For the longest time, the prevailing wisdom in the paddock with regard to exhaust was the bigger the pipe, the better. Bigger pipes let engines breathe better and, in turn, make more power.
That’s not necessarily the case. To find out as much as we could about the dynamics of getting exhaust out of an engine, we spoke with John Grudynski, CEO of HyTech Exhaust in Irvine, Calif. HyTech has been building exhaust headers and systems for more than 30 years, working with the likes of Honda Performance Development and Kia’s Pirelli World Challenge racing team. The company also has worked in series as varied as Indy Car, sprint cars, Australian V8 Supercars, Formula Ford, Formula 2000 and Formula Atlantic.
Grudynski has a lot of experience in the industry, but he admits to the Socratic Paradox, that as much as he has learned throughout the years, there is still that much more that is still unknown.
“I think it’s a black art, and that there are so many variables that we don’t know how things work,” Grudynski said. “We all have our theories and our ideas, but there’s still a lot that happens that we don’t know.”
One of the reasons there are still so many unknowns that the characteristics of a header and exhaust are dynamic and changing. As the engine rpm changes, what the engine needs in terms of exhaust design also changes. The industry has theories, computer design programs and instrumentation to measure performance, but exhaust design still largely comes down to trial and error.
“I’ve seen the computer programs that design this stuff and most of them aren’t close enough, in my opinion,” Grudynski said. “We know what works for a Formula Ford because we’ve been doing those since the 1970s. So we know the header that works really well for those, but if you use the computer program, it’s way off. It leads you in a bad direction. That tells me the programs are OK. They’re generic. They still don’t give you what the motor really likes, and really the only way to tell is through development. Build the stupid thing and build it where it’s adjustable and then just throw it on the dyno and have at it.”
It helps if you begin to look at engine exhaust as two elements instead of one. First, there are gases, the end result of the combustion of hydrocarbons. Second, and equally important is the sonic element of exhaust. The two elements have conflicting traits, which likely is one of the reasons exhaust technology is so difficult to pin down.
“You have the exhaust pulse, or wave, and then you have the exhaust slug of the particles,” Grudynski said. “They travel a different speeds, because the exhaust pulses in a normal engine, they travel at about 300 feet per second, but the sound wave is traveling supersonic sometimes, so you have those waves playing against each other, and they are going back and forth over each other.”
Another phenomenon within the exhaust system is what is known as reversion or a reversion wave. When the exhaust valve opens, a pressure pulse and wave are released. As soon as it hits the first opening, there is a reflecting wave, which is good because it aids in scavenging. But when the wave travels all the way to the collector, it comes all the way back in the form of a negative wave, which brings with it moving particles, inert and contaminated gases that cannot be burned again. That negative wave will travel back into the cylinder and it can actually make its way out through the intake manifold, which negatively affects the intake process.
To combat reversion, headers can be fitted with anti-reversion chambers. Early methods for controlling reversion were located in the header flange. Now they’re most often located in the primary tubes. From the outside, they look like small bulges, but there is more to them than that.
“There are no moving parts. It’s basically a chamber,” Grudynski said. “The pipe that comes in from the motor sticks in about an inch, and the other one just gets butt-welded on and it’s kind of like a one-way valve.
“It’s kind of a misunderstood thing, and I don’t get all of what happens,” he continued. “I mean we try to figure this stuff out, but like I said, there’s so much we don’t know about what’s going on because it’s constantly changing. It’s not like it’s steady state, where you could monitor all that stuff. It’s just that with heat and temperature and speed, the gases are just doing crazy stuff. The chamber helps clean everything up.”
Sophisticated dynos show that anti-reversion chambers help smooth out the torque curve. On the track, the car accelerates better, and on-off throttle modulation is much improved, which gives the driver a greater measure of control.
Another hot topic of debate is the collector, how it should be built, the diameter it should be, and so on. Grudynski spent years developing HyTech’s collectors to the point where they now make all of them pretty much the same way. Rather than making them as big as possible, HyTech made the volume as small as possible without hurting power output. By stepping the collector down in size, you can keep the gas speed inside the collector higher. Collectors that are large for the sake of being large aren’t necessarily better.
“That just kills the speed of the gases and that’s what you don’t want to do, because the faster you can make the gas move, without hurting it, it’s going to scavenge better, it’s going to help the reversion wave better,” Grudynski said. “It just amplifies everything a little bit and makes everything work better.”
Exhaust is often the last thing you think about when building an engine, but it’s complex and it can add power if done right. When sourcing an exhaust system for your racecar, don’t just go the “bigger is better” route. Turns out, there’s a lot more going on in your exhaust system than you may be aware of.