Long Revision
Lethal Performance upgrades its GT350 project with Kooks exhaust
By Steve Turner
Pushing the performance of its project cars is Lethal Performance’s calling card. Soon after leaving Weikert Ford in a cloud of smoke, Team Lethal embarked on an aggressive modding plan for its Shelby GT350 project car, including bolt-on upgrades, a 10-second run at the drag strip and, eventually, a Whipple supercharger.
When you start pushing that much air through a high-winding engine like the Voodoo 5.2-liter engine, maximizing the airflow is crucial. Lethal started out by opening up the exhaust with a cat-back exhaust, but the time had come to open it all the way up by replacing the factory manifolds with full-fledged long-tube headers and a supporting exhaust from Kooks Headers and Exhaust.
“As with most of our builds we like to run a set of long-tube headers and an off-road mid-pipe in place of the stock manifolds. Those mods have been proven time and time again not only to increase power and torque, but to help with engine efficiency especially in boosted applications,” Jared said. “So it was no surprise that when Kooks approached us about their new system, which includes the headers, midpipe and mufflers and we jumped at the opportunity.”
“We’ve been doing excellent business with Kooks since we started back in 2005 and love the products they make so we're proud to run their stuff on our GT350 project car,” he added.
That system (PN KOO-1154F310; $3,239.97) features 1 ¾ x 1 7/8 x 3-inch stainless steel headers built to resist cracking from those flat-plane vibes. It is supported by a 3-inch X-pipe and a 3-inch cat-back. This system is available with high-flow cats, Green cats and without cats for racing applications like the Lethal Shelby.
“We were trying to develop a system that made horsepower and torque while getting rid of the dip in the factory systems power curve, while maintaining the distinct sound that the Shelby has become known for,” Chris Clark Director of Sales and Marketing at Kooks said when the system debuted.
To see how this system installs on the Lethal GT350 we made the trip down to Power by the Hour in Boynton Beach, Florida to follow along as technician Donnie Renfrow spun the wrenches for this stage of the build. After this upgrade and a new McLeod clutch (more on that later) it will be back to the track to see what the latest upgrades did for this project.
“I think it’s time we get this car back to the track—the road course and the drag strip. Last time at the road course before the Whipple install Ford Performance, driver Billy Johnson took the car for a few laps and logged its performance,” he said. “So we’d love to see the improvements that the Whipple Supercharger setup yielded us. We’ve also got a best of 10.98 down the quarter-mile on nitrous with a slipping clutch so a solid pass with the blower and a new heavy-duty RXT clutch is definitely in order.”
When we last left the Lethal Performance Shelby GT350 the car had received a 2.9-liter blower from Whipple Superchargers and developed over 819 horsepower and 600 lb-ft of torque. That was through the stock manifolds, a Lethal Performance mid-pipe and the stock cat-back system.
The Kooks 2015-2016 Shelby GT350 5.2-liter Complete Off-Road Competition Exhaust System (PN KOO-1154F310; $3,239.97) is made up of 1 ¾ x 1 7/8 x 3-inch stainless steel headers, a 3-inch X-pipe and a 3-inch cat-back. And, unlike the factory system, it comes in multiple pieces to allow for ease of adjustment during installation.
Power by the Hour technician Donnie Renfrow starts by removing the battery and battery box to allow for more access to the header fasteners.
Next Donnie removes the wheels, secures the brake rotors to the chassis and removes the brake rotors.
Then he removes the cat-back exhaust system to free up the engine and make way for the new exhaust.
With the engine and cooling module supported, Donnie uses a jack to lower the K-member after removing its fasteners and the strut nuts. Then he removes the cast motor mount brackets from the block for maximum access to the factory manifolds.
Donnie uses an impact to loosen all but the least accessible manifold bolts, then he snakes the factory manifolds out of the engine compartment.
Though the hand-welded short-tube headers on the Voodoo engine aren’t married to a catalytic converter on one side like the Mustang GT manifolds, they still feature some production compromises. Most notable are the crimped pipes on the driver-side header, which are presumably there for clearance.
With all the stock parts out of the way, installing the headers is really a pretty simple job. Donnie snakes the Kooks long-tubes into place and hand-tightens the fasteners.
He prefers to use the factory studs and nuts, which requires a bit more tedium, but delivers trouble-free operation down the road. This means you can’t get a ratchet on most of the fasteners, but with a bit a patience it doesn’t take too long to bolt everything up.
With the headers in place, Donnie pieces together the mid-pipe leaving all the fasteners hand-tight.
Working his way back, Donnie installs clamps and pipes, supporting the system with jack stands when necessary.
He then installs the under-axle pipes on the factory hanger and joins them to the mid-pipe.
While the Kooks system does not have internal valves, its mufflers do feature mounts for the factory motors so they can be plugged in to keep check engine lights at bay. You’ll want to swap them from your stock or other mufflers to the Kooks mufflers before installing them.
On the home stretch it’s time to hang the Kooks mufflers and star lining up the system.
This bolt-on brace keeps the axle-back pipes in place and the Kooks logo is a nice touch.
Donnie leveled an aligned the pipes and worked his way back from the headers tightening the bolts and clamps along the way.
The Kooks system is also designed to work with the factory’s valance-mounted tips, which are engineered to stay cool during extended track session. They were removed for the previous cat-back, but you can see how the air gap here would promote cooling of the tips.
With the system bolted up, Donnie reinstalled the factory oxygen sensors in the mid-pipe and the headers, with the latter using extension harnesses supplied in the Kooks kit.
All that was left to do was reinstall the factory tips. If you are swapping the stock parts in favor of the Kooks system you won’t have to worry about this. Then it was off to the dyno for a fresh Lund Racing calibration to dial-in the combo for the free-flowing exhaust.
When we last left the Lethal Shelby GT350 it had put down 819.87 horsepower to the rear wheels thanks to the addition of a Whipple 2.9-liter supercharger, a Lund Racing E85 calibration and a set of Lethal Performance cat-delete pipes. After adding the compete Kooks exhaust the car picked up another 21.79 horsepower at the rear-wheels, which is pretty impressive considering the cats had already been removed.
