2.25" vs. 2.5" X-Pipe?

[J.Belknap]

Scavenging is ALWAYS good. Where the xpipe is installed (collector LENGTH) will greatly affect how it works. Properly fitted will make the exhaust QUIETER and SMOOTHER. Not drone. If adding an xpipe only creates drone you're doing something wrong. There are no "kits" though so you'll have to get busy with a welder yourself. Make sure the collector lengths leading to the X are EQUAL and short as possible. Having the X before the cats works REALLY GOOD but makes it harder for the cats to work.

Proper construction will mean a GUSSET plate top and bottom joining the two parts of the X. Here is a pic of a Dr. Gas xpipe that I have for example. I have not tweaked the inlets and outlets YET. In fact it is still sitting in a box in the garage.

Originally Posted by bcredliner

Does the X pipe replace the H pipe and is the pipe 2.5" at that point?

What H-pipe? 60mm at that point. A slight neck down inlet then expansion after the X helps velocity (scavenging)

[bcredliner]

I disagree that increasing scavenging is always good to achieve improved performance.

Scavenging works best at low RPMs when there is the greatest exhaust pulse gap that creates more following vacuum. As RPMs increase the pulse gap decreases and the benefit of scavenging reduces. Scavenging benefit increases if the exhaust pipes are smaller because the smaller pipes increase the exhaust velocity which creates a greater pulse and corresponding vacuum that follows.

The problem is that the smaller the pipe the greater the back pressure. When scavenging is not good is when scavenging increases back pressure. In the case of forced induction such as turbo charging that use the exhaust to function, the pulse is already smoothed out so the pulsing necessary to create the vacuum needed for scavenging benefit is gone.

Drone is caused by resonate frequency. A change in resonance can result in drone. I agree that drone should not be due to the installation of an X pipe but there is that potential. That's why my suggestion is to do something that someone else has done with no drone outcome. FYI-I thought an X pipe made the exhaust louder. My only reference were Youtube videos that sounded louder to me but could easily been where the microphone was placed. I agree, it would be far less likely to create a drone if the result of an X pipe is a quieter and smoother sounding exhaust.

I think, if you can achieve the sound you want by lowering back pressure it is always a better route.

In our cases, yours because of the supercharger and mine because of nitrous, we have too much back pressure even with the Dinan exhaust. I have decided electric cutouts are my best investment. It will result in a greater performance gain due to decrease in back pressure. I won't get the 20% increase that could come from headers but it will be far more than I could expect from an X pipe even if I don't fully open the cutout flaps. Good cutouts will cost about $450, the welding about the same as an X pipe but I think the cost benefit ratio will be far better than an X pipe.

I love the sound of Dinan exhaust and don't want to change that. Not saying the X pipe is not a significant benefit to a closed exhaust system but since I have not read anything that reported any significant measurable improvement in performance my conclusion thus far is that the cost benefit outcome is not good.

[TiAgX5]

It's always been my understanding that equal tube length (left/right) are a MUST to get an X-pipe to function correctly. The V8 E53 has longer exhaust tubing from the left cyl bank to the X-pipe location then the right cyl bank. This causes L/R pulses at certain RPM to reach the merge point at the same moment, causing a high pressure spike (the exact opposite of scavenging).

Unequal length exhaust tubing means H-pipe is the go-to for better sound and no power loss.

For an X-pipe to function correctly, the exhaust path from every exhaust valve to the merge point MUST BE IDENTICAL. That means equal length headers, collectors and exhaust tubing upstream of the X-pipe. Anything short of that, and the X-pipe is more likely causing a power LOSS.

[bcredliner]

That seemed logic to me but tech information that I have found indicates equal length tubing is not necessary in 2 planed V8s.

I have spent some time this afternoon getting more informed concerning X pipes. What I found to be important is an X pipe that has smooth inside walls, the optimum size is 2.75" but 2.25" to 2.75" is nearly as effective, in 100% of applications the sound is reduced and smoother as mentioned. 60% realize some gain in HP, the average being 5-8HP. In 40% of the cases HP did not change either way. The best location is as close to the engine as possible so the scavenging effect happens sooner but that won't matter if rest of the exhaust will not handle the increased velocity and it won't matter that much anyway.

Regardless, I wouldn't install the pipe in between the exhaust manifold and O2 sensors even if I could. The X pipe is better but the choice to use an X pipe or an H pipe is often governed by where it will fit, ground clearance etc. Scavenging increases velocity of the exhaust which means narrowing down the exit tubes out of the X pipe reduces the benefits. This is all based on engines producing less than 600HP. My sources were aftermarket exhaust companies, and printed articles in well known car magazines.

The design of an exhaust system is complex. I found it is possible to achieve better performance results with a well designed performance exhaust system than open headers-that was news to me and a big WOW. To do so it is necessary to know the maximum flow of the heads and construct a system that matches that flow--that would include an X pipe. The result would be maximum velocity from engine to tailpipe. I don't know of anyone that has verified the outcome of specific exhaust changes on an X. Until there is we are assuming results measured elsewhere are applicable. I think that makes some sense to do so but I would make the purchase of an X pipe solely with the goal of a quieter and smoother sounding exhaust.

FYI-I don't consider seat of the pants endorsements. I might if I found a few that started with OMG you won't believe the difference, I wish I had done this years ago.

If we consider horsepower and reduced noise the X pipe is always effective as it always reduces noise and never decreases HP.

[jcp240z]

You mention equal length tubing. From what I have gathered in the 4.6is the shorter run has a smaller diameter (50mm) vs the longer run (55mm). I believe this was to make the two equivalent in length.
This is for the downpipes to the resonator.

[bcredliner]

Originally Posted by jcp240z

You mention equal length tubing. From what I have gathered in the 4.6is the shorter run has a smaller diameter (50mm) vs the longer run (55mm). I believe this was to make the two equivalent in length.
This is for the downpipes to the resonator.

If that is the case the differences in tube diameters would not make them equivalent in length. The smaller side would produce a greater velocity which would be a bad thing to do.

[TiAgX5]

I should have ended the sentence in post #23 ....."X-pipe is more likely causing a power LOSS"..... with, across the RPM range.

Many exhaust manufacturers that use X-pipes with unequal length tubes and report HP gains have dyno tested the X-pipe location so the scavange effect is occurring right at peak HP RPM.

These manufactures usually will not provide full RPM pull sheets comparing straight pipe vs H-pipe vs X-pipe results. In most tests the X-pipe will show a few more hp at peak RPM but an actual DROP in HP/torque at lower RPMs (peak HP #s sell exhaust systems/exhaust parts. Few buyers look into average HP/torque thru the RPM range) .

On tracked vehicles an X-pipe tuned for peak HP is a plus (the engine is kept near the optimum RPM to use the HP peak). Street vehicles operate thru the RPM range, a loss of HP/torque at lower RPM for a few more HP near redline is not a tradeoff I would accept.

[J.Belknap]

Lots of bad information. Not going to argue, pointless. Ultimately it's a "feel good mod" for most anyhow.

[bcredliner]

Originally Posted by TiAgX5

I should have ended the sentence in post #23 ....."X-pipe is more likely causing a power LOSS"..... with, across the RPM range.

Many exhaust manufacturers that use X-pipes with unequal length tubes and report HP gains have dyno tested the X-pipe location so the scavange effect is occurring right at peak HP RPM.

These manufactures usually will not provide full RPM pull sheets comparing straight pipe vs H-pipe vs X-pipe results. In most tests the X-pipe will show a few more hp at peak RPM but an actual DROP in HP/torque at lower RPMs (peak HP #s sell exhaust systems/exhaust parts. Few buyers look into average HP/torque thru the RPM range) .

On tracked vehicles an X-pipe tuned for peak HP is a plus (the engine is kept near the optimum RPM to use the HP peak). Street vehicles operate thru the RPM range, a loss of HP/torque at lower RPM for a few more HP near redline is not a tradeoff I would accept.

The general consensus is that an H-pipe equalizes exhaust pulses. A X-pipe both equalizes the exhaust pulse and improves scavenging. Depending on the application and the rest of the exhaust system components, the addition of an X pipe may, can, might improve torque and or horsepower. The following video provides a demonstration: The XPipe Explained by Ed Hanson at Ed Hanson's Muffler Service. - YouTube

There are some applications like a low torque high winding engine, where the addition on an X pipe may decrease the performance of an application. Since getting from point A to point be is a combination of torque and horsepower the X-pipe could decrease torque even when it increases horsepower resulting in it taking longer to get to point B.

Since a H-pipe does not improve scavenging, an X pipe, assuming on that particular application it improves performance, will generate better numbers than a H-pipe, except in low torque and high winding application where the X-pipe is more likely to be a negative. There are also cases where a H-pipe will generate lesser numbers in those same applications.

The benefits of scavenging depend on the pressure of the exhaust flow combined with the amount of vacuum that follows an exhaust pulse. As RPMs increase the pulses are closer together resulting in decreasing vacuum. Less vacuum means less scavenging which means any benefits of scavenging decrease as RPM increase. At the top of the RPM range a X-pipe is the least, if at all, beneficial.

The best numbers because of an X-pipe will come from the farthest upstream the exhaust system it is possible to install the X-pipe.

It is important to know how exhaust changes impact torque and horsepower as the best changes will be those that are most effective where you need improvement, i.e--torque get you going, horsepower keeps you going.

I have been unable to find dyno numbers that are meaningful. The ones I found either changed more than just the X pipe between runs or the difference was within normal dyno error.

[bcredliner]

Originally Posted by J.Belknap

Lots of bad information. Not going to argue, pointless. Ultimately it's a "feel good mod" for most anyhow.

the only thing I would count on is that it will change the sound.

If you are saying the information I am posting is bad, I think you should explain what and why the information is bad. I don't see differences as arguing unless someone gets angry.