Do I have enough backpressure?
 

Hey, I got a s5 N/A, so I am not concerned about 6 ports cuz they are fine. But my exhaust from block on goes like this-
Headers (2 1/4 each pipe from the block to 2.5 collected)
Straight pipe back (no cat) 2.5 inch
to the y-pipe which goes down to 2 inch on each side
So I have no cats and a header and the rest.
My car actually seemed faster without the headers, when I put them on I seemed to lose some torque. My friend said maybe I dont have enough back pressure. Do I have enough or could it actually be faster if I put in a high flow cat?
So basically I have headers, straight pipes back to the mufflers, and my car seems less torquey.

added info
 

your exhaust system is spot on for n/a as far as diameters etc. (must be LOUD). Your lack of torque may be do to 6 port and/or VDI systems not working properly. What did you do for these systems? Leaving them wired open you WILL notice less torque in the lower to mid RPM were you'd spend most of your time driving.

BTW, theres no such thing as "not enough back pressure". Its having too large of a pipe/tubing that kills VELOCITY that will make an n/a motor loose power, not back pressure. And anyone who argues this is a fool.

~Mike................

i thought the S4 engines are the only ones that ran on back pressure

i thought the S5 ran off the air valve or somethign like that :confused:

Maybe your friend is correct. IIRC there is relationship between backpressure and torque. But I forgot since I studied those theories long time ago.... But in my case it was opposite. When I put header and presilence on my 88 NA I felt more torque on mid to top.

I know with most 4 bangers that are n/a you will notice a drop in low end tq when you free flow the intake/exhaust system. But you get top end hp and the tq usually is the same it just takes longer. There is a realationship between to two that the more back pressure you have the faster the tq and hp come on but they flat line very quickly as where no exhaust pressure or low back pressure equal low tq on the lower rpm band but higher hp on the top end. Now for turbo you just want no backpressure or as little as possible. Hints why most headers/exhaust systems you see for non turbo cars are 2.25 or 2.5 inch in diamer vs 3 inch systems you see for forced induction/big block apps. If you dynoed the two you could see what i was talking about. I dont know however if this is the same for n/a rotaries.

Yea, s5 6ports operate on the air pump. And yes, my exhaust is loud as hell, it kills my ears and my head. Sounds great, deep as hell, but very very loud.

I just got my exhaust done today I did a dual exhaust no Y it comes seperate off the header straight back to mufflers ,man is it loud .My 5th and 6th works off air pump with home made valve to open at 3800 ,and I feel like I gained power at the top and bottom .

Hmm, well I do have a SMALL exhaust leak that I am gonna weld up today, but I doubt it will make much of a difference, its a little tiny hole at the y-pipe.

mm... when doing anything that can increase/decrease power in your car, it changes the powerband.

The way i always look at the butt dyno is that you are far more sensitive to drastic *changes* in the power curve rather than overall increases. So, if u block up something out back and change your powerband, it might feel more "torquey", when in reality its actually less torquey someplace else and then drastically changes to where it should be.

Anyone follow? Am i totally off base?

My bottom line is the butt dyno lies.

jack you are right on, but a properly calibrated butt dyno works really well ;)

Well, is it just me or do headers always sound like they are leaking when they are on a 7? I heard that the strong exhaust pulses just make a lot of noise in the header, so it sounds like it might be leaking but its not. Anyone else hear of this?

thinner exhaust tubbing makes more tingly noise, when i make headers i use the thicker gauge steel

NOT BACK PRESSURE!!!!
scvanging and velocity