Decat has made NO difference

[FlamingMonkey]

Yep noticed a difference on ma Willy with the Decat

[Martin]

Shit and i thought Northy had the slowest one.. Sounds weird, mine was deffo a bit faster with de-cat.

[J o n]

the cat used before was Jons and that's fine as I used it to get the car through the MOT and once done i put the cat on (2 live, both do's were with same box..) it SEEMED to be quicker, but I think it only felt faster due to being louder and having less torque... is it possible that it's just losing back pressure and running slower due to larger than stadard exhaust bore?

i' thinking of having the original zorst copied but in stainless and a same backbox but in stainless and the pipe coming straight out instead of pointing down... prolly wont tho, will just get cams instead... reckon the decat will do something then, at the moment it deffo aint. strange, cos on the magnex it made a big difference, but thats on a standard mid section and stainless backbox...

[J o n]

Shit and i thought Northy had the slowest one.. Sounds weird, mine was deffo a bit faster with de-cat.

he prolly did until i started to put light mods on

[Purple]

When the original cat broke on mine, I had a straight pipe replacement but that lost torque everywhere.

I quickly changed that to a mid-muffler (about the same internal bore) and the back pressure on that was a a lot better but feels a little constricted at top-end revs than original.

IMHO, I think the original cat was very well suited to the original Williams exhaust system.

[J o n]

well if it's flowing a bit too freely at the moment and I get cams that will certainly stop that problem as it will be pushing it all through quicker and in larger volumes???

[northy]

lol...get a standard exhaust pal..then you will be able to keep up with me.

[KingStromba]

Exhaust


The exhaust system is the other half of the airflow process that is either overlooked, or overdone. Exhaust restrictions occur at the valves, exhaust manifold, connecting pipe, catalytic converter, exhaust pipe, and muffler.

If you are just starting to modify the motor, don't bother with exhaust upgrades until you have opened up the intake first. After you install a K&N and some better ignition components, you might enjoy opening up the exhaust. If that is too obnoxious, consider a 2.5" muffled cat-back system.

As the modifications begin to stack up, meaning you've installed a larger throttle body, K&N filter, intake plenum, lower intake manifold, and more agressive camshafts, you might want to consider a 3" muffled exhaust. This should be all you will ever need until you supercharge/turbocharge the motor.

The effects of back pressure
A muffler is an ambiguity in the performance world, as it can have both good and bad effects on power output. Back pressure is essential for peak power in almost any application, even 6000hp alcohol-burning Top Fuel dragsters have some built-in back pressure. Before we begin this discussion, take note that 1 atmosphere is the pressure that air is at naturally (uncontained) at sea level.

During the combustion process, when the exhaust valve is open, all of the compressed (and depeleted) air-fuel mixture spills out from the cylinders as fast as it can, through your exhaust manifold, into your catalytic converter(s), the muffler, and finally out the tail pipe. The problem with this is, just before your exhaust valve is about to close again, your intake valve opens up, allowing the fresh air-fuel mixture to rush into the cylinders. This is called overlap, and one of the things you take into consideration when choosing a cam, because it can be used to your advantage.

If there was no valve overlap, it would be 100% impossible to completely irradicate all of the spent gases from the cylinder. This has a two-fold effect on power output, depending on what RPM the motor is running at. At low RPM, this effect actually increases torque, because the least amount of compression is lost during the intake stroke, and the ratio of intake to exhaust gases is high. Unfortunately, as the RPMs increase, there is increasingly less time to evacuate the exhaust gases during the exhaust stroke, and more and more depleted air-fuel remains in the cylinders when the exhaust valve closes. The motor becomes incredibly inefficient near its readline. A motor designed for high-torque applications, such as towing, tends to exhibit less valve overlap then normal. The type of cam used in this application is often called an "RV" cam, because a recreational vehicle doesn't need horsepower as much as it needs low-end torque to get it moving.

Your car would also run terribly if there was too much valve overlap as well. When exhaust gases rush out of the cylinder, they create a low pressure area in the cylinder and the exhaust system, sucking the intake charge right into the cylinder, and right back out into the exhaust system. This is called scavenging. When that air spills out, so does the fuel it was carrying, so the O2 sensor reports a rich condition to the computer, often causing further decreases in the amount of fuel the computer injects. At low RPMs, this effect is most pronounced as there is sufficient time to suck out a significant portion of the intake charge, reducing torque. As the RPMs increase, however, the extra velocity imparted to the intake charge increases the amount that squeezes into the cylinder after the exhaust valve closes, as the valve closes so quickly at high RPMs that barely any intake charge escapes through the exhaust system.

No doubt you see the dilemna posed to designers when they choose a cam for the motor - the right combination of power must be achieved at the intended RPM range of the vehicle - if this is a tiny four cylinder, which must spin high RPM to make any power at all, you have to design in a higher amount of overlap. If the vehicle is mostly intended for low speed towing, you design in a smaller amount of overlap for more low-end grunt. Overlap is one of the reasons why the four-cylinder Acura Integra GS-R makes 170hp at almost 7000 RPM, and your 3.0L makes 171lb-ft of torque at 2000 RPM.

You can't control your overlap without changing the cams, but the effects caused by changing the back pressure are the same. When you reduce back pressure, it is equivalent to increasing valve overlap, and when you increase back pressure, it is the same as decreasing the amount of valve overlap. That's why some people will say, "you need a muffler for torque", or, "you'll have more high-end, but less torque, if you run straight exhaust". They are right, but a muffler's purpose is to reduce sound output, not horsepower! By reducing back pressure in an exhaust system, you increase high-end horsepower at the cost of low-end torque.

You can compensate for this by increasing the velocity of the intake charge. Increasing the intake velocity has the added side effect of increasing back pressure, because there is more air to be evacuated during the exhaust stroke. Note that if you increase intake velocity past the limits of the exhaust system, the gains you achieve are diminished to the point of being non-existent. That power will be there when you do upgrade the exhaust system, which is why something as simple as upgrading the exhaust system can result in huge horsepower gains.

You can decrease back pressure by increasing the size of your exhaust manifolds, or using separate tubes for each cylinder (called headers). Increasing the size of the exhaust pipe and decreasing its length also helps, as well as installing high-flow catalytic converters and mufflers. Of course, you could just cut them off. Although it is illegal in the United States to remove the catalytic converter in a registered vehicle, the muffler is a different story...

Putting tinkertoys in their place
I don't know about you, but, when I see a $25,000, four-cylinder Honda or Eclipse with a 3.5 inch tailpipe injustice the muscle car spirit of America, I burn inside for the opportunity to put them in their place.

What better way to do that then to beat them at their own game - put a big straight-pipe on our imported 3.0L six-cylinder?

Considerations to make before proceeding with this modification:

Your high-end power will improve tremendously.
At the same time, you will lose some low-end torque.
Yes, it's loud, but this engine is not a big block, people. Verifying the operation of your rev-limiter at 2 o'clock in the morning will no doubt irritate people, but with a little restraint the added noise is but a small nuisance.
People will ask you all the time if you swapped a V8 into your car. The natural sound of the SOHC is incredibly bassy, and unlike the thin bass tone a well tuned import has, with an open exhaust this motor sounds mean as hell for a six-cylinder!
And finally, the muffler does not filter hydrocarbons! You will not fail emissions because you have no muffler.
It's okay to yank that muffler!
In regards to pollution, people contribute more to the cozy greenhouse by voicing their opinion on muffler removal as actually running a car without one does. Let me explain how the catalytic converter works, and you will understand what I mean:

When combustion is complete, the exhaust contains CO2 (carbon dioxide) and CO (carbon monoxide). Carbon monoxide is the poisonous gas that is formed when combustion does not occur completely and only one oxygen atom combines with the carbon atom. Carbon dioxide is produced when combustion is completed cleanly. Many years ago, engineers discovered that introducing fresh air (oxygen) to the exhaust gas in the presence of catalytic substances (such as platinum), would further complete the combustion process and bond the extra oxygen atom to the carbon-monoxide molecule, turning it into the much friendlier carbon dioxide molecule. For this bonding to occur, the ambient temperature inside the converter must be around 1200-1400 degrees. If the catalytic converter was a filter fine enough to catch little errant pollutants, IT WOULD MELT. Your car would also have no power as it would struggle to push air through the tiny little holes. Stop and think about how big an oriface the tailpipe is, even on a Geo Metro. How ya gonna shove all dat air through a cheesecloth? Huh?

Point is - if your catalytic converter is not a filter, then your muffler sure the hell is not a filter.

[J o n]

lol...get a standard exhaust pal..then you will be able to keep up with me.

lets not get ahead of ourselves here lad, u pulled 1 car length in 1 test, i'll be back tho! lol

we'll do more 1st gear starts next time ;)

[willy 0329]

i think the magnex sys has the same dimentions as the std sys i have this on mine with a ktec silenced de cat i noticed the difference when i put the cat back on for mot felt like i was driving with hand brake on