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Monday, August 10, 2009

Why fit another exhaust to my vehicle?

Why fit another exhaust to my vehicle? There are several reasons, style, performance, sound and "the old one needs replacing". Style cannot be argued against as most standard system's exit through a boring small, often rusty tail pipe. Performance is a harder one to equate, as putting a performance big bore kit exhaust system on a standard engine is unlikely to produce much benefit. Sound is hard to give an opinion on, and one person's noise is another's music.

Changing to a performance exhaust system because the old one is worn out is arguably the best reason, as many performance exhaust system are not that much dearer than the manufacturers replacement and on some models even cheaper. Do not compare a non genuine replacement exhaust from your local free fit garage prices with manufacturers own or performance exhaust systems, as not only are the majority made out of thinner or lower grade material but from our experience reduce performance. Most performance exhausts, including Scorpion, Piper, Magnex and Musketier are made out of stainless steel, which is superior in every respect, especially against corrosion, which is the main killer of exhausts, so when comparing prices and value for money make sure you are comparing like with like.

To give you an example of a non genuine exhaust system, we fitted the rear silencer to a customer's car, he insisted on none genuine to save money. At the same time we carried out a full service and a day after he was back complaining we had done a bad service, as the car was sluggish at high throttle openings. We rechecked everything and every thing was OK, so in desperation we fitted a genuine rear box, performance restored.

A few more words are needed regarding the Piper and BTB exhaust systems. We like the Piper units as they are equal to all the others as regards performance but the big benefit is that you can get about 6 different tail pipe designs, round, oval, twin, slashed etc etc.


As said above the majority of so called performance exhausts will not increase performance, especially on a standard or near standard car, sure they look good and maybe make a lot of noise, that makes you think performance exhausts are more powerful but on the dyno the results say the standard exhaust unit is very good. It is only when you start to make a lot more power that the exhaust system needs to be changed for a true freeflow unit, due to fact you are burning more fuel and so have a lot more exhaust gas to get rid of. We found this a particular problem on our supercharged cars, the power would go up and then level off as the exhaust just could not pass the gas, causing exhaust back pressure, which builds up and pressurizes the exhaust ports and so stops the gas coming out of the cylinder, this in turn means the cylinder does not get rid of all the burnt gas and so contaminates the new charge stopping it burning as it should and so reducing power. We found that for every 1 lb of boost there was 1 lb of exhaust back pressure. The solution was to make an exhaust system with a much bigger bore exhaust pipe all the way from the front to the exit, the size being 63 mm. (Photo: BTB Pipe Diameter In Comparison To A Standard VTS System)

A case that proves this point is we had a supercharged Saxo VTR in our workshops, it was a car that had had our kit fitted by another garage. After correcting numerous other faults we dyno tested it and it went to 130 bhp at the wheels and just stayed there from 5000 rpm to the rev limiter. We took off the "performance" system and fitted the BTB unit, 150 bhp without any other alterations. I know many of you may say, "Well I don't need one of these my car is not supercharged". Yes, I would agree up to a certain point. This problem is not because it is supercharged but because the engine is producing a lot of power and hence a lot of exhaust gas, this same problem will come about with a normally aspirated engine if the power level is increased a lot. We have done no tests, so have no figures to say at what level of BHP this back pressure problem will start but I would guess at anything over 30 BHP increase, and the bigger the BHP the more of a problem. It is something worth considering when you have just bought your car, as many start off by fitting a new exhaust as the first job. What you should do is think where your tuning mods are going to stop, as many have plans to fit an air filter, ex manifold, cams, head job etc. If you think your tuning is going down this road get the BTB don't get any other as if you go for big power later it will mean you need to change your exhaust system again.

Catalytic Converter (Exhaust Cat)

Another reason to fit the BTB unit is a bit more technical, most people want to take the catalytic converter off as they sap so much power, the problem is taking the catalytic converter out and substituting a straight pipe is not really correct. OK, this will certainly give you the power back the exhaust cat takes away through restriction but this causes another problem. When exhaust gas comes out of the cylinder it produces sound/pressure waves that travel up and down the exhaust system, if these S/P waves can be controlled correctly they can be harnessed to draw extra gas from the cylinder and so make the engine more efficient. These S/P waves are controlled by the cam timing and the pulses go down the exhaust until they meet the first expansion chamber, silencer, catalytic converter (anything that causes the size of the exhaust bore to rise significantly). As soon as the gas reaches this expanded section, the S/P wave stops and goes back to the cylinder head, as this wave hits the back of the valve it bounces back and causes a suction pulse that helps pull more gas out the next time the valve opens. The problem is the distance from the first expanded section to the cylinder head is critical, to get this pulse action working correctly, if this distance is not correct the wave goes back at the wrong time and hits the gas coming out of the valve and so stops the gas coming out rather than helping it. This is what happens when you take the exhaust cat out, you no longer have an expanded section in the correct place and the S/P wave will go down the system until it reaches the first silencer box before returning. Before catalytic converters were fitted, this was the reason all cars had an expansion boxes in a similar position to where they now fit catalytic converters. I know some one will say, what about the VTR with the catalytic converter on the down pipe, or the old mini with only one silencer at the rear. The complications of how these S/P waves travel isn't quite as simple as above. Lets assume the correct distance from the valve head to the first silencer should be 80 inches, providing you double or treble the distance or half it, quarter it, the S/P waves will still work their effect, so having the catalytic converter on the down pipe or just one silencer at the back can still be right, what is practically certain is that taking the exhaust cat out and substituting a straight pipe will not be correct and so you will loose some power.

The BTB system we make is from the manifold back, with a box put in the place the catalytic converter would normally be (It also looks like you have a catalytic converter fitted if anyone has a casual look) and so keeps the S/P waves correct.
The BTB unit is a more expensive exhaust unit than the others on the market as they are much bigger, with a lot more stainless steel needed to make them, one benefit is however you do not need to buy a de cat pipe.

Having said all of the above about taking the exhaust cat out is not a perfect solution, it is still better to remove it, than leave it on. The reason the exhaust cat causes the back pressure problem is it does, is it's construction. As said before a good condition car does not need an exhaust cat, the reasons for the manufacturers fitting them is that if the car was out of tune and running rich, the gas would be very polluting and so the cat then comes into play. So for most of it's life it is doing nothing, except under cold start conditions. The way the exhaust cat works is that the gasses are forced through a honeycomb of small holes that are coated with special metals which form a chemical reaction to purify the gasses. It this forcing the gas through the small holes that creates the back pressure.

BTB System Shown In Comparison To Standard VTS System
Many people ask about the legality of removing exhaust cat, especially in regards to passing the M.O.T., all the law says is that the exhaust gas exiting the tail pipe must conform to a certain standard, any car that's in good condition and regularly serviced will pass the exhaust cat test, without one fitted, even cars that have been modified quite a lot usually still pass. For those that don't we can provide management to overcome these problems.

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