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Author Topic:   90-HP 4-Stroke with Carburetors
Barney posted 10-16-2003 09:07 PM ET (US)   Profile for Barney   Send Email to Barney  
I take it that a number of you guys don't like carbureted 90 four strokes. Is there some current data to compare them to 90 2-strokes? Do you have this position based on your experience and is it fairly current? We have some performance numbers in the Post Classic Forum on the Mercury 90 4-stroke on the 170 Montauk.

Course, I want to understand this because I own one of these 90 4-strokes which I am thrilled with. Maybe we need a drag race or something, off of the trailer of course. I'm willing, bring on your 170 Montauk 2-stroke. The loser buys lunch. Jim

13DAUNTLESS posted 10-17-2003 09:39 AM ET (US)     Profile for 13DAUNTLESS  Send Email to 13DAUNTLESS     
I too would like to see that as I plan to purchase a 170M within the next few months and am undecided on the 2-stroke vs 4-stroke issue.
Bigshot posted 10-17-2003 11:37 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Top end is gonna be close, holeshot is where the 4-stroke lags. This table turns when you weigh down the boat. My 4-stroke has the same holeshot with one person as it does five. A 2-stroke chokes when you add weight.
Moe posted 10-17-2003 12:04 PM ET (US)     Profile for Moe  Send Email to Moe     
I think the only thing better for the 4-stroke would be if Merc hurries up and adds EFI to the 90 HP. I love it. Starts right up and runs perfectly immediately, even when cold. Better fuel efficiency too. Those, along with torque, are the main reasons Tabasco went to the 115 (so he says ;-)


Knot at Work posted 10-17-2003 01:11 PM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     
I would love to watch it. Tell me where Jim and I will float on over.

"Hey Does that have a HEMI?"

I love my 4-stroke. Reliable quite and speedy.

Sips gas really slow like a Bar girl sips beer in the Phillipines.

I cannot hear my motor when running over my 5 gallon self made Bait well.

Barney posted 10-18-2003 09:31 AM ET (US)     Profile for Barney  Send Email to Barney     
I agree the EFI will be a good improvement. But, I will say if you have to have a 90-HP, don't write off the carbureted 90 4-strokes. The Mercury 90 4-stroke is pretty good.

Also the drag race will be with a full load. Still waiting for a contender. Jim

jimh posted 10-18-2003 11:28 PM ET (US)     Profile for jimh  Send Email to jimh     
EFI engines are more sophisticated and lend themselves to more control of the combustion process by microprocessor. This permits the engine designer to have more control over the exhaust gas emissions.

As regulatory control over recreational outboard motor exhaust gas emission is increased, use of simple carburetor induction will probably begin to decrease.

The EFI approach seems to have completely taken over the automotive engine. The last car I can recall that had carburetors was a HONDA Prelude from the early 1980's. Thus when I see a 4-stroke outboard motor with carburetors I think of engineering from c.1980, not engineering from year 2003.

Since outboard motors are usually operated in fairly temperate zones, they work rather well with simple carburetor set ups. We usually are not trying to start and run our outboard motors at temperatures like minus-10-degrees. Thus you don't need to pre-heat the air to them or to modulate the choke as the engine warms up. All these things are typically done in automotive applications to compensate for the limitations of the carburetor.

From a performance standpoint, the EFI engine just starts better and perhaps runs better without a warm-up. It does not have, as far as I can tell, any intrinsic advantage that would make it run faster or get better fuel economy than a carburetor-equipped engine of similar horsepower.

Barney posted 10-19-2003 09:35 AM ET (US)     Profile for Barney  Send Email to Barney     
Also, Mercury as worked on the cold start problem with the carbs. On the 90 4 stroke, if it doesn't start up on the first try, the sequence is to turn the key on for 30 seconds and then start it. Mine has always started using this sequence. I had to read the owners manual a couple of times to pick that up. Of course it's not been really cold here yet. Jim
Barney posted 10-19-2003 10:48 AM ET (US)     Profile for Barney  Send Email to Barney     
jimh, Thanks for the thread fixes, the search function will pick it up better. Reads better also. Jim
Moe posted 10-19-2003 12:14 PM ET (US)     Profile for Moe  Send Email to Moe     
EFI engines are more sophisticated and lend themselves to more control of the combustion process by microprocessor. This permits the engine designer to have more control over the exhaust gas emissions...

It does not have, as far as I can tell, any intrinsic advantage that would make it run faster or get better fuel economy than a carburetor-equipped engine of similar horsepower.

Improving emissions is improving efficiency. The emissions of unburned hydrocarbons are molecules that didn't get to burn or burn completely and thus produce power and thus miles per gallon. Carbon monoxide instead of carbon dioxide indicates incomplete combustion. Emissions from carbureted cars and motorcycles can be lowered simply with a catalytic converter. There are carbureted bikes still in production that meet CARB and EPA standards. Admittedly these aren't quite as stringent as those for cars, but that is mostly in the area of oxides of nitrogen, until recently. Fuel-injected engines offer better control of mixture, and increase the efficiency of converters, but that's not the only reason for EFI. Corporate Average Fuel Economy (CAFE) is the main reason you don't see carbs on cars any more.

I'm not an engineer, but my hobby for a number of years has been motorcycle performance. That includes significant time on chassis dynos with fuel/air meters and eddy current brakes for steady state part throttle loading, in addition to WOT performance determined by acceleration of a drum. Most of that experience is with tuning carbs, but a little of it includes programming electronic fuel injection. Although motorcycles (and cars) have to deal much more frequently with throttle position changes, and have to address that through accelerator pumps on carbs, the WOT and part-throttle cruise is similar to the requirements of outboards. Where fuel economy is of no or slight concern, it's easy to make power by throwing fuel at the problem... to a point. But getting that power efficiently, i.e. having a setup that gets good performance in all areas of operation AND gets good mileage, is certainly more of a challenge.

With a carb, you only have a very few number of circuits to adjust and each, primarily the main, covers a HUGE number of combinations of engine speed and throttle position, not to mention air temperature and barometric pressure (density and oxygen content) etc. With a carb, you can't just tune for an average air/fuel mixture for all of these combinatins because of the danger of engine-damaging detonation that can occur with the mixtures on the lean side of average. So you jet for an average on the rich side, which means in most cases, you're pushing fuel through the system that will not contribute to power, and that reduces economy. With EFI, you're can much better tune much smaller ranges of operation, eliminating wasted fuel in many.

I won't even get into the advantages of port injection when it comes to reducing the condensation and aggregation of fuel droplets in an intake manifold, be they from a carb or throttle-body injector. The smaller and greater the number of droplets from a given amount of fuel, the more surface area available for combustion, and it's only surface area that combusts. Injectors excel at this.

I might agree with the statement that EFI won't make more, or at least significantly more, power at WOT. But I'll strongly disagree that EFI doesn't improve part-throttle performance or overall economy. I've seen too much evidence to the contrary.


Moe posted 10-19-2003 01:22 PM ET (US)     Profile for Moe  Send Email to Moe     
Let me add, during this first caution of the race, that EFI does produce significantly more power at altitude. If the engine may be operated at sea level, it has to be jetted sufficiently rich to prevent detonation. When it is then operated at altitude, it is significantly richer, and loses power to this, as well as to the lower air density and available oxygen. The latter is all that affects an EFI motor, since it can maintain a proper ratio. Carbed engines can be jetted leaner for higher altitudes, but then cannot be operated at lower alitudes without risk of overheating and damage.


Perry posted 10-19-2003 04:06 PM ET (US)     Profile for Perry  Send Email to Perry     
The Honda 90 has carbs and meets the ultra low 3 star carb emmissions which is the cleanest technology available. Cold starts are a simple; activate choke...turn the ignition key...crank…and the engine starts! High idle for 5-15 seconds depending on temp and I'm cruising away from the ramp. Hot starts are a simple twist of the key and 1-2 seconds of crank time immediately to a smooth idle. Honestly, my carburated BF90 has started, idled, ran and performed identical to a similar 4 stroke fuel injected motor EXCEPT with the following differences.

1. I have to lift a choke lever and hold at the desired high RPM for cold starts.

2. I've had to synchronize my carbs at the 20 hr. break-in service.

3. A carburator adjustment CHECK is called for every 100 hours with or without possible adjustment.

A friend of mine has an identical Whaler as mine but has a Suzuki 90 EFI on his boat, I have the BF90. Performance is almost identical as is fuel consumption.

Moe posted 10-19-2003 05:39 PM ET (US)     Profile for Moe  Send Email to Moe     
Interesting comparison, but of two different motors with many other different design factors, not the least of which is a derated larger displacement engine (90 vs 115) vs a maximum rated smaller displacement engine (90 vs 75). Let me quote Honda's perspective on EFI from their 115/130 web page:

• Programmed Electronic Fuel Injection
4 Fuel Injectors and a Common High Flow Throttle body deliver the precise amount of fuel/air to each cylinder.
The result is easy starts, instant throttle response with superior fuel efficiency.

Having owned a VTEC, I certainly admire Honda's engineering, and doing more with less.

Barney posted 10-19-2003 06:13 PM ET (US)     Profile for Barney  Send Email to Barney     
Moe, You are tough. I can't play on the say wordsmithing ballfield. Wordsmithing, is that a word? Anyway, the Honda Marine site also says this about the Honda 90: "Their highly fuel-efficient 4-to-4 induction system uses one carburetor for each cylinder. And that means power—with quicker and smoother acceleration than engines with large throttle bodies and long intake runners." So we are down to the difference being superior fuel-efficiency (EFI) versus just highly fuel-efficient (4 Carbs). We need data that we do not have to compare. What is the difference between superior and high efficiency? I guess it's the best there is, EFI, versus the next best thing, 4 Carbs. And if you can't have and outboard with EFI, don't sweat it if all you can get is 4 Carbs on the beast. Jim
Moe posted 10-19-2003 06:56 PM ET (US)     Profile for Moe  Send Email to Moe     
LOL! Wordsmithing indeed! The point is that sequential port EFI is intrinsically more efficient than carbs for many reasons. Besides the level of sophistication of the EFI, other factors play into efficiency. And the amount of time devoted to tuning is much greater with the EFI in order to take advantage of that benefit.

I wasn't saying the Merc 90 is a bad engine because it has carbs. Just saying it could be better with EFI.

lhg posted 10-19-2003 11:08 PM ET (US)     Profile for lhg    
Regarding the 75 and 90 HP Merc 4-strokes, I think Mercury is locked in by Yamaha's decision making on this engine. As most know, Mercury was way ahead of the others with EFI outboards, the first ones going back as far as 1987 with the 2.4 liter 260HP EFI racing motors. So we know Mercury believes in the technology. A look at Mercury's new 2004 catalog is revealing. All of Mercury's own, 100% produced engines are indicated to be "Smart Craft" enabled,, including the 30,40,50 & 60 HP EFI 4-strokes. But yet the 75-115 & the 225 engines do not accept Smart Craft, probably because they are Yamaha designs. Note also that Yamaha's version of Mercury's 30,40 & 60 HP engines are carbed. So it does sound like Yamaha is making the decisions on these 90's, staying with the carbs. My guess is that sometime soon, Mercury will bring out it's own 4-strokes in this HP range, definitely with EFI and SmartCraft. But I think first they're focusing on getting the 135-250's out.

It appears that in the near future, all brand of 4-strokes over 30HP will be fuel injected.

jimh posted 10-20-2003 12:11 AM ET (US)     Profile for jimh  Send Email to jimh     
I don't think that use of fuel injection implies computer control and sophisticated feedback, but it often does accompany it.

I don't know if most outboards with EFI have exhaust gas sensors and use a feedback loop. I think Yamaha had a model that does that, the OX-66 engine was it?

Very good information re more sophisticated fuel mixtures, etc. Also good report on current carburetor technology.

That old Honda Prelude ran very nicely--even though it had a mess of carburetors, hoses, plumbing, etc.

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