in simple terms, this is normal.
you removed your cats and took away back pressure.
so naturally your very low RPM's will suffer, but your higher RPMs will benefit.

few years ago, i did an experiment; took the whole exhaust off my bike, and ran just headers. low rpm suuuuucked!! but wide open throttle, felt much better response.

 

I really haven't noticed it any louder after warm up. It sounds a lot tin-canny/raspy on a cold start. After like 2 minutes it goes away.I have video where I start on cold then rest is warmed up(old video.. going to replace it soon):


<iframe src="http://www.youtube.com/embed/OpVAyF78z3E" allowfullscreen="" frameborder="0" height="349" width="560"></iframe>

 

Yeah, but I don't think any more than 100-200 RPMs more than without A/C. However, the sound difference is HUGE. It's almost like I'm accelerating while stationary, lol. I kinda like it, though. Makes the Subaru STI exhaust look like a girl. (No pun intended!)

 

Yep, yep. I know exactly what you mean about the cold start-up rasp. If you remember, the stock exhaust did the same thing on cold stats; it sounded nice, though. I always loved morning starts. The FI exhaust NEVER has rasp. Not even on cold starts. It's a sexxy deep growl that my street can hear, lol. And I love it. Especially after about two minutes of cold-start idle, it actually delivers a bass that makes your ears kinda drone like you get with a boom-box.

All in all, after driving more today, I'm not regretting anything about the HFC's, because my car is still more powerful than stock, and that's all it takes for me to be happy! Heh.

 

Thanks for your confirming input, Stricken! That makes more sense.

 

Yep I'm getting the FI exhaust for sure soon! I don't care if I have to wait 3 months for it to be made. ...with a UpRev tune after, it's all good!

 

You won't be disappointed, man! I anticipated it every day, and when it was finally on, it was worth every waiting day and every penny! I'm thinking about doing a very detailed written review on it... I know there's lots already, but one more wouldn't hurt. I'm going to mention everything, lol.

 

I keep hearing but I'm not understanding. How does an increase in backpressure increase torque?!

 

Well, it takes an exhaust person to really be able to explain that. It has to do with the balance of backpressure and velocity of the exhaust; and velocity plays the bigger role. The HFC's do increase and add torque on the higher RPM's, but they decrease some in the lower RPM's due to increased airflow. It's complicated, but it's true: You loose a slight bit of torque in the lower range. There's a thread somewhere here which explains how backpressure/velocity work together.

 

You mean this thread? If my understanding is correct the article states that back pressure is always bad. The reason you lose low end torque with HFC and wider exhausts or dual exhausts (e.g. FI x-pipe) is because the increase in exhaust piping volume reduces exhaust velocity rather than back pressure both of which rob torque. Increasing back pressure never increases torque. Low velocity at low RPMs on the other hand reduces the scavenging effect which is why having a large exhaust pipe is not always the be all end all of exhaust tuning.

 

Yep, exactly that! Interesting thing, though: I spoke to Fast Intentions yesterday, and the guys there said that their HFC's are proven to add power all across the RPM band and not reduce it anywhere, so they said that I'm not loosing any low-end torque after all.

 

^ Hmm... not sure if I believe that or not. Seems the census is losing bottom end

 

Well, I'm not too sure exactly, but all I know is that my car is faster in all RPMs will the HFC's and exhaust. Although I don't have any dynos to accurately confirm anything. But the feel is good enough, I guess.