Project Scion tC

Lots of shiny piping

Even with supercharged cars, where air is pushed into the cylinder and all the pumping work comes from the crankshaft, an unrestrictive exhaust can give significant gains by reducing backpressure, which increases charge scavenging through the cylinder. Backpressure, the pressure of gases trying to pass through the exhaust, is an indicator of how much resistance exhaust gases have to push against as they exit the cylinder. With less resistance, the piston is able to push out the combusted gases faster which, for the brief moment when both intake and exhaust valves are open (at the end and beginning of the exhaust stroke and intake stroke), allows more fresh air to be sucked in. This is because when the hot high-pressure combusted exhaust gas leaves the cylinder, it causes a mild drop of pressure inside. This helps breathe more fresh air in before the intake valve closes for the next combustion cycle. The faster the exhaust blows out, the more the pressure drops. That's scavenging. This differs from turbochargers, which benefit more from an open exhaust - less back pressure means more exhaust energy to spin the turbine.

The main reason behind our huge gains from replacing the exhaust is the elimination of the close-couple catalyst on the stock headers. Both the cat and the pinched primaries will be a restriction to exhaust flow.

The main reason behind our huge gains from replacing the exhaust is the elimination of the

Although Project tC came with a stainless steel TRD cat-back exhaust, we decided to replace it with a Magnaflow unit, which eliminates the stock resonator. The new exhaust uses slip fittings and clamps to hold each connecting section together, so variations between vehicles won't cause fitting issues. The drawback is that, if not properly installed, the slip joints can rotate or lose their seal. Unfortunately, our exhaust has done that because the factory rubber exhaust hangers are rather loose and tired, causing the piping to occasionally rattle against the rear heat shield. So we removed the heat shield.

On the bright side, the Magnaflow exhaust replaces much of the factory B-pipe that other cat-back exhausts don't replace. Scion uses another catalyst underneath that's integrated into the factory B-pipe. We took a hacksaw and cut off the pipe three inches behind the cat, so it will slip into the new exhaust. The result is a tC with one catalyst, the one cops look for.

Megan Racing probably did not plan for the header to be used with the TRD supercharger. The heat shield that TRD provides with the supercharger will have to be discarded, leaving the supercharger exposed to more heat soak.

Megan Racing probably did not plan for the header to be used with the TRD supercharger. Th

We also installed a Megan Racing 4-1 T-304 stainless-steel racing header and 2.5-inch S-pipe with the help of Mark at MD Automotive in Westminster, CA. Megan's header eliminates the close-coupled cat on the stock headers and uses a slightly more efficient collector to join the primaries. Although the welding isn't spectacular, the fit is great for both parts and the elimination of the first cat and reduction of backpressure meant we were able to pull another 11 wheel-hp.

That brings our total to 230 wheel-hp with quarter-mile times in the range of 14.8 seconds and trap speeds at 96mph, compared to our previous best run of 15.1 seconds at 93mph. Not bad for a budget econo-box.

Installation is straightforward, with the exception of removing the nut and stud nearest the passenger side, which holds the exhaust flange. Without removing the alternator (there isn't much tool clearance), we ended up just chiseling off the nut turn by turn. It's easier to remove the stud and replace it with a bolt when you get the header on.

We had to ding the number four primary to get added clearance for the supercharger housing.

We had to ding the number four primary to get added clearance for the supercharger housing

On the recommendation of Megan Racing, we also added an adaptor from Road Race Engineering to the rear oxygen sensor. This acts as a buffer between the sensor and the exhaust flow so the sensor doesn't detect the air/fuel fluctuations, which are much more severe now that the close-couple cat is removed. This little trick seems to work well since we haven't seen an 'engine check' light come on during all of our testing.

Up Next

As stated, Project tC is near completion. It doesn't make sense to spend much more money. We'll head out to the track and measure our Scion against a mildly-tuned Acura RSX Type-S. In the meantime, we're looking to find someone to tune it, insulate the tubular exhaust header, (which sits just beneath the supercharger) and make some mild suspension adjustments to further reduce understeer.

By Josh Jacquot

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