My suspicion is that the free flow exhaust and cone air filter are incompatible with your engine. They have probably messed up the scavenging effect needed for engine efficiency by removing back pressure.
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You have two options as far as I can see:
1. Take your car to a professional rolling road and see if the engine and carburettor can be tuned to work with the parts you've fitted.
or
2. Re-fit the original filter and exhaust and see if that solves the problem. If it does then the aftermarket parts you fitted are obviously no good for your car.
You are on the right with your conclusion but not quite right with the cause.
'Scavenging' and 'Back pressure' are not commonly used terms and so they are often not understood well , so lets see what they are and what effect they have on an engine - relative to fuel economy., and then we will look at low restriction air filters.
'Scavenging'.
At the end of each firing stroke and expansion the combustion chamber needs to be evacuated of the burnt air/fuel mixture in preparation for the fresh new mix coming in.
Residues of this old burnt gas takes up space in the chamber and this space obviously cant be filled with new air/fuel mix.
This charge dilution results in less power and economy.
It is a dirty inefficient engine.
So what you have to do is make sure all the old combustion residue is gone.
It of course has to go out the exhaust pipe.
Now as soon as the exhaust valve open this very hot charge which wants to expand and fly out the exhaust port down into the pipe.
In a very short time though this expansion power is lost and some residue remains.
This is where scavenging comes in.
Scavenging is the term given to the cleaning action of the chamber of burnt gases.
Ideally you want a complete scavenge.
The scavenging process is created by a few various things and they all do their part in contributing to this effect.
First off as mentioned you have an initial burst of pressurized exhaust gas that wants to leave the small chamber that has high pressure to a lower pressure environment , namely , the exhaust pipe.
As this exhaust gas is traveling inside the manifold (like a train) it is pushing air in front of it , and also crating a vacuum behind itself.
This small vacuum helps suck more waste gas from the chamber.
During this short period when scavenging has to take place both the intake and the exhaust valve is open.
This is known as 'overlap'.
Just as you get a train effect of pushing and vacuum in the exhaust side you also get it on the intake side.
Air flowing down the intake will push past the now open intake valve , through the chamber and out the exhaust port.
The longer the overlap duration the more time scavenging has to take place.
However , camshafts with long overlap durations have very poor low speed (RPM) efficiency.
Due to excessive overlap duration air speeds are very low at low RPM's.
That is why a when you hear a race engine idling it has that characteristic 'Rump Rump' sound.
At low engine speeds it can barely even tick over properly without stalling.
In a normal car when everything is running rite scavenging can be completed and engine efficiency can be quite high even at low engine speeds.
Now say we change one of the variables that creates a good scavenging effect and replace it with a bad variable.
Keep these points in mind and we will move on to the next topic....
'Back pressure'
'Back pressure' is the resistance to exhaust flow caused by the following.
Resistive mufflers - either from small size , bad design , or failure.
Resistive pipe work - either from small size , bad design , or failure (crushed bends)
Automotive accessory outlets praise the effects of HUGE pipes and mufflers with no restriction , so big you can stuff an obese turkey up the tailpipe.
While these sellers often claim outrageous performance gains from using such a product , in the real world it doesn't translate to increased performance , in fact , often the opposite happens.
Poor economy , lower mid range power , excessive noise etc.
BUT , we know that race cars usually run quite large pipes and low restriction mufflers.
How can it be right and wrong at the same time....?
Is having back pressure then a good thing or a bad thing....?
The answer is both YES and YES , and NO and NO.
It ALL depends upon the application - but now for some more technical theory.
Depending upon the shape of the combustion chamber , wedge , heron , hemispherical etc , will determine the ratio of intake versus exhaust valve size.
To put it another way - The exhaust valve is always a certain percentage smaller than the intake valve , and this percentage is determined by the design of the combustion chamber.
Now take for a real world example a UNO 1300 , Ritmo , 128 blah blah ...OK.
FIAT in their wisdom tended to use exhaust valves TOO big.
They have (even in their twin cams) used a percentage figure now known not to provide the best fuel consumption versus power output.
I have never found a reason why they did this , perhaps they reasoned a larger exhaust valve would stay cooler and less likely to burn over a long time....
who knows...but thats what they did.
So in most FIAT engines (not sure about the latest ones) fuel economy isn't all that great.
Consider the economy figures from Jap cars that are of the same engine size and make similar power.
Usually they will get better fuel economy with less emissions.
My 1L Suzuki Swift (geo metro) gets 4.5L/100km and makes 55HP.
My UNO book says it will get 5.0L/100km and make 45HP from the same engine size.
It should be noted that my Swift weighs 150kg more than the UNO but has a top speed 20kmph higher.
--but thats beside the point.
So what FIAT had done was make the exhaust side of the cylinder head flow well , too well by comparison to the intake side.
This promotes OVER SCAVENGING.
OVER SCAVENGING happens when the chamber is vacuumed of all its reside , and also a lot of fresh intake charge is also sucked out the exhaust during the overlap period.
This means unused , unburnt raw fuel and air mix is going out the exhaust pipe.
This looses fuel economy , and you certainly cant make power from gas going out the exhaust pipe.
OK...
Now , when we come along to our FIAT example and we put a lower restriction exhaust on it (with no other REAL modifications) we are INCREASING this already overdone scavenging effect that FIATS have.
The result . less economy -- its all going out the tail pipe unburnt.
In summary....
Low restriction exhausts will only IMPROVE MPG on a car that has unusually restrictive exhaust ports or small exhaust valves.
In the dark old days (50's and 60's) this wasn't uncommon , and after-market pipe and muffler sellers had a boom time because their products really did add 10% power and 25% on the MPG.
Low restriction air filters.
This really is only a problem with non EFI cars, or in other words cars fitted with carburettors.
Carburettors are intricate devices that are pressure and volume sensitive.
From the factory they are set up (aka jetted) to suit the engine it is fitted to.
This is done with the full exhaust and intake system connected , including the air filter element.
As Air filter devices have some sort of resistance to air flow , this resistance must be in place when the air fuel mixtures are set through jetting.
Simply put , Jetting is a procedure where they select main fuel jet , emulsion tubes and air correction jets.
These , when right , provide a continually self adjusting mixture dependent upon air flow and vacuum levels through the carburettor across the whole RPM range.
When you change to a LOWER (or higher) restriction air filter it will affect the levels of vacuum within the carby compared to what it experiences normally , this in turn will alter the air/fuel mixtures being delivered to the engine.
This means, with a low restrictions air filter the standard carburetor jetting is NOW incorrect and cant delivery the right mixtures required for full power and full economy.
gW
