Muscle Mustangs & Fast FordsHow To Engine
Stepping up the performance of Project Stocker with a trick exhaust from Kook's Custom Headers.
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Mustang owners love the sound of a hot V8. That's why one of the first items they change is the exhaust system. Hey, we all know headers and free-flowing mufflers look great, sound cool and they most certainly make your Mustang faster, right? Yes, sometimes, but finding the right set of tubes can still be a difficult task.
Fact is, there are many levels of aftermarket exhaust systems including different types of headers, collectors, H-pipes, X-pipes, straight exhaust tubes, mufflers and even tail pipes. Combine that with the variety of engine combinations and types of power adders and trying to figure it out can leave you completely exhausted. Thankfully, MM&FF is here to keep you energized and sounding good.
It's likely that you've worked hard in the garage and at the track to find the best engine parts that make up the combination under your hood. So why leave performance on the table by picking a sub-par exhaust system? Smart racers know that a well-designed exhaust system will complement camshaft, cylinder head port design, rpm range and induction to help the engine reach maximum efficiency.
Not so long ago stock 5-liter Mustangs came from the factory with small tubular headers of the unequal-length variety. These engines made great low-rpm torque and the small headers provided plenty of flow for a stock 225-horsepower 5.0. However, they only work well up to about 275 horsepower and then it's time to upgrade.
What goes in must come out and that's why your Mustang will go faster with an exhaust system that complements the rest of the engine. Ultimately, you want to match the header pipes, collector size and the exhaust system (if you use one) so that the engine reaches peak efficiency. The question is, how do you do that?
We wondered the same thing so we ventured north to New York to spend the day with George Kook, Jr. of Kook's Custom Headers. Kook explained quite a bit about the latest advancements in exhaust technology and helped us along with a new exhaust system for our NHRA-legal project Mustang.
Aside from looking good and sounding cool, a quality set of headers will perform better than stock because they will improve cylinder scavenging. In lay terms, scavenging is nothing more than removing the burned gasses from the cylinders. An efficient exhaust system will increase the level of scavenging, which, in turn, helps to draw more intake charge into the cylinder during valve overlap. As a result, the engine gets more air and gas to compress and this almost always increases horsepower and torque.
Cylinder scavenging occurs every time one of the exhaust valves open. As a valve opens the burned gases begin to exit into the header pipe. The piston will be on the upward swing and will force most of the gases out of the cylinder. However, when the piston nears TDC, the intake valve starts to open letting in the "new" air/fuel mixture, even though the exhaust valve is still open slightly. This is called valve overlap and in most cases it causes the outrush of exhaust to actually draw more incoming air into the cylinder. So, by making the exhaust pulse stronger, you get more draw on the incoming charge of air and gas.
But wait, there's more to this story. Just after the exhaust valve closes the exhaust trapped in the header pipe loses any push from behind. This causes the pressure in the tube to drop. At the same time the exhaust gases in the tube begin to cool and lose velocity. When this occurs, the exhaust gases stall until the valve opens again and more gases enter the pipe. Fortunately, the other cylinders are constantly pumping, and forcing gases past the stalled tube in the collector. This creates a pressure differential in the collectors which helps draw on the tubes with low pressure.
When it comes to late-model Mustangs, there are quite a few header designs to choose from. They include unequal-length shorty, equal-length shorty, long-tube and Tri-Y. Shorty headers are the most common because they fit with the stock exhaust system and are often the lowest-priced aftermarket headers. However, when you're looking to make maximum power, the only choice is a well-designed long-tube header.
On the low end you'll find one-piece (non-slip apart) multi-purpose headers. These inexpensive headers can be had for about $150, but it's likely that they may not fit properly and they will rust away after a few seasons. On the other side of the coin are high-end custom headers. Some custom headers are simply better designed and built, while a true set of custom headers are built right on the car for your specific combination. These headers usually fit the best and are made with better materials. Anyone who has installed cheap headers knows that you usually get what you pay for and that better-built headers are worth their weight in gold.
With this information we set out to find a new set of headers for Project Stocker. Kooks' currently supplies headers to some of the quickest Mustang racers in the country so we had plenty of faith in his selection for our Stang. "We design our headers for optimum driveability, track conditions and power," stated George Kook, Jr. "This includes drag racing, road racing and street driving. We have headers for all Mustangs, from old ones up to 2001 Cobras and GTs. The type of header you choose should be based on the combination of the car and engine. Areas we look at are horsepower, cubic inch diameter, stick or automatic, tire size, vehicle weight and type of power adder, if one is used."
When doing our exhaust system research we first looked at the rules for our class and we consulted with our engine builder, Jim Kuntz. We race in NHRA Stock Eliminator, where any exhaust, including open headers, is allowed. However, the NHRA has been looking into quieting the cars by implementing a noise suppression devise (such as mufflers) in the near future. So, we're designing our new exhaust system keeping the big picture in mind.
Our old exhaust consisted of Hedman 15/8-inch headers with 35-inch tubes and 3-inch collectors. The headers were run wide open and we've used them for about six years. In the past we experimented with collector extensions and mufflers, but we have run best with the standard collectors open. We spoke at length with Kook Jr. and he asked me a battery of questions before determining the proper exhaust system for our combination.
First off, I told him about the engine. The Kuntz & Company-built small-block was built to take advantage of the NHRA Stock Eliminator rules. Therefore, the engine is based off a stock 5.0 roller block that has been treated to .040-inch overbore. Kuntz used a stock crank, stock rods, and Venolia stock replacement pistons inside. Compression is 9:1 and the cylinder heads are GT-40 iron casts with Manley replacement valves, Manley springs, a 3-angle valve job and stock Cobra rockers. If there is any one component that I'd consider "modified," it's the Lunati Stocker cam with stock .480-inch lift, and, let's just say, lots of duration.
In Stock Eliminator, air must enter through the stock throttle body and flow through a bone-stock intake, too. Due to the long-runner design of the Ford EFI intake, the engine is operated between about 4500 and a maximum of about 6500 rpm. Because of this tight powerband, our exhaust system differs from what may be considered optimum on a street car that must make good power from idle to about 6000 rpm. I also told him that we use a G-Force 5-speed and that the car weighs roughly 3100 lbs.
Generally, headers with shorter tubes will complement an engine with a tight powerband, while longer headers work best over a broad power range. Another rule of thumb is that smaller engines, or should I say lower power engines, like smaller tubes and higher horsepower engines like larger tube diameters.
In most Ford applications, especially those that use a power adder, there is no problem getting air and fuel into the engine, but getting the burned gases out is another story. Stock 5.0- and 4.6-liter Mustangs come equipped with either shorty headers or log-style manifolds, unlike the long-tube design found in racing. While the factory exhaust is OK, it's not always the best performance choice. When Ford Motor Company designs an exhaust system they have to factor in more than just peak power. The headers must fit tightly to the engine in order to make assembly line installation quick and easy. In addition, the headers must be designed so they can go over 100,000 miles without leaking and they must be quiet enough to pass all the governmental noise regulations. And we haven't even touched on emissions yet.
In contrast, aftermarket headers need only fit in the car (without too much hassle), though they better make more power than stock. Some aftermarket manufacturers, like Kooks, can build custom headers designed for your exact combination, and if you can afford it, this is the best way to go.
For our application Kook selected a set of his custom, off-the-shelf headers with a step design and a 3-inch Spike collector. The headers feature an individual "slip" tube design meaning the eight tubes can be separated from the collectors to ease installation. The tubes are approximately 31 inches long and they go from 15/8-inches to 17/8-inches about eight inches off the header flange.
"A step header will not always give you more power," stated Kook. "It depends on the application. Stock-type street cars with full exhaust and low-duration cams like a straight header. However, cars with less restrictive exhaust and big cams will usually benefit from a step header. The trick is to build the headers to maintain proper heat retention, not necessarily back pressure as most people think. Heat retention throughout the system improves scavenging and adding a step in the header tube size allows the exhaust to expand and maintain velocity as it travels down the tubes."
Kook also explained the importance of equal length headers because it keeps the exhaust pulses at the collector timed properly to get an equal draw on each cylinder.
Collector technology has come a long way in recent years as professional header manufacturers have realized different ways to increase scavenging by shaping and modifying the collectors. The most common collector is just called a collector. The purpose is to collect the gases from the individual pipes and guide the exhaust flow into one larger pipe to even out, or balance, the total flow. Having a collector also makes it easy to attach an exhaust system to the headers.
The hottest item to hit the header market in recent times is the merge collector. A merge collector is a radical design having all four header pipes merge or neck down at a 15* angle and then expand out like a trumpet or a funnel. Merge collectors have a small neck that appears to choke the exhaust and cause a restriction. But this is not the case. In actuality, the merge collector does make the exhaust gas bottle up, but then the pipe expands rapidly to improve flow.
Merge collectors seem to work in virtually all applications including Pro 5.0 and Pro Stock. The down side is that Merge collectors are expensive, costing about $400-$800 for a set. And finally, you may notice that most merge collectors are made from expensive stainless steel. Stainless retains heat and that's one reason it's not necessary to coat stainless headers or collectors.
A better alternative for the racer on a budget is the spike collector. The spike unit is a collector with an actual spike welded into the center of the collector, at a point where the four tubes converge. "The spike guides the flow to the center of the collector and that increases flow," stated Kook. This spike setup is less expensive than a true merge collector, but it offers similar results.
For years now racers in many classes including NASCAR, drag racing, Indy cars and off-road have experimented with trick exhaust systems such as H-pipes, X-pipes, two-into-one headers and various collectors and pipe lengths. The purpose of this is to optimize the flow of exhaust by creating a balance between the different banks in a V-configured engine and add to the scavenging ability of the exhaust system. Ultimately, a properly tuned exhaust will increase torque and horsepower.
Most recently though, Mustang owners have used X-pipes to connect either side of the engine's exhaust. Some designs are bolt-in and work with catalytic converters and mufflers, while others are run as open exhaust systems.
According to Kook, "The X-pipe will smooth out the mid-range powerband on most street cars and on some race engines. An X-system works best on naturally aspirated engines and nitrous engines, but not necessarily on supercharged engines. In a high-horsepower supercharged engine the X might become a restriction."
Kook selected a 3-inch diameter X for Project Stocker that we tested with great results. We installed the X-pipe "as is" and found an instant increase in mid-range torque. This is quite helpful in any racing application because the engine recovered and accelerated quickly after gear changes.
Shortly after putting the X on the car I found that the engine had richened up, necessitating a change to my tune-up. Backing the fuel pressure off slightly cured the problem and got the engine running fine. Yet another byproduct of the X was the funky sound of the exhaust. Onlookers said my car sounded like everything from a V6 to a Winston Cup Stock car. To my ear it sounded like it had a much higher pitch, sort of like a 4-valve mod motor.
In the future I plan on shortening the outlet tubes on the X, which should move the power curve up. To determine the best length, I'll try the old trick of painting the pipes and then making a run to determine how far the paint burns off. The idea is to then cut the pipe at that point and go racing.
Aside from providing better exhaust flow and increasing the power of our 302, the Kook's headers were the best fitting headers I have ever seen. They cleared the Lakewood bellhousing and also the starter on the passenger side and they did not interfere with the steering shaft on the driver's side or the frame either. Total installation time was about one hour, and that included removing the old headers.
In addition, the header pipes and collectors sit parallel to each other under the car, which makes hooking up an exhaust system a bit easier. Once again, you get what you pay for.
At The Track
We were anxious to hit the track and see the results of our new exhaust system. For once our timing was impeccable as we hit the track just two days after bolting up the new pipes and found great weather and top-notch traction. No snow, no rain and no slippery starting line.
Our track side adventure took us to Maple Grove Raceway just outside of Reading, Penn., where we competed in the 17th Annual Pep Boys Nationals (formerly the Keystone Nationals). This year's event was a "Class race" meaning we'd be running heads-up against all the other cars entered in I/Stock. Then, after racing heads-up we'd run in the main eliminator, which consists of mostly bracket racing.
The fine folks at NHRA Pit Control got us a dandy pit space and I unloaded and headed for tech inspection. Immediately after firing the engine Stocker drew a crowd, as the other racers wanted to know what I did to the engine. No doubt they took notice of the strange sound coming from the X-pipe exhaust. I rolled over to the lanes and prepared to make my first qualifying run. Due to the scheduling, we only got one qualifying run before going into Class Eliminations; therefore, I needed to make the first run in bracket mode.
Note: Stock racers often put the car on "kill" for heads-up competition and it's tough to bracket race in that mode. Typically, "bracket mode" dictates a lower launch rpm, lower front tire pressure and no ice on the intake.
When it was time to run I heated the Mickey Thompson slicks with a short burnout. Then I was guided into the stickiest part of the track by my pal Mike Scott. Scott has been a great help on the line and I trust him to get me in the groove. After moving me over to the left just a bit, he gave me the thumbs-up and I staged as shallow as I could. Next, I brought up the rpm and matted the gas once the tree dropped. It was then, with the engine pegged on the rev limiter, that I noticed a huge audible difference. At sight of the last amber bulb, I dropped the clutch and felt the car hook and go. It lifted the nose in the air, but not excessively. The tach needle dropped and then shot back to my shift rpm. I grabbed second gear in the G-Force 5-speed and kept the momentum going. Before I knew it I was in fifth and the stripe was approaching rapidly. I clicked shortly after passing the finish line and coasted to the time slip booth at idle. I had a pretty good idea that it was a good run and the slip confirmed it. Stocker ran 11.41 at 115.86 mph in I/Stock trim with a corrected altitude of just over 1000 feet.
My next stop was at the scale and then fuel check, where they OK'd my VP fuel. Then, after parking the car in the pits it was time to check out the NHRA trailer to take a look at the qualifying sheet. The 11.41 was 1.08-seconds under the 12.50 index and that was good enough for the seventh spot on the Stock Eliminator sheet.
I was pleased with that run until I glanced up and saw that Jim Roberts blasted to a quicker 11.37 with his I/Stock '69 Camaro. The Camaro is powered by a 255-horsepower (factory rated) 350 small-block and was running like a champ. During our last meeting I ran an 11.50 to Roberts 11.75, and now he was returning the favor.
It was pretty obvious that I needed to find a little more performance so I hauled butt back to the trailer to see what I could find. Like most Mustang racers, I busted out the obligatory bag of ice and also pulled a little ballast out in order to get the Mustang closer to the 3110-lb. minimum. By the time Stock was called back to the lanes the weather was a bit worse (2300-feet corrected). Roberts and I selected lanes (I went right, he went left) and headed towards the starting line. I was really pumped up, more than I've been in a really ling time.
Roberts had a quick car and he's also a great driver, but I was ready. Before I knew it we were staged and the tree was dropping. I saw the last amber and snapped my left leg into action as quick as I could. Roberts did the same and his Camaro blasted off alongside my Ford. The run was surreal as we accelerated with our cars virtually locked by the door handles. I yanked second and he was there, then third and all I saw was Camaro. I don't remember looking at the tach or the track, or anything but that Camaro, for that matter. All I saw was the green fender on Robert's Camaro and believe me, it was there in a big way. Fourth gear came and went and before I knew it I shoved the G-Force forward to engage high gear.
This all took place before half-track mind you, and at that point Roberts and I were mere passengers. We marched on lock in synchronicity, but then something happened. It was then, in high gear, that the Jim Kuntz horsepower came on strong. My Mustang began to inch ahead of the Chevy and I was holding on, yelling, "come on, come on ... " I couldn't believe what was happening, but I was pulling away, or should I say inching away, from that green Camaro. It was the best feeling. Then finally, after what seemed like an eternity, we crossed the line and my win light came on. I was beyond charged. I was yelling, pumping my fist and my adrenaline was maxed out. This was the best drag race I ever had!
Back on the return road we got the slip and what a run it was. Roberts strapped a .513 lamp on me (I was .527) and he ran a stout 11.386 to my 11.341 at 116.29 mph. Yes! Margin of victory .030-second--Ford wins.
That Thursday was simply awesome. Not only did I win Class Eliminations, but I produced my best run ever, which answered the question of how the step headers and X-pipe worked.