Lvangundy

I came somewhat close to making a decision on putting new mufflers on my LS yesterday after spending a couple hours researching exhaust theads on CL and other sites.

Don't have the $$ but I was getting a quote to see where things were. Mod bug is splitting me in two.

Dynomax, Full Boar, Magnaflow, or Viper 1 (?) were all about $100 each (x2) installed. I've wanted a Greddy PE for a long time, but the price is way out of what I want to spend for 3-5HP. I'm looking for the deepest sound for the price...Sound clips of borlas make me want to pick those up but the peformance gain is minimal and the price is up in the clouds.

If I don't go with this I'm going to return the car's current setup back to stock and just cruise around for a while an decide on what to do. Otherwise it's $200 for a couple months of sound fun.

The conclusion I got from everyone else's ideas were to get a straight-through design and 2.75" piping. Peter Scott's soarer site also made me think to put the centor resonator back on because it's not as restricting to flow as the 2 rear mufflers and resonators. Nobody here has any dyno results except for a GS$00 with borlas which produces like 1HP or something. Now that I have the G-tech I can do a few more comparison tests.

The 'back pressure loss' concept still eludes me. I don't have a good understanding of how this works and many of the things on PS's soarer site confused me comparing back pressure loss or gain. The phrase 'back pressure loss' itself doesn't make sense to me.

TomWahjudi

Lvangundy,

If I were you, I wouldn't put anything larger than 2.25" piping. Unless you are talking about the single pipe after the Y. Also, the X-pipe configuration (stock configuration) actually yields more horsepower, that's why I said 2.25" max (twin 2.25" if you want to look at it that way). Even this might be a wee bit too big, 2" will be just perfect but high performance 2" mufflers and resonators might be harder to find.

Another aspect to watch is the bends. Using mandrel bends will also ensure that cross section area will stay constant throughout. This will help quite a bit even if the piping is only 2". Another item in my list is cat delete This will make 2" piping is big enough.

I have yet to see the exhaust routing under the car, but it seems that in order to muffle exhaust noise without posing too much restriction, several mufflers seem to be mandatory. I'm thinking four straight through (perforated, no louvres !) oval mufflers (two for each bank) like Magnaflow or Ultraflo. Maybe if there is enough room, each bank will also get a resonator. The X-junction will come from Burns Stainless, what a piece of art.

As far as "back pressure", just think of exhaust system as pulses instead of constant flow of gas. One pulse should travels fast enough that it creates vacuum behind it. This vacuum "sucks" out the next pulse, make it travel at higher speed. This phenomenon is called scavenging effect. Too big of an exhaust pipe will slow down the pulse speed, making scavenging effect less effective. Now, if you have too small of cross section, it chokes the exhaust pulses.

Stock exhaust tends to be on the small side due to many reasons, two of them are probably due to manufacturing cost and ease of manufacturing. This produces effective scavenging effect on low rpm, not so good at high rpm. What we want is something that doesn't hurt low rpm but boost high rpm torque.

Back to back pressure, logically bigger pipe poses less back pressure. That's why some people out there say that you don't want too much back pressure, but a little back pressure doesn't hurt. This fits the scavenging effect theory. You don't want too big of a pipe (don't want zero back pressure) or too small (too much back pressure), instead you want just enough size of pipe (some back pressure) so you can create this scavenging effect.

How big is too big ? Well, I don't have the proper education to give you the exact answer, but if I may use my experience with turbo DSMs, I will say that 2" is about enough. There are turbo DSMs running 11 second 1/4 mile with full 2.5" exhaust. This is a 2.0l motor. The LS400 has exactly twice the displacement. Give or take a few things, keeping low rpm torque happy (besides 1UZ-FE has redline of "only" 6000rpm anyway), sans cats etc., etc.

So, it's 2" for me. 2.25" if wall thickness is a little thicker and if 2" straight through, oval, high performance mufflers are hard to find. 2" piping is cheaper and easier to fabricate, some people in here can probably attest to this.

Anyway, don't give up just yet and keep us up to date

Lvangundy

Thanks for the reply.

Yes I've been doing a little research on the whole back pressure thing and there's another discussion in the LS400 forums on the same topic.

If a 2.0 liter engine is using 2.0" piping and the LS is a 4 liter why not use a 2.5" or bigger on the LS 4.0? Is it because the 2.0 is turbo and is producing exhaust 'power' equal to or more than the 4.0? In that case without the turbo on the 2.0, the exhaust pipes should be 1" or less based on a quick ratio estimate.

The confusing part is that most of the stock LS exhaust is 2.25 (I think). So where can we gain on performance through piping? So far nowhere.

JBrady

Tom, I agree with most of you post... good points. The 2 sections above I want to add a little perspective to. Backpressure, a condition of positive pressure at the exhaust port, is a bad thing. Larger pipes actually increase backpressure at low gas flow. It seems counter intuitive at first but so does the venturi effect! Your comments about pulse scavenging are good but an important consideration is system volume and velocity. As the system bends, expands, contracts, merges and diverges the flow changes pressure, speed, temperature and accoustics. A good way to think of an exhaust system is a series of springs. Each "section" reacting like a spring. The goal is to get the ALL the "springs" working together and not against each other. Flow restrictions like sharp bends, constrictions, protruding edges and the like are just flat bad news.

As far as a systems flow capacity goes, this is dependent on BOTH the total exhaust volume and the tuning of the system.
A properly designed small diameter system can carry more volume than a poorly designed large diameter system. I do agree that 2" pipe (apx 1.875" ID) can support 2 liters at 6000rpm making 150hp. The question is will 2.25" pipe hurt performance? Again, the answer yes or no depending on design.

TomWahjudi

John,

Yes, I agree with you. It is a complex system, talking about accoustics and dynamic gas flow. I hope my explanation and yours don't confuse Lvangundy even more. I can see that mine actually just opened up unnecessary field, ooops

Anyway, my concept of 2" piping neccesitates mandrel bends, removal of cats and straight through mufflers that don't have reduction in ID. The system should be constructed as straight as possible. I'm thinking about angled mufflers but that might look too boyracer-ish for an LS400 I'm all for high gas velocity, as long as it doesn't turn into turbulent flow. Let's not go into Reynolds number and such

Although it is probably better to use 2.25" if the engine is equipped with "scavengers", i.e. equal-length, long, tuned headers coupled with very mild cams and head porting (conservative, not talking about 3000rpm-idle-speed, hogged-out ported heads).


Lvangundy,

Maybe I shouldn't have said anything about turbo DSMs. Turbocharged engines behave rather differently since the turbine wheel negates the scavenging effect after the turbo. On turbo engines, the pipe size past the turbo becomes "bigger is better". Ever seen Supra Twinturbo using full 3.5" piping for exhaust ? And, no, that's not the exhaust tip size My '95 Eagle Talon TSi AWD uses a catback exhaust that goes from 75mm (3") to 85mm (~3.3") by the muffler inlet, now that's on a 2.0l inline 4

I doubt that LS400 uses 2.25" piping. Even if it's so, the ID is probably highly constricted or there are a lot of necking down. Not to mention the maze-like path inside the mufflers and/or resonators.

Also, assume that I'm talking about twin 2" piping when it comes to LS400. Each bank (2.0l) will be served by one 2" piping. It seemed that you were talking about single 2" for both banks. If that was the case, you were right, it becomes very choking.