I just had the tubi cat back + fabspeed headers installed. Here in the UK both pieces are 'cat free' and the system depends on the OEM central cat section which is retained. The bottom line is that I got some very dubious results at a dyno on Saturday and will have a re-test this week at a more sophisticated Porsche center. I had a consultation session with them today and they advised that sometimes the Porsche engines prefer some back pressure (I assume it occurs when cats are installed especially on the headers).

I will let you know what the results throw up later this week especially now that I have the Revo-technik stage 2 re-map installed.

Anyone with experience of dyno results on the above configuration?

Terry

I think Ken had installed a Tubi on his 996 and actually recorded a power loss. I did a little searching and couldn't find any dyno info on your combination.

Terry,

The following is from the Dinan BMW web site, and though the technical paper is written for the BMW E46 M3, Steve Dinan's comments seem generic to me:

"I’m fairly certain that many of you have been exposed to a “bench racing legend” that would have you believe that increased backpressure will improve low rpm power and that low backpressure will increase high rpm output. Nothing could be further from the truth. An exhaust system is sized for maximum flow at wide-open-throttle and peak rpm. All exhaust systems are “oversized” for lower engine speeds (rpm), as backpressure is so insignificant that it can’t even be measured. Less back-pressure always results in more power at higher rpm, with no negative effect on lower engine speed performance. The amount of power that can be extracted from an
engine at a given rpm as a result of exhaust design is really limited by the exhaust manifold or header. After the header, less backpressure is always better."

No offense to Mr. Dinan, but he is way oversimplifying the matter. Too big a post header exhaust can indeed hurt low end power, as the energy in the exhaust stream is lost to thermal cooling due to an overly large "column of air" found in too large a tube. We have documented this time and again with our chassis dyno.

Also, cylinder scavenging effects can be lost with a mismatched header and cat-back system on a normally aspirated engine. A header can be tuned with a certain backpressure found in the "cat back" or "header back" portion of the exhaust. Going too big or small with that secondary exhaust portion can disrupt the tuned-in scavenging abilities of the overall system, resulting in a loss of power.

__________________
Regards,
Todd Sager
President
A.W.E. Tuning
http://www.awe-tuning.com
1-888-565-2257

Todd;

With all due respect, I think that you and Mr. Dinan are in agreement.

He stated that the functionally important part was the design of the manifold and header system, beyond that, the less backpressure the better. You are both saying that the correct design for the header is paramount to increased output.

Stan

I recently spoke to the guy at Milltek here in the UK who stated that after significant research it was proven that the optimum system for a cayman is: headers with cats (because the cayman engine likes back pressure) + cat-back with no cats. I guess when it comes to cars there is no such thing as one size fits all. He also stated that the header is the most important element to get right and is the place to start with any upgrades for the CS...

Nope.

According to that quote, he is saying that the bigger the better on a catback: "After the header, less backpressure is always better."

I am saying that in a lot of cases, the entire header and catback system is designed to work together to optimize cylinder scavenging abilities.

The flow through the header is affected by what is going on downstream. Go larger or smaller downstream and the flow dynamics in the header will change, too, sometimes for the worse, producing less power.

Therefore, you *can* go too big on the catback portion and disrupt the exhaust system's overall scavenging effects, as the header was designed with a certain cat back flow in mind.

If he had stated that "Throughout the entire header and catback systems, less backpressure is always better", I would have agreed with him.

However, you can't just max out the size of the catback portion after the header in pursuit of less backpressure there and get an automatic reward of more power. Which is what he is suggesting.

__________________
Regards,
Todd Sager
President
A.W.E. Tuning
http://www.awe-tuning.com
1-888-565-2257

Todd;

I didn't read it that way, but of course your explanation is right. I think he is over simplifying and perhaps leading himself to mis-interpretation.

Many times it is difficult to explain the compicated nature of these things without stating the technical details. You have been very astute in detailing the technical aspects affecting exhaust flow.

Stan