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. | 
