J_Krypt

I just recently got a custom cat back system done onn my 99 L. I know that replacing the stock muffler with a high flow muffler deletes a lot of my back pressure and that this is the reason I have lost a lot of my bottom end power. Are there any mods that i can do besides turbo, to get this power back? Please help, thank you!!

VRT MBasile

iTrader: (17)

what size is the piping on the cbe? Typically there's a balance between pipe size and low end torque

J_Krypt

I got 2 1/4" size piping

SubySal

It's not backpressure that's the issue. Backpressure is always a bad thing. What your talking about is a scavenging effect. Properly setup exhausts will have some slight vacuum when the exhaust valve opens on a given cylinder. This vacuum aids in sucking out spent gases from the combustion chamber allowing more of the fresh A/F mixture to enter through the intake there by producing more power.

Exhausts are also a balance between velocity and volume. If the pipe is too big the velocity falls off and performance suffers and if it's too small it can't flow the volume and performance suffers.

Right now it's sounds like you have an unbalanced system. This happened to me as well. Since I was having the cat-back made I replaced the header and cat-pipe first with Borla and Random Tech respectively leaving the cat-back OE for the time being. Needless to say I didn't expect the initial results. Power fell off dramatically. I was worried because I didn't even think the cat-back would bring the power back. I was wrong. Once the cat-back was installed it was great. The car really transformed. The catback is a 2-1/4 system with two Magnaflow mufflers. One of which acts as a resonator. Now between the exhaust and CAI which really just consists of a pipe going from the OE airbox to the fender it pulls like a truck all the way to redline.

What I think may be happening with your system is that the small OE header and restrictive OE cat-pipe are flowing at a very different rate then your new free flowing cat-back causing the exhaust gas flow to stall resulting in a power loss. Usually if this happens then the scavenging effect is greatly reduced. Of course another contributer could be a partially clogged cat. The thing I would try and do now is increase the flow of the remaining parts of the exhaust system. If nothing else is wrong the power should come back and then some.

Good Luck!

J_Krypt

Thx a lot i was thinking about doing the same thing, Borla header nd Random Tech High Flow Cat.

J_Krypt

Any other suggestions too?

J_Krypt

Will either of these help? http://store.summitracing.com/partde...5&autoview=sku

http://www.yourhotcar.com/prod/Surge...p_Combo/544396

SubySal

I couldn't tell you anything about those boxes. I stay away from that stuff. I just don't trust them.

Sometime down the line I'd like to get some Delta cams at which point ECU tuning will have to be addressed to get all I can out of the car. I had planned on a I-speed reflash or something similar. Hopefully by the time I need it they'll have some reflashes worked out for the mods I have in my model year.

loccomoffo

Those black boxes look realy bad , never trust anything that promises huge gains and doesn't explain the princliples behind it. They look like all they do is trick the engine management into dumping more fuel through the injectors, either through manipulating o2 or MAF sensor output.

As for scavenging, this is achieved using tuned headers, ie 4 into 2 into 1 pipe, 4-2-1 or tri-Y. The lengths of the Ys are selected to match the specific engine. As the exhaust gas (pressure pulse) travels past the Y it creates a vacuum on the next exhaust pipe to exhaust ie pulling more mixture into that cylinder and making the engine lose less power as it pushes the exhaust gasses out. The tri-Ys increase power down low where you use it most on the street.

I never really understood why you need back pressure in the exhaust?? The only reason I can think of is that if it's too low and your valve overlap (when both intake and exhaust are open at the same time) is too large then you'll lose mixture into the exhaust thus reducing power.

Anyone has an explenation for why back pressure is a good thing??

J_Krypt

My dad said bigger fuel injectors and a better fuel pump might help with this. Any comments?

loccomoffo

JKrypt, I had a read of a few online articles and it seems that back pressure is never a good thing, what you're trying to achieve is good gas velocity. Once the exhaust pipe diameter gets too large for a particular engine capacity/revs, you lose velocity and the gases 'stall' the flow. Here are a few links if you want to do further reading:

Destroying a myth
http://tinyurl.com/2adddn

Back pressure, Exhaust velocity and scavenging
http://tinyurl.com/2nlh9f

Anyway so it looks like you just need to get more exhaust volume flowing to regain your low end torque I'm not sure how far the injectors can go, but they should cope with more intake air volume. Mybe a more free flowing intake will sort you out? The ECU will take care of the rest

Also there's a wealth of info in this great ozzi online mag
http://autospeed.com/cms/A_2711/article.html

J_Krypt

Thanks for researching this for me!! Thanks for helping me.

Chocoholic005

Here's a good read on custom exhausts:

http://www.eclipseforums.org/ftopicp...9.html#3110619

loccomoffo

Chocoholic005, nice write up i've PDFed it for everyone here as that forum is viewable via regoistration only.