Worth looking for an EFi / Gi engine over a carb version? Or better to seek duoprop?

[ontheplane]

Hi there,

Looking at boats still!

Vast majority of boats the price range I am looking at have carb engines of 4.3 / 5.0 or 5.7 litres capacity.

Now some of them are injected - single point or multipoint.

In my simple world, I would have thought a 4.3 multi-point injection engine kicking out 225hp would be much more economical than a 5.0 carb engine kicking out the same - and with the easier starting etc etc these would be the ones to go for.

However looking at very old test data - here :- http://www.fourwinns.com/upload/Documents/Catalogs/2000/FastFacts_2000.pdf - This doesn't appear obviously to be the case - a 5.0Gi (injected) 250hp seems to burn exactly the same as a 5.7gl (carb) 250hp at the same rpm and gives the same speed - the only difference is between single or duoprop drive - does anyone have real experience of this at all as it flys in the face of everything I thought was true.

Certainly it seems to be true that the same boat, with the same engine fitted with a DP (duoprop) drive will do a couple of knots more at the same rpm and fuel burn than a boat fitted with a single (SX) drive, and is therefore more economical - this is backed up in the figures shown there and things I have read before.

So, am I better looking for something with a Duoprop than an EFI, do people think it makes much difference, and if I found something EFi, would I pay a bit more for it, or is it not worth it?

Peoples experiences please??

 

[wipe_out]

I think the first thing to remember is that the engine torque is what's being used to 'push' the boat through the water.. Depending on the boat this is more or less relevant and largely the reason larger boats are diesel only because diesel engines while not having specifically high HP numbers do produce huge amounts of torque..

So a 4.3 might have the same HP numbers as a 5.7 but the latter will produce far more torque and so be more capable of pushing the boat.. The 4.3 would have to be pushed a LOT harder on the same boat than a 5.7 so there would probably be little fuel consumption difference..

As for the carb/EFI debate like everything there are pro's and con's.. While EFI is probably a little more efficient and has turn key cold starting its also a lot of electronics that could potentially have issues and be expensive to fix.. Carb is simple and mechanical and with the right procedure very easy to start cold and can run just as sweetly as an EFI engine but maybe slightly less efficient..

DP outdrives are great, they have a good bite on the water and no prop walk but there is really nothing wrong with non-DP drives either.. DP props are expensive to replace where single props are cheap if they strike something..

From my experience I got a really nice 2002 sports cruiser that had a broken VP 5.7 on an SX drive.. I replaced the complete engine from the fuel pump to the exhaust risers including a new Holley 4BBL carb.. After it was fitted I spent a day adjusting the automatic choke and getting it right.. Also replaced the 3 blade prop for a 4 blade to get more bite on the water.. She runs like a dream, starts easily and runs very smoothly.. You wouldn't be able to tell it wasn't EFI..

So my advice would be find a boat you like that fits you budget and get on the water.. If it means going a couple of knots slower at full speed because it hasn't got all the latest developments so what.. If your budget does stretch to having all the bells and whistles then that's great too..

 

[ontheplane]

And as you say quite rightly - there isn't a lot on a carb engine I couldn't fix myself - the efi's might be a different story - haven't thought of that before!

Good point about the DP props being mighty expensive if they have a chat with the bottom... sx's (alpha's in Mercruisers) are a lot cheaper.

None of them are going to be cheap to run anyhow!

 

[ontheplane]

Anyone else have any views on it?

 

[AndieMac]

I agree with wipe-out regarding the torque issue, low down grunt wins every time when your boat is loaded up going to sea, in the size boats we are now talking about.

The main difference I noticed going from a carbed 5.0 litre to mpi 5.7, besides the easy starting and great hole-shot, max. revs on the old 5.0 litre was 4200 rpm and the 5.7 is up to 5200 rpm, cruise revs spinning a 17" prop has gone from 3000 rpm (20 knots) to 3350 rpm (24 knots) spinning a 19" prop (better torque) for what appears to be similar fuel consumption.

 

[tinkicker0]

Give me a well matched carb setup any day. Only downside with a carb is it is less flexible over the entire rev range than injection. However when the carb is running in the correct range, beats mpi hands down in my book for quality turning and burning.

 

[julians]

My vote would go to getting the MPI engine, a lot more modern, it just works, no fettling of carbs, going out of tune etc etc.

I've had a mercruiser MPI engine (5.0mpi) for the last 7 years, and its been pretty spot on, a couple of electronic sensors have failed over the years, but they're cheap to replace and just screw/clip on, and didnt leave me stranded anywhere.

If you did go MPI its worth buying a rinda code scanner, so you can quickly get to the bottom of any electronics issues.


I've also owned a carbed 5.7 V8, and a efi 4.3 v6, and whilst they were generally reliable, the carbed engine did need fairly regular tuning to get it to start nicely and run well.


IMO MPI is a vast step forward over the old efi and carbed models, simply just for lack of maintenance, ease of starting, and improved fuel economy at all points in the rev range (except may WOT).

 

[ontheplane]

The main difference I noticed going from a carbed 5.0 litre to mpi 5.7, besides the easy starting and great hole-shot, max. revs on the old 5.0 litre was 4200 rpm and the 5.7 is up to 5200 rpm, cruise revs spinning a 17" prop has gone from 3000 rpm (20 knots) to 3350 rpm (24 knots) spinning a 19" prop (better torque) for what appears to be similar fuel consumption.

That's interesting, and a bit puzzling for me.... So an MPI 5.7 (probably 280hp (or even 330)) will spin a 19" prop rather than 17" (that makes total sense) but will run at 3350rpm using the same fuel per hr as the old carb engine did at 3000rpm?

That gives rise to a question...

I thought that on a 20' cuddy a 4.3 carb would be underpowered a bit. So I would have been looking for a 5.0. I have owned a 5.7 before (albeit in a 28 footer) and that was frightening on the fuel. I therefore thought that getting the same sized boat with a 4.3mpi would use less fuel and give same performance as 5.0 - however using the above info it does suggest that the MPI motors are MUCH more economical than the carbs - but as suggested earlier the answer may not be to go down a size and have the mpi, but stay at the same size and go for mpi to get better economy i.e. look for a 5.0 or 5.7 mpi

Thanks guys, any more input gratefully received.

 

[[2068]]

Carbs are great for people that love to tweak carbs.

But if you want something that starts and runs like a car engine (i.e. you turn the key: it starts) then MPI is the way to go, and has been for decades in the automotive world. The alternative is the noble art of throttle pumping (too little, nothing happens, too much and the distinct odour of petrol gets added to your sandwiches, and sometimes doesn't work at all in cold weather). Fuel consumption with MPI is substantially better than with carbs.

As to single or duoprop, it depends on how much "grip" you want to have on the water. Fuel consumption isn't really a reason to go either way. Nor is replacement costs on hitting the bottom: you should not be hitting the bottom.

 

[AndieMac]

That's interesting, and a bit puzzling for me.... So an MPI 5.7 (probably 280hp (or even 330)) will spin a 19" prop rather than 17" (that makes total sense) but will run at 3350rpm using the same fuel per hr as the old carb engine did at 3000rpm?

That gives rise to a question...

I thought that on a 20' cuddy a 4.3 carb would be underpowered a bit. So I would have been looking for a 5.0. I have owned a 5.7 before (albeit in a 28 footer) and that was frightening on the fuel. I therefore thought that getting the same sized boat with a 4.3mpi would use less fuel and give same performance as 5.0 - however using the above info it does suggest that the MPI motors are MUCH more economical than the carbs - but as suggested earlier the answer may not be to go down a size and have the mpi, but stay at the same size and go for mpi to get better economy i.e. look for a 5.0 or 5.7 mpi

Thanks guys, any more input gratefully received.

The 5.0 - 5.7 - 6.2 litre are all the same weight, just bored and stroked from the same block, it's nice to have the extra power at hand if needed, or throttle back and the bigger capacity engine just does it easier.

I don't think the difference in fuel consumption over a season will be any kind of a windfall with the few engine hours the average under 7 metre boater clocks up, perhaps it just makes the re-power cost a little more palatable

 

[MapisM]

Give me a well matched carb setup any day. Only downside with a carb is it is less flexible over the entire rev range than injection. However when the carb is running in the correct range, beats mpi hands down in my book for quality turning and burning.

Mmm... that surely doesn't match my experience.
I've had a 25' with a 5.7 carb (Yamaha, one of the very few they built), a 22' with a 5.7 carb (merc), and a 27' with a 8.1 MPI (merc).
And believe it or not, the latter burned on average just about as much as the other two...

 

[wipe_out]

It's also worth remembering that the question of fuel burn isn't simply related to the engines displacement.. For any given amount of power produced an amount of air/fuel mixture is required at an ideal ratio somewhere around 14.7:1..

I would imagine all the V8's (and probably the V6's) have similar efficiencies in terms of head designs, port shapes, air flows and frictional losses etc..

So if a given boat is run at the same constant speed and is running a near enough correct air/fuel mix then they should all consume a very similar amount of fuel to produce the power needed to sustain that speed.. The difference being that the smaller engine is working a little harder than the larger engine.. The larger engine would have more in reserve and so would accelerate faster and have a higher top speed if the throttle was pushed down..

EFI does increase efficiency in that if they have exhaust sensors that can automatically adjust the air/fuel ratios but I am not if or when marine EFI engines started using exhaust sensors.. The ones that don't have exhaust sensors would use a static fuel map that would not adjust automatically for changing conditions and so would be as efficient as they designed it on their test rig making it not that different to a carb setup..

At the end of the day, irrespective of the fuel delivery mechanism "more power"="more fuel" and "how you run" is a far bigger factor than "what you run" with regards to fuel burn..

I disagree that carbs are difficult to make run right or to start in cold weather but agree that EFI makes it very easy because you just have to turn the key..

 

[ontheplane]

Thanks to all..... not conclusive then!

My thoughts are a small engine working hard (but not more than 3/4 of max) is usually more economical than a large engine not working very hard.

Exceptions might be with modern diesels in cars where they run in super high gears on the motorway to get unfeasable economy.

Would love to hear from anyone with a 20' cuddy running a 4.3GXi or MPi to know what the economy is like especially if they have also run carb engines

 

[julians]

My old boat (a Sea Ray 200 select - 21 foot long) had a 5.0 mpi engine (mercruiser not volvo, but I guess the figures would be the same), and would use ~30-33 litres per hour at a cruising rpm of around 3000rpm, this would typically equate to ~25mph, the actual speed varies with conditions.

 

[tinkicker0]

Thanks to all..... not conclusive then!

My thoughts are a small engine working hard (but not more than 3/4 of max) is usually more economical than a large engine not working very hard.

Exceptions might be with modern diesels in cars where they run in super high gears on the motorway to get unfeasable economy.

Would love to hear from anyone with a 20' cuddy running a 4.3GXi or MPi to know what the economy is like especially if they have also run carb engines

I have a 3.0l carbed in a 20ft cuddy and get 5mpg at 6kts and 5mpg at 30kts.

Above those two speeds, fuel economy is horrendous, dropping down to as low as 3mpg. Why you think a 4.3 6 banger is underpowered in a 20ft cuddy I have no idea. A real life gps derived 35knots is fast enough for me and for my bank account. The previous owner used to tow toys and water ski behind it.

 

[tinkicker0]

Mmm... that surely doesn't match my experience.
I've had a 25' with a 5.7 carb (Yamaha, one of the very few they built), a 22' with a 5.7 carb (merc), and a 27' with a 8.1 MPI (merc).
And believe it or not, the latter burned on average just about as much as the other two...

And you had a couple of spare carb bodies with different choke sizes and a box of jets to really dial it in?

As I said " a well matched and set up carb running in the correct range beats any mpi for quality turning and burning in my book".

I also said that out of these ranges a carb is less flexible. We actually used to swap carb sizes according to how technical a track was, back in the day to get the best from it. Modern race teams do the same with remapping the fuel curve on injection, usually at each part of the track. It must be remembered that the number one priority of a mas produced mpi is to allow lean burn stratified charges for better emission performance, not for max stoic efficiency.

The OP needs to look at his envisaged use to choose the best motor. 6 passenger loaded boat, towing toys, bigger motor is better.

2 passengers going at cruising speed and above, smaller is better.

 

[MapisM]

And you had a couple of spare carb bodies with different choke sizes and a box of jets to really dial it in?

'course not, but both the 350 carbed small blocks which I've got ran pretty well, anyway.
Particularly the yammie, which I specced for one of the very few new boats I've ever bought.
Therefore, I would think that it was tuned just fine by the engineer which commissioned it (at that time, it was the first Yam outdrive ever installed by the yard).
In fact, it pushed a 25 cuddy at 40+ kts, with enough oomph for deep water starts with a monoski.
And I never had starting problem, at any temperature (neither with the Yam nor with the Merc).

But the 496HO with MPI, with almost double the ponies and on a heavier boat, burned just about the same LPH, on average. That's all I said.
Sure, the very efficient hull, capable of 60+ kts, meant that at the average lake cruising speed (30 to 35 kts) the 496 was much more far from its limits than the small blocks.
But I definitely expected that boat to burn significantly more than the previous two, and it was a pleasant surprise to see that she didn't.
I would think that the MPI had a lot to see with that.

 

[ontheplane]

Also, I have to say, I don't think I have the skill to "set it up" that precisely either - whereas an MPI should be almost self-adjusting.

Thanks for all the input though guys - I'm finding this very interesting keep it coming

 

[wipe_out]

Also, I have to say, I don't think I have the skill to "set it up" that precisely either - whereas an MPI should be almost self-adjusting.

You really don't have to set up anything, the engineers who build the engine/carb/efi combination would have done it all for you.. The only time it would need changing is if something in the engine changed or was modified significantly, this is common in cars but not so much in boats.. Also the EFI systems are only "self adjusting" if they have exhaust sensors that feed back the AFR to the ECU, without sensors they just run off a map that is the electronic version of the jets and emulsion tubes used in the carbs..

Like I said initially.. Find the boat you like that is at the right price and if it starts and runs well on the trial then buy it.. Don't get bogged down with these intricate details that will have you waiting longer or paying more for you boat..

 

[lynall]

Sx drive with carb 4.3 on my sprint, tad over 30 knots full chat with clean bum.
13 gals per hour flat out.
7 gals per hour 20/21 knots.
1 1/2 gals per hour 6/7/8 knots tide depending.
Changed prop again this weekend after hitting another rock at our marina, took me all of ten minutes with leg up.
Engine and leg 2009 so recent, but if i had a choice would have gone with injection, BUT have had water in fuel issues recently, easily fixed, not sure what that would have damaged on the injection version?


Lynall