| Exhaust article 3: Crossovers |
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One of the things that you will see on a professional exhaust system and virutally any exhaust system that comes from a factory that is attached to a V8 is a crossover. But why do you need a crossover, can you get by without it, and is it worth the trouble? This is really quite a complex issue (like most exhaust issues... you'd
think that pushing gas out of the engine would be a simple thing, but
it isn't). The use of a crossover greatly benefits a V8 motor due to the fact that in every V8 you will have one cylinder in each bank that will fire within 90 degrees of the crankshaft rotation of another cylinder in the same bank. You can't help it, it just happens with that many cylinders. The upshot of this is that twice during the firing order you will have two cylinders pushing gas into your exhaust system at nearly the same time at the same side. What you get from that is increased pressure for that particular pulse, and it affects manifolds and headers both... in a manifold immediately, and in a header when it hits the collector pipe.The crossover is an attempt to allow some of the pressure from the "high" side to bleed off into the "low" side in order to equalize things and make it easier for the exhaust to breathe. The H-pipe is also an attempt to correct this issue. Apparently a crossover works better in some cases. Sometimes you might see a channel pipe as well, which is running on the same concept. By the way, not only do you need a crossover, but the exact PLACEMENT of the crossover can be essential as well. Oh, those crazy gas physics! Incidentally, this is mostly a V8 thing. V6s don't need a crossover, although apparently a V6 "sounds" better with one. I6s are going to suffer from the increased pressure problem but there's no way to fix it since there is only one exhaust channel, and I4s not only have only one channel but never fire two cylinders at once anyway. If you have read the other two exhaust articles here and here then you will know that a surprising amount of your engine's energy is being used to do nothing other than push exhaust gases around. Anything that makes that task easier translates into more power on the ground. A crossover positively impacts a V8 motor without question. Then we get to the thorny details of placement. This is complex and has a lot to do with the length of resonationg frequencies and the hertz or cycles per second of the exhaust pulses, but the short end of the stick is this: A crosspipe is good for the above described reasons. While it doesn't seem like it is much of an issue for a crosspipe to be too far away from the engine, it CAN be too close to the motor which will result in the high pressure that is being "bled off" between the two halves of the system to make its way back up the exhaust and cause a higher pressure situation at the valves, which of course is what you are trying to avoid in the first place. Men with huge, bulging foreheads and permanently attached pocket protectors toil away long hours late at night on supercomputers to calculate the exact placement of crosspipes. Luckily for us there is another method, one that comes down from the "old school" rat racer crew. I suspect that this works because of a lot of experimenting and trial and error, but the results that it gives is dead on accurate: Install the exhaust as straight pipes. Paint the pipes with spray paint. Take the car and and drive it like you stole it. When you raise the car back up, near the engine the paint will be burned off the exhaust. At some point the further you get away, the paint will be unburned. At that point is where you install the crossover. Neat! A side effect of both X-pipes and H-pipes is that the exhaust tone is "mellowed" and droning is reduced. As such a second X-pipe is sometimes installed immediately before the mufflers in order to help eliminate droning. This pipe does not affect performance good or bad, it just helps with the sound. An H-pipe at that location would do the same thing. You can read the first article on exhaust HERE. And the second article HERE.             Comments (0)
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