Just for good measure I've just bought a cat bypass pipe to do a dyno run at the same time as stock system and H&S system to post the results.
Just to clarify, my intention is to get a good free flowing Stainless Steel system (NOT INCLUDING HEADERS) that will.
(A) Posisbly free up a few horses.
(B) Reduce weight
(C) Sound good and is track legal.
(D) Last the remaining lifetime of the car
Also hopefully this should also compliment any further planned mods.

 

Please don't forget to let me know when you are testing so I can come down and watch/help!

 

I actually wrote and posted a response last night which doesn't seem to have made it onto here - and just as well IIRC. I think Simon, you accused me of being more than stubborn.

Aaaanyway, this gist of it was this. If we look at the 'proof' re decats on this page, we have one owner who gained 14 RWHP - but was probably around 20RWHP over what a stock 964 would usually be anyway. Why? Tony Taylors MODIFIED car made 13 FWHP (Not RWHP). I look forward to PWW's results - because as I said in 5 years....

When I look at all the workings above, I am struck by the naivety that people think that they can put a figure that will always be the same on the 'gain' one would get with a decat...whatever the application....so whatever I have done on my car I will get 14HP......Why do you think that my GHL is no longer on the car if it adds 20HP regardless of application????????????????

PWW, I hope that whatever gains you get on your standard car witht he H&S system is reflected also in the mods that you do in the future. My experience has been that this really doesn't necessarily happen.

Finally Simon I would just like to point out that I have spent a lot of personal time, money and effort on here trying out mods and seeing if they work or not - and posting the results as I found them. I don't do this because I expect something back, or feel that Rennlist owe me anything as a result - but I don't think that it is acceptable that you should be suggesting things that have no base in fact. On decats or on my 'motives' for not believing that 14rwhp is a bit much to expect......PWW can show the dyno gain for his decat and then we can discuss further.....

 

Christer

If you have taken what I have said in the wrong way, I am sorry. I stated at the beginning of one of my posts that I didn't want to upset you and if I have, I unreservedly appologise. It was not my intention.

However (why there is always a however ), I still cannot understand your viewpoint about the cat situation. Are you saying that on a standard car, if all you do is a decat, you will see no difference? Or are you saying that in every case that there is a proven gain due to decatting, the gain is actually due to something else? Are you saying that Tom and Tony have misunderstood their gain and that while the only change that they made to their cars was to decat it, the gain was due to something else?

Here is my view and I would like to say that it is only that, a view based on what I have seen and read. I fully accept that I might be misinterpreting the data, which is the whole purpose of such discussions.

First things first. As a generalisation, I will say that decatting gives you a power gain and it seems that in most cases, you can expect about 14bhp. I have based this on the following on comments on this thread, 2 other rolling road days and verbal or written claims from a fair number of people, some of whom have agendas (such as tuners) and some of whom are only after bragging rights!

HOWEVER, I think that I have said time and again that any mod to the exhaust side will only give a benefit if the limitations of the current set up hasn't been reached.

The using standard injectors and Motronic DME. I hope we can all agree that due to fueling limitations, there is a maximum power you can get from that set up. Now, if you achieve that without a decat, then decatting your car will produce ZERO gain. The same applies at almost every level of tune. Take a standard engine, add bigger injectors and MOTEC (or similar) and if you can get to the point where the heads/cams/valaves won't flow any more, then a decat will add ZERO power.

I have to admit that I have never seen a dyno run (before and after) from a standard car where all that is changed is the cat. My views on this are based on the following.

1. Cats are one of the biggest restrictors of flow in the exhaust system. I believe that this is well proven and documented and in non standard cars, it is proven that decatting makes a difference.

2. If you remove some of the restriction in the exhaust, the whole set up is capable of flowing more air.

3. If more air is flowed, the DME adjusts the fueling and therefore more power is produced.

Therefore based on very simplistic principals, decatting a standard car should produce a noticable gain.

Please can somebody tell me where the flaw to that agruement is?

 

Simon, please read my posts carefully. I am not saying you gain nothing, I think the closest I have come to saying that is 'pretty much jack'.....I do believe that there is *some* gain to be had but whether it is 14HP (RWHP or FWHP or do you not care which?) remains to be seen. I am waiting for PWW to post the decat only numbers so we can discuss when we know what we are talking about.

I would also add at this point that dyno graphs are important because one can see what happens throughout the power curves, what is the gain in torque at 4000rpm for example? HP is not everything as we all know.

 

So this is looking good for the guys who can get rid of their cats, what about those of us stuck with a cat?
Mine has a G pipe, but needs teh cat for the MOT and I don't really want to have to keep putting things on and off, bit tiresome...and and heat, do you guys all require heat?

Kevin.

trying hard not to upset the applecart and spend more money on "that car"......

 

I have to admit (yet again) to not having the direct 964 experience but I have seen high flow cats advertised. I cannot comment on their performance.

However, in most cases I have come across (in other makes of car), sports/high flow/race cats usually give the same performance gains (or otherwise ) as a decat does. This would lead to the obvious question of why do a decat rather than a sports cat and the simple answer is usually about £400!!!

Now this really is an area I would want to see some real proof. While it is, IMO, a no brainer that a straight through pipe must flow better than a cat (!), how good a performance cat is does require evidence. The way they work is to have a reduced number of cells and a better internal layout but there is no way of verifying how good one is except on a rolling road. I have also seen very dubious claims from manufacturers of such cats. However, I have seen really good ones as well.

 

Since the power of the 964 was predetermined by Porsche and the entire system configured to produce that power level it seems unlikely that changing just one part of that system will produce significant gains. Hence just changing the cat isn't likely to produce a great deal of extra power.

Having said that the cat is the most restrictive part of the exhaust and getting rid of it is likely to produce some gain. You also lose about 8kg which takes 1 or 2hp to carry about. The chances of a 15yr old cat working properly are slim.
All the evidence suggest a gain will be made, if you make other mods the gain can be relatively large and you lose weight. Since a cat bypass costs next to nothing to do it's a no brainer really. It's the cheapest hp you can get.

If you're going to make changes to the car it would seem perverse to just do a cat bypass but when done in conjunction with other intake and exhaust improvements it's essential to acheive the full gains available.

 

Crikey, talk about lighting the touch paper, I think I best take the liberty of settling this "cat" issue once and for all.

After testing a whole heap of exhaust and intake combinations on Thursday (to be published later) I came in on Friday morning with the same question in mind, so since the car was still on the dyno whilst we arranged publicity photos I could not resist refitting the cat to run another couple of dyno sessions for the camera, one with the 9m intake and one with a stock 964 afm and intake. For everyone's benefit the results are on the attached graph, representing flywheel hp corrected to DIN standards, measured with the same control of variables (engine temp, oil temp, air temp, tyres pressures, etc.) and optimisation of the Motec system for best power & torque.

What you have on the graph are the four best runs with each configuration showing that the cat lost 5 hp with the standard airbox and air filter or 14hp with the 9m intake. This appears to reinforce my view that engine tuning has to be undertaken with a complete understanding of the requirements of the whole engine system and not (as seems to be the concensus) as a sum of individual gains. What you can see is that the engine responds best to overall airflow improvements at both ends of the system at the same time, although if given a choice of one or the other intake modifications should take priority.

 

Colin

Nothing really suprising there! The real thing that it proves is, as you say, tuning is an overall approach. Unfortunately, while your figures are interesting, they do not answer the question about how much power inrease you get on a standard car by just decatting. But, having said that, it seems to me to be achademic as I don't think anybody really only does the cat!

The real question that needs answering is the one about headers. I accept your figues but I don't undertand them. The 964 header system would seem to me to have some very fundemental flaws and everything suggests that good designed headers will give far more power than the standard ones.

I can only assume that the issue is that you are coming up against other limiting factors, such as head design or cams (or both).

 

311.1 to 316.1bhp I make that a 5hp gain for decatting the system.......

Kevin.

 

Thanks for all the work Colin - although that is not a standard car either....I am still waiting for PWW's dyno chart.

BTW Colin, my car seems to be somewhat sick - I will PM you and start a new thread on that one....

 

OK, although the car has got Motec fitted the fact that it was tested with the standard AFM, airbox and air filter gives a pretty accurate indication to the maximum potential gain that removing the cat would have on a standard car with a standard ecu, the motec has no advantage in this condition other than it making it possible for me to quickly optimise the fuel and ignition for the full throttle run. At this power level injector size would not factor in the measired gain, therefore we can conclude that the power difference from a cat bypass is in the order of 5bhp with a standard car given that the engine is optimised with and without the cat. In practise many folk may simply dyno their 964, fit a bypass and then re-dyno and find 15hp - but does this mean that the cat made 15hp? IMO the answer is no, all it means is that the standard mapping of the engine is more suited to the non-cat condition, thus I believe that my tests are a more definitive answer than you would see from testing the same car on Motronic unless you went to the trouble of remapping it for every test.

On the standard engine both sets of headers make more torque from 3000 to 5000rpm and more power from 6000 to 7000rpm, but in the run between peak torque at 5000 and peak power around 6000 the stock system is runs them very close so the gains seen are marginal.
On a modified engine with long duration cams and individual throttle bodies the stock system shows up its weaknesses, especially in the 3000-5000rpm range where on our first 9m 4.0 litre for example we found a peak gain of 100Nm and an average gain of 50Nm. I assume that this is because the headers are scagenging the cylinder better and thus increasing the low speed VE but we have a lot more work to do in this area before I can quote definitive answers which should come soon once we finish the development of our "ultimate" 9m 4.0 litre engine that just happens to be destined for a little black 964 speedster....

 

Colin

So does a cat bypass need an O2 sensor and will it f**k up the engine if there isn't one?
Rob

 

I think we are now getting a little bit into symantics! While I can accept and very much support your assertion that it all depends on the state of tune, if you take a standard car and do something to it that makes 15bhp difference, then it has to be that part that made the difference, on that occasion.

Maybe, it would be a fairer comment to say that a decat can produce between 5 and 15 bhp more power, depending on the current state of tune of your car.

I hope that makes everybody happy