Michael Benno

For the dual exhaust cars (86.5+), most of the X-pipes I have seen have the the cross-over up front connected to the exhaust manifolds/headers.

However, I do remember seeing at least one photo of a 928 with the X between where the cats would go and the mid mufflers.

In looking at the two designs having the X in the rear seems to offer more gradual bending of the pipes and allow for more room for cats/glass packs up front.

I am just wondering if placement of the X matters to get the evacuation efficiency and subsequent HP bump.

Just curious.

PS: I am hoping this thread won't devolve into yet another debate about catalytic converters

Mikebte

Catalytic Converter asside.
I have heard having an X pipe will help in the flow of exhaust. I have not heard any pros on the locations of the pipe.
I was going to do an X pipe, but did duel in single out muffler instead.
IMHO I don't think it would help unless you had an excess of power.

Also known as a balance pipe, they work to help with scaveging and pressure balance.

What is your current exhaust set up? Are you replacing it all or adding into a current set up?

GregBBRD

Rest assured that my header system with complete exhaust which has the x-pipe in the rear, in back of the cats, did not happen by accident.

Many, many hours were spent in the development and testing of that system. Multiple configurations and locations of the cats and x-pie were built and tested. Many configurations. X-pipes. H pipes. Front. Rear. Virtually everything we could figure out was tested. The "scrap pile", when we were done looked like a scene from the movie about the 300 Spartans, except there were pipes instead of bodies.

The best result is what you see in the pictures that have been taken of my system.

What works on your own exhaust system may be completely different.....this is what worked for me.

Andre The Giant

Greg, do you have the link to that picture ?

The Forgotten On

Another reason to have the x pipe in the back instead of ahead of the cats is to allow them to heat up faster to reach their optimal operating range faster.

Michael Benno

Yeah that is what I thought, 'what the heck does GB's system look like?' ... A little googling yielded some photos. Here is a photo from Greg's candy store:



And here is a full install from the recently discussed Zyclamrot car. You can find the full thread here.





Hey Greg, what is the purpose of the "H" bridge at the front? Is that an effective balancer or were you considering O2 mounting options?







This is definitively a very nice system and it's clear that a lot of prototyping must have been done to get it perfect. It looks like Magnaflow mid mufflers and final muffler and I think it's 2.5" tubing.

Found some additional details and some sound clips of this system: https://rennlist.com/forums/928-foru...ml#post7606768

hacker-pschorr

As Greg said, the location is important for optimal performance, also every system is different.

Louie Ott spent a considerable amount of time designing the X-pipe for the stock manifolds, later refined by the guys at Motorsports in Utah (which is the current version available to us all).

Yeas ago the guys in Utah spoke about moving the X around finding the optimal location for the stock manifolds on a 32V car.

Michael Benno

I am almost embarrassed to discuss my exhaust setup after showing pictures of Greg's. But some pictures are posted below. (disclaimer, I dont know much about exhaust systems, so corrections and opinions appreciated).

This was installed back in 2004 by one of the previous owners. It's not the worst setup in the world. It sounds really good, ableit a little loud but not drone, and it definitively has that unbalanced rumble of an american v8. I think there is room for improvement in sound (more euro, less american) as well as performance.

The front starts out with 2.25" straight pipes coming off the stock headers. The O2 sensor is mounted to a small "H" bridge, but I am not sure how effective this is for balancing the pulses or for HP gains. But is it similar to GB's solution so far.

The main 2.25" pipes are mated directly to the OEM GT mid mufflers, which seems odd to me since those were set up for 2" inlet/outlet. So I am wondering if those are constricting.

After the mid mufflers there is a 2" Y pipe into a magna flow final magna flow muffler. My understanding is that the the "y" is constricting because it is smaller diameter and merging the flows into a single flow is bad.

So now you can see why I am looking at an X-pipe and some overall exhaust system changes. Maybe I start over, or maybe it makes sense so edit some of the piece in the system.

James Bailey

Yes it is ugly .... look up mandrel bent tubing as a starter !!

Andre The Giant

WOW, thank you Micheal.
I would have thought that the X-Pipe was to restrict less airflow and that would provide more HP, and that is why to me logically it made more sense to install up front replacing the H pipe. Is the H in the H pipe a hollow pipe or just a solid support, because if it's hollow you would think that the exhaust flow would be turbulent and slowed down while coming from both sides and passing through the H portion ?

But at the end I see that the best position for the X pipe is at the rear because of the testing done. But I just need to understand and that is why I am asking.

ptuomov

I've been thinking about the exhaust issue because I've got a house on a steep private street in a posh (by Boston standards) small town. This is my current thinking.

In my opinion, there are three things that matter. Pulse tuning, steady flow capacity, and sound.

If you run cats, you want to run a cross-over of some kind upstream of the cats. This is because of steady flow capacity and almost certainly also for pulse tuning reasons (but that's in principle depends on the engine).

If you run cats and you also (smartly) run a cross-over upstream of the the cats, pulse tuning considerations are almost completely irrelevant downstream of the cats. Downstream of the first cross-over and cats, the steady flow capacity and sound are all that matters in practical terms. The first cross-over and cats act like David Vizard's "pressure wave termination box".

Downstream of cats (and the first cross-over), assuming that the headers etc. are well designed for the application, going too large or free flowing in the exhaust doesn't hurt power. However, after some point going larger no longer helps power. Unnecessarily large pipes are loud, hard to muffle, and often sound like crap. In my opinion, you therefore want to stay as small as you can without hurting power. Not going excessive on the pipe size and selecting mufflers that flow almost as well as straight pipe of the same size is the right recipe here.

A second cross-over downstream of the first cross-over and cats won't change the power because of pulse tuning, but it can really alter the sound. It helps combine the sound waves into a single soundscape and also breaks down any resonance from a long pipe, as long as the resulting pipe sections aren't all equal length.

In my opinion, the stock resonator locations near the rear axle should have large mufflers or resonators. Those help break any long-pipe resonances and also use the allowed space efficiently for sound suppression. Weight is also in a good place there.

There are some nice things about the 928 exhaust packaging. The 928 packaging allows one to design a trombone resonator after combining the pulses. This is done naturally without any Rube Goldberg's machines if the pipes are merged either explicitly or inside a muffler on the driver side after unequal flow path lengths. The unequal flow paths phase shift the sound and combining after a phase shift smooths out the sound.

There's also another reason why one should have a rear muffler. If you don't have a rear muffler, the long tail pipe will act as a resonator. The long tail pipe suppresses some frequencies (when it acts as a quarter wave resonator) and pass some thru loudly, and often that's experienced as drone. If you run a rear muffler and "just the tip" after that, you'll kill many unwanted resonances.

jhaverly

I have a stock exhaust on my 1994 GTS with the exception of a rear muffler bypass. While I am sure I could get a few more ponies by making it more efficient I love the resulting sound of this small mod. When it starts I hear the beautiful rumble. If I have the windows down it harkens back to my teen years messing with the sound of my V8. As soon as the window goes up and I'm driving down the highway....nothing. Quiet as can be. Best $150 and 20 minutes I have spent on a car

dr bob

Obviously different folks have differing opinions.

I had a chance to drive the GTS that sits in Steve Geiger's garage before he bought it, part of a PPI for him. His car came with the RMB. My thinking when driving it was that it was too loud on even throttle and pretty darn obnoxious on a pulling throttle. My opinion.

My formative auto years were spent driving interesting cars faster than I should have, at least according to all those judges I got to meet. By the third or fourth "performance" car, I was spending way too much at the county clerk's office. For purely economic reasons, exhaust systems from then on were made as quiet as possible. No more second glances from officers as I cruised by, no big aural announcements of throttle position for everyone nearby to witness. "Loud" is at least as much of a homing beacon as "arrest-me red" or "speed yellow", it turned out. Moving to black cars exclusively, with extra-quiet exhaust, allows me to drive much faster. Less flash = more available speed. I've managed to pretty much avoid judges since then. At the same time, I learned to thoroughly appreciate "quiet" performance.

Not saying I don't enjoy the rumble of a well-tuned exhaust system, as I most certainly do. I'm just not a fan of "loud" for no good performance reason.

Again, just my opinion.

bd0nalds0n

My car came with an RMB and the small resonators. Nice around town but annoying on the freeway. I went with an Ott x-pipe, got a set of the big resonators from Mark A, and installed the platypus muffler. Much more civilized and it still sounds beefy. With the 80s sound dampening the cabin picks up way more noise than my 2006 BMW 750 or 2010 Audi S4--and that's after adding Dynamat everywhere I could stick it.

I would like to switch to a GT muffler but they're pretty dear in price when they occasionally show up on ebay.

SteveG

I have to agree with Dr. Bob on the "flash" aspect, i.e., what I will call cosmetic effects. Spinning tires and "roar" can be rewarding to the ear, but are actually wasted HP, I think. Re exhaust note, my ears must be getting on in age because I don't hear what Dr. Bob heard on my car and it now has S3 cams and has a sharper tone than before. It is a little over the edge now, for me. Conclusion, there are a lot of variables to this equation. Michael needs to listen to as many combinations as possible to make the decision.