merge collector on 91 LS400

[spf_lexus]

I just picked up a second 1991 LS400 for a daily driver and want to keep it an N/A application. I decided to start an exhaust upgrade and just picked up 2.25'' mandrel bends that are pre cut, and plan for 2 summit race 2.25'' in/out offset mufflers and plan to keep the exact Y-pipe design. I am starting mine at the end of the first Y pipe after the cat, but need a few tips on making my merge collector. I ordered a Jones Y-pipe from Summit.com but the more I looked at the design it looks like the merge point is too wide. I have a decent welder and a miter saw. Basically what do people do when they weld the 2 halves togerther? I am having a hrd time visualizing the collector at the bottom, do you need to bend it to slip over?


Also, if I plan for 2.25'' mandrel, should I be running 2.5'' or 2.75'' main pipe? I decided that 2.0'' mandrel wasn't quite rewuqires because this LS has SC400 gears inside and the bottom end mimics an LS with a torque convertor so a slight loss down low is ok.

 

[JBrady]

The stock Y pipe with 1.97" primaries feeding a 2.36" center pipe is the same dimensions used on the much more powerful GS400 and 430s. Some of the supercharged GS4xxs have made close to 400rwhp on the stock Y pipe. I must recommend the stock Y part with its formed scavenging collector (very hard to duplicate design) over MOST aftermarket or custom efforts. Unless your goals are over 300rwhp I would look elsewhere. Even a very nice racing Y such as a Burns Stainless will IMO not out perform the stock Y over the stock RPM range for a street driven heavy Lexus.

I have discussed with S&S Headers a possible 2.125" Y pipe into a 2.5" centerpipe using a similar to stock Y design which may offer a slight power boost without low end loss on the VVTi engines. I would still be hesistant to change the part on a first gen 1UZFE that is largely stock.

Now, the LS400 has a bottleneck center pipe back design. I would replace the center pipe with a 2.5" that splits into dual 2.25s on back. This is especially true of California spec cars that have the 3rd center cat. You can use the Jones Y pipe in reverse. I would cut it before the crush bend part and weld on the mandrel from there. I would keep the center 2.5 pipe length the same as stock.



To further improve the stock system look to the manifolds and stock cats. If you chose headers such as S&S you can also source metal substrate cats that can fit between the headers and the stock Y pipe. Fabrication required.

 

[spf_lexus]

thanks for the reply JBrady. I am keeping everything stock until after the 3rd cat. do you think this particular y pipe will work efficiently? i thought the merge collector desigh was crucial to keep exhaust gas velocity up. do u know the mainline ID that runs between the two Y sections? I changed the order from 3'' into 2.25'' to 2.50 into 2.25''



BTW, my other LS400 is getting tuned right now and is shooting for 370RWHP, and it has a dual 2.5'' x 3'' main exhaust with it's center cat deleted and the stock front cats/Y-pipe in tact. Everything is routed proportionatly to the stock configuration. Should I be considering a burns stainless Y with 2.5'' to match the rest of the system? I am sure a set of S&S headers would open up a lot of power considering the power I am getting out of the stock ones but being realisitc w/ aftermarket cats and the burns Y, how much could the front Y section alone net me?In a nutshell the system is a JIC mandrel bent center cat back with replaced magnaflow mufflers and no resonators.

 

[spf_lexus]

I just got my mandrel 2.25'' bends in the mail, with 2 edelbrock sdt mufflers, the jones Y pipe and plan to start this next weekend. Just curious but should I shave the tail sections off the Y pipe, leaving just enough mandrel bend to weld to? The actual merge is well made but each ''leg'' is crumple bent. It looks like just 1/2'' of metal to weld onto, should I mod the pipe or leave it alone?

The car is staying N/A with the exhaust and SC400 closer ratio trans being the only mods.

 

[JBrady]

I would cut off all the "crush" bent part and then uses an expander to create a slip fit over that section for the receiving pipe. Once welded this creates a very strong joint and assures no outflow restriction.

Also inspect the area of the Y pipe where the single fits over the merged twins. Often there is sharp metal protruding into the gas flow. Use a grinder to remove any such metal to leave a smooth transition.

 

[spf_lexus]

I noticed the inside merge where both legs intersect has some nasty welding spatter, i'll clean it up with my trusty dremel. As for slip fit, is there any tradeoff compared to end-to-end welding? the slip will have very little to grab, but its close.

Sean

 

[JBrady]

I noticed the inside merge where both legs intersect has some nasty welding spatter, i'll clean it up with my trusty dremel. As for slip fit, is there any tradeoff compared to end-to-end welding? the slip will have very little to grab, but its close.

Sean

Slip fit is stronger. Slip and weld. Even 1/4" of slip fit creates a double sided joint that supports itself (if correctly formed). Butt welding is fine but requires skill to keep pipes perfectly aligned and no welding protruding into the pipe internal area.

As far as the internal sharp edges I spoke to here is an exageration to show what I mean. This would be looking from the 2.5" center at the two pipes forming the Y. Notice how the edges of the pipes fit inside the 2.5". Even 1/8" protrusion creates a big flow restriction.