for, and it is less than 1L differnece in size from the 4.1 I am using right now, which I think may not be to big of a change in gph. This boat actually originally had a V8 in it that was locked up when I bought the boat, and I did modifications to put the I6 in that can be pretty easily changed. As I see it, the only thing I would need to add to put in a 305 is a dual horn exhaust pipe instead of the single that is on it. However, if the 305 sucks a significant amount of fuel more than the inline, it would not be worth doing. In any case, thank you for your advice."
 
"Your economics completely baf

"Your economics completely baffle me.

if you don't recover the cost of the engine in fuel, it's a <u>losing proposition</u>.

You will be out more money than you would be if you just continue using the boat as it is now.

Besides, the most advanced marine engineers in the world could not predict how your particular fiberglass hull will react at different speeds, weight biases, and torque levels. Whether or not a 305 would use more or less fuel than your inline 6 is a total crap shoot. I would bet on a V8 using more at equal boat speeds.

How efficient an engine is in a truck rolling down the highway producing 30HP at the flywheel is no predictor of it's efficiency pushing a boat with 100HP at low cruise, or 230HP at WOT continuous. It's apples and oranges."
 
"going to more inches will NOT

"going to more inches will NOT save you fuel dollars. instead, make the engine work easier. Swap the upper drive to one with a 2:1 ratio and change the prop to make WOT w/ full load. If you still have the original (V-8) 1.5:1 gears, you'll notice even more improvement.

Modernize the I-6 ignition will help, too, if you haven't already done that.

The only other option is 'slow down' which is much better than staying at home."
 
How about a 488? A 4 cyl that

How about a 488? A 4 cyl that has more power than your 6. And you won't have to swap gears. What kind of fuel consumption are you talking with the 165?

Also do you have trim tabs? They help your boat get on a plane a lot faster than without.
 
"Hy stat,
if my economics ba

"Hy stat,
if my economics baffle you, its because you didn't pay attention to the rest of my post. Fuel consumption is not the only factor i am looking at. Ease of finding parts and more power are also factors. There are a lot more of the 305's out there than the 250's, because they were in production longer. Simply put, if I can get more power without a huge difference in gas usage, great. However, if the consumption is too much, I would just as soon grin and bear it. As far as the cost of the engine, I can find a used GM 305 for 400$ or less (already called around), and since I plan on rebuilding it anyway over the winter, I have time to buy the parts I need to change it over for marine use a little at a time. These are simply questions that either someone knows the answer to or not. there is no need to get condescending.
for makomark, thank you for those suggestions, I will look into an upgrade kit for the ignition, but I doubt there is one (I tried once to see if there was a fuel injection kit for this engine, and there isn't, which goes to what I was saying about it being easier to find 305 parts).
To PugetSounder, I thought about that, but a 488 or 470 is strictly a marine engine, as far as I know, so I could not find a cheap automotive block and convert it. I do not have trim tabs, and because a kit is pretty expensive, even used, I was thinking about a hydroplane fin.
I cannot give you an exact figure for gph because I have never actually timed it. I have two fuel tanks on board, and usually only use the smaller 21 gallon tank. I do not like to let the level drop below a quarter tank. If I fill the tank 3 quarters of the way, I burn back down to a quarter tank puttering around the lake. This is a lot of start and stop, not continuous running, so it is hard to say I am burning exactly so much in a given time period. But we're basically talking about 10-15gallons for a days fishing, and I typically go once a week. At close to 4 dollars a gallon, you can see how that adds up, and why I don't want to go crazy with engine size. }"
 
"parts are no more difficult t

"parts are no more difficult to find for a 250 I6 than a 305. Napa has virtually every internal engine part in stock. All marine manifolds etc are readily available.
I doubt you'll save $400 in fuel, let alone the thousands it would cost you to actually make the swap properly and safely.
305's have nowhere near the reputation for reliability and longevity that the Chevy I6 has. "
 
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