Macca

Camlob. The work looks tidy and neat enough.
Your power gains are most likely around 10-14bhp and due largely to Cat Converter removal. Throttle response will have improved and this I think is also what you describe. The headers may help a little at the top end but because your system is originally a Bischoff system (I believe because of model year) you were already starting ok with the factory headers. I think with race headers you will actually loose torque in mid range over stock until you get those new cams and motec or live remap in place. Your limiting factor will then be exhaust valve size. Unfortunately the NVR cars have small exit valves unlike the VR cars which are 3mm larger. I think if Colin and Steve chime in here they will tell you that a X system/merge is best for making horsepower, Bischoff is marginally better than Gillet fitttings for high flow and that headers are a Nth degree mod for many as there affects are unknown until the engine chracteristics and parameters have been confirmed through tuning. Once you are flowing more air then they will benefit. Before during and after dyno would be very useful. You should consider getting her on the dyno now and at least establish whats happening with the engine before you open the top, put in the cams etc and do new timing.

I really enjoy the way you have gone about your modifications. You are doing things first that were last on my list (I always thought Id jump straight to 9m stage two but have had a rethink in that regard of late due to deciding to go for an additional vehicle to satisfy those needs), but now I am starting to contemplate these things its facinating learning what you are doing. Im going PSS10 but never thought of JIC. So now I have learnt about JIC. Not sure Ill go that way (no support in NZ) but wouldnt have thought to research it until I saw you mention it. Same with ERP/Tarret toe links (which I think I will do). I like most here need heat (it gets cold in winter!) and already have the Dach X catless pipes and RSR mufflers with Motorsound tips so I wont go down the header route but the Cams are something I would definately consider if they can be proven to make a reasonable diifference. Unfortunately my budget for the 993 has finally got (self imposed) thin.... It would be good if you get hard data on trhis before and after you do it....there are many who would like to understand your results, benefits etc

Best of luck with the rest of the mods. You're well on the slope now!! We watch in anticipation. What other engine/exhaust mods are you considering and what are your ultimate expectations of engine out put from the NVR engine?

Cheers
M

Bill Verburg

why?

Bill Verburg

Explain the mechanism, why?

Bill Verburg

You can't reasonably compare an open system w/ megaphone and a muffled system.

Bill Verburg

One of the nicest 993 exhaust setups I've seen, by 9M


993RSR, if you are going unmuffled this is a great setup


997RSR, see the similarity to the 993RSR setup?


for headers to really work well they need cams to work w/ stock smog cams just don't allow headers to work

AOW162435

It was my attempt at humor. I guess I failed.


Andreas

Stealth 993

It's along used trick, it causes a lot more exhaust scavenging. It will start to actually cause a pressure drop around the exhaust valve, & pull the exhaust out. This is most apparent in the low & mid range.

I know it's used a lot on the 928's, & it really helped my old M3.

Bill Verburg

Do you know why it's used on 928s? Do you know why it was used on your M3?

Please explain?

Stealth 993

Bill, are you baiting me? They use it for more exhaust scavenging. Do you want a white paper on it?

trophy

I actually thought that joining banks on 911'a was not effective. Hence why Porsche never does it with any of their race engines (as per the pics above of 993RSR and 997RSR headers)

I do know it is very popular with detroit iron. From memory (and I am no expert) it has something to do with firing order, ie. Odd fire vs even fire engines.

Stealth 993

You can't really compare a race engine to a street engine. The race engine is made to operate in a narrow RPM band, so they do everything to optimize that range. Joining the 2 banks is for low & mid, I bet the Porsche Rally cars might have them joined.

A lot of race engines are totally dead in the lower range & very hard to drive slowly & often will stall out in lower RPM's with light throttle.

I also can't think of any street Porsche Aircooled car that doesn't have them joined.

vincer77

X-pipes and Y-pipes do in general improve midrange torque. By carefully selecting the cross-sectional area of the crossover, the pressure waves in the exhaust are converted to low pressure waves that do indeed scavenge the cylinder during the overlap period, extracting spent gases from combustion and actually helping draw intake charge into the cylinder.

An exhaust pulse that leaves the cylinder when the exhaust valve opens, will travel down the header pipe and will reflect as a negative pressure wave (also known as rarefaction waves) and travel back to the cylinder. If the wave arrives at the exhaust valve during the overlap period, the negative pressure draws gases out of the cylinder. The rpm that this occurs is a function of the velocity of the pressure waves and the length of the header pipe. The wave reflection occurs at an area discontinuity such as the end of the pipe, a step in the header pipe or a collector. The magnitude of the reflected wave is related to the area ratio of the discontinuity. Large area ratios causing large reflections and vice versa. An open header pipe will reflect a large amplitude rarefaction wave, whereas a header pipe terminated in a collector will have a smaller amplitude wave.

An engine setup with "zoomie headers", i.e. headers without a collector, are "peaky" motors since the pressure waves and resulant rarefaction waves are very strong, but only around the "tuned-rpm." The rarefaction wave is very strong since the area ratio of the end of the pipe is essentially infinite (no, it does not reflect a wave of infinite magnitude). A collector will actually reduce the area ratio, and the resultant rarefaction wave will be weaker. But wait you say, don't we want a strong wave? Yes, and at the tuned rpm, i.e. at resonance, the wave is plenty strong to aid in scavenging, but "off-the-pipe" the wave can actually hurt scavenging and reduce power - resulting in a peaky power band.

In a merge collector, the outlet of the collector is reduced in size which reduces the area ratio of the collector. The area changes within the collector actually broaden the rpm range that rarefaction wave helps in scavenging, resulting in a broader power band. The key is to reduce the size of the collector outlet to improve power band, but keep it large enough to not increase frictional losses due to the flow of exhaust gas.

So what does this say about X-pipes and Y-pipes? The same thing occurs in the cross-over. The timing and exact mechanisms are more difficult to understand, but the result is that in most applications, an X-pipe or Y-pipe improves midrange torque. It provides additional rarefaction waves that are beneficial to scavenging in the midrange.

That all said, I am not convinced that a 911 is an ideal candidate for a cross-over as the rear engine exhaust configuration makes it difficult to build a good-flowing cross-over. Most of the cross-overs I've seen have radical 180 degree bends. I am not convinced that trying to turn the flow 180 degrees, and actually having exhaust flowing against each other is overcome by the benefit of the additional scavenging. There is alot of anectotal evidence that it helps, but I have not conducted any testing to support this.

I really like the 9M setup that Bill posted. It looks like there is room to package cats which I would like to have in a street system.

trophy

As I said I am no expert, I remember reading once, hence why I was unsure. Again with all mods, the only way to prove if they work or not os to do some before and after dyno testing in similar atmospheric conditions and on the same dyno.

If possible I will do that with my new fabspeed headers, so I can see the results.

And for those who say the headers are not worthwhile upgrade, It will be part of a series of modification when the endpoint is reached everything should then be working well.... (ECU, CAMS etc...)

Thanks Vince, very interesting reading, as I said in my post i am no expert, just someone that likes to learn.....

Bill Verburg

First you need to understand a fundamental difference in the exhaust requirements between odd fire and even fire engines
90 deg crank, odd fire 90deg block, US V8 also Porsche 928
Left__collector: B-B-S-S-B-B-S-S
Right_collector: S-S-B-B-S-S-B-B

or another way to hear it L-R-R-L-R-L-L-R

180 deg crank, 90deg block V8 Ruf V8, Ferrari V8, Porsche RS Spyder V8 ,even 120 deg crank Porsche H6, 60 or 180* V12 etc
Left__collector: B-S-B-S-B-S-B-S
Right_collector: S-B-S-B-S-B-S-B

or L-R-L-R-L-R-L-R-

On an odd fire engine power and torque are increased by joining the 2 sides because otherwise each collector would need to be much larger or suffer from congestion due to the consecutive pulses of gas it receives. This does not happen on an even fire engine. On the odd fire engine the position and or geometry of the connection between sides doesn't matter a whole bunch.

Second you need to understand the different processes that are occurring in the exhaust, the 2 most important being gas flow and acoustic pressure waves.

gas flow is not continuous or homogeneous, it is lumpy and chaotic. The best flow is from reduced turbulence and wide separation of the exhaust lumps. In a Porsche 6 there is a clump of hot, expanding gas delivered to each collector every 240 deg. of crankshaft rotation(a cyl. fires every 120* but the firing alternates sides), the clumps of hot gas expand and slow down as they pass through the pipes, fewer bends and adequate pipe size reduce the slowing but they tend to pile atop each other as the revs build, another thing that happens is the flow becomes more turbulent as it moves through the system, fewer bends turns and transitions reduce turbulence. The most efficient exhaust w/ the lowest pressure build up and the least pumping losses will make the most hp. This is achieved in an even fire engine by keeping the pipes as straight and unconnected side to side as is possible.

the other big thing is the acoustic pressure waves that are set up in the pipes, very similar to those in a musical instrument. These can be useful but only under a set of very restrictive circumstances. The pipes have to have the correct geometry, this includes length, width, transitions, terminations. The pipes have to have something to work w/, that is the cams which have to have enough overlap duration to suck fresh charge into the cylinder due to partial exhaust port vacuum, the desired effect only occurs at a narrow range in the engines rev band and there is a corresponding range where the pressure at the exhaust port isn't the desired low but rather high, impeding flow.

In a carefully crafted system these co-factors of gas flow and acoustic signals can work very well to enhance power and torque but the key is carefully crafted, not *****-nilly joining as on an odd fire engine. This is not to say that any odd fire doesn't benefit from the same gas flow/acoustic properties as an even fire, just that the connection between banks only needs to be somewhere, where or the geometry is of no importance.

Further there are several different ways to construct these systems depending on whether hp over a limited range of revs or torque over a wider range of revs is desired.

for HP on an even fire engine you keep the sides separate for torque you join them but you join them in the way that the 3 pipes come together in a separate system. This double collector setup is called a tri-Y or 180* system.

here's a great example, the 3 pipes from one bank come together in the first collector and then the 2 sides are merged in the second collector, there is a whole science to the design of the collectors and Burns does make some great ones. What the tri-y does differently is smear the low pressure range out so that the signal is spread out more but is not as strong at any one point of the rev range, this broadens the torque curve


lastly a muffled system has a different termination or end than an unmuffled system, I mentioned earlier that transitions and terminations alter the acoustic signals and generate turbulence. The minute a muffler is slapped onto the best header in the world most of the acoustic benefits are lost, there are tricks that can be used such as extending the inlet pipe into the muffler cavity but this is only of limited usefulness.

For a muffled system w/ smog cams the best design goal is free flow, minimized turbulence and light weight, for an unmuffled system you can take advantage of the acoustic signals to generate low pressure at the exhaust port over a narrow range(keep them separate) or wider range(triY)

Bill Verburg

they can but do not always, the type of engine and other factors determine whether they do or don't, the cross overs you are referring too can help but they can also hurt, an odd fire needs a crossover an even fire does not.

all true on an open system not so on a muffled system unless steps such as using extended inlets are taken

Correct zoomies are the ultimate in 'keep them separated' and they work equally well on odd and even fire because there is no collector, these are designed for the best high rpm hp in a very narrow rev band

correct again the merge smears out the acoustic signal and reduced turbulence from the gases that are passing through and out,

again not so fast, it all depends, a cross over is used primarily to alter the volume of sound at the exhaust outlet, a *****-nilly connection at an arbitrary point is more likely to hurt both flow and internal acoustics as it is to help.

correct again, turbulence and impediment to flow is generated at every turn, transition, connection and termination

All exhaust is a compromise, the 9M setup just has a very nice set of compromises in a muffled system. For an unmuffled I'd go w/ a straight shot out the back w a megaphone,
like Matt is using on his