QJet myth?... running on small primaries= better economy

[Howard Sterndrive]

I gotta think the common idea that a QJet's smaller primaries somehow give better economy is not supported by fact.

Been doing some reading about BTUs, BSFC and how much gasoline it takes to make HP.

Many points of the interwebs seem to agree that if you take a carbureted pushrod engine and measure the gas flowing to it in GPH, you can multiply that number by about 12 to figure out how much HP you are making at the crank.

Seems to work for my motor. I burn 16 GPH @ WOT...which is 192HP and my V6 is rated at 185HP.

So at cruise, I'm burning 5GPH at 27 mph/3000 rpm......=60HP... yeah.. seems about right

Is there any way higher air velocity through 2 small venturis can make the engine more efficient?
Because that's all I can see would be different from a QJet to a 2 BBL....faster air through the throat....

 

[dubs283]

Re: QJet myth?... running on small primaries= better economy

your engine is most efficient when it operates with the stoichiometric ratio, which is 14.7:1 (14.7 parts air to 1 part fuel) - 14.7 being one atmosphere, air pressure at sea level

this is basically saying that throughout rpms, regardless of how much fuel you are burning (rpms), the fuel delivery system is set up to deliver as close to the stoichiometric ratio of air:fuel as possible

this of course is different at varying altitudes as air pressure at 1000 ft above sea level is less than at 10 feet above sea level, hence, different size main jets - you are still trying to maintain the 14.7:1 ratio - which in most cases is virtually impossible

in mercruiser service manuals, this scenario is printed to basically say, no matter where you are, what you are running, and what you are using for fuel delivery, you can forget about fuel economy - we have set up our fuel delivery system as best we can to allow for best operating scenarios for the stoichiometric ratio

now with superchargers, you are basically "tricking" the engine to think its bigger than it is by adding more "atmospheres" to the air:fuel ratio, but thats a different discussion

 

[skargo]

Re: QJet myth?... running on small primaries= better economy

I have a 1967 Pontiac Lemans, currently powered by a Pontiac 350, although I have a built 455 ready to go in.
Anyway, I removed the 2 barrel which has 2 huge "barrels" and replaced it with a q-jet clone. It definitely gets better fuel economy IF I stay out of it and keep the secondaries from opening.

 

[achris]

Re: QJet myth?... running on small primaries= better economy

The small secondaries are more for better run quality at low engine speed than fuel economy. The faster the air moves through the venturi the better it's ability to pick up the exact amount of fuel to maintain that 14.7:1 ratio, and as this is one of the main factors in fuel economy, it also makes sense...

This is one of the big reasons smaller hp engines of the same engine block family (like the Johnny 90 as opposed to the 140 of the late 70's) have better low end torque. This mainly applies to outboards, but in 4 strokes the rationale is the same.

Chris.........

 

[swaycleveland]

Re: QJet myth?... running on small primaries= better economy

I gotta think the common idea that a QJet's smaller primaries somehow give better economy is not supported by fact.

Been doing some reading about BTUs, BSFC and how much gasoline it takes to make HP.

Many points of the interwebs seem to agree that if you take a carbureted pushrod engine and measure the gas flowing to it in GPH, you can multiply that number by about 12 to figure out how much HP you are making at the crank.

Seems to work for my motor. I burn 16 GPH @ WOT...which is 192HP and my V6 is rated at 185HP.

So at cruise, I'm burning 5GPH at 27 mph/3000 rpm......=60HP... yeah.. seems about right

Is there any way higher air velocity through 2 small venturis can make the engine more efficient?
Because that's all I can see would be different from a QJet to a 2 BBL....faster air through the throat....

mr sterndrive,
i hate to high jack your thread but i have a question for you
you and i are running the same motor/outdrive (i run a 21 4 winns)
at 3000 rpms i run about 24 mph (gps) and i run about 8 gph
i have a 14and a half x 19 pitch prop (stock)
new plugs wires filters timing is 10 btdc

any idea why i get less speed and gph then you
the boat has less then 100 hours on it
id love to pick your brain a bit if you wouldnt mind

 

[wire2]

Re: QJet myth?... running on small primaries= better economy

......
Is there any way higher air velocity through 2 small venturis can make the engine more efficient?
Because that's all I can see would be different from a QJet to a 2 BBL....faster air through the throat....

Higher velocity air will atomize liquid gasoline into smaller droplets, allowing more complete combustion.

Picture a paint spray gun; at recommended air pressure you'll get complete atomization, to coat a surface evenly. Reduce the air pressure, and the paint will come out in larger drops, making blobs hit the surface. Just like low velocity in a carb will draw larger drops of gas into the combustion chamber.

(Increased air velocity will create finer atomization but the extra air will blow the paint around on the surface; not an issue in a carb.)

 

[Howard Sterndrive]

Re: QJet myth?... running on small primaries= better economy

(i run a 21 4 winns)
at 3000 rpms i run about 24 mph (gps) and i run about 8 gph
any idea why i get less speed and gph then you

my boat is only 18 feet/ approx2250 lbs
and my engine is a 3.8L/229, not a 4.3

Like the 305, the 229 was designed to sip fuel in it's automotive smog version

All part and parcel of trying to maintain the 14.7:1 ratio at part throttle I suspect.... I always wondered why they made small displacement V engines....totally makes sense with a carb and the velocity in the venturi re. atomization

glad I asked the iboats crowd.... all makes much more sense now.

 

[swaycleveland]

Re: QJet myth?... running on small primaries= better economy

howard
would you mind if i message you

 

[Aloysius]

Re: QJet myth?... running on small primaries= better economy

It's all about fuel atomization, which the small double venturis on a quadrajet deliver. 14.7:1 is "stoichiometric" where supposedly ideal mixture is achieved. True, it's the cleanest setting, but maximum power is about 13:1, and fuel economy is 17:1. That's why you have power enrichment circuits on carbs.

Fuel injectors, even throttle body, atomize fuel VERY efficiently. Liquid fuel doesn't burn..it MUST be mixed with oxygen..oxidized.

so why is 13:1 better for power? 2 reasons. First, it compensates for the poor atomization in the carb AND condensation of fuel between the carb and the cylinder. Second, it keeps the fuel/air charge cool and DENSE during its voyage between carb and cylinder.

Smog heads? Designed for lower combustion temperatures to reduce oxides of nitrogen, but that's another lesson.

 

[rodbolt]

Re: QJet myth?... running on small primaries= better economy

the Qjet primary cluster is actually a triple venturi setup, its done to help atomize the fuel into a vapor for better combustion.
as far as supercharging, your not tricking jack your simply adding more air and fuel but if your runing gasoline you still MUST keep the ratio of air to fuel between about 13/1 and about 15/1.
lower than 13/1 and you post about melted pistons, higher than about 15/1 and you post about black smoke and lack of power.
an articulated rod internal combustion engine powered by gasoline as a fuel has a very narrow window of physics to work in.
its simply an air pump.
we place X amount of air with X amount of gasoline and when it ignites it produces X amount of gas.
its very simple.
to much air and its lean, not enough and its rich.
to much fuel for X amount of air and its rich, not enough fuel for X amount of air and its lean.
a carburator,while simple in design, is a comprimise at ALL engine speeds, loads and altitudes.
EFI can overcome quite a bit due to atmospheric sensors,engine load, throttle angle sensors and crank and cam position sensors and mass air flow sensors.
add in pre and post catalyst O2 sensors and you can hold the air/fuel ratio at the correct ratio for a precise burn ratio at any engine speed or elavation you desire within the physical limits of the articulated rod internal combustion gasoline powered air pump.
yes virginia, its that simple.

 

[wire2]

Re: QJet myth?... running on small primaries= better economy

....
lower than 13/1 and you post about melted pistons, higher than about 15/1 and you post about black smoke and lack of power.
....

Um... the other way around, Rod?

 

[spdracr39]

Re: QJet myth?... running on small primaries= better economy

mr sterndrive,
i hate to high jack your thread but i have a question for you
you and i are running the same motor/outdrive (i run a 21 4 winns)
at 3000 rpms i run about 24 mph (gps) and i run about 8 gph
i have a 14and a half x 19 pitch prop (stock)
new plugs wires filters timing is 10 btdc

any idea why i get less speed and gph then you
the boat has less then 100 hours on it
id love to pick your brain a bit if you wouldnt mind

Well there are many factors to this such as weight/load of the boat, altitude, water conditions, trim position, style of cylinder heads based on vintage of the engine, Manufacturer of the carb. you are using, dirty air breather ......

Difficult question to answer needless to say.

 

[rodbolt]

Re: QJet myth?... running on small primaries= better economy

yep, to early, however you get the drift.
to much air for X amount of fuel and its lean, to little and its rich.
some fuels can tolerate mixtures as lean as 150/1 and as rich as 5/1.
gasoline cannot.
take ethanol, its octane is arbitrarily pegged at 100 for automotive use.
its burn ratio is about 7.1/1.
7 parts air to 1 part fuel.
gasoline is roughly 14 parts air to 1 part fuel.
means without modifying your gasoline engine and you try to run 100% ethanol you will run it incredibly lean and smoke a piston or a valve in short order.
its also why you lose power when adding ethanol to your fuel and not modifying,or the ECU being able to, any thing on the engine.
its a simple law of physics and the fuel being used.
its also why MOST gasoline engines use a throttle shutter to control air into the engine and MOST diesels do not.
gasoline and alcohol are constant volume fuels, diesel,kerosene and a few others are a constant burn fuel.

 

[JustJason]

Re: QJet myth?... running on small primaries= better economy

It's not so much about fuel efficiency as it is more about throttle response and runability.
The problem with fixed venturi carbs is just that... the venturi is fixed.

So if you run smaller ones from the get go, the engine won't fart on it's face by getting an influx of to much air when you hammer it from a dead stop.

If you had 2 identicle engines, 1 with say a 600 CFM qjet, and the other with a 600 CFM 2 bbl, or a 600 CFM racing 1 bbl carb, you'd feel the difference immediately as to how the engine responds under light, medium, and full load.

 

[Aloysius]

Re: QJet myth?... running on small primaries= better economy

That's why SU's worked so well..variable venturis.

The nice thing about fuel injectors is that they atomize the full so well..all the throttle valve/plate does is meter air.

 

[rodbolt]

Re: QJet myth?... running on small primaries= better economy

SU's work well, until they get low on oil

 

[JustJason]

Re: QJet myth?... running on small primaries= better economy

The other thing with the qjets is they actually run rich on the primary side. It needs to flow enough fuel to feed up to 8 cylinders up to 3000-3500ish rpms. It's the opening of the secondaries that leans the motor out a bit for better economy.