I am not sure I follow you on this. We know that decat and cup pipe produces reasonable gains, so long as a few other bits are done as well. For instance, my car has big throttle body, hot film, drilled airbox and K&N filter plus a AMD tuned chip. The car produces cira 300bhp depending on whose RR it is tested on. Withoit the decat and cup pipe, I would expect to lose at least 10-15bhp.

Furthermore, I am told by almost all tuners that I will get no more power if I do the headers as I have reached the limit for fueling, using standard injectors.

The problem is that my set up gives me a 105 db static noise test and means I cannot do very many trcakdays. What is hoped is that the H&S system will reduce noise and not reduce power.

Do you have a different view to the above?

 

Hi Simon

I wasn't aware that Paul was going to do or had done the rechip, big throttle body, hot film conversion and filter change - I must have not read the thread properly.

Of course, if he was just putting on the back bit of the exhaust with no other work then I would say the gains would probably be minimal.

I totally disagree with your comment that 'We know that decat and cup pipe produces reasonable gains' unless as you say 'some other bits are done as well'.....and if they are as described by you above....then I would not describe them as just 'a few other bits'....

In your case, if you are at 300hp then I agree the fuelling may soon become a problem, but as I say I wasn't aware that this applied to Paul's car.

I await before and after dyno results eagerly - especially with headers if possible.

 

If I can get 10-15bhp I'll be more than happy. Christer how can all exhaust manufactuers claim this sort of increase without headers if it wasn't true. We all know the cat and primary silencer are restrictive so i would expect to see somewhere around these figures.
I do however note and agree on the header issue as I picked up another 23bhp on my Ram SRT-10 by fitting equal lenght headers although this did also include cat bypass with the headers.Also although its not a race car I'm building then surely any weight saving is going to be a bonus.

I agree I dont know if its going to be louder,as loud or even quieter than proven systems but I'm willing to find out and if there is an issue being that its a UK company I'm hoping they can come up with a solution..
Your comments are appricated though as are anyone elses who want to chime in.

 

Are you serious? Do you really believe all of the chip manufacturers etc. who say "you will get xx bhp with our chip"?

I think manufacturers are.... often optimistic with their claims about power gains.

 

Just to clarify at present my car is Stock!
That means no Chip no hot film no throttle body.....these mods will follow so I can get to that 300bhp figure.
The exhaust is just the start and I plan to have RR sessions done as and when mods are fitted.
This way gives me and forum members a true indication of BHP to cost ratio. Perhaps this has already been covered in the past after all I am a newbie here!

 

Drop in Chip gains I never believe in. But if you helping to de-restrict and help the engine breath better then surely there has to be some gain. Re-mapping when other mods are made is another tunning mod altogether, hence 9M and the motec, am I correct in that?

 

I think it's more complicated than that - although I confess I don't understand the details and I won't pretend I do. Just "de-restricting" isn't as simple as it sounds - having a straight through pipe will not make your car faster (I don't think). For years, Yamaha motorcycles have had valves in the exhausts which open / close at certain revs, effectively varying the length of the pipe. If straight through was always better, this wouldn't be necessary.

I agree about remapping etc. though - it's a very complicated business which I think is why people like Colin are so popular. Otherwise, we could all drop in a new chip, bolt on an exhaust and Colin would have to sell his race car.

I would certainly be interested in a step by step upgrade with before and after dyno plots at each step. I am not convinced that has been done before - lots of people get dyno plots but there was once a long (and fairly heated) thread about whether it was necessary to get dyno plots before a mod when you knew your car should make 250bhp anyway. I was on the "of course you need before plots" side of the fence....

 

I've been convinced by a man whose dyno tested various exhaust and header setups on an 964 RS with a stock engine ( but intake exhaust chip mods) that once you've gone and fitted a catbypass and cup pipe there are no gains to be made using different headers.
Once you start internal engine mods it's a differnt matter though.

Mine has a similar set up to Simons and dyno'd at 285 with the stock exhaust and 298 with a catbypass and my straight through type exhaust silencers.

 

As already stated, it would be great to see dyno plots of a before and after with the H&S system without headers.

 

I asked myself the same question recently following a test seaaion of the new 9m 4.0 litre engine fitted in a customers 964RS (billet heads, stroker crank, 103 pistons, throttle bodies, Motec, etc - don't worry, I'll come back to the 4.0 litre engine on another thread after I have done more testing). What I can confirm however is that on this engine the stock 964 heat exchangers with the usual cat bypass performed poorly in comparison with the similar 3.8 litre unit in my 993RS which runs on stock 993 headers and N-GT style exhaust. Always ready to look for answers, we removed the rear bumper and entire exhaust systetm and fitted a set of equal length headers that I used to run on my 3.2 Carrera. The gain was more than significant - like 50Nm across the board and around 90Nm at 4500rpm. So, if the exhaust can make sucha a significant difference on the 4.0 litre, imagine what it might do on a standard 964 - could we possibly see the same results? Could this be the basis of a relatively simple 964 Motec +2 upgrade?

Fortunately a couple of customers have helped out in finding the answer, one who is having Motec fitted to his stock 964RS and another who has supplied an alternative RSR style equal length header system for his 4.0 litre engine, the owner of the RS being more than happy for me to try different exhaust options on his car (at my own expense) just in case I found some easy power gains. Obviously the Motec system allows us to optimise the engine for each full power run, so fuel/ignition mapping does not factor in the measured differences. Whilst having the car on the dyno I also took the opportunity to try different intake set ups: the stock AFM/airbox/filter, a custom fabricated AFM bypass tube for the stock airbox and finally the 9m "long tube" Motec intake with K&N style filter.

I am still in the middle of the tests so I do not want to show the dyno results or quote specific numbers just yet, however the results are suggesting that provided the engine is optimised for each configuration, whilst there is significant power to be gained from removing the cat on the stock system there is a lot less to be gained beyond that point with a stock engine. On the intake side a significant gain comes from replacing the stock assembly (AFM, airbox & paper filter) with the 9m intake (Motec only, remember) and this whole gain can be broken down to the individual increases when retesting the engine whilst replacing one part at a time. No surprise though that the largest proportion of the intake system performance gain is in substituting the AFM.

My conclusion is that the relationship between an exhaust system (read headers) and engine is a complicated interaction that must be engineered as a whole, so the rather open question of the thread title should probably be redefined as "what is the best exhaust system for a standard engine?". IMHO the simple answer is that there is little to be gained in doing anything other than removing the cat (if allowed, local laws, etc) and thus direct any extra funds at a good free flowing air filter & optimising the management system.

For a modified engine (like Christer's or the 9m 4.0 litre) there is a completely different answer.......

 

FWIW, a friend's 964 USA Cup Car dynoed at 275rwhp with the "standard" cup exhaust which means cat, primary, and secondary bypasses as well as the completely open airbox and different DME. It is WAY too loud for the street.

c

 

Colin....like what?????

For a standard engine car, I would say that the GHL I have is one of the best options that I have at least seen dyno plots for.....Decatting only (with nothing else done) on a standard car I have not seen dyno results for....

In any case, look forward to hearing more from the Hayward Experiment!

 

Colin

That's great info and supports what I have been trying to say (I think!!). However, you have made me realise that my thinking has been a little too simplistic.

I think the issue is this. Once you are able to flow enough air so that the injectors cannot handle any more fueling, anything else is pointless (with standard set up). Using standard injectors and the Motronic OEM DME, that point can be reached without headers, so headers won't add any more.

Having said that, I do wonder, theoretically, if there might be a little gain in torque in mid range as I am unaware at what point you hit 100% duty cycle. I would not expect that gain to be worthwhile enough for the expense involved.

Once you are into larger injectors with a fully mapable ECU, then the limit is set by how much air the standard head, cams etc will flow. Based on other makes of car and with no direct experience of Porsches, my money would say that the real benefits will come from the shape of the curve rather than top end BHP. But that really is just a guess.

 

IMHO testing headers on a stock engine even with remapping of the DME is a fruitless excercise because irrespective of the actual hp gains you make/lose, there would always be more to be had if you changed the injectors and ecu for Motec or the like. The conclusion I came to is that alternative exhausts are merely making the best of a bad job, whereas with Motec we can find the best of both jobs.

Again in my view, the stock 964/RS engine mapped properly on Motec with a free flowing intake & filter is pretty damn close to the optimum performance that the engine will make, so if you directly compare results of different exhausts at this point you are measuring the real hp differences between the designs. So far my results indicate that if you compare the stock headers with no cat & the 2 stock silencers with 2 different headers, the maximum hp gains are very small at the top end, although to be fair there is an improvement at 3000-4000rpm (but nothing to shout about).

So why would a set of headers work on Christer's engine or our 4.0 litre? The answer is in the camshaft design, where to get the air flowing through the engine faster you need to run a longer duration cam with more overlap at TDC. In this case a properly working header actively promotes the scavenging of the cylinder and initiates a stronger intake "pull" on the port, which in turn results in a higher intake velocity and better cylinder filling. As I see it the stock headers in this situation are poor at scavenging and have too much backpressure, thus slowing down the emptying of the cylinder and hence delaying the onset of intake flow. The effect is most noticeable at low to mid rpm where the torque curve rise is slow, at high rpm when the engine gets a move on the intake resonance takes over and scavenging is less critical. Take a look at the attached curve of the 4.0 litre same engine with stock exhaust against headers and look at the low end torque gains.

 

Just a qick question without hi-jacking this interesting thread.

Are there any disadvantages with using a cat-bypass on a stock engine? It seems like a thing to do to get some more power, but does it affect the engine in anyway?