In a few days I am going to drop by my friend who runs an exhaust shop.

We are going to chop the exhaust from the cats on back. Yes, there is no flange so we don't have a choice. I could change it out with high flow cats but I don't want problems with emissions on this car, especially when concerned that it might set off some error code.

We are probably going to go true duals all the way back to a set of Borla Mufflers with 2 1/2 " pipeing connected with an H- Pipe.

My question is how much horsepower can I expect from this modification? Any dyno graphs?

 

looks good on paper. LMS gained 20 horse power without cats, but with silencers on each pipe. my estimate would be 10-15 horse power.

 

If you are not going the FI route (which is doubtful since the IS is still so new), 2 1/2 " pipes might be a tad too big and you may end up losing power due to lack of sufficient backpressure.

Why the choice of H-pipe instead of an X-pipe?

Here is a very interesting article regarding the merits of an X-pipe: http://www.vetteweb.com/tech/0307vet_xpipe/index.html

I just did a custom setup on my car - you may find this interesting based on the questions you are asking - https://www.clublexus.com/forums/sho...6&page=1&pp=15

 

Unless you're running two cats, the exhaust won't be true dual. Along with this, you won't see much more of a performance gain. Keep in mind though, you'll need some back pressure so you don't want to open things up too much. The weight added by running a true dual and the cost, would negate most benefits, unless you have supporting mods. Depending on what you finally go with, I'd say you'd gain about 15hp +/-. Good luck though.

 

Ya, I just thought about the 2 1/2" might being too big. I might go 2 1/4" instead.

My friend said H pipe is better, sounds better and performs better; hes been doing it for over 40 years so I didn't question him.

Are the horsepower increase you guys are stating wheel horsepower or brake horsepower?

 

Listen to mine and read that article I posted a link to and you will have doubts about the H-pipe

I will dyno my car sometime this summer and will let you know how much I gained.

 

I think the IS has 2 cats already, which connect together with a Y, then leads to a large resonator.

His plan is to remove the Y and the resonator, then send pipes from each cat to a muffler.

You could also do this with the stock mufflers.

 

Yes, but how restrictive is the stock mufflers?

It is very counter productive to increase airflow to decrease it towards end.

 

When I went to larger pipe and free flowing mufflers on my car(not and IS yet) I lost some low end torque. But with a supercharger I could get it back with a smaller pulley. Also, V6's don't sound that great when they are loud IMO.

 

4 cats not 2

 

Unless you're going to remove the cats, I don't see you losing enough back pressure to rob you of any power. While you do need some back pressure to maintain torque, the cats, especially if there are 4 of them should provide enough back pressure to keep your torque from dropping, even if you did go with 2.5" pipes. Other than changing the exhaust note a bit, the performance difference between 2.25 and 2.5" will be small. By the time the exhasut reaches the back of the car, it has cooled down enough not to need larger diamater pipes. There are of course exceptions, like if you've got significant mods and moving large amounts of air, then of course you'd wan't the larger diamater.

 

Anyone know if I swapped out the 4 cats with high flow cats, if it will set off any codes?