When I was thinking about buying a 2.5'' or 3'' SpeedTech,John told me he dynoed both on his K24/18 car and they made the same top end and bottom HP but in the mid range only,the 3" was slightly better...That was done at EPL....Also he said he ran faster at the track with the 2.5" set-up over the 3''...John D. is a great guy....Just passing on what info I was told.......

 

HI,How do your stock exhaust tips fit??Mine stick out very far and they are bolted on as far in as they can go....

 

They do stick out about an inch further than stock...

 

Agreed, John was very helpful when I purchased mine. I went back and forth on the decision as well. Tony mentioned that he got 30#' more with the 3" FWIW.

 

Nickolas is correct. The 3" system dynoed higher, but in a very small window. Up to approx. 3500 RPM's the 2.5 and 3.0 were identical. At about 3,500 RPM's the 3" began to make more power and it did for about 1,500 RPM's. At about 5,000 RPM's, the 2.5 and 3" again became identical to redline. What we found was that low end and top end were the same with both sizes - 1,500 RPM's in the mid-range are improved with the 3" system.

As Stef suggested, we have also found in testing that a system is only as good as the lowest common denominator. Put 2.5" tips on a 3" system and it dynos like a 2.5 - The OEM tips are even more restrictive than our 2.5 - we have to manufacture a custom pipe, smaller than 2.5 for them, so any system that uses the OEM tips is even more restrictive.

All that said, I can not tell the difference via seat of the pants between our 2.5 and 3" systems. Even thought the 3.0 dynos higher in the mid range, I can not detect/feel any difference in driving the car. They both perform very well and allow you to clearly hear the turbos spooling, indicating they are flowing very well and have very little to no restriction.

 

You are correct John, with the Cargraphic the exit pipe is reduced to 2.125 / 54mm which is odd as the exit pipe on the stock exhaust is 2.375 / 60mm.
I'm sure you'll feel the difference between the 54mm and your current 60mm system.

 

Hey Guys,,,Their is more to it then just outlet tail piping and tip size.....It depends on how modified the engine is,power output and the build characteristics of the rest of the system....When I just had a flash,GIAC,wastegate mods and switched from a stock exhaust, 2.375 tail piping,to the CG with 2.125 tail piping,,,,It was a big increase in power....Yes,I know the 200 cell cat size made a difference,,,,,which is some of my point...I made 506 HP on a rear wheel dyno jet.When I swapped to the k24/18g set-up,I made 530 HP on the same dyno with almost same conditions.When I switched to the Milltek,it made a difference at this level over the CG...BUT...If I put that Milltek on with the flash set-up I do not think it would have helped over the CG...... John D. above tested on his own car which is at AWD 500 plus HP, both the 2.5'' and 3'' tips that also have some piping to them with the same size ODs as the tips...Thank you John for those results....That they made almost exactly the same HP in the whole range ,only in a small window in the midrange was better with the 3'' size over the 2.5'' size...Same exhaust system used which was a 3'' one....If I may Steff,,,when you compare your systems for flow,,,their is not that big of a difference and the difference is due to changing the whole plumbing,not just the tip size....Thanks for those results ....My point is that,,,if you exhaust..lol.. the limitations of one piece that doesnt mean that that is the only piece that is causing it,so it might not be benifical to change that size....There are tuners still running the Milltek with those size outlet tail pipes and tips that are making good HP[was considered big way back when...lol..]....There are many good and better systems out there for a specific range,,,,But to say its all in the short tail pipe outlet OD and tips,,I disagree,,,,Just my thoughts to share......

 

It's not all in the short tailpipe outlet diameter and tip size indeed but the diameter determines the flow characteristics nevertheless.

On our 997 Turbo sport exhaust with 6 inch 100 cell cats and 84mm tubing ( about 3 3/8 inch ) the maximum backpressure goes like this :

Stage 2 with 76mm tailpipe outlet : 0.12 Bar maximum backpressure
Prototype with 70mm outlet : 0.14 Bar
GT2 model with stock 65mm outlet : 0.16 Bar
Stage 1 with stock 60mm outlet : 0.18 Bar

Also don't focus on the diameter but calculate the cross section of the particular diameters.
The cross section of a 60 x 1.2mm pipe is only 61% of the cross section of a 76 x 1.2mm pipe.
So the smaller the outlet pipe the more restrictive.

Hope this info helps.

 

HI,Thanks for info.....Yes ,I agree smaller is less restrictive,as said above,when that is the bootle neck in the system ..BUT thats if nothing else is not the bottle neck before that in the system for the power output..As I tryed to explain above.....As,you have different sizes for different levels...Also,those are the flows and sizes for the 997 turbo stuff,,,,,,which you do not have that much of a spread in the 996 turbo line....Its all about ideas and sharing info......

 

Fixed it for you...LOL