I have a pretty good understanding of how a performance exhausts system works to create power. I always thought the point of dual exhaust was so the exhaust pulses from one set of cylinders did not interfere with the other bank of cylinders. But now after looking at performance aftermarket exhaust systems for some cars I see that most use an ‘H pipe’ to equalize the pressure between the two pipes or an ‘X pipe’ to merge the two exhaust pipes to even out the exhaust pulses. The companies claim this improves the power gains.
If you are combining the two exhaust streams anyway, is there any reason to divide it back into two pipes instead of using one larger pipe? (Other than looks of course)
I’ve understood the crossover to make the apparent sound of the exhaust quieter as the pulses come from both sides rather than distinctly from one side then the other. Also having the crossover farther back probably has less interference between cylinder banks than a system that joins near the manifold.
I’ve owned a car with an inline engine that essentially had a dual exhaust, a '90 BMW 325. IIRC the inline six has two manifolds for the front and back half of the engine and I’m pretty sure the firing order alternated between front and rear manifolds. Dual pipes went under the car to the catalytic converter which has dual inputs and dual outputs. The two pipes continue to a muffler with similar dual connections and ends in dual exhaust pipes.
One large pipe is just as good as two smaller ones. The reasons for running a dual exhaust is that often you cant fit the very large single pipe that would be required and it is often less restrictive to run two parallel mufflers compared to a single muffler.
Many race cars run a single 4" exhaust pipe that exists below the passenger door. Not very practical on the street… Makes more sense to run a dual 2.5" or 3" for street use.
“H-Pipes are a good addition to dual exhaust systems. They help even out the velocity and back pressure between the two pipes which increases performance.”
“In the same way that headers improve performance over the stock exhaust manifolds, a performance H-pipe or X-pipe will improve the exhaust flow further down the path.”
“X pipe crossovers are designed to enhance performance by merging exhaust pulses in one area and improve scavenging, resulting in increased horsepower and torque.”
All those links simply claim that connecting the two sides of the exhaust is better but do not really explain why it should be separated into two pipes again.
The last link claims that true duals make better high rpm power while a single exhaust makes better low RPM power but still recommends an X or H pipe. This sounds plausable but they don’t explain why. They also present the ‘some backpressure is good’ myth as fact, which makes me suspect of any information they post.
This makes sense but since the cross sectional area increases exponentionaly with an increase in diamiter it seems unlikely that you could run into this problem very often. To double the area of a 2.5" pipe you only need a 3.5" pipe an extra 1/4 inch clearance around the pipe.
Oops, I missed the part about the mufflers on my first read through, I guess thats a good reason. However, just as an example, the Stillen ‘true dual’ exhaust system for the G35 sedan (the stock exhaust is single) has an X pipe and dual pipes but they lead into a single muffler
The lengths of the individual pipes comprising headers are “tuned” for optimum scavenging. This relates to pulses of exhaust pressure in the headers and pipes. ( http://www.elephantracing.com/techtopic/headers.htm ) On a V-8 engine, there is not even spacing of these pulses in each side’s header, because the firing order does not consistently alternate right-left-right-left-etc. The H-pipe or X-pipe helps compensate for this, by joining the left and right sides together in such a way as to let the pulse on one side help with the scavenging flow on the other. The fore/aft placement of the crossover pipe affects how well this works.
