gmacrae

So... whats your reason for not running a 2.5" chambered centre muffler? Im open to ideas

When the cats are gone the noise level will go up a bit more, i want a more complex/effective centre muffler to compensate. If the exhaust system's flow is going to be interrupted (as it will be with a more complex muffler), i'd rather it happened nearer the front where the exhaust gas is hotter/lighter, and before it gets a chance to resonate/reverberate down the rest of the piping. If the stock front y-section is 50mm, ill leave that alone.

JBrady

The Rush headers are longer and larger in diameter. I have been told the low end torque is down somewhat with them. The S&S would be easier to install. The hardest part of a header install is removing the stock manifolds due to the heat shields.

There are no direct bolt in headers for these cars. The S&S and the Rush headers both bolt to the engine but require exhaust mods to connect them.

JBrady

The center resonator needs to be straight through. A chambered muffler will add backpressure and reduce the ability for the Y to scavenge. Both will reduce power at all RPM points.

The stock middle resonator is designed to be at a harmonic point in the exhaust. Replacing it with a 2.5" of the same design and location is advisable. The stock center pipe is 60mm = 2.36" but the stock resonator is more like 55mm = 2.17". This was done on all LS400s from what I can see in pictures and what I have measured on my 99. Increasing to a single 2.5" pipe will provide more peak flow capacity without drastically slowing the gas speed. It is also a widely available size and will slip over the 60mm pipe very nicely since the OD of the 60mm is 2.36" and the ID of a .065 wall 2.5" pipe = 2.37"

Another option is to use the center resonator from a GS400 which is a 60mm part instead of the LS400 which uses a smaller 55mm resonator.

Here is a picture of my LS400 center pipe/resonator. Notice how the 60mm pipe is necked down for the center resonator. This is an easy and fairly benign way to improve capacity. You could even just replace it with a 2.5" pipe and no resonator and see what that does to tone and drone.

gmacrae

Ok cheers... Now im going to hunt for the longest straight thru muffler i can find, possibly make 2 into 1 or make one from scratch, im guessing i should be able to get one in there thats roughly 800mm long.

Jbrady, i'm assuming you've looked at the internals of the stock y pipe and how each pipe is squashed down to half its size just to make the merge? Do you really believe this is an efficient design? I know the toyota engineers know much more than we do but they're also trying to make things to fit a budget and keep mass production simple - to me, this Y piece seems like it was made without performance in mind.

Im not trying to hijack this thread here, hopefully the thread starter is learning something too

Gene

JAndersson

No problem. This is interesting. Just keep on posting your ideas and thoughts

steveski

Interesting post about removing the center resonator and replacing with 2.5" pipe. I did exactly that only because it was dragging on the entrance to my garage as I have a steep grade into my garage. It markedly changed the sound.What really changed the tone was when I swapped the 3" x 7" tips for 4" x 12" tips. Sound got deeper and lost some of the lub lub sound.

JBrady

Steve, other than sound did you notice any performance change when you replaced the center resonator with the 2.5" pipe?

JBrady

Yes, I am very familiar with the specific construction of the Y and yes it is surprisingly good for performance.

To understand why consider what is going on. You want a merge that will flow very well in one direction (away from the engine) while flowing poorly in the opposite (back towards the engine). It must also have enough flow capacity.

Since all of the exhaust must pass through the 60mm center pipe that flow capacity is either sufficient or it is not. I say it is. Let's leave that question to another post and lets go on the premise that it is enough capacity.

Now, each 50mm pipe has an internal cross section area of 2.65 inches. The single 60mm center pipe has 3.91 inches. So, both 50mm pipes have a combined area of 5.3 inches which must be accelerated when merged into the 3.91 inch area center pipe.

The question becomes what is the best way to accelerate the gases in the Y transition? We want it to be very smooth and flow away from the engine very nicely but not in reverse.

Lexus/Toyota utilizes what I have named a nozzle collector for this Y section and it works great. If I were to build a larger diameter Y system I would like to copy this design.

Look very carefully at the design. As each 50mm approaches the merge each pipe is formed into a D shape or half circle. This half circle is not half of a 50mm but is in fact half of a 60mm. When both 50mm pipes are fit together they equal one 60mm. The 60mm pipe center pipe is expanded to fit over the two formed ends to complete the transition. VERY clever.

Think about it. In order for the two 50mm to Y together into the single 60mm the gas must be accelerated. People get confused here and think this creates backpressure and therefore is bad. Dyno testing almost always shows that merging gases like this benefits the power curve. The reason why is complicated so lets also leave that to another post. Bottom line is Y and X transitions work to make power and this one does as well.

So, since the gas MUST be accelerated it comes down to HOW and WHERE to accelerate the gas. Now, if the pipe size being used is sufficient in capacity for peak power gas flow (and the stock size is) anywhere below peak flow the pipes are more than large enough. So, for most engine RPMs you have plenty... even too much pipe capacity This is why going with even larger pipe reduces velocity which hurts power. The goal is high velocity at low pressure.

By creating the half circle shape that is half the capacity of the final pipe the stock Y accelerates the gas right at the end of the 50mm pipe. The result is a collector with no added volume which is GREAT. Even a racing merge collector has a collector volume increase. This means that the volume after the merge and before the single pipe is larger than the single pipe AND the direction of the gas is not straight into the single pipe. Also, a merge collector flow very well back towards the engine. The factory nozzle collector transitions the gas very smoothly and the gas is from each primary is parallel and in line with the single. All of this is in favor of the factory style.

Here is a picture of this TYPE of merge. The picture is of my LS400 splitter Y which is after the single center pipe. Since it does not have the heat shield you can see the construction. Notice the way the single pipe is stretched larger over the two half circles. Looking inside you can see the design. This design is less effective flowing in revers due to the sharp edges and shape at the corners.

gmacrae

Cheers for explaining that Jbrady. I definitely see where you're coming from now. I guess with when the exhaust pulse exits one side of the Y into the merge that creates a good pressure drop right at the merge so it makes sense that this design is great for scavenging.

Now im wondering if there actually are gains to be had by increasing the size of the centre pipe to 2.5". I guess the acceleration forced by the stock merge could be aiding scavenging a lot. Increasing the centre pipe volume could decrease acceleration and therefore reducing scavenging enough to make it a waste of time... hrmmm

JBrady

I have not yet modded my pipes. The pictures of merges that I had built were intended for this but the more I understood the stock parts the less inclined I was to change them.

That said, look closely at the picture above in post #18. Notice the 60mm pipe comming out of the center resonator... it is necked down to about 55mm. This gives an internal flow area of 2.035". That is pretty small for 290hp (99 LS400 VVTi). The GS400 had a constant 60mm (internal size 2.23") and was rated at 10hp/10tq more. The exhaust is the only significant difference between these engines. Going to a 2.5" pipe give internal size of 2.37". That should be a very tolerable change that gives a nice increase in flow potential. Like anything else changes must be done properly with the pipes having smooth interal features. Too many exhaust shops stick pipes together without concern for this.

Now, eliminating the center resonator also changes things. Personally, I will probably replace mine with a 2.5" resonator. On the early cars it may be a good plan to source someones discarded 60mm resonator from a GS400. It has been a LONG time since I looked at a 1st gen exhaust system and some had resonators and some had catalysts. If someone can confirm either way if the 1st gen necked down like mine that would be good to know.

Here is a close up of the neck down at the front of the Y on my car.

LiCelsior

iTrader: (32)

yep its necked down like that on the 1LS. i scraped a hole where that little heat shield or cover and i had to remove it.

Boostdls

i know this post is OLD, but just so i understand it.. It is best interest to go from the stock Y (merge pipe) to a 2.5 resonator and 2.5 all the way out? or just to the next Y (split pipe) and then go 2.25 all the way out or to a 2.25 muffler on each size?

CAN SOMEONE DUMB THIS DOWN FOR ME... hahahahahah

gmacrae

It is an old post but there's some great info in here... but yea that did seem to be the conclusion, leave the stock front y-pipe!