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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
1100Sport,

I think we are mostly on the same page.

I have wondered if the Carrera and 930 headers are differed in size. Thanks for that info.

So the 3.2 Carrera headers are larger ID than the 930's? The SSI has about the same tube ID as the SC and turbo's?

On an SC the SSI's will make more HP than the stock headers. Not so on a 3.2 except briefly at about 3000rpm.

An exhaust change has been reputed to be the biggest reason for the HP jump from the 3.6 964 to 993 (250 v 272). That SC/3.2/3.3/3.6 C2 cross over is very suspect. (I think AFM to MAF is part of it though one 3.6 expert says not so much.)

The arguments for equal length (SSI) seems to be they are going to help pre-boost power.

Pre-boost is before 2000rpm.

The 3.2 N/A data I have seen dose not seem to support that the SSI improves normally aspirated operation before 2000rpm. Thus, how can we expect it to help on a turbo.

As you noted NA 3.2's do however respond well to increasing tube ID over that of SSI's 1.5".

This is supported by the SW HP numbers where SSI's made 10hp less at 3500 and 9hp less at 6000rpm than 1 5/8's-1 3/4's. Lets call it 10 hp.

Unfortunately larger tube size lowers boost response on a turbo.

Also, on a turbo, SSI's are only equal length on the primary tube so it is no longer a fully equal length system unless a split turbine is used.

With a turbo we are nearly doubling air flow through the intake side when running 1 bar. I do not know if we can interpolate from this that if a 1.5" on a 3.2 costs 10 hp that on a 3.3 930 it might cost around 20 hp?

I think it gets back to low overlap cams are not as effected by back pressure at low rpm nor benefit as much from savaging.

I do accept that going to a K27-7006 with its larger hot side and slightly smaller and older compressor than a 7200 is good for about 20hp mostly do to less back pressure.


We are all just guessing at what the differences or advantages might be at this point but that is how we learn.


I am thinking it mostly comes back to a couple of things.

True equal length are better at getting the gasses out any motor, NA or turbo/ and should result in lower back pressure and thus more HP.

A smaller volume header (including WG circut) will pressurize quicker for faster boost response.

It will be difficult to match the factory's quality and heat exchanger system.

Larger tube ID's will better support higher levels of HP.

Smaller tube ID's will reduce header volume and aid boost onset.

Past that, it comes down to getting a good execution of these basic's as to a system design and hard dyno data that is repeatable as to which is best for the intended use.


In principle I currently believe:

That a properly sized true equal length 930 system (Brian's, Prototec, Mode, Vision, Marco's...) will make for solid HP gains at most operating points on boost and possibly transition.

Smaller volume shorty headers should result in earlier boost onset. M&K or Vision if they still do so converted 993's are my favorite.

I do not care for the WG circuit on the B&B style headers as I suspect it leads to over-boost potential on some cars nor the collector however, there smaller volume and elimination of the funky stock crossover are potential improvements.

Stock or converted 993's will probably be best choices for heat.
Old 01-13-2010, 02:22 PM
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