Hi all, finished my procharger 6psi and am loving the power and the low end power. I have a cat back JIC mandrel 2.5'' from mid cat back and I didnt loose much low end from it but here's my question:
-If I went thru and replaced the first "Y" and went 2.5 from header back utilizing the stock "y" locations would there be a significant drop in low end? I have a lot of low rpm air being pumped and i figure it will help the flow better.
I plan on going to Mufflers West here in Temecula CA and have them replace the stock y w/ a race merge collector and bore out the mid cat ID to accomodate the 2.5''. Hoping for opinions/suggestions
thnaks all
Shawn, first of all congratulations on supercharging your LS400. You have already learned much. Now, keep in mind your BOV experiments and costs with what I am going to say on exhaust. Your instinct will be to go big thinking it will work the best. Let me STRONGLY recommend you keep your stock parts so you can put them back ON the car if you aren't happy with any changes you make.
The stock Y pipe works much better than it would look and is NOT a restriction below 250hp and very little up to and over 300hp. So, installing larger pipes will only allow more flow ABOVE the stock restriction power levels. Below those levels how could a bigger pipe help?
Now, you have a centrifugal supercharger. These do not make much low RPM boost but are moving more air than no supercharger at low RPMs. That is why you get more power even without noticing any actual boost. At the intake valves there is MORE pressure than without the supercharger compared to stock.
You also have a torque converter which allows the engine to rev more quickly than stock. Both the TC and SC give better low RPM power. Still, the power levels are below the stock Y pipes ability to flow. Therefore increasing the pipe size will reduce velocity and therefore make the engine work harder to PUSH out the exhaust at low RPMs. How much difference will you notice? Hard to say because each design and construction is different. What you will probably notice is that the engines FEELS softer, somewhat sluggish and less sharp that with the stock Y pipe.
So, how much could you gain in the higher RPMs with bigger pipes? Again, depends on many variables. May be very little.
My recommendation is if you want to experiment (R&D, research and development) would be to get 2 bolt flanges that match the catalyst outlets and have whatever system you have made created from there. This leaves the stock Y pipe intact and easy to re-install for back to back testing. You may find that the stock pipe makes the car more fun to drive. Of course it is possible to create a better pipe than stock but it is NOT easy and takes a bunch of testing an solid understanding and experience.
The stock pipes use twin 1.97" mandrel bent stainless pipe that Y's into a single 2.36" center pipe. This as I said above is pretty darn efficient at 300hp. Trying a racing merge pipe using 2.25" pipes into a single 2.5" pipe on a GS400 with S&S headers showed 1 rwhp gain (ONE horsepower) over the stock Y pipe. Now, this bigger Y pipe really made the car feel LAZY. Going back to stock makes the car MUCH more fun to drive.
So, spend some money and try some mods. It is a good way to learn. Exhaust systems are similar to musical instruments and small differences and changes can make big differences in performance. Pipe size is only ONE variable out of MANY. Racing designed parts are for high RPM power and a street car needs power from idle up. I always recommend the SMALLEST pipe sizes that will support the desired peak power. I would rather make 5 or even 10hp LESS peak power for a sharper crisper feeling engine. In fact this will usually make a FASTER car!!!