450Thumper

I just installed an Invidia downpipe, high-flow cat, and custom cat-back with Otto cans. The shop used 3" pipe from the cat back. The car has a deep throaty sound. They did an awesome job.

My question is about back pressure. How important is back pressure in a turbo engine? I really can't tell if there is a performance gain in the car. I still have planned an aftermarket K&N air filter to boost intake when they are available. I am worried that all I accomplished is a louder car with no real performance gains. Any thoughts?

450Thumper

Thanks for pointing me to the post on back pressure. I understand the flow of the various pipes, but I guess what is troubling me is that the car actually "feels" slower with this new system, which is basically wide open from the downpipe to the mufflers. I am waiting for some programming to become available to boost the turbo some, but I am still concerned that I may not see the benefits that I was hoping for with this costly upgrade.

The other strange side effect is that the car tends to want to run at lower rpm's, near idle, when cruising. It is an automatic with triptronic shifting. When it runs at lower RPM's, the engine sounds like a lawn-mower in the cab. When you let loose on the gas it sounds awesome, but you can hear the turbo "squeal" over the exhaust at about 3800 rpm.

Well, thanks for the input. I guess I will wait for an accessport to become available for some better tuning.

Nickel

Exactly. The turbo itself provides all the backpressure that you need. The best possible exhaust on a turbo car is no restriction after the turbo.

-Nick

Xaqtly

Ding! I'm surprised this didn't come up earlier. The more free-flowing your exhaust is, the more power it will make. This is specifically where it differs from naturally aspirated cars. In any case if you still want to keep it sounding relatively civilized, what you can do is get a custom made turbo-back exhaust system, 3" all the way back, with long, straight through components like the mufflers, cat if you want it, maybe a glasspack midpipe. I'm talking 20" long or more. As long as all the components are straight through, you won't lose too much power to airflow restriction, and the longer your mufflers, midpipe etc. are, the quieter it will be.

You don't want to do 3" all the way back straight piped. It will be louder than your ears can handle. A lot of people think turbo cars are quiet when they're uncorked but that's only under throttle. At idle or driving around town it'll make your ears bleed if it's not muffled at all. If you really don't want a cat, I would bet a couple nice 20-24" long straight through 3" mufflers and a 20-26" long 3" glasspack midpipe somewhere in there would do the trick, and as long as all the rest of the piping and the downpipe were all 3" there wouldn't be much restriction at all.

I did this on a Nissan NX2000 I had a while ago, I had swapped a Japanese SR20DET turbo motor into it and I put a full 3" exhaust system on it - it immediately gained 30 wheel HP on the dyno. Granted the stock exhaust was likely more restrictive than the LGT's stock system, but it's a good example of why turbo cars need free flowing exhausts. My current project car has a similar exhaust setup and it's very livable on a daily basis, and it flows very well.

smithby

Ditto, I have a turboed Honda civic with a 2 1/2" dp to a straight 3" exhaust back to the bend in the frame for the stock pipe bends. Then "Y'ed" into the stock muffler. The other end dumps to ....

The car is a little load, but boy does that IHI turbo spool. 214WHP and 179ftlbs. 1.6L

450Thumper

I added a resonator after the cat and it made a big difference. The car still has a deep throaty sound, but I can now hear the engine, which I couldn't hear before.

I do feel the additional power. I should get the K&N filter within the next two weeks. K&N just gave it a part number and they are in production. I am still waiting for ECU programming to become available. I heard next March.

smithby

Do you mean you replaced the second cat with a resonator type muffler ? How much did it cost ?

Mine has 4100 miles and with the radio turned down there is more exhaust sound. I still have the stock system.

450Thumper

I added the resonator after the high flow cat to tone down the noise. It is not a cat. There are three stock cats, two on the downpie and one mid-car. I removed all of them and placed a high-flow cat mid-car and placed a resonator after the mid-cat. Without it, the car is too loud in th cab. On the freeway, I could barely hear my cell phone.

I am going to get rid of the stock up-pipe and go with a cat-less STI or aftermarket up-pipe to see if I can get the turbo to spool quicker.

John