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Old 08-04-2004, 01:40 AM
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Carburator

Hi,

I am driving Nissan Terrano 2.4l (Petrol/Manual) 4X4. I find the fuel consumption is too high. I am thinking of modifiying the engine to improve the fuel consumption. Could someone advice me on how to go about it. One of my mechanic friends suggested to replce the original carburator with a carburator for 1.6l engine. Apparantly this process will not involve too much of modufications as the carburator mountings and etc are exactly same. I wonder if this will work. I do not understand the logic behind this. How would the combustion and other processes tuned...


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Old 08-04-2004, 03:43 AM
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I believe that putting a block of wood underneath your gas petal to keep you from pressing the petal all the way to the floor would be the same as installing a smaller carburetor, and would have the same effect on gas mileage.

The best way would be to get a smaller(lighter) engine and a lighter car.
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Old 08-04-2004, 10:50 AM
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I can't see how a carb from an engine which is almost half the size could be made to run on your engine. Without re-jetting it's going to be really lean - I doubt if it'd run at all! I don't know much about these engines. Is there a later model of the same engine with fuel injection? If so you could (in theory) swap all the components (including head if necessary) and make it work.
A more expensive solution would be to put in electronic F.I. system that you build yourself - it all depends on how much you're willing to spend.
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Old 08-05-2004, 03:42 PM
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BenzBoy, If a carb that is meant for a 1.5 litre engine flows enough fuel for a 13:1 air fuel mixture at 100 CFM of airflow when on that engine is put onto a 2.5 litre engine, and the 2.5 litre engine is sucking 100 CFM of air through that carb, how much fuel will the carburetor deliver?

What are the conditions under which a particular carburetor operates that change when that carburetor is placed on different size engines?

Why would you believe that a smaller carb on a larger engine would automatically run lean?
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Old 08-05-2004, 05:21 PM
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Terje: If that's the case and the carb from a 1.6 can deliver a stoichiometrically correct charge to a much bigger engine, an give better economy, surely there must be a problem with the original carb to begin with. In any case, I wouldn't expect a 2.4litre engine to have the same headflow as a 1.6, but as I said I don't know anything about these engines so maybe I'm wrong.
Had Nissan thought that a carb from a 1.6 would work on a 2.4 also, surely they'd save on buying / developing two separate carbs and just invest in one?
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Old 08-06-2004, 05:44 PM
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BenzBoy, you failed to answer my questions.

Second, I never said that the smaller carb would give you better gas mileage only that it would be equivalent to placing a block under the gas petal.

I was instead hoping to receive your answers to my questions.
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Old 08-06-2004, 06:02 PM
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OK, my assumption was that the smaller engine would have a lesser intake flow - this is a big assumption as I haven't a clue what this 1.6 litre engine is or what it's put in.
My thinking was that if the carb is jetted for a smaller headflow then once it's on an engine that flows more it will not be able to supply enough fuel. If the headflows of both cars are the same then the carb would probably work.
Also, I was thinking about the reason why the 1.6 has better economy - it probably produces less power, and is perhaps installed in a small car rather than a big 4x4.
Maybe a 1.6 carb on a 2.4 is a popular conversion and runs just fine - I have no idea. However I took the initial question at face value which seemed assume that a carb from a more economical engine can be just bolted onto a bigger engine and hey presto, you got 40 to the gallon - as you know there are far more factors to consider.
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Old 08-06-2004, 11:45 PM
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About 3 or 4 weeks ago I would've made the same assumption, "if the carb is jetted for a smaller headflow then once it's on an engine that flows more it will not be able to supply enough fuel", but it's not quite that simple.

Imagine this, a larger engine should have more power, and therefor suck harder. This harder sucking causes more air flow past the venturi, and thus my well cause more gas to flow than was intended.

This logic is just as valid as the logic that says that the smaller carb is meant for a smaller engine and therefor should flow less gas.

The fact is however that a carb is a device that measures the airflow through it, and meters the correct fuel for that airflow. A larger engine that flows more air will suck more fuel that will maintain the air/fuel ratio.

The only problem with the smaller carb is that it requires more intake manifold vacuum to produce an airflow than a larger carb, and this makes it harder for the engine to suck that much air. Because it is harder for the engine to suck the air, it won't get as much air, but it also won't get as much fuel and will run fine, but have less power than before.

The fuel efficiency of the vehicle depends on the weight of the vehicle and the efficiency of the engine. All engines are made to be as efficient as they can be, but if the car weighs twice as much, then it's like driving two of the smaller cars and will therefor use twice the gas. Also, the larger engine has more internal friction which will use more energy and thus require more fuel.

The problem with putting a larger carb on a smaller engine is that the vacuum produced by the smaller engine at lower RPMs and lower power requirements and at idle may be so low that the larger carb might not meter those smaller amounts of fuel accurately enough to produce a reliable idle.

The amount of airflow produced by an engine is alot more a function of the throttle position than the size of the engine. A very large engine doesn't suck very much air at idle, just as a small engine sucks alot of air when it's floored.

So, a smaller carb on a larger engine will run fine until the vacuum in the intake manifold has increased to the point where the engine cannot produce more vacuum and that's all the air and fuel that engine will get. But to produce more power it needs more air and fuel. So the smaller carb will effect the top power. Pretty much just like placing some sort of limiter on the gas petal to prevent you from pressing the petal to the floor.

The other problems with putting a carburetor on a car for which it was not designed is that the vacuum characteristics of the vacuum ports may not be the same as the carb which was intended for this engine, and thus the equipment connected to those vacuum ports may not perform as designed under the conditions they were intended to perform. But this problem has more to do with the engine for which the carburetor is designed than with the size of that carburetor.

The answer to the original question asked is that the smaller carb will not give better fuel economy, but it will cause the engine to perform with less power, and flooring it with the smaller carb will be equivalent to driving it at partial throttle with the larger carb.
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Old 08-07-2004, 04:41 AM
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Thanks for setting me straight - it makes sense! Now I get where you were coming from on the block of wood thing - pretty much the same effect.
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Old 08-07-2004, 06:50 AM
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Hi Terje,

Thank you for your very comprehensive explanation. I am not a technical guy. But, I understand what you are saying. Can I say that, if power is not my concern than I can get fuel efficiency by using smaller carburator? I do not want to use the block of woo dthough! I hope this will not bring me into other technical problem. I plan to try out this idea in 3 weeks time. It will cost me around US$100. If you strongly feel it will be a waste of time and money, please warn me.
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Old 08-07-2004, 12:22 PM
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Terrano, I feel that it would not be a waste of money to conduct this experiment and to post the results here to tell everyone about it.

But, it will not accomplish your goal other than that when you floor it, you do use more fuel per second (more air also, but that parts free) to generate the extra horse power.

What I mean is, if you have a heavy foot and like to accelerate all the time you will use more gas than someone with a light foot who speeds up slowly.

Also, those people you see on the freeway who speed up, then slow down, then speed up again will consume a lot more gas than those who drive at a constant speed and try to leave the gas petal in the same place during their whole trip.

Make sure you completely test the gas mileage of your engine with your current carburetor first, and keep good notes. And also, make sure you tune the new carburetor properly before conducting those tests with the new carburetor.

Last edited by Terje; 08-07-2004 at 12:29 PM.
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