The following are my thoughts on how the smoking issue and aftermarket exhaust components are related. Flame away...
The 2.3 L DISI turbocharged MZR engine is boosted by a Hitachi K04 turbo that is suspected of having faulty turbo seals. There are numerous reports of replaced turbos for this problem and an excellent write up including pictorial evidence in the Forced Induction section of the mazdas247.com forum. It would interesting if those that have replaced their turbos with stock or aftermarket units had experienced smoking that had stopped upon replacement of the turbo.
This missive addresses the belief by some that the aftermarket exhaust modifications (DP, RP, CBE) somehow cause the engine to smoke. The smoke in question is a bluish smoke indicative of burning oil. Contrary to most peoples understanding, smoke of any kind is particulate (solid phase material) and not gaseous.
Changing the stock exhaust does not change the nature of the materials leaving the exhaust side of the turbo. Immediately after the exhaust exits the turbo however, there are dramatic changes with a different exhaust system. An aftermarket performance exhaust typically will have larger diameter tubing, high flow or no catalytic converter, and a high flow muffler and resonator system. All of these elements work to allow the exhaust to flow as freely as possible out of the motor. They do not however, change the material before it leaves the motor.
The 2.3 L DISI turbocharged MZR engine that comes in the Mazdaspeed6, Mazdaspeed3 and the CX-7 is outfitted with a restrictive DP containing two catalytic converters and connected to a restrictive exhaust system. A three-way catalytic converter has three simultaneous tasks:
1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2
2. Oxidation of (non greenhouse gas, but toxic) carbon monoxide to less harmful (greenhouse gas) carbon dioxide: 2CO + O2 → 2CO2
3. Oxidation of unburnt hydrocarbons to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O
All of these exhaust elements result in increase exhaust gas temperatures relative to a typical aftermarket exhaust components. Replacement of any or all of the stock exhaust components will result in:
1. Faster exhaust gas flow
2. Faster expansion of exhaust gases
3. Cooler exhaust temperatures
a. more efficient exhaust gas expansion
b. shorter residence time of hot exhaust gases in the system
If our engines were burning oil from leaking turbo seals, it is likely that this hydrocarbon particulate could be oxidized to gaseous material all or in part during travel through the stock exhaust system for the following reasons:
1. reduction of the NOx gases will produce oxygen which will react with hydrocarbon particulate
2. Both CO and CO2 oxidizing materials that can oxidize particulate hydrocarbon
3. Oxidation of particulate hydrocarbon increases in efficiency with temperature, i.e. r = kt, where r is rate, k is Boltzmanns constant and t is temperature
4. Increased residence time in the exhaust components will promote oxidation of particulate hydrocarbon materials.
Replacement of the stock exhaust components will result in less residence time, cooler temperatures, and reduced or eliminated catalytic converter function resulting in less oxidizing media. The cooler temperatures afforded with improved flow (aftermarket exhaust) will allow for condensation of hot exhaust products to solid particles or liquid droplets, in other words, smoke.
In short, an aftermarket exhaust will make your car smoke only if the engine is producing the materials (burnt oil) for smoke to form. In our case the oil is most likely coming from turbo seal problems. In any case, the exhaust does not cause issue resulting in the smoke, it just causes you to see it.
