How does exhaust gases/flow react to an x-pipe? Could it count as twice the flownumber, or atleast near that, after the cross section? And muffler flow is even more interesting. Could a street muffler in an x-pipe configuration be considered to perform as well as a race muffler in a standard dual exhaust due to split gas flow/mass? I know that it helps draw in more fuel mix in cylinders in the intake stage but I just thought of this today. Alot of questions hehe Thanks
Yes, you dramatically increase the muffler volume that is "seen" by the exhaust gas, and therefore reduce restriction. I doubt that it's a 100% improvement in flow, however. WHICH "street" muffler and WHICH "race" muffler? Too many possible choices. I would say that in general an "X" pipe system is likely to be more efficient than an "H" pipe system; which in turn is much better than completely separated (except for the exhaust crossover in the intake manifold, if applicable) dual pipes. I suspect that even better would be a variant of the "X" pipe where you just merge to a length of single exhaust of appropriate diameter, and then either split again; or continue with the single exhaust using a truly huge single muffler. A single exhaust of similar volume will have less surface area than an equivalent dual exhaust; therefore less gas friction against the pipe walls and less heat rejection to the tubing. But who uses a single exhaust if they can shove a dual under the chassis? Not to mention the problems with bending tubing of such large diameter.
Thanks for the reply. The first reply gave answer to both my questions thanks! Wich type of muffler was just secondary but letts say a street/strip muffler that flows arround 350 and 2.5" x-system contra a 3" dual system and a more race type muffler that flows 500+? Quote: Not to mention the problems with bending tubing of such large diameter. /end quote Or the chafing against the ground with that 5" house drainpipe :grin:
For the time being, don't think of the exhaust as a continuous flow. Instead, just imagine a single "pulse" traveling out of one cylinder - for example cylinder #1. The pulse, which is a rather dense packet of fumes, shoots out the exhaust valve and into the header. It flows through the frontpipe into the "X". As you can visualize, the pulse (or packet or whatever) gets split in half (approximately), so that half of the pulse now travels down each tailpipe and through each muffler. Since the pulse is much smaller in each pipe, each muffler will provide a lot less resistance to flow. You effectively double the flow capacity of the exhaust system! Here's an electrical analogy from high school physics. In a parallel circuit, 1/2 of the current flows down each part of the circuit. If you replace a single 10-ohm resistor (muffler) with two 5-ohm resistors (mufflers) in parallel, you will get 25% of the total resistance that you would have gotten compared to a single 10-ohm resistor! Stated differently, you have 75% less resistance! (The reason that I am changing the resistance to 5 ohms on each side is that since the pulse was cut in half by the "X", the muffler will provide less resistance than if it had to handle the entire pulse by itself). It's not a perfect analogy, but you get the idea. This is why you get a different sound, and better low-RPM torque and gas mileage - the engine sees less "resistance" in the exhaust system at low RPM / low-flow conditions. When you reach higher RPMs the exhaust system will start to act more like a continuous-flow than a series of pulses (because the pulses are happening so much more quickly, and the exhaust pipes are much fuller which tends to self-dampen the pulses), which is why you might not notice so much of a difference in high-RPM power. -Bob Cunningham
Bobc455: I agree with you on your concept, but you're forgetting another important matter. Those pulses also reflect and travel back up the pipe as a negative pressure wave. That's right, you're exhaust system actually flows both directions. It just happens so fast, you can't see it or hear it. So what happens is that pulse travels down the pipe until it sees a volume change. My reading has found that it takes a volume change of about 8-10 times (per David Vizard and others agree) for the pulse to "think" that it has found the end of the pipe, so it turns around and sends a negative pulse back up the pipe. So then that negative pulse hits the back of the intake valve, or the header collector (depending on where it started) and bounces again back out the pipe. So, if you can time the pulse to be heading out the pipe at the same time that a new pulse is coming out the header (or exhaust port) one will pull the other along, providing extra scavenging. That's what "tuned exhaust" means. So, an X-pipe is simply a change in volume that causes a reflection. If that change is positioned correctly, you can increase the performance of the engine overall. But, it also decreases back-pressure, which helps, too. As for the idea of merging into one pipe, yes. It's called an expansion chamber. If you merge two pipes into a large chamber, the exhaust system "thinks" it is seeing atmosphere. So anything after that is somewhat irrelevent. Again the placement and volume of that chamber is critical. There are computer programs to tell you where to put these events in a given exhaust (and intake) system. Have fun.
Jeff- What happens as the engine changes RPM? As the RPMs climb, the time between exhaust valve events changes - wouldn't that mean the speed of the pulse would have to change? Seems like that would only work in engines with a very narrow RPM range. -Bob C.
Exactly. You have to tune the exhaust for peak power in a given window, the other areas will fall where they may. A big V-8 is not as sensitive as a little two-stroke, so it's not as noticable. But if you're building a race motor, I would consider lengths of the exhaust AND the intake system when determining or "tuning" for a given RPM. You need to match those lengths with your cam timing, converter, rear gear, tire size, etc. This is really for race motors. I wouldn't bother with all this for a daily driver or a street car. Have fun.
