I have hundreds if not near a thousand hours of dyno time designing intake systems. Of the 30+ intake systems that I have designed/tuned on the dyno, the end result were products that made more power and torque across the RPM band AND increased gas mileage. However, each intake design took more than 3 iterations (typically 7 iterations) and at least 8 hours dyno time for each vehicle. I have never had a released design that resulted in a decrease in mpg.

Bottom line, I have a ton of experience designing/dyno'ing intake systems but near zero hours dyno tuning exhaust systems.

 

Well,.. backpressure is one thing, and velocity is another. You can think in both.

Alot of people have backpressure way out of wack,.. almost into mythical proportions.

Some people say reducing backpressure is evil for the car,.. or that it causes valve burning and so on. This just isn't the case with modern cars, and specifically the LS 400.

The optimum fuel to air ratio is 14.7fuel to 1part air. This is referred to as some word I can never remember, but it is a chemical balance that is true.

Anyhow,.. a car without an automatic computer to adjust to air flow and other variables, would have negative effects when you drop backpressure via opening the breathing through the exhaust. Less air gets in the combustion chambers (as the backflow isn't as strong as it was..) - This introduces a lean condition and well, it is less efficient. Problems all around.

But, this doesn't happen with the LS. You put on a recommended 2 1/4 tubing, with your pick of muffler and such, and the backpressure will drop. That is the result of freeing up the exhaust. It isn't as restricted. But,.. the car will adjust to this change. The fuel will adjust, the computer will change dynamic mappings. It will not run lean as an old car would.

Velocity is just that -- How fast the gas is going. I believe the optimal number is 240 feet per second, maybe higher now.

Larger piping, more volume, less pressure, right?

Smaller piping less volume, higher pressure, right?

Yes, but not for the reason you might be thinking, or maybe you are.

Exhaust gases lose pressure with velocity. That is how the pressure is decreased,.. the gases are moved faster, and it lowers the overall pressure in the system.

This is why it is key to have a optimum exhaust velocity -- You don't want high pressure which will slow down the escape of the exhaust, and create a number of problems, scavenging, overlapping and other misc. engine/combustion cycle references that you can read up about.

Bottom line, is at low RPM you exhaust produces X pressure and travels at Y speed. At high RPM the exhaust produces X times 10 pressure and travels at Y times 5 speed. (Numbers are made up.. in no way represent any equation).

So we have a problem..

In a perfect world, the exhaust would grow as the RPM range increased, and shrink as it dropped. This just isn't feasible obviously -- So you have to come to a compromise between the two.

The stock exhaust is obviously not the best compromise, it is good, but not the best, hardly ever is. That is why aftermarket systems offer some benefits, but it is also why everyone always says keep it at 2 and 1/4 no bigger -- Because that is the maximum tubing for good low end performance and big enough to not choke the high end.

This is huge science,.. and I don't pretend to be an expert -- but yes you can lose gas mileage with exhaust modifications, but you can also gain. It is one of those 'done' right, thought out things.

 

So when people here recommend 2.25" piping, does that mean from the cat back, but essentially copying the stock paths?

 

yes, pretty much except u have ur shop smooth the bends and use a proper x-pipe not a y-pipe like the factory uses.

 

2.25 unless it has forced induction. the larger the pipe the slower the exhaust gases move and the more scavenging happens in the flow causing a loss in power from the pulses as they stagnate in the large cavity. Also the larger the pipe the louder the resonance bouncing off the inside of the pipes.
This will give you an idea , it is slightly high compression but not forced. A way of finding out what is needed is take the total diameter of piping and it should equal to the liters of the engine. It is not a theory just i guideline i made up.
So we have a 4 liter with 2.25 from the factory on both exhaust manifold equalling 5 inches total. So it is slightly larger to accommodate the slightly higher compression.
While my 1mz-fe es300 has 2.5 as it has lower compression and scavenges with anything bigger. While my civic with a 1.6 zc with slightly higher compression for increased torque has 2 inches.
I said this before never mod an LS with aluminized piping use stainless steel or wait and save the money to afford to mod it. It sounds like crap and it will cause people to laugh at you as the tin can goes down the road as it looses the deep resonance of a stiff exhaust pipe holding the notes together.

Do it right , do it once ,not just for the sake of being different.

 

No , the stock paths are horrible and made to make the exhaust more quiet but cause more turbulence and reduced efficiency. This is truly the stupidest design ever made for an exhaust but that is the usual Toyota rule of thumb. Make a perfect car and stick a maze of pipe up its ****.
The factory uses "T's" on all merges. They cause you to loose at least 10-15 hp and an equal amount of torque as you are wasting power over coming the pressure exhausting the old gases instead of turning the wheels.

Y's are very nice X's are nice as well but to design the exhaust properly it needs to have curves so that all merges or X's are after and before a straight flow not in the middle of one.


i am too tired to remake the diagram i made years ago showing the changes but it makes perfect sense once seen of where the problems are.

 

First of all the stock system is 1.97" (50mm) from each catalyst in mandrel bent low grade stainless pipe. The first Y section is actually a very good design (except for the funky slash cuts right off the cats in 60mm) and is nearly identical on the 300hp/310tq GS400 and the 300hp/325tq GS430.

The center resonator is smaller than the 2.36" (60mm) pipe after the Y and again after the center resonator. Replacing it with a 2.5" center resonator may be a slight benefit.

The real problem is where the center Y's back into duals. The LS has middle mufflers and they are heavy and restrictive. The splitting Y itself is not a good design. The GS4xx is much better and has dual straight through resonators instead of the middle mufflers.

Last, my airbox mod does NOT reduce fuel economy. I average higher than most. I drove from Houston to upstate NY and averaged 29mpg running 70-80mph and have had multiple 500+mile tanks of gas.

 

Most aftermarket cheap X pipes are not as good as the stock Y pipe. Save your money. A quality Y pipe to split the center 2.36"/60mm pipe back into dual 2" or 2.25", a pair of resonators and a pair of good flowing mufflers is all that is needed on these cars unless you plan a serious power increase.

Lexus uses the 1.97"/50mm pipe for the duals from the front cats on all models up to the 4.3 engines. This is more than enough for most of our cars especially the earlier ones that were rated at 250hp but are more like 230hp.

I hate the word backpressure because engines do NOT need pressure at the exhaust port. Any pressure must be overcome and that reduces power PERIOD.

The reason velocity is good for low RPM operation is it helps prevent the gas from reversing during the end of the exhaust period for each cylinder.

A big pipe slows velocity and alows this reversion to happen easier. You want the SMALLEST pipe that will flow the needed gas at peak RPM and power.

BTW, I ran a test on my 1999 LS400 by removing everything after the center pipe. The car was insanely loud and performed worse. A full system will help our cars operation over the full RPM range.

 

I deleted the middle resonator as well as the other two resonators - muffler guy ran I believe 2.5" pipe between the Y-pipes and 2.250" pipe to the MagnaFlows and it sounds better.

I asked him if I should go with true duals into an X-pipe and he said it would not be worth it - said there would be no gains except maybe sound. Cool that he is honest and didn't take my money. I am staying with the OEM Y-pipes and no resonators at this point.

 

at least i would replace the OEM Y-pipe with a good quality X-pipe or aftermarket Y-pipe.

the OEM y-pipe is basically 2 pipes SQUASHED into 1, which is why its so restrictive.

 

It seems to be wide open and free as before it felt restricted.

Its not crazy loud...when I start it I can hear it, but then it quiets down and makes no noise until I give it some gas. I like it

GRRROWWWL

 

PD, the stock Y is actually pretty good for peak flow and very good for low to mid RPM use.

The merge is created by forming each end of the 50mm pipe into a half moon in a 60mm size. The shape changes but there is minimal reduction in flow area. This is actually a desired characteristic that race designers such as Burns stainless create with their Y and X designs. This creates and acceleration from the total feed volume (50mm x 2) into the smaller single 60mm.

As I said, on fairly stock cars keep the stock Y pipe.

 

i have an HKS intake with a custom fabber intake tube. twin 2.25" pipes from cats back, no middle mufflers or cente rmuffler and stock rear mufflers. soon hopefully to have an x pipe and some good looking rears...

 

right now i have the open airbox mod. soon im going to find out where my exhaust leak is and of course fix it! but most likely going to run 2.5 piping from the cat back to either 2 Spintech mufflers or Stock GTO mufflers from a friend of mine. deleting resonator, mid muffler and adding a Xpipe.'

edit: after reading, might change to a 2.25 set up.

 

You have to post up a vid if you get the GTO mufflers...I was looking at those too.