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Old 21-10-2009, 01:56   #1
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Bigger Carb = More Power?

I was ordering a new carb for my 5hp four stroke outboard and the dealer had a 6hp carb available but needed to order out for a 5hp.

He suggested buying the 6hp unit, stating that it would fit and that I'd get more power.

Is it really that easy?
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Old 21-10-2009, 02:02   #2
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No, it's not that easy.

Order the right part.
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Old 21-10-2009, 02:12   #3
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I did order the right part, but not because I didn't believe the salesman, rather because I figured there would be a cost, probably both in terms of usability and life expectancy.

However I'm still intrigued to know if there was any truth in the statement.
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Old 21-10-2009, 02:23   #4
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Fundamentally power comes from how much air/fuel mixture you can shove through the engine from inlet to exhaust. Let's leave aside nitrous oxide and such for the time being.

Imagine the inlet to the exhaust as one pipe. The way to get more air though is to turn the engine faster (higher RPM) or compress the air on the front side (turbo or supercharging). You still have to get it out the back side.

A normally aspirated (non turbo) engine relies on ambient pressure to "push" the air through the inlet into the combustion chamber. As the piston goes down the cylinder the pressure drops and ambient pressure outside pushes air through the carb.

As it goes through the throat of the carb (venturi effect) a low pressure allows ambient pressure to push fuel from the carb bowl into the air. The size of the throat and the size of the fuel nozzle ensures the proper mix.

By putting a bigger carb on the engine you open up the inlet but have done nothing to improve the passage by any valves, the head and ports through the combustion chamber and out the exhaust.

There may be a slight improvement due to a freer breathing (bigger) inlet but you could acheive those gains with a low resistance air filter.

There is a risk with switching to a bigger carb of changing the mixture as the carb depends on the pressure drop through the throat to draw in the fuel.

At a minimum you would have to increase the exhaust capacity to want to see any improvement.

All of this is caveated by the total design of your engine. There could be gains but nothing you would write home about. The 6 hp carb could also be an alternate but you'd need to see the parts book to believe that one.

If I was stuck in Vanuatu and the only carb available was one for a 6hp. I'd go for it but in general those guys at Honda, Tohatsu, Mercury etc. are pretty smart fellahs...
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Old 21-10-2009, 02:46   #5
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Back in the days of the 2stroke outboard, at least one manufacturer used the same engine for both the 9hp and the 15 hp (I think those were the sizes)

The only differences between the two engines were carburettor jet size and price.

The only guarantee that you have with just fitting a bigger carburettor is that it will use more fuel!
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Old 21-10-2009, 03:17   #6
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Actually it sometimes is just that easy. Some manufacturers have sold the same engine as two different HP ratings by doing nothing more than putting a smaller carb on one.

Current Mercury 8 & 10 HP are one case. Old 2 stroke Mercs and Evinrude 9.9 and 15.

Not sure of your particular application but it is very possible.

George
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Old 21-10-2009, 03:54   #7
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I don't think your single pipe analogy completely holds, as the combusted fuel is a bigger volume due to heat and liquid fuel now burned and is a gas, it's now under high pressure and leaves the combustion chamber fairly rapidly. Look at the valves in the engine. The intakes are bigger than the exhaust valves.

At least in the car world unless it is an all out racer the carb is undersized for the engine, probably for fuel economy and engine longevity. (Or was when cars had carbs.)

John

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Originally Posted by Ex-Calif View Post
Fundamentally power comes from how much air/fuel mixture you can shove through the engine from inlet to exhaust. Let's leave aside nitrous oxide and such for the time being.

Imagine the inlet to the exhaust as one pipe. The way to get more air though is to turn the engine faster (higher RPM) or compress the air on the front side (turbo or supercharging). You still have to get it out the back side.

A normally aspirated (non turbo) engine relies on ambient pressure to "push" the air through the inlet into the combustion chamber. As the piston goes down the cylinder the pressure drops and ambient pressure outside pushes air through the carb.

As it goes through the throat of the carb (venturi effect) a low pressure allows ambient pressure to push fuel from the carb bowl into the air. The size of the throat and the size of the fuel nozzle ensures the proper mix.

By putting a bigger carb on the engine you open up the inlet but have done nothing to improve the passage by any valves, the head and ports through the combustion chamber and out the exhaust.

There may be a slight improvement due to a freer breathing (bigger) inlet but you could acheive those gains with a low resistance air filter.

There is a risk with switching to a bigger carb of changing the mixture as the carb depends on the pressure drop through the throat to draw in the fuel.

At a minimum you would have to increase the exhaust capacity to want to see any improvement.

All of this is caveated by the total design of your engine. There could be gains but nothing you would write home about. The 6 hp carb could also be an alternate but you'd need to see the parts book to believe that one.

If I was stuck in Vanuatu and the only carb available was one for a 6hp. I'd go for it but in general those guys at Honda, Tohatsu, Mercury etc. are pretty smart fellahs...
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Old 21-10-2009, 20:04   #8
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I don't think your single pipe analogy completely holds, as the combusted fuel is a bigger volume due to heat and liquid fuel now burned and is a gas, it's now under high pressure and leaves the combustion chamber fairly rapidly. Look at the valves in the engine. The intakes are bigger than the exhaust valves.

At least in the car world unless it is an all out racer the carb is undersized for the engine, probably for fuel economy and engine longevity. (Or was when cars had carbs.)

John
Points taken on the design of the engine being rated for 2 different carbs. Could be the case here.

In regards to the exhaust gasses...

As the gas expands it pushes the cylinder down. You like to extract all the power of the expanding gas before the exhaust valve opens but the exhaust valve usually opens before BDC to allow the expansion to get the gas moving out of the cylinder.

On multi-cylinder engines this is timed so the previously exhausted gas create a low pressure helping the next one out. This is what "tuned" exhaust does. It is also what low restriction exhaust does.

As a kid the first thing you did was put on an aftermarket high flow air filter. The next thing you did was headers and low resistance mufflers. Then you looked at carb and manifold, porting and polishing the heads or replacing the heads with one with bigger valves.

No matter how "hard" you push there is only so much fluid that is going to pass through the pipe. In the worst case you "supercharge" the inlet and then the burnt gasses don't get out of the cylinder because the exhaust is too small.

On a single cylinder 5 hp outboard I don't think we are out to win any drag races - LOL. If someone said I don't have the 5hp carb but I have the 6hp carb and I was in a place I couldn't get the 5hp carb for 6 weeks. I'd go for it.

In general as a mechanic and engineer for 30 years I always lean towards putting things back the way they were designed.
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Old 21-10-2009, 21:42   #9
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Carburetors work by the venturi principle, using air flow to suck gas in. The carb is sized according to the amount of air the piston can pump. If you don't flow enough air, you don't get enough fuel. Higher rpm's would mask most issues, but oversize the carb enough and you won't be able to idle. I don't know if 6 vs 5 hp rated carb makes enough difference, but I'll wager at the very least you'd be doing some serious tweaking to get the carb just right.

OK, perhaps not that much tweaking. Mercury 4 hp through 6hp engines apparently share the same displacement. I'll bet they share the same exhaust system, so your parts guy was probably right--the difference in power is in the carb.

Brett
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Old 21-10-2009, 22:14   #10
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Points taken on the design of the engine being rated for 2 different carbs. Could be the case here.

In regards to the exhaust gasses...

As the gas expands it pushes the cylinder down. You like to extract all the power of the expanding gas before the exhaust valve opens but the exhaust valve usually opens before BDC to allow the expansion to get the gas moving out of the cylinder.

On multi-cylinder engines this is timed so the previously exhausted gas create a low pressure helping the next one out. This is what "tuned" exhaust does. It is also what low restriction exhaust does.

As a kid the first thing you did was put on an aftermarket high flow air filter. The next thing you did was headers and low resistance mufflers. Then you looked at carb and manifold, porting and polishing the heads or replacing the heads with one with bigger valves.

No matter how "hard" you push there is only so much fluid that is going to pass through the pipe. In the worst case you "supercharge" the inlet and then the burnt gasses don't get out of the cylinder because the exhaust is too small.

On a single cylinder 5 hp outboard I don't think we are out to win any drag races - LOL. If someone said I don't have the 5hp carb but I have the 6hp carb and I was in a place I couldn't get the 5hp carb for 6 weeks. I'd go for it.

In general as a mechanic and engineer for 30 years I always lean towards putting things back the way they were designed.
Yeah, you're following the path of most bang for your buck, but as the there is no direct path from intake to exhaust due to valve timing, changing exhaust components has little to do with how well air/fuel gets into the cylinder. So while not as cost effective as changing exhaust components, putting on a better carb will increase HP even if you don't touch the exhaust side. (assuming the carb isn't huge to begin with)

I'd also replace with original if possible, but I bet you probably see somewhere between no change and a slight increase in power with the other carb.

Lt. Brett: At idle the throttle plate is essentially closed and air/fuel mix is controlled by the idle mixture screw.

My first car had a 6 pack, 3 two barrel carbs, sitting in the trunk, I popped the four barrel off and tried the six pack. Normally the primary is partially opened first then the other two started to actuate. The person that had put this together also had a set screw in the linkage to open all three at the same time. When I tried this mode acceleration was poorer, as you said too much air, it definitely ran much better staged. The car ran fine, just flooring it opening all the throttle plates all the way made an interesting whoosh sound, the car would start to slowly accelerate, then finally take off. Don't remember if the 4 barrel was faster, but the 6 pack was definitely cooler in high school.

John
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Old 22-10-2009, 00:56   #11
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Mercury 4 hp through 6hp engines apparently share the same displacement. I'll bet they share the same exhaust system, so your parts guy was probably right--the difference in power is in the carb.

Brett
Interesting. The Outboard is a Tohatsu, so probably shares the same characteristics under a different name.
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Old 22-10-2009, 14:57   #12
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Bigger Carb = More Power?

If the inlet restriction is the carb, then yes.
If the inlet restriction is at the valve or due to the cam, then no.
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Old 22-10-2009, 15:37   #13
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With all due respect to the above posts, It very well COULD be as simple as more air/fuel flow (bigger carb) If you ever owned a chevy with a 283 v8 "in the day" there was a huge difference in HP if it had a Rochester 2 barrel carb or an AFB 4 barrel carb. (regardless of which heads it had) Many outboards use the same block for HP ranges of 8-15 hp etc. I would imagine ONE way of restricting HP would be fuel flow. Look at the specs on the engines and see if the Bore/Stroke/Cubic Cm's are the same ...BOY... S&S said it much more eloquently than the rest of us! :>)
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Old 23-10-2009, 00:48   #14
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Yep - S&S said it most eloquently.

Final thought, no disrespect to Lt. Brett. Nothing in a naturally aspirated engine sucks. Ambient pressure pushes the air charge. The bigger the pressure drop as the piston moves down the cylinder the "faster" the air charge goes in. The pressure drop across the venturi and the idle bleed holes work the same unless it's a pressure carb.
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Old 23-10-2009, 04:44   #15
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my nissan 4 can deliver 6 hp with a re-tuned carb. But it wont add much.
The prop on the 4 is a 7.7 x7 and the 6 is 7.7 x 8. So for the benefit of the added hp the prop needs to change.
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