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Dammit
Kyalami


Joined: 23 Sep 2016
Posts: 1839



PostPosted: Tue Oct 09, 2018 8:27 pm    Post subject: Reply with quote

I seem to have given offence, which wasn't my intention.

I was asking in the spirit of genuine curiosity whether you had something that was rather more authoritative then what I'd found using Google - there is much written about these cars that simply isn't true when you dig into it.

Anyway, no chest prodding intended I assure you.
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Martin996RSR
Spa-Francorchamps


Joined: 08 Dec 2016
Posts: 270



PostPosted: Thu Oct 11, 2018 6:16 am    Post subject: Reply with quote

Sorry Dammit, I probably went a little far.

Basically though, you don't need to read the papers that I did in order to be able to guess that a tight radius 180 bend in an exhaust pipe not far from the collector is going to be more restrictive than an only slightly bent/mostly straight pipe.

Add in the joint between the two pipes at the point of fastest gas velocity (the outside edge of the bend) and you're adding turbulence that probably does more to rob power than any gains achieved by any scavenging effect.

That's why it's obvious that an X-pipe exhaust on a 996 is less efficient than the alternative.

A better solution would be to put a balance pipe between two non-X-pipe cat pipes. You would get the X-pipe sound but none of the losses. There is a Motortrend Youtube show that has done a load of exhaust testing on V8s and they've shown a balance pipe to give negligable gains on the dyno.

If anybody still wants an X-pipe (wel they do look cool I suppose) then get one custom made with the biggest radius bends that you can. My research done whilst trying to figure out the least loss exhuast system for a 996 taught me that bends in the exhaust for 58mm pipe needs to be at least 8 inches to not suffer from serious losses, and there are more gains to be had by making the radius larger.
 
  
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Dammit
Kyalami


Joined: 23 Sep 2016
Posts: 1839



PostPosted: Thu Oct 11, 2018 7:58 am    Post subject: Reply with quote

Interesting- what are your thoughts on manifolds?

I need to get those next, and of those available I am likely to pick the Cargraphic 1.75” primary ones- but that’s based on diameter rather than radius of bend. Porsche (for the 3.8l X51) pick a three into one 1.85” design, rather than the long separate primary/equal length design of the Cargraphic.
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Martin996RSR
Spa-Francorchamps


Joined: 08 Dec 2016
Posts: 270



PostPosted: Thu Oct 11, 2018 8:18 am    Post subject: Reply with quote

The aftermarket manifolds I've seen are all very similar in design and all subject to the same restrictions:
1. The need for ground clearance, so having a tight radius bend very close to the port (a real no no for decent exhaust performance)
2. The need for the collector to be before the point at which the stock exhaust joins the stock cat pipes. - The primary pipes need to be longer and straighter than the packaging challenges allow in order to maximise torque in the rev-range that road cars are driven in.

After market manifolds give a slight increase in HP at the top end beause their greater diameter gives more flow at high RPM, but they almost certainly rob a little torque lower down the rev range where the smaller diameter stock manifold is most efficient. I wouldn't mid betting the same car would feel slightly quicker on the road with the stock manifold and slightly quicker on the track with the after market manifolds.

What this boils down to, is that in my opinion you'll probably get the same benefits with the cheap Chinese stainless ebay manifolds as you'll get with the posh Cargraphic ones, and for a road car that isn't driven by a hooligan you may well be better off with stock manifolds.

Of course, much of this is conjecture and ideally a forum member will get some serious dyno time and do some back to back testing.
 
  
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Dammit
Kyalami


Joined: 23 Sep 2016
Posts: 1839



PostPosted: Thu Oct 11, 2018 9:00 am    Post subject: Reply with quote

I did buy a pair of the Chinese manifolds, but they measure 1.65” so I had a crisis of confidence. They are 10% of the cost of the Cargraphic ones however.

My engine will be 3.7l with a rev limit 1,000rpm higher than stock, so (hopefully!) it’ll want to move a lot of air at higher revs.

The radius from the exhaust port is a *****- no getting away from the ground clearance issue. That said, I wonder what GT3-RS manifolds look like, design wise?
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Martin996RSR
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PostPosted: Thu Oct 11, 2018 9:43 am    Post subject: Reply with quote

Given the capacity increase and rev limit increase you may well find there are gains to be had from the larger bore Cargraphic exhaust then.

What are you doing on the induction side? From what I've read there are real gains to be had from bigger diameter throttle bodies.
 
  
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Dammit
Kyalami


Joined: 23 Sep 2016
Posts: 1839



PostPosted: Thu Oct 11, 2018 10:07 am    Post subject: Reply with quote

48mm ITB’s fed from a GT3-RS plenum, driving the resonance flaps from Syvecs.
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demon
Spa-Francorchamps


Joined: 19 Apr 2011
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PostPosted: Thu Oct 11, 2018 11:29 am    Post subject: Reply with quote

Pop Corn

Kudos re. the rather mature and balanced exchange at the top of this page BTW

Backstory: I've ended up with cargraphic 'sport' backboxes, top gear 200 cell x-pipe and OEM 3.6l headers coated by zircotec


Crawling under the car I don't see astounding evidence of the zircotec coating anymore... and as much as I appreciate the top gear part, I'm looking to fit as high grade a CAT as I can afford - hence I was 'future planning' to compliment the backboxes with 1.75 cargraphic headers and CATs (you've no managed to make me question the headers again!... and the x design!)

Q1: do we think there is any magic impregnated benefit to my oem headers having been "plasma ceramic coated"?!!

Q2: why the bloody hell does no one see to do an equivalent design to the OEM headers, just in higher grade material?!!



BTW - have you had a look at the Supersprint 996 headers.....?
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coullstar
Barcelona


Joined: 15 Sep 2015
Posts: 1254
Location: Aberdeen/Torphins


PostPosted: Thu Oct 11, 2018 11:39 am    Post subject: Reply with quote

I'll be honest I went x-pipe for the sound and no other main reason. Havent really felt any difference in power either way so Im happy.

I totally appreciate the benefits of a balanced system.
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Stotty
Silverstone


Joined: 17 Dec 2009
Posts: 135
Location: Surrey


PostPosted: Thu Oct 11, 2018 11:47 am    Post subject: Reply with quote

coullstar wrote:
I'll be honest I went x-pipe for the sound and no other main reason. Havent really felt any difference in power either way so Im happy.

I totally appreciate the benefits of a balanced system.


Sound is the primary reason I want 200's.

What's the difference in sound between straight and X?
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coullstar
Barcelona


Joined: 15 Sep 2015
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PostPosted: Thu Oct 11, 2018 12:27 pm    Post subject: Reply with quote

Smoother and slightly higher pitch with the X-pipe.
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Martin996RSR
Spa-Francorchamps


Joined: 08 Dec 2016
Posts: 270



PostPosted: Thu Oct 11, 2018 1:18 pm    Post subject: Reply with quote

Dammit wrote:
48mm ITB’s fed from a GT3-RS plenum, driving the resonance flaps from Syvecs.


Crikey! That should really breath well. I was going to comment that raising the rev limit by itself would probably on hurt engine longevity without an upside, as I've seen plenty of dyno charts in stockish M96s where it's all over by 6.5k. With that induction set up and capacity increase you will probably be looking at GT3 engine power levels(as I'm sure you know).
 
  
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Martin996RSR
Spa-Francorchamps


Joined: 08 Dec 2016
Posts: 270



PostPosted: Thu Oct 11, 2018 1:29 pm    Post subject: Reply with quote

Dammit, my top tip for the best all around exhaust is the After Hours style. It's probably loud and uncouth, but i would bet it's the least restrictive. Here is a video of a dyno comparison (done by the manufacturer so obviously to be wary of) https://www.youtube.com/watch?v=N3od84Ps3uU

It shows a torque advantage of just over 10 lb/ft.

The best exhaust would be a custom made one that had long collectors and then fed directly into an after hours style loop exhaust without any cat pipes running across the back of the car. This would maximise weight losses and minimise exhaust pipe losses. It probably wouldn't cost much more than a Cargraphic set up either.
 
  
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Jamesx19
Spa-Francorchamps


Joined: 10 Jul 2015
Posts: 357
Location: Brighton


PostPosted: Thu Oct 11, 2018 1:56 pm    Post subject: Reply with quote

Without back to back testing of manifolds or X-pipes Vs crossover pipes on the same engine, on the same Dyno, on the same day, it's impossible for anyone to be sure what works best.

I understand that scavenging the exhaust system is as much about low pressure waves creating a depression at the back of the exhaust valves (at a useable RPM) as it is about gas velocity and exhaust pulse's.

High end, very expensive computer software can get you close to the right dimensions of primary and secondary pipes but often as already stated by Dammit, the physical fit may cause compromise.

I suspect that as a general rule, based on nothing more than reading about the subject in some books by reputable people like David Vizard, longer primaries are probably better than short. Larger ID primaries will work better for higher reving engines. Too large and low end torque may suffer, but I bet it's not as much with modern fuel injection as it would have been in the past with carburettors.

All complete supposition of course as I don't have a Dyno of my own!
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Martin996RSR
Spa-Francorchamps


Joined: 08 Dec 2016
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PostPosted: Thu Oct 11, 2018 2:11 pm    Post subject: Reply with quote

Jamesx19 wrote:
Without back to back testing of manifolds or X-pipes Vs crossover pipes on the same engine, on the same Dyno, on the same day, it's impossible for anyone to be sure what works best.


I respectfully disagree. Try this thought experiment: Take a straight pipe and test the flow rate. Now take a pipe of the same diameter and put a 180 degree bend in it with a five inch radius, and make the total length of that pipe another 10 inches longer. Do you think the second pipe will flow better or worse than the first?

Of course the second pipe will flow worse. The only thing we can't know without testing is how much worse - and it's there that the dyno testing will help us. It could be that the x-pipe robs you of 10lb/ft or it could be that it robs you of only 3, but I guarantee you that it doesn't add any power and I guarantee it's heavier.
 
  
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maldren
Österreich


Joined: 07 Oct 2016
Posts: 923



PostPosted: Thu Oct 11, 2018 3:16 pm    Post subject: Reply with quote

For what it's worth, not sure I agree about Fuel Injection vs Carburettors either, the exhaust side is all about getting spent gasses out, FE and Carbs are at the other end. There's a bit of overlap of course but efficient exhaust systems matter to all engines.

There have been enormous improvements in computer modelling and measurements in recent years but Anand and Roe's Gas flow in the internal combustion engine was an excellent guide in my student days (and they were my lecturers in Manchester). No longer available but you may get it through your library.
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Jamesx19
Spa-Francorchamps


Joined: 10 Jul 2015
Posts: 357
Location: Brighton


PostPosted: Thu Oct 11, 2018 7:34 pm    Post subject: Reply with quote

Hi Martin,

It's an interesting subject isn't it. I am aware of the view that states there is flow loss in a pipe with a bend that has a tighter inner radius than 2x it's diameter.

Question is,
1.) How much flow do you need at useable rpm's. ( There maybe greater restriction in the CAT or silencer box)

2.) When the exhaust gases encounter the X, instead of having to continue following the same pipe, they are able to split into the second half of the system, hence there is now double the exit volume available.
Also, when this occurs there will be a change of cross sectional area that sends a negative pressure wave back up the system to the valve which causes a depression at the back of the valve, scavenging more exhaust gases from the cylinder.

Instead of each Bank of 3 cylinders acting independent of each other (almost 2 engines joined together at the crank) all 6 exhaust gas pulse's affect each other, not just 3.

IF this all occurs at a useable RPM, then an X pipe MAY produce a worthwhile result.

You can have an educated guess, but Idon't think you can say with certainty that one system is conclusively better than another until you test them back to back in the circumstances I described earlier.


Any sort of pressure waves in the exhaust will affect air going in as well. At low rpm's large bore pipes designed to work at high rpm dont have enough gas going through them to get the required velocity and may actually contribute to a flow reversal of air, leading to "standoff" above the mouth of the carburettor. This is caused by air going in and out of the carb, before finally being sucked into the engine. Each time the air goes through the carb in which ever direction, it picks up fuel from the jet, leading to overfiueling and consequent loss in torque/power. Fuel injection limits this effect by only injecting the right amount of fuel regardless of how many times the air has pulsed in and out of the Venturi before being sucked into the engine.

Again, I'm happy to be corrected. These thoughts are only my understanding of the real experts work on the subject.
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asterix_the_gaul
Suzuka


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1999 Porsche 996 Carrera 2

PostPosted: Fri Oct 26, 2018 9:35 pm    Post subject: Reply with quote

So, I'm in two minds at the moment, I've been thinking about the 200cell straight cats, and you are supposed to get a mild performance increase but I'm assuming that's going to be at the top end and you would loose torque lower down the rev range? I guess without back to back dyno you wouldn't know?
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Martin996RSR
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PostPosted: Sat Oct 27, 2018 10:44 am    Post subject: Reply with quote

By having less of a blockage in the exhaust (200cell vs 600cell) you should achieve a performance gain. It stands to reason. If you go for the bigger than standard bore pipes then you may see a decrease in low end torque as past the optimum point, the bigger the diameter pipe, the slower the exhaust gasses travel. It's the slowing down of the gasses that costs you torque.

So in short; get the 200 cell cats and stay with the standard bore ones if losing low end torque is a concern (which it probably should be on a road car).

If anyone offers you advice on this topic and uses the word 'backpressure' then ignore them. 'Backpressure' is to exhaust engineering as cheese is to moon geology.
 
  
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Phil 997
Brands Hatch
Brands Hatch


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2009 Porsche 997 Carrera 4S

PostPosted: Sat Oct 27, 2018 10:56 am    Post subject: Reply with quote

I had the topgear big bore on my gen1 , it worked very well, but agree with Martins theory about torque loss in low mid range . Now I never noticed it on the butt dyno as so much had been done to the car v stock but I would have loved to try the std bore 200 cells for a direct comparison Thumb Thumb
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