To say that our 2005 GTO is one bad mofo would be an understatement. Starting out with an incredibly stock yet strong-running '05 LS2, our Project Head Poncho has proven to be a consistent and fearsome runner both at the track and on the street. Nothing can touch it from stoplight to stoplight, as the car just plain hooks and goes thanks to its grippy drag-radials, capable IRS, and incredible power on demand. With nothing more than a few suspension bushings and an excellent Vigilante 3,200-rpm torque converter, we've dived well into the 12s. I mean, think about it: How many cars can you buy today that can handle as nicely as the Aussie-made GTO and still nail down a 12.62 at 109 while looking good doing it?
So to up our own ante, we took our Head Poncho to the next level. Because we've been systematically making upgrades to our GTO with what we believe to be the most effective mods for the buck, adding some horsepower to the mix is next on our list. We're going to upgrade our car step-by-step, like we've done so far, so we can see what each modification makes as far as power and e.t. goes. For us, our goal is to keep the GTO naturally aspirated for maximum reliability and to help keep our portly coupe's weight within a reasonable level. To get there, we went in two steps.
Chapter 1: SLPPerformance Parts
What you read on the Internet can only go so far in the real world. No two cars are alike and no two dynos are alike, so we've decided to kick off the first chapter in our bolt-on lovefest with the low-buck mods that have historically proven to be cost-effective. Then, we'd record the results on a chassis dyno for empirical numbers. Thanks to the kind folks over at SLP in Toms River, New Jersey, we were able to spend an entire day testing out what works and what doesn't to not only show how much power can be made, but also to dispel some of the myths we've heard through the grapevine. Thankfully, SLP uses a dual eddy-current SuperFlow AutoDyn chassis dynamometer that gives dead-accurate and repeatable results.
First up, we took a look at the factory airbox and noted how it was drawing air from a 90-degree plastic elbow. It looked like a restriction, so we gave it the old college try and pulled it out. Our hunch proved correct as horsepower went up slightly from our baseline of 334.9 at 6,100 rpm to 339.8 at 6,050 and torque went up from 330.1 lb-ft at 4,600 rpm to 333.4 lb-ft at the same engine speed-that's a gain of 4.9 rwhp and 3.3 lb-ft of torque. Best of all, this modification was free. Air/fuel ratio was very safe at this point, being a smidge on the fat side at 11.93:1 at peak horsepower.
Next, we decided to do the tried-and-true drop-in K&N filter. Using part number 33-2314 ($48), we simply replaced the factory paper element with the pre-oiled high-flow unit from K&N and were rewarded with a solid gain of 2.0 hp at the wheels, but torque only went up by 0.2 for a new high of 341.8 at 6,100 rpm and 333.6 at 4,600 rpm. Then, just for shifts and giggles, we removed the air filter element completely and bolted the airbox back together. Here, we found 344.1 at 6,100 and 334.6 at 4,600 rpm-which meant that the K&N filter was only costing us 2.3 rwhp and 1.0 lb-ft in rwtq. For the safety of having an actual high-quality air filter element in place, we'll gladly give up the pony or two to prevent flying birds from becoming one with our combustion chambers.