The single 60mm is split into twin 50mm pipes to the twin silencers. These look like standard mufflers with a VERY unusual side entry rear exit configuration. Notice the heat discoloration at the intersection where the split occurs. Heat discoloration shows up where there is a flow restriction. Notice also the heat discoloration just after the pipes enter the silencers. This indicates the silencers are a large restriction to flow. It is AMAZING the engine makes the power it does through this congestion
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Close up of the way the pipes split to the silencers. Each "receiving" 50mm pipe is crushed in half, squeezed next to the other and welded INSIDE the 60mm "feed" pipe. This places the rough edges into the flow. This is a VERY restrictive intersection.
Kind of hard to see but if you look closely you may make out the sharp edges and half moon shape of the 50mm pipes inside this 60mm section. This is looking in the direction of flow from the center single 60mm pipe as it splits into the silencers. Again, this looks like a good one way anti-reversion section... unfortunatly in the WRONG direction
It is also a good assumption that this same construction is used under the heat shield on the Y pipe seen above. In that location it is not as bad as this but still is a marginal intersection.
The stock catalytic converters flow pretty well. The core is made up of a thin metal spiral that has corrugated dividers. The end result is a part that is very similar to a large number of straws all aligned. This allows the gasses to flow directly through the core with minimal restriction. Here is a picture looking through the core. Notice the light that can be seen THROUGH the core. The entire core has this structure and as you move the converter against the light source you can see through the entire core:
As many here know, I have preached about the importance of high velocity / high scavenging exhaust systems to maintain low/mid RPM power while increasing high RPM power. To that end I have been working with an engineer to computer model various designs. I have also had VvTi 1UZ-FE specific flanges custom cut. At this point I have a number of the parts so I thought I would post a few pictures to keep everyone up to date:
Here is a picture of the various parts, included are three high velocity merge Y-pipes of which all use 2.25" primary pipes, two have 3.0" outlet pipes and the last has a 2.5" outlet. This will allow me to real world test several configurations to see what makes for the best stock replacement sizes. Also notice the 2 bolt flanges for connecting to the catalyst outlets and the prototype 3 bolt flange to bolt directly to the header flange. The 2 small parts are O2 sensor bungs that I will weld into the primary Y-pipe tubing for after cat O2 sensor mounting.
The shape and contour of the Y-pipe design is important to promote velocity and reduce pressure (venturi's law). Here are a few pictures showing this detail. Notice the smooth transition, the lack of welds or uneven edges protruding into the flow stream. You want the flow to be un-restricted and as turbulence free as possible. You also want to minimize the area where the 2 flow paths meet. A large area slows velocity while a small area maintains it and can even aid in scavenging from side to side.
Interestingly, the flanges I had cut have a 60mm OD (outside diameter) center hole (the same size as the stock center pipe) and this is almost exactly the same size as the ID (inside diameter) of the 2.50" pipe end on one of the Y-pipe merges. This shows that even using a single 2.50" will be a good upgrade and the 3.0" parts are MUCH larger still. Testing will show which is best.
Since the stock LS400 exhaust system sucks... I decided to design a complete system from the cats back and keep the stock system intact in case I need to return the vehicle to stock. This will also allow me to remove the system (put the stock back on) and make jigs and send parts off to have custom pipes bent so I can build other copies for those interested. Obviously this is NOT the cheap way to do things but the end result should be very worthwhile.
First, I need new flanges to connect to the catalysts. I drew up the specs and had new flanges laser cut. Next, I need a REAL high velocity scavenging Y or X pipe. I chose to go with a "long X" design which is essentially 2 high flow Y-pipes with a length of pipe between them for maximum scavenging. I then needed high flow mandrel pipe bends to design the layout.
For muffling I want the car to be quiet, virtually unnoticeable at cruise and with nice low growl under acceleration. To accomplish this I decided to run a center resonator as the middle of my "long X", high flow turbo mufflers in place of the stock silencers and also in place of the stock mufflers.
The silencer replacement is tricky and I seached many many muffler manufacturers looking for a decent replacement but found none. The silencers are very strange in design and look like a conventional muffler with their inlet oddly going into the SIDE of the case The space location for the silencers is very tight and this creates the need to fabricate an unusual fitment replacement. Attached is a pic of the stock silencer section for reference. (BTW, SC400s, many anyway, have similar silencers only the outlet is the case side)
Here is a mock up assembly of the new high flow parts.
Shown are the flanges, the mandrel bends, the 2 back to back high veloctity Y-pipes that will combine to form the "long X". The long X will have a straight through resonator on the final assembly but I will experiment with straight pipe in both 3.0" (shown) and 2.5" to determine the optimum size for this output (torque curve).
Also shown are the non-crush wide band clamps. These are both high flow and easily r&r'd to allow different system configurations to be quickly and economically tested.
Once the final configuration is established I intend to have the parts copied to create a nearly seamless system that will look great but most importantly make POWER while keeping the Lexus "steel fist in a velvet glove" personality
Here are a couple of mock up pics of the replacement for the silencer section as seen in the first picture above.
Things to note are the smooth transitions, the nice Y-pipe split and the high flow turbo mufflers replacing the VERY restictive stock silencers. Of course pipe size is increased from the stock 1.96" (60mm) to 2.25".
My first set up will include transitions to keep the stock rear mufflers and that will allow me to easily open up the system or quiet it down by mearly removing or installing the stock rear mufflers. This is only 2 bolts and 2 rubber O-rings to do... very simple... 10 minutes.