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Review - 991 TT Akrapovic full system exhaust

 
Old 05-14-2016, 09:22 PM
  #31  
speed21
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Originally Posted by goin2drt View Post
I have also heard EP is nice sounding. Last comment. So you mention R&D. When I get my 100cel cats and IF I get zero CEL then what is the beef with Akra and the comments of bad R&D. I have not heard that their 100cel cats are any worse (or better for that matter) than any others.

There is no exhaust that can run catless without a tune or you will get a CEL.
That is what I was doing with the Akra so that doesn't make them deficient in R&D. That makes me a knucklehead for running catless without a tune.

I will be running the 100 cel next, JUST LIKE YOU, and provided again I do not get a CEL then it seems to me that there are no R&D issues. Then it would just be personal preference on looks and sound, no?
1. I mean insufficient R and D because Europipe have proven it is achievable to produce a non CEL 100 cat and openly state on their website their 100 cats are CEL free. So surely Akra can do it? Fwiw I have no beef with Akra at all, more-so amazement how they of all companies with such an amazing facility have yet to offer a system for this car that is genuinely cel free. There is no question about Akra's manufacturing skills and their quality as being superb but what happened with their 100 cats? Surely they have the in- house expertise to get it right? The fact Akra openly state on their website their 100 cat requires a tune is not comforting. Reliability is key for me, and perhaps others here. I have zero interest in running an aftermarket tune in this car as it goes so hard as it is. I'm not into drag times, just drive mostly in sporty environments and want to enjoy the car without cel issues. I also respect Akra for being honest and up front, unlike many others (I won't name names) that make claims of zero drone and no cel yet certain users report otherwise. So Kudos there - It's very important for me to be dealing with an honest company.

2. I understand there is no cat-less system that can run with a stock tune without throwing cels but I want 100 high flow cats in the system and they NEED to be reliable - not MAYBE reliable. I had a flawless 4 year run on my past 997tt with a Europipe and the S has so far proven the same, so until Akra can openly state cat reliability then I must wait in amazement. That said user reviews such as yours are always appreciated and provide feedback to consider.

3. I wish you luck and truly hope you can report no cels into the future but remember they have warned you. The personal looks and sounds I'm sure are there and Kudos to Akra for that as well......but without reliability then it falls short for me. Hope they can get it right one day as I'd love to have a second option to choose from.
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Old 06-12-2016, 12:27 AM
  #32  
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Recently I have been enquiring about an exhaust for my F car and learnt quite a bit from the conversations I've had about exhaust design that might help shed some light speed21.

The "cleaning ability" of the cats is dependent on surface area, the amount of precious metal catalyst deposited on the matrix and the time the gasses are in "contact" with the precious metals. In order for Europipe to achieve the results they have, Stef uses an oversized Emitec matrix that he then has custom coated with a higher level of precious metals than HJS (this is my best understanding). So the Europipe cats are huge and efficient. Additionally Stef uses perforated matrices which allow gases to flow sideways and spend longer inside the catalyst.

In terms of exhaust fluid dynamics, it appears that performance is related to the velocity of exhaust gases throughout the system. As the gasses cool the further they go trough the system, they actually slow down and cause a back pressure effect. This is why some exhausts get narrower the further from the engine you go. The most notable point where this happens is the catalytic converter. Exhaust gasses come from the headers at high temperature and confined to the tightness of the header diameter. They then hit the large catalytic converter and suddenly expand and slow down. This facilitates the "cleaning effect" of the catalytic converter itself but now poses a bit of a "wall" of gas ad far as the exiting header gasses "see".

The technical director of Kline explained that the perfect catalytic converter FOR PERFORMANCE would be the same width as the other pipework, in order to maintain the same velocity of gasses.

So it seems like there is a tradeoff between "emissions performance" and "power performance" when it comes to cats. This is why I am presuming that many race exhaust systems seem to use very narrow looking catalytic converters.

For the OP, when you were catless, were you able to shoot blue flame from the exhaust? I ask because that appears to be a giveaway sign, and catless is a no no where I am.


drvik

Disclaimer: I am not an engineer or exhaust expert, this is from my own understanding based on private communications. I too am disappointed that bigger is not necessarily better!
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Old 06-12-2016, 01:19 AM
  #33  
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Thanks Drvik.
Back pressure and airspeed (inside the exhaust pipes exiting the engine post cat) whilst interrelated in an exhaust system are still two separate things altogether and, both CAN be measured to prove they will be of any benefit.
In terms of performance on a turbo most would/should know by now a reduction in back-pressure is the most important ingredient required to achieve a legitimate gain on the exhaust end. The airspeed going into the cat is restricted to the cubic capacity of the engine and the exhaust gasses exiting such. You can speed up the airflow downstream the cat by using smaller tubes but that isn't going to suck the exhaust gas out of the engine any faster than it's coming out of the exhaust ports upstream, nor will create any additional HP. It may make for some fancy marketing talk though, as is the case with the X design on a turbocharged engine, but that's all just clever semantics to fool the masses who may not know any better.

The back-pressure which Stef has measured post his cats show a significant improvement over the stock cats. The testing has been done....hard argue the science there. He also states those figures openly on his website unlike all others who don't have any info on back-pressure post cat whatsoever (funny about that eh?).
I think the ultimate tell all would be for Kline to put up his own back-pressure results post his cats so he can prove whatever point he is trying to make about performance being better with a smaller diameter cat. Frankly I'm not buying his story there at all and whilst it stands to reason the airspeed will be higher in a smaller diameter cat (elementary), as it would be in a smaller diameter tube, that still doesn't negate the back-pressure issue which in essence is the key to gaining performance on a turbocharged engine where the use of aftermarket exhausts are concerned.
Using Kline's large into small pipe theory, the bottleneck would be the juncture point where large meets small and from there the airspeed understandably will accelerate and back-pressure increase accordingly in line with the restriction of the smaller tube the air is being forced into - bit like how a pressure washer nozzle works. A church organ creates sounds upon much the same principle where the smaller tubes (or flutes) of the organ produce a higher pitched sound the smaller the diameter of the tube is, and the larger diameter the lower or bassier the note produced becomes.
Of course the air speed within the larger tubes will be respectively lower than the airspeed within the smaller tubes where pressurized air is being forced into it and the airspeed will increase accordingly the smaller the tubes are. Again. Elementary!
I have seen fancy exhaust systems on high performance NA engines where the pipe diameters along the system vary along the exit route of the system. Again we all should know back-pressure is a necessary ingredient in an NA engine. Playing around with pipe diameters there is one thing but essentially adequate back-pressure levels must remain to ensure the scavenging effect is maintained to optimum.
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Old 07-08-2016, 09:59 AM
  #34  
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