Project SRT-4: Part V

Inspiration for our side-exit exhaust comes from the 1970 AAR 'Cuda. Year One, Inc. sells the original replacement exhaust tip for that car. Based on the shape of this tip, it looks like the 'Cuda had a far more convoluted exhaust routing than our car does.

Inspiration for our side-exit exhaust comes from the 1970 AAR 'Cuda. Year One, Inc. sells

As far as we know, no one makes a tool to measure exhaust backpressure, so we built our own. We started with a glycerin-filled, 4.5-inch, 30-psi pressure gauge from McMaster-Carr (www.mcmaster.com, part number 3708K402). We then took an 18mm fine-thread bolt that happens to have the same threads as an oxygen sensor, some copper tubing, a few brass compression fittings and some vacuum line. Voila! Instant backpressure gauge.

We had a machine shop drill and tap a hole in the bolt for the brass fitting. Then we attached a few feet of copper tubing to the fitting, followed by several feet of rubber vacuum line leading to the gauge. The copper tubing can handle high-temperature exhaust gases and the rubber line allows you to put the gauge wherever you want during testing. All our testing happened on the dyno so we just ran it under the car and in the passenger door.

We removed the first O2 sensor and installed the bolt and brass fitting in its place. This allowed us to measure backpressure before the catalytic converter. If you're smart, you'll zip-tie the O2 sensor out of the way before you start the car. If you're like us, you'll let the zip-tie short out on the alternator's positive terminal and then burn yourself when you try to remove it. Remember, O2 sensors are heated. Heat will burn you.

When you're done burning yourself on the O2 sensor, you'll realize it would be really nice to know how much backpressure there is after the catalytic converter as well as before it. This would give you some idea how much of a plug the cat is and allow you to judge how much power might be found by replacing the factory cat with a high-flow piece from a company like Magnaflow. Many modern cars put the second O2 sensor after the cat. In those cars, this would be easy to figure out, but on the SRT-4, the second O2 sensor is in the middle of the cat between the two blocks, making this test worthless. Apparently, our friends at the California Air Resources Board mandate this placement so we can blame them for making the test impossible.

Before and after. Perhaps the only drawback to a side-exit exhaust is that the tip lies directly in the debris path of the drive wheels. That means every piece of sand and grit kicked up by the front tires hits the shiny Magnaflow tip. Below is what a new tip looks like. On top is our tip after about six months of use. Note the shiny one is much longer. None of Magnaflow's 3-inch slash-cut tips are short enough, so we had to cut it short.

Before and after. Perhaps the only drawback to a side-exit exhaust is that the tip lies di

On the dyno, the Magnaflow exhaust made 5.3 psi of backpressure, while the stock one made 7. The improvement is actually slightly better at the exhaust ports, though. The turbine creates a ton of backpressure (we're told it's more than 35 psi), but here's the magic: the pressure ratio across the turbo stays the same when we change the exhaust. If you add atmospheric pressure, the pressure in the manifold is about 50 psi (35 psi plus about 15 psi atmospheric pressure) and downstream at our pressure gauge, it's 22 psi (7 plus 15), so that's a pressure ratio of about 2.25:1. Now, take 2 psi of backpressure out of the exhaust, as we almost did, add atmospheric pressure back in (5 plus 15 plus 20 psi) and multiply by 2.25 and you get 45 psi. Subtract atmospheric pressure again and you only have 30.5 psi in the manifold now.

If you got lost in all that, here's the point: We lost 2 psi in the exhaust, but lost 4.5 psi in the manifold where it matters. Hurray for turbos! On our dyno, the exhaust was worth 4 hp and 4 lb-ft of torque at the wheels. The final numbers are 239 hp and 261 lb-ft of torque with the Magnaflow exhaust.

Before trying this kind of dyno testing yourself though, you need to know one more thing about that ECU-controlled boost. A sudden drop in backpressure from the new exhaust will cause a sudden spike in boost, which will scare the ECU and pull all your boost away. Over time, it will give your boost back, but you have to teach it by driving hard. We found two fourth-gear pulls on the dyno were enough to rebuild the boost map. A fourth-gear pull to redline on the freeway would do the same thing, but we're not that stupid.