Carburetors v. Fuel Injection

Optimizing the performance of Boston Whaler boats
mike
Posts: 23
Joined: Wed Jan 20, 2016 7:34 am
Location: Florida's Space Coast
Contact:

Carburetors v. Fuel Injection

Postby mike » Wed May 19, 2021 9:36 pm

Among Yamaha two-stroke-power-cycle engines of similar age and horsepower, what difference in “performance” and fuel economy will occur between engines with carburetors and engines with fuel injectors?

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Carburetors v. Fuel Injection

Postby jimh » Thu May 20, 2021 7:44 am

Engines of the same horsepower produce the same power output, so “performance”—whatever that means exactly—will be the same.

Two-stroke-power-cycle outboard engine fuel economy at low engine speeds is generally not very good, and there might be some improvement with electronically controlled fuel injectors over carburetors.

A fuel system with fuel injectors is generally a closed system with controlled venting, while carburetor fuel systems are open venting. This results in generally better engine starting with fuel injection than with carburetors if the engine has sat unused for a long time and is starting cold.

I don’t recall any Yamaha engines of the same model year and same horsepower that were available with a choice of fuel induction with injectors or carburetors.

What model year and horsepower engine are you actually asking about?

Also, do not confuse direct injection methods like the Yamaha HPDI or high-pressure direct injection with the much simpler and much less sophisticated “fuel injection” that just replaced a carburetor with electrically controlled fuel metering into the air intake stream.

Yamaha made many great carburetor outboard engines.

biggiefl
Posts: 897
Joined: Thu Jul 11, 2019 1:31 pm
Location: south Tampa Bay area
Contact:

Re: Carburetors v. Fuel Injection

Postby biggiefl » Thu May 20, 2021 10:03 am

The Yamaha OX66 and early VMAX two-stroke-power-cycle engines were just EFI versions of their carburetor engine brothers.

As Jim mentioned, do not confuse a HPDI with an ordinary EFI.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

mike
Posts: 23
Joined: Wed Jan 20, 2016 7:34 am
Location: Florida's Space Coast
Contact:

Re: Carburetors v. Fuel Injection

Postby mike » Thu May 20, 2021 2:45 pm

BACK STORY
I just purchased a 25-footer with Whaler Drive.

The engines are twin 1998 Yamaha 200-HP two-stroke, perhaps TXRW models.

My Yamaha 250 Ox66 engine has been one of the best engines I've owned.

Q: What is the difference in [fuel] efficiency between the 1998 Yamaha carburetor engine and the 1999 OX66 engine?

ASIDE: In my motorcycles, lawn equipment and everything but cars, I always run the fuel out after use. I don't have to do that with fuel injection.

BACK TO BOATING: I understand 200-HP is 200-HP. I thought an engine with fuel injection might respond differently than an engine with carburetors.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Carburetors v. Fuel Injection

Postby jimh » Fri May 21, 2021 8:58 am

The Yamaha Ox66 engines were unique and were probably the pinnacle of classic non-direct injection two-stroke-power-cycle outboard gasoline engines due to their use of a closed loop fuel induction control mechanism. Most EFI outboard engines run in open loop mode. The Ox66 had an exhaust sensor that helped control the fuel air mixing more precisely.

The closed-loop control system would tend to improve fuel economy compared to open-loop EFI and traditional carburetor fuel induction.

ASIDE: I appreciate the concern about fuel economy. The fuel price at my local fuel dock rose 33-percent to $4.15 per gallon from $3.10 per gallon last Fall.

Feeding twin 200-HP engines will probably consume twice as much fuel as a single 250-HP Ox66, and the sudden rise in fuel prices in the last month won’t help.

jimh
Posts: 11659
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Carburetors v. Fuel Injection

Postby jimh » Fri May 21, 2021 9:23 am

Engines are rated for horsepower at or near their maximum engine speed, so when running in that top RPM range the power output should be as rated.

A difference between two engines of the same rated power may be the engine speed range at which the engine can produce that rated power. An engine with a very wide engine speed range over which it will produce its rated power will tend to give better “performance” than an engine of the same rated peak horsepower but can only produce that peak power in a narrow band of engine speed and usually at the very top engine speed allowed.

As a general rule two-stroke-power-cycle engines tend to be able to produce more power across a wider engine speed range than similarly naturally aspirated four-stroke-power-cycle engines. This is due in part to the design of most two-stoke outboard engines to use ports instead of overhead valves and the pumping action of the piston pushing air into the combustion chamber. This advantage exists no matter how the fuel is mixed with the air, so differences in fuel induction should not affect the power band RPM range.

In four-stroke-power-cycle engines there is no pumping action so superchargers or turbochargers are used to increase the power band.

dtmackey
Posts: 760
Joined: Thu Sep 28, 2017 9:29 pm

Re: Carburetors v. Fuel Injection

Postby dtmackey » Fri May 21, 2021 9:41 am

biggiefl wrote:The Yamaha OX66 and early VMAX two-stroke-power-cycle engines were just EFI versions of their carburetor engine brothers.


For clarification.

The Yamaha VMAX model outboards have been offered in 4 flavors over the years: Carb, Fuel injection (OX-66), Direct Injected (HPDI) and now in 4 stroke.

They also used the VMAX model designation in their motorcycle and snowmobile lines as well.

D-