by MazdaManiac

The exhaust system on the RX-8, much like the intake system, is fairly well thought-out and properly engineered for the task at hand – an unenviable task at that. Constructed in three main sections, the exhaust system must remove waste heat and gasses from the engine without impeding their exit from the tailpipe, all the while keeping the heat and gasses away from the passenger compartment and other driveline components and simultaneously reducing the exhaust noise to an feeble, buzzy rumble. The exhaust manifold is a progressive-taper, multi-layered “log” style manifold with ample shielding and carefully planned bends. It has large ports – larger than the exhaust ports themselves on the motor – and a relatively large 2.5inch (64mm) outlet that terminates in an asymmetrical 3-bolt flange. It also houses the front wide-band oxygen sensor, which is responsible for controlling target air-fuel ratios in closed-loop operation. The exhaust manifold must simultaneously channel the immense thermal energy output of the rotary exhaust to the remainder of the exhaust system while keeping radiated heat and noise to an absolute minimum. Moving that heat along is also important because the next link in the chain – the catalytic converter – is dependent on that heat for proper operation. The manifold also has a fitting for the secondary air system which, by way of an electric Bosch centrifugal pump, forces air into the exhaust stream during cold-starts to help reduce emissions by effectively causing a second combustion cycle in the exhaust manifold.

The catalytic converter is a three-way, two part ceramic monstrosity that, as big, heavy and restrictive as it might be, is still less than half the size of the converter that was employed on the RX-7. Its two sections alternately clean and trap combustion byproducts, theoretically converting them entirely to carbon dioxide and water. Mounted between these two sections is yet another oxygen sensor. This one, however, is the conventional narrow-band design that has been found on most cars up to now and is essentially tasked with watching the catalyst do its thing and report to the PCM on the progress. It does so with a vengeance.

After leaving the catalyst, the exhaust gasses are funneled through a small, in-line resonant silencer, which also reduces the internal diameter of the exhaust path to its final 2.25 inches (57mm) before terminating in a flexible, gasketed, spring-loaded swage joint.

The now rapidly cooling gasses make their way down to the enormous 1.5 cubic foot (32 liter), six-ton muffler/trunk warmer where they are expelled through twin tail pipes with polished tips. This muffler is very effective at reducing exhaust noise and producing a pleasant and neighbor-friendly tone, but it also will melt the ice cream in your groceries before you leave the supermarket parking lot, despite the heavy steel shield between it and the trunk floor. It also weighs six tons – did I already mention that?

Much of Mazda’s hard work and careful attention to detail in maximizing the RX-8’s performance went, unfortunately, right down the drain with this exhaust system. Not because it isn’t well built or effective, but because it is required – by law – to strangle the motor nearly to death and do so for a warranted period of time that surpasses the life of the vehicle itself. This meant that Mazda engineers had to re-think the tuning of the engine management system as to produce an optimal output to support the catalyst in its proper function, seemingly without regard to its effect on total performance. This assessment is probably a bit unfair because it is unlikely that these engineers would have simply surrendered their passion for this car – a passion that is obvious in nearly every other detail – just because they were suddenly straddled with Draconian emissions and warranty requirements. However, the effect on power output because of these restrictions cannot be ignored. Mazda was actually faced with the terrible reality of having to downgrade their published engine power output estimates from 247 horsepower to 238, mainly because of the effect of this emission retuning. Most owners realize that even this revised estimate is a bit optimistic and dynamometer testing shows that the actual power output is often considerably less than this published value. This engine management retuning also has an adverse effect on fuel economy because the catalyst, while dependent on heat for proper operation is also sensitive to excessive heat and must be cooled at part throttle cruise by, of all things, dumping fuel into it. This catch-22 has been the motivating factor in a long series of update services to the PCM programming which have been applied alternately by a series of Terminal Service Bulletins (TSBs) and emissions recalls over the entire production life of the RX-8 thus far.

So, all of this leaves the enthusiast with a bit of a dilemma – how to increase performance in a meaningful way without upsetting the balance of this emissions-strangled system.

The resolution is that you can’t do it without compromising one or the other. Simply slapping on a “CAT-back” exhaust system – as one would on other, lesser automobiles – will produce only minimal gains in power with the original equipment catalyst and its requisite tuning still in place. The improved tone and reduced weight of an aftermarket exhaust system might make the investment worthwhile for some, but you shouldn’t expect a neck snapping performance improvement from just replacing the muffler and the plumbing that leads to it, despite the claims of the various aftermarket exhaust manufacturers. Even deleting the last parts of the exhaust system entirely will only yield a very noisy 7 to10 horsepower.

Then, there are the so-called “high flow” catalytic converters that replace the factory converter and promise less restriction and, therefore, higher performance that, when coupled to a free-flowing CAT-back exhaust system might amount to a few more ponies. That choice, however, must be balanced with the reality that a “high-flow” catalyst will violate your 150,000 mile emissions warranty and probably decrease the efficiency of the emissions system, which may land you in trouble with local enviro-police in your area. Also, it probably wont fool the PCM and its oxygen sensor nanny which will, eventually, notice the difference in catalyst efficiency and turn on the check engine light.

Then there is the option of deleting the catalyst entirely in favor of a mid pipe (sometimes referred to as a “test pipe”) and forego any sort of emissions cleaning at all. This is what is done on racecars for good reason, but racecars are not driven on the street and are not subject to local and Federal emissions laws – yet. Mid pipes are available with plumbing for that second oxygen sensor (which would then scream bloody murder because it has nothing to monitor) and in a more flow-friendly 2.5inch inside diameter to match the outlet of the exhaust manifold. Coupled with one of the 2.5inch exhaust systems, a mid-pipe will yield the greatest gains in power and, when matched to appropriate engine management tuning can increase the output as much as 20 horsepower or more. It will also turn on that annoying check engine light almost immediately after you go down that path unless you have access to one of the flash-based engine management choices  such as the Cobb AccessPORT.

The original equipment exhaust manifold is very difficult to improve upon. Most runner/collector type exhaust headers will increase noise and heat in the engine bay with minimal – if any – increase in power. Perhaps, if coupled with the above-mentioned exhaust system modifications, a well-designed exhaust header might add single digit increases in total power output.

At the time this was written (six years since the introduction of the RX-8 into the automotive marketplace) there were precious few header systems available for the RX-8 and they were very expensive compared to their level of performance. Most of the race teams that are experimenting with exhaust headers as a replacement for the original equipment exhaust manifold are building their own units from scratch. In an arena where every last iota of performance left on the table may make the difference between winning and losing, such investments make sense. On the street, headers for the RX-8 are, for the most part, just bling.