Hooter

I am wanting to lower my v-tec engagement point. Everywhere in these forums, it says a TP is needed to handle lower v-tec. If the cat is already handling the volume at 8000, why is a TP needed to handle the volume at 4000 ?? I must be missing something here. It makes me no difference either way. I just want to understand.

Aurex

With a cat in place it will create or increase exhaust back pressure. I assume that due to the increased back pressure in the exhaust from the cat, there is insufficient pressure at lower rpm to over come the back pressure. At higher rpm there is sufficient pressure coming from the motor to overcome this.

A test pipe removes the restriction which decreases the amount of exhaust back pressure. Which also means that the vtec point can be lowered.

I wouldn't question the need for a test pipe to lower vtec below 55/5600 rpm. It has been dyno proven many times that a test pipe is needed to lower vtec beyond 5500 rpm.

Gernby

The issue isn't related to flow. It's about harmonics. Everything about the S2000 intake and exhaust was designed for peak performance from 6K to redline. There were a lot of sacrifices to achieve that.

It's complicated to explain, but the OEM cat causes a strong reflection of the exhaust pulses, which travel back up the exhaust stream to the engine. The speed of the reflected waves (plugging pulses) and the distance they will travel back to the engine dictates what RPM will suffer the greatest performance hit. In the case of the S2000, the performance hit is at about 5K. When VTEC is engaged at 5K, the plugging pulses arrive back to the motor at exactly the time that the intake and exhaust valves are both open, so exhaust gases are actually pushed back through the cylinder into the intake manifold.

Below is a dyno plot showing the same car with the same tune, but 1 is with the stock exhaust and the other is with one of my exhaust systems (patent pending). The VTEC engagement was at 3600 RPMs. The dip in torque is enormous with the stock cat.

wadzii

I had a car on the dyno a few weeks ago that had been tuned with a test pipe, vtec around 4000. Customer wanted to put the cat on and test in hopes of reducing noise on the highway, results of that graph looked similar to what gernby posted. Huge losses from 4000 to 6000, very little change past 7000 rpms.

Adding a test pipe alone wont gain much, but adding a test pipe and tuning the car to lower vtec makes a huge difference in the car.

Hooter

Does a test pipe add much noise ? I like the sound of my CAI but don't want to sound like a frickin bumblebee.

Gernby

That will depend on the exhaust that you have. The stock exhaust would be slightly louder, but it would still sound stock with no rasp or drone. Any other exhaust, and it will be signficantly louder with a test pipe. Just don't consider getting a "resonated test pipe", since they perform almost as badly as a stock cat. Basically, it's not just the catalysyt that causes the performance hit. It's also due to the size and location of the chamber that houses the catalyst, so putting a resonator there is stupid.

Hooter

Everyone says a TP smells bad due to incomplete burning of gases. I wonder why exhaust didn't smell that bad before converters ?

noodels

Good info, no point in just TP then without a tune !!

dantediss

Question .. what about a high flow cat ?? Just curious for emissions. I myself have a test pipe but i havent installed due to debating between the test pipe or a high flow cat

Car Analogy

Um, they did.