Editor's Note: This is the fourth installment in CHP over the last few months from Mark Stielow, the builder of many badboy, Pro Touring-type '67-'69 Camaros. He's building a new '69 Camaro, the Thrasher, and is showing us how he created these super-performing ponycars. This month's story details building a minimum restriction, but maximum quiet, Borla exhaust system and how he modified the stock dashboard to accept aftermarket Auto Meter gauges.
The last few months have been spent doing groundwork for the Thrasher, so I'll review to make sure everyone understands what we're up to here. My latest car, a '69 Camaro called the Thrasher, is being built to run a few different events (One Lap and the Pony Express 100 for example) and drive on the street. The drivetrain is loaded with a 450hp, 406ci small-block, a Centerforce clutch setup, a T56 Borg-Warner transmission, and a Currie 9-inch rearend. The suspension is equipped with Koni shocks, my own design SED spindles, Landrum springs, Hotchkis swaybars, Global West bushings, BBS wheels, and BFG tires.
I've built cars with trick, unobtainium parts in the past. They are cool to look at and touch, but when they break, you can't find a replacement part to save your life. That's why the Thrasher is being built with off-the-shelf parts. It should run just as fast as the trick stuff, if not faster, because of how the parts are combined, and I'll be able to fix it if we break something in Resume Speed, Missouri. Having built the engine and outlined the basic chassis work in previous articles, this month you'll learn how to tuck a full 3-inch Borla exhaust system up under a slammed '67-'69 Camaro and maximize ground clearance. You'll also see how I installed a full set of Auto Meter gauges in the relatively ugly '69 Camaro dashboard.
GETTING SOME EXHAUST
As much as I really dislike exhaust backpressure, an exhaust system that leaves me deaf after a day of hammering it down the highway hardly makes sense. I want the best of both worlds, and through experimentation I think I've found it. The exhaust system on the Thrasher has almost no backpressureand emits a slight rumble at idle and cruise but lets 'er rip when I stomp on the throttle.
The entire system is very simple but completely custom-built to tuck up underneath the car. I started with a set of race-style 17/8-inch primary tube Hedman Hedders (PN 65020) that dump into 3-inch exhaust pipes with a balance tube where they come closest to each other. Just in front of the rear axle (under the rear seat area), two 3-inch inlet and outlet Borla XR-1 Sportsman mufflers soften the bark, and then the exhaust snakes its way over the axle to exit at the rear of the car.
So it would last forever, I chose to build the system with T304 stainless steel tubing, which is more than double the cost of mild steel. If you are on a budget, you could use mild steel, which is also easier to bend and weld, and have it coated inside and outside by HPC to get the maximum life out of the system.
Another reason to consider mild steel tubing over stainless is that stainless "grows" much more than mild steel as it increases in temperature. It is common to see a stainless exhaust system grow 1/2-inch or more over its entire length. This might not seem like much, but often it's enough to tear an exhaust hanger apart after a few hundred miles. I have learned this lesson the hard way and now use a "swing" type exhaust hanger that can move with the tubing growth yet still retain the exhaust. Also, don't mount any hangers too close to the mufflers, as the intense heat will likely shorten the life of the rubber mount considerably--these babies get hot! And, as a final note on the hangers, I use a "blade" type late-model factory hanger at the tailpipe, so as the exhaust tubing expands and the length of the system grows, the blade just slides in its rubber receiver mount on the frame.
Because the components used on this car are aftermarket, sometimes they need a little "persuading" to fit properly. This was the case with the headers. My exhaust guru specialist, Richie Hernandez, used a jack and an acetylene torch to heat up the primary tubes around their bends to reduce the amount the headers hung down under the car. While this is cool to do because it will give me more ground clearance and clean up the underside of the car, understand that the second we started whittling on the headers the warranty was out the window. So, get somebody that can heat the tubes up without blowing through them, kinking a tube, or worse. If done right, it is worth the effort, as it got each collector up about 11/2 inches (and I'll stress "right"). Richie did a great job because he knows what he's doing--if I had to do it, I might have ruined them.
I don't use flanges on my headers, especially on stainless steel systems, because they seem to all leak, crack, or somehow fail after a series of heat cycles are put into the system. I found some trick exhaust clamps that are used on big rigs to seal a slip-on collector to the exhaust system. They are available from Summit Racing Equipment, made by Dyno Max, are called stainless steel exhaust clamps (PN WLK-35978), and cost $8.50 each. These clamps are great because they seal the system but allow some movement, which is important. I'm yet to have one fail. CHP