Gas consumption 4.3L carb VS 5.0L MPI

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the1nfamous

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I'm currently looking at my first boat, buying it with one of my friend, and we're looking at a 18-20'' open deck boat, at first we were looking for a 4.3 MPI or not, but we came across a baja islander with a 5.0L MPI that we really like and we're wondering if there's a big difference with those engine with gas comsuption. We dont plan on doing high speed runs with that, just cruising and doing a bit of wakeboard..

Thanks guys

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Sorry, can't offer too much useful advice, as my experience was with a 5.7 carb V8. However, in principle, if the hull is the same, and a similar weight, the difference in fuel consumption between the 2 engines at say 30 mph, will be pretty similar. It is just that the bigger engine will ultimately push you faster, and use more gas as a consequence at the higher speeds. When choosing a boat, I don't think the difference between these 2 engines will be the decider. It is more likely to be which do you prefer, and has been maintained best, and you think is better value.
 
The bigger engine will push you at the same speeds for less revs and thats where you will save fuel, so the relative consumptions won't really be that much different.
I have a 5.7 with a fuelflow computer and at 3000rpm it uses 32 liters ph and at 4500rpm its up at 65 lph, guess what rpm I cruise at!
Those last few knots cost you dearly, so if you can cruise at around 3-3500rpm thats the sweet spot with most gas engines, what will determine your cruise speed on two identical boats is the hp and what prop the engine can swing. A lower hp engine will have to work harder to give the same speed on similar sized boats.
My boat weighs 2 tons and maxes out at 36knots, the one in the pic should cruise happily with a 5.0 V8 at 3500rpm at around 36 knts.
 
My v/p 4.3 v6 with a nikki carb gives me about 6.5 gph, to be truthfully thou I some times I had really good fuel consummation and the same journey another day it's been a lot more fuel. We purposely now don't go over half throttle and get the cruise speed to around 22. Knots.

Our recent visit torquay-Teignmouth back to torquay then across to Brixham and back to Torquay I used that day around £35 worth of fuel for that day
Just remember sea conditions will drastically improve or your mpg or on a rough sailing your mpg will lower a lot
 
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oldgit said:
Wow thats one economical boat you have there to have only used 20L of fuel for a 40 mile trip. :) .
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Yeh I had to think twice as well, I had a full 120 litre tank and at the start of rhe day and the end of the day I ended up with the petrol gauge needle sitting between 3/4 and half a tank, so is my maths out a little or does that make it 25-35 litres used, I stuck ten quid in this week and the needel now shows just above the 3/4 mark on the gauge.

If my figures are correct I think turning the fuel mixture in has helped as the original base setting on the carb made the spark plugs very wet and black, after testing, my spark plugs currently have a dry gray to charcoal colour, which is a correct if the mixture is set right, all my previous engine I have tuned via the colour of the spark plugs.
 
markcw said:
Yeh I had to think twice as well, I had a full 120 litre tank and at the start of rhe day and the end of the day I ended up with the petrol gauge needle sitting between 3/4 and half a tank, so is my maths out a little or does that make it 25-35 litres used, I stuck ten quid in this week and the needel now shows just above the 3/4 mark on the gauge.
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I don't think the fuel gauge is going to be accurate enough to work out your fuel consumption like that.

On my 4.3EFI 19ft I use around 1 litre per mile at a cruise of 20-30mph. I've calculated that from the amount of petrol I have put in over a period of time, with the distance travelled taken from GPS. Its still only a rough estimate, but seems quite consistent from the amount of fuel I need to put in for a particular trip.
 
The mixture screw will have absolutely no effect on overall fuel consumption as its only effective at idle, as soon as you increase the revs the idle circuit drops out and the primary jets which are fixed take over, then as the revs and airflow through the carb increase the (also fixed) secondary jets kick in and then its like pouring the fuel in with a hosepipe!
If the carb is jetted correctly as it will be by the manufacturer then the plugs will have the correct colour after being run for a few minutes under load.
Marine carbs normally run on the rich side as the excess fuel aids cooling by keeping cylinder temps down and you can smell the unburnt fuel in the exhaust.
I had a V8 in last year that had burnt most of its exhaust valves as the electric fuel pump was worn out and wasn't delivering enough fuel. The owner opened the throttle to get more power but all that did was give the engine more air causing it to run very lean.
 
Thanks spanner man, I'll leave my engine as it currently set, has been great this year so far, fuel consummation had been better than expected. And she runs a dream, may be my newly fittied carter fuel pump improved things, my only wish is if I could improve the running of the boat at speeds between 6 and 12 knots, boats not happy at this speed and bogs down but soon it hits 15 knots she begins to go up on the plan and it's nice, up to 5 knots is smooth and can stay at this speed all day but it's to slow, don't know if a prop change will give me better usability, I don't need the prop to give me 46 knots as I have no interest to go at that speed, cruising at 21 knots is perfect for us, may be I should start a tread to see if I can get help with this issue and how to resolve it.
 
Sold a few of those islanders over the years they are pretty good on fuel and are pretty dam quick! 60 mph if memory serves me correctly that's with a 23p laser prop.

we've fitted smart tabs to a lot of them just to make them a bit more usable at wakeboarding speed.
 
We went from the 4.3 to the 5.0MPi a couple of years back and to be honest you probably won't see a big difference in fuel consumption between the two, especially if you're doing a lot of wakeboarding. The 5.0 has more grunt so gets people out of the water with less effort, and when you're sat on the plane at normal skiing speeds neither engine is doing a huge amount of work anyway. My advice - go for the 5.0 because it's the nicest and sweetest engine ever and is much smoother than the 4.3. Good luck.
 
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