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.
With the stock airbox out of the way, it was then time to step up our game to a full air induction kit. Since K&N happens to offer two intake kits for the LS2 GTO, we figured it was worth giving each a try. The first system is the Typhoon kit (part number 69-7201TP, $321), which uses a polished section of aluminum pipe that is connected with silicone fittings to the mass air meter on one end, and the throttle body on the other. It includes an air dam that prevents hot air from the radiator's cooling fans from getting into the engine through K&N's trademark stack-style air filter that sits right behind the driver-side headlight. With it bolted in place, we found a sizeable gain of 12.4 hp and 6.7 lb-ft of torque at the wheels, compared to our dead-stock baseline. Considering that K&N only rates this kit at a 10.25-rwhp gain, we'd say that they are being 20 percent conservative. So, our new power peaks were 347.3 hp at 6,100 rpm and 336.8 lb-ft at 4,700 rpm at the wheels.
Not wanting to stop there, we then sampled Summit Racing's high-flow mass air meter designed specifically for all LS2 engines. It is the same size from the outside, but it modified for increased airflow and is retuned for more performance. Installation was a five-minute deal and once we were finished, we managed to wrangle 352.9 hp at a lower 6,000 rpm, and torque took a nice-size bump upward to 340.0 lb-ft at a higher 4,800 rpm. The air/fuel ratio was still safe at 11.64:1, which incredibly, was a bit richer than on the baseline run when the car was bone stock (11.93:1.) This translates into a gain of 5.6 rwhp and 3.2 rwtq over the stock meter when equipped with the K&N Typhoon kit. Airflow is certainly making a positive effect on our Head Poncho.
With plenty of dyno time still on the clock, we then tossed on the K&N FIPK system (part number 57-3053, $285) and retained the Summit mass airflow meter since we liked what we saw. With just the simple change from aluminum to HDPE piping, we were now looking at a dyno sheet that said our GTO was making 352.7 hp at 6,000 rpm and 341.4 lb-ft at 4,800 rpm at the wheels. So, the FIPK was just as good as the Typhoon intake as it stayed within 0.2 hp and 1.4 lb-ft of torque. On our project car, the difference was a wash, so either intake kit would work equally well. It would simply be a matter of personal taste as to which one would suit you better for looks and budget.
Plugged In To Performance
Our next item to test at SLP was its handheld tuner, sourced from Diablosport. Its Predator handheld unit is one of the most sophisticated on the market and, because it is an SLP-specific piece, it comes with several custom tunes already programmed into it to complement its own line of Bobcat performance packages. Because our Head Poncho did not have a Bobcat engine package, we were still able to take advantage of the Predator's innate abilities to alter fuel and timing tables for more performance and increased fuel economy. It took about three tries, but once Hank figured out what our GTO liked, we were able to find an additional 4.8 rwhp and 5.8 rwtq for a new peak of 357.5 rwhp at 6,000 rpm and 347.2 rwtq at the same 4,800 rpm.
With our mechanical mods completed, it was time to unstrap our GTO off of the SuperFlow dyno and put it on the lift over at SLP's installation shop, where technician John Forner helped perform the installation of our next piece of high-performance equipment, which was an SLP underdrive pulley. SLP's unit is a machined piece of steel beauty that is precisely manufactured to exacting tolerances. Precision is important for an underdrive pulley, and installation must be performed with a new bolt whenever the damper/pulley is removed from an LS engine. If you don't do it, that pulley will come right off and make a nice run for the border, right under your car and to the curb. Thankfully, SLP's part number 100226 ($249) is a proven piece in many race applications. On our GTO, it took about two hours of time to install. After it was torqued into place, we headed back over to the SuperFlow dyno and watched the rollers spit out peak figures of 365.2 rwhp at 6,000 and 349.9 at 4,800 rpm, for a gain of 7.7 hp and 2.7 lb-ft at the wheels. This goes to prove that, time and time again, underdrive pulleys and LS engines are indeed best friends.
Chapter 2: TT Performance
With our GTO fresh off the SuperFlow dyno and equipped with all our latest bolt-on goodies, it was time to mosey on over to TT Performance where proprietor Matt Sorian was able to take us in for our next round of modifications: the exhaust system. Obviously everything upstream was now opened up, so it only made sense to free up the backside of things. We had heard of some people getting incredible gains from cat-back exhausts and headers while other enthusiasts found no gains at all, so we decided to find out for ourselves what the real deal was.
Now obviously, going from one type of dyno to the next is not the best way to perform back-to-back testing, but in this day and age, many of us enthusiasts have come across both the eddy-current type and the inertia dyno at least twice in our lives. As the market for chassis dynos continues to grow, we see more and more shops using different types for their particular customers. Some prefer a load-bearing dyno as it can help tuners work with turbocharged cars, while others prefer the simplicity and repeatability of the inertia dyno. Either way, you can't go wrong, but it's always good to test your car unchanged between two shops. Knowing that SLP uses an eddy-current SuperFlow dyno and that TTP uses a DynoJet 248c inertia dyno we made a baseline run to reestablish ourselves at TTP in Passaic, New Jersey, with no changes made to the car. The runs were made within a few days of each other, so the car was still on the same tank of gas and the weather was similar.
With the Head Poncho strapped on the rollers at TTP, we baselined ourselves with 338.2 hp and 324.1 lb-ft at the wheels. This is down compared to the numbers that we picked up at SLP, but now we knew where we stood between the two dynos. So, our first mod was the replacement of the cat-back exhaust. Looking at all the various exhaust companies that offer a system for the GTO can be dizzying. Most are of high-quality construction, but we went with the 3-inch cat-back offered by Stainless Works. The advantage with using the cat-back from Stainless Works is that it is a fully adjustable system that can help you get a better fit under your car. Because no two cars are alike, the slip-fit design of the Stainless Works system (part number 05GTOTMCB, $867) allows you to clear items that you normally couldn't with a fully welded system. In addition, the X-pipe crossover system and turbo-style mufflers offer less noise and interior resonance compared to some other systems on the market.
Back at the track, our new best e.t. is a 12.286 at 110.8 mph. Compared to where we were prior to our bolt-ons, that's over a three tenths reduction in e.t. and a full mph over our previous best of 12.620 at 109.6. Make way for the Head Poncho!
Once in place, we found a slight gain of 3.0 rwhp and 1.2 lb-ft of torque for a new peak of 341.2 rwhp and 325.3 rwtq. We weren't too surprised of the small gain, because on 2005-2006 GTOs the factory dual exhaust isn't very restrictive at our power level. So, this proves that on a mildly modified LS2 GTO like ours, only a slight gain can be realized with a cat-back on a bolt-on car.
Next up is our upgrade to the last link of the exhaust chain. This means it was time to ditch the factory cast-iron exhaust manifolds and catalytic converter pipes and to switch over to higher-flowing long-tube headers and catalytic converters. Here again, we went with components by Stainless Works to complement our cat-back. Part number 05GTOHCAT ($XXX) comes with 1.75-inch primary tubes and two high-flow catalytic converters by Random Technologies. Installation was typical for a long-tube header in a GTO in that the steering rack had to be disconnected and lowered out of the car while the headers were installed from underneath. It is important to note that once in final position, the headers cleared everything perfectly, including the steering shaft, which has proven to be a common spot for interference with some other brands. Before we cranked the car up, Matt wanted to put a quick tune into the car to help make up for the increased airflow and to prevent piston-killing detonation. Based on his experience, he was able to plug in a tune with his HP Tuners software in just a few minutes. Back on the dyno, our GTO was now pounding out 350.1 rwhp and 336.7 lb-ft of torque, bringing the total gain from the full exhaust upgrade to 11.9 hp and 12.6 lb-ft of torque. Best of all, the car did not get much louder because we still kept our GTO equipped with catalytic converters and the street manners were not degraded one iota.
There is no better way to find out how bolt-ons affect your ride than to take it to the track. So, we slapped on our Nitto Drag Radials and headed over to Atco Raceway for a little test-and-tune fun. With a strong burnout and plenty of mat-the-pedal adrenalin flowing, we were able to lay down a solid 12.286 at 110.70 mph with a 1.650 short time. In total, the bolt-ons allowed us to drop our e.t.'s an incredible 0.334 seconds and we picked up a full mile per hour.
At the end of the day, you want to know how your hard-earned dollars turn into miles of grins and lowered e.t.'s. We hope that our Head Poncho has given you some hard facts and, more importantly, some motivation to get on the dyno and onto the track. In the end, our bolt-on assault was a very rewarding experience for two reasons: we learned what parts make the most power per dollar and, ultimately, were able to dispel many myths that have been running around on the Internet and at the track pits. See you at the strip.