 We also opted for a high-flow...  We also opted for a high-flow MagnaFlow catalytic converter (right). It's longer and has much smoother transitions internally, where it counts. |
 This shows the difference...  This shows the difference in size between the stock 3-inch and the giant 4-inch Mufflex tubes. As you can see, the area is dramatically increased. If your exhaust is restricted, this will cure it in a hurry. Tubing is aluminized steel for the new system. |
 Dyno Testing - The dyno chart...  Dyno Testing - The dyno chart reveals the baseline pull produced 180 rwhp and 245 lb-ft of torque. After the exhaust was installed, the first pulls revealed that the engine was running lean: The result was 194 rwhp and 231 lb-ft torque. Though peak power was up by 14, peak torque dropped 14 lb-ft from the baseline. Adjusting the fuel regulator for higher pressure resulted in 195 rwhp and 257 lb-ft torque, an increase of 15 rwhp and 12 lb-ft of torque over baseline. |
 The photo doesn't do it justice,...  The photo doesn't do it justice, but when anyone looks up under the back bumper, they always have some kind of exclamation of amazement at the size of the tailpipe. Not being an import, this isn't false advertising. This pipe size remains constant for the entire length of the Mufflex after-cat after it's adapted from the catalytic converter. |
The final detail on the headers was that some of the AIR passages were blocked by welding or coating slag, so another zip of the grinder on the inside of the header tubes released a handful of small white balls that were blocking the AIR tubes. As it is our objective to have this car pass California emissions tests and the AIR system has proven to cost minimal if any power, we want it working correctly. Installation of the headers went as planned: All bolt holes lined up properly and mounting the shorties was simple, especially since we have deleted the air conditioning on our GTA (retaining air conditioning will require jacking up the passenger side of the engine).
Since we've heard positive reports on the catalytic converters made by MagnaFlow, both for their ability to flow air on a flow bench and their competitive price, we ordered one. The MagnaFlow high-flow cat converter is designed to replace the OEM unit, but the 3-inch-id pipe that goes into it is intended to slide around (outside) the OEM Y-pipe. Our Hooker Y-pipe is a beautiful mandrel-bent piece and is actually 3 inches instead of the 2.875 (or so) of the crimped and mutilated stocker. As you can imagine, you can't slip one 3-inch-id pipe into another, so we trimmed the end of our MagnaFlow cat converter to butt against the end of the Hooker Y-pipe and dragged out the welder.
With the Hooker headers and MagnaFlow catalytic converter installed and adjusted, we were finally able to move on to mounting the Mufflex 4-inch after-cat exhaust. We did have to do some minor "convincing" of the area directly above the Panhard-bar brace to get the giant pipe into place above the rear axle, so the fine adjustment tool (FAT: i.e., mallet) was used to "adjust" the area where the pipe passes between the panhard-bar brace and the floorpan above the rearend. The installation was otherwise bolt-on and trouble-free, and upon starting the GTA, we were pleasantly surprised to hear the sinister rumble of the optional SpinTech muffler, which was somewhat quieter than the Flowmaster muffler that is usually supplied.
Keep in mind that everything is relative, and with the 4-inch-diameter system, you won't be sneaking up on anyone regardless of which muffler option you choose. If you like the louder exhaust, go for the Flowmasters, but if you want to serve up helpings of crow to your opponents without sounding like you have a top fuel engine under the hood, the SpinTech should get the nod.
Some of the details required before the GTA was test driven were to loosen and adjust our big-rig-size exhaust and jockey it around so it wouldn't be pressed against anything because at certain rpm, it would resonate the car. We also had a minor issue with the new headers and our new set of spark plug wires. Yes, Murphy's Law is enforced upon us more than you know. The headers want right-angle short boots. We had a few straight-style boots for use with the stock manifolds, and they were laying on the headers, a sure recipe for burnt rubber and bad spark. We simply replaced the two oddball straight wires with some decent-looking others for now, but they will obviously need to be replaced with a new set when we don't have a magazine deadline to meet.
Back To The Dyno - After a quick trip around the block to make sure nothing important fell off the car, we made our way back to MD Dyno where Mark loaded the GTA onto the Dynojet to see what all of our time and energy had brought us. After some pulls to settle things in and get the GTA good and warmed up, we gave the ol' 350 a few good tugs to find that, true to Bruce Hawkins' prophecy, we had gained 14 peak horsepower but lost almost 14 lb-ft of torque, and the power below 4,400 rpm, where this car normally does all it's best work, dropped off substantially. From idle to 4,300 rpm, the horsepower and torque were both well below baseline, but we were also running quite lean, which makes sense with the reduction in back pressure realized by the new exhaust system. The car feels faster, as without any bottom-end power, the spike at 4,400 rpm feels like the car is coming alive, especially with the roar of the new headers and exhaust, but unfortunately, this is all happening while bicyclists are casually passing us.
Looks like this is a job for our Holley AFPR, so we reached under the manifold and gave it a quarter turn, although at this juncture, we weren't trusting our gauge anymore to give us the real fuel pressure. It was just what the doctor ordered, and after a few more pulls, our GTA's torque jumped 26 lb-ft over the previous lean pull, while at the same time, the torque peak went from 4,400 rpm down to 3,550. We tried even higher pressure, but power dropped off.
Remember not to worship the false god of peak power as it often doesn't tell the story of how well the car drives in other areas of the powerband. Looking at the baseline versus the first pull with just the exhaust system, the GTA actually gained 14 peak horsepower, but the loss of torque and the death of our powerband below 4,400 rpm made the Pontiac noticeably slower. After increasing the fuel pressure, our powerband has now resurfaced and exceeds the original all across the chart, which is exactly what you want. Peak power is up 15 hp from the baseline, but most importantly, the powerband and torque band no longer dip below the baseline, and throttle response is instantaneous.
Conclusion - Our GTA is now a ball to drive and puts out about the same torque at the rear wheels as some of the classic Pontiac musclecars. Power and torque come on right off idle, and the sinister growl has heads swiveling like fans at a tennis match. All our buddies who had given us static about "the grey mullet mobile" are now transfixed by its arrival with tires smoking, and they are chomping at the bit to drive it. Are we happy? Absolutely! Are we satisfied? We'll see.