S2 exhaust vs. RS2 exhaust manifold

[90quattrocoupe]

Ok,
I have been reading the different forums, S2forum, AudiWorld, and etc. for a while.

What makes the RS2 EM better than the S2 EM?

I have read that the 1 & 5 exhaust have more heat in the S2 when you start boosting the power.
Does the RS2 have larger diameter runners? Is this why the studs have to be shortened when you install it on a S2, AAN, or ABY motor?

Or do the studs have to shortened because the runners run closer to the head?

Is the collector larger, in order to except the RS2 turbo?

What makes the RS2 EM better?

They are both cast, does the RS2 EM have the same cracking problems as the S2?
Anyone done one out of stainless? Not the long tube ones, but just matching an RS2 manifold.

Thank you,
Greg

[91_S2]

Greg,

I can only answer some of your questions. I have been sort of looking for an RS2 manifold for a while. I will let others expand on the advantages.

1. Do the RS2 x-manifolds crack? Damn straight they do. A lot of the used ones have hairline cracks. Need to look out for that if you ever buy one.

2. Are there SS spaghetti x-manifolds? Yup. I know of 3. One guy in the states had the CAD drawing all finished and ready to go. He was on the S-car list. There is one guy in Sweden where I live. There is another in Finland.

If you are interested, get back to me and I will see if I can dig up the threads.

Rich

[Pisobiker]

I'm not sure about runner's internal diameter, but I don't think there should be significant difference.

Where I can find the most important flowing differences is:

1) Collector: RS2 one have a much smoother one with 5 to 1 passage made directly out of casting and allowing better and less turbulent flow;

2) Runners of Cylinders 1 and 5: RS2 ones go directly in the collector while the others (3B, ABY, AAN etc.) end in the WG duct at opposite directions each other. This means flow coming from them impacts directly onto each other and WG duct wall, loosing much of theyr kinetic energy and converting to heat (and back pressure on exhaust valves).



Turbine flange and studs pattern is the same both for the std K24 and RS2's 7200.

All cast iron manifolds sooner or later crack. Most probable part on RS2 manifold is inside the collector, but only very "abused" ones crack all the way to the outside letting pressure escape. Most probable part on S2 (etc.) manifold is on runners 1 and 5.

About the studs, some of them need to be shorter because of clearance with the runneres, as you have suggested.


Hope this helps


Marco

[audiman]

The RS2 manifold is NEEDED if you want to use a larger turbo, the stock S2 one has no clearance for large compressor housings, for example my K26/27 wont even fit an S2 manifold, but it just clears on the RS2 one as the runners are in a different position

[Floddy]

A picture where you can see the difference:

[Glen Chamberlain]

I take it the top one is the RS2 one?:confused:

[Floddy]

Yep, nice guessed.

Forgot the small detail of writing that in my post...

[90quattrocoupe]

Thank you,
You are right, a picture is worth a thousand words. You can see how the RS2 manifold merges all the runners, whereas the S2 has the 1 and 5 runners hitting each others impluses. I am sure this probably effects 2,3, and 4 also. Kind of a bottle neck.

I have seen the cad of the RS2 manifold, but alas no one has made one up.
I would think that at the prices that the manifolds are going for these days, it would be profitable to make some up out of stainless.

Thank you,
Greg

[Graham]

If you could get the engineering drawingd of an RS2 exhaust manifold i might be able to get a St/St fabricated one made up at our in house boiler shop, all i would need is the drawing specs. or if there is other options with regards changing the design could you explain what they are.
Cheers
Graham

[leo]

I hope it's not so Off Topic, but what do You think about custom made collector for S2 ? Is it better to make it equal-lenght pipes or is it not very important?

Is it better to have an RS2 collector or custom made (properly- diameter, size, lenght etc.) one?

[91_S2]

I think in theory I think you would want equal length. And unequal may not make much difference.

But if are going to make a custom EM anyway, why worry. Do it right and make them equal length.

Just my opinion.

Rich

[Pisobiker]

Sorry to be always the one messing things up with theory....
but theory allows to better understand how things are supposed to work, then choosing the way one prefer to go.

Since the turbine efficiency is much variable trough it's rpm range and also continous changes in collector pressure dissipate more energy, from a theoretical point of view equal lenght exhaust runners allow even distribution of pressure pulses thus more "stable" turbine speed.

Unfortunatly making the runners equal in lenght means you need to increase total overall lenght thus loosing some spooling up speed and throttle responce.


That's the theory. Getting practical:

Equal lenght runners allow 100% use of turbine power delivery to the cold side, so this is the choice for top hp and track use;

Shortest possible runners allow best possible spooling up of turbine and increased throttle responce, so this is probably a better choice for a all around use of the car.

There might of course be some "compromise" layouts. Your choice.


Marco

[91_S2]

Unfortunatly making the runners equal in lenght means you need to increase total overall lenght thus loosing some spooling up speed and throttle responce.

Marco

Marco, isn't the pressure wavefront moving at the speed of sound? I could be wrong there but I think it does.

At 1000 centigrade (EGT) sound travels at about 94 000 cm/sec. For a runner that is 10 cm longer, the time needed to travel the extra distance is about 0,1 mS. Just as a comparison, the reaction time of normal people is about 200 mS. So we are talking about something that is 2.000 times faster than human reaction time. I don't think anybody is going to notice that in terms of faster spool up and better throttle response.

Rich

[Pisobiker]

Rich, we are also talking about "pressure" and not only kinetic energy.
If you have a header+runners where total internal volume is doubled, you'll have double the time to build up same internal pressure (thus spinning the turbo impeller).

Marco

[91_S2]

Marco,

The cross section is more or less constant isn't it?. If we double the length, we double the volume. Increasing the length from 10 cm to 20 cm increases the propagation time of the pressure front from 0.1 mS to 0.2 mS.

I don't think an increase of that magnitude will make a noticable difference throttle response.

Rich