Red rooster

Generally , cat removal will not give much in bhp terms, but does help with low speed response.
The most restrictive part of the exhaust is the primary silencer , hence the cup pipe ! There are many threads on this.
Header design is affected greatly by camshaft specification. If the motor is running standard cams then alternative headers will not be a best buy and can even be a negative !
For big bhp it is not difficult to fit larger flow injectors and recalibrate the standard Motronic DME for correct fuelling .
Optimised Motronic mapping probably gives more bhp per $ than any other modification .For example Porsche took a standard 964 motor and played with ignition timing to make an RS !!!
With all this stuff its toooooooo easy to spend loads of money for very little gain !

All the best

Geoff

ThomasC2

OK Geoff, I've a cup-pipe today and I understand the a cat-bypass wont give me "the big boost" but maybe a little more flow.

But my question was mor if it's suitable to run an engine set up for a cat, without a cat? I ask this because I've heard someone who said that it's not a good idea to run a cat-engine without a cat, the enginde wear would be grater or something like that...

Red rooster

Thomas ,
I have been running my C4 for ten years + with no cat . What I did do though was retain the O2 probe and run the DME in standard closed loop configuration.

A lot of cat bypass pipes have no O2 probe hole and then you have to fit a DME jumper to run in open loop . Not good !! and could lead to engine wear due to incorrect fuelling .

Hope that helps.

Geoff

AVoyvoda

"however the results are suggesting that provided the engine is optimised for each configuration... there is a lot less to be gained beyond that point with a stock engine"

Colin,
This is fascinating. Because, many have attributed the increase in power between the 964 and 993 primarily to a better, more efficient exhaust on the later model. If, as you imply in the above and following post, the 3.6 engine is more-or-less optimal with the 964 exhaust, where does the difference in power between the two models come from (ignoring varioram and cat)?

We are all very interested in the results of your experimentation with various exhaust systems, but you seem to imply that fitting a 993 exhaust to a 964 engine is a waste of time.

tonytaylor

Ignoring the varioram the 993 makes 272 hp but the ECU is programmed to run on super unleaded IIRC.
If you fit a MAF instead of the flap AFM and "chip" to run on super unleaded I think you'll find most of the extra 22hp

ThomasC2

Thanks Geoff, crystal clear!

Red rooster

The 993 exhaust layout was developed for OBDII function where it became essential to seperate the two sides of the motor exhaust flow. Each cat could then have its own before and after O2 sensors.
For Europe the 993 exhaust does not have the seperator plate before the cats and runs one O2 probe for OBDI .
That seperator plate could make Colins experiments less relevant for those of us this side of the Atlantic !
As has been said the 272 993 runs more timing , slightly different cams/valve sizes and a better total inlet system. That would account for most of the 22 bhp and not the exhaust.

All the best

Geoff

NineMeister

Looking over my previous results I decided to eliminate a lot of questions by testing all three exhaust systems in one day on the same car that has been fitted with Motec. The results have made interesting reading and I will post them seperately in a later thread, but for now the summary is as follows:

Equal length header A, 9m intake = 343bhp@5990 & 436Nm@4650
Equal length header B, 9m intake = 346bhp@6000 & 429Nm@4770
Standard system*, 9m intake = 339bhp@5910 & 429Nm@4690
Standard system*, AFM bypass, std airbox, no filter = 325bhp@6070 & 420Nm@5125
Standard system*, standard AFM & airbox, no filter or lid = 322bhp@6070 & 416Nm@4720
Standard system*, standard AFM, paper filter, airbox & lid = 316bhp@5810 & 412Nm@4600
As arrived, remapped motronic with G-pipe & cat = 301bhp@5770 & 392Nm@4600

* = standard heat exchangers, straight pipe cat bypass, standard rear and side silencers.

Lots of interesting comparisons to make from the above, my favourites are the £250 9m intake for the Motec conversion making 23bhp & 17Nm over stock against a £2500 header set which made 4bhp & 7Nm - so don't throw away your standard silencers just yet!
The tests also confirm that the standard intake is restricted, but given the power levels measured would find it hard to justify changing the air flow meter on the stock DME unless other changes were made to get more than the 316bhp that the Motec found.

Red rooster

Colin,
Got any pictures/details of the 9m intake ? Had a look on your site but couldnt see it.

All the best

Geoff

964rh

Geoff

My cat bypass has got the provision for a O2 sensor but when this was fitted by the PO by a well known UK specialist they didn't install the O2 sensor and, from what I can see the DME has a jumper wire/plug installed (looks like a looped wire and plug with Porsche part number).
Is this the setup your describing in your post?

I thought this was the proper way of running without a cat but the engine wear warning is something I did not know about.

Should I install an O2 sensor onto my car?

Thanks

Rob

AVoyvoda

Thanks Colin, outstanding.

Christer

Well, regardless of this my GHL added 20hp and tq to a standard car on the same day on the same dyno at JZ Machtech - and they were entirely neutral and sceptical of this system. In fact, they tried to gently dissuade me from buying it in the first place!

NineMeister

And you won't! However I can tell you that it comes free with every 9m Motec conversion.....



Motec runs Alpha-N mode with MAP correction hence 9m intake has no provision for an AFM (so it could not be used on a 964 with a conventional Motronic DME).

Chris M.

Colin I'm assuming the above numbers are from a 964 and not a 993 since you mentioned the G pipe. A 964 can make 301 hp with just a Motronic remap, G pipe, and cat bypass? Impressive.

c

Red rooster

Rob,
The 964 system was designed to be cat with an O2 sensor. The recode for non O2 sensor operation was an emergency measure for markets that didnt have lead free gas. Running with the open loop set up means you have to *** around with the flap bypass adjuster to set idle ,part load fuelling !
I would install an O2 sensor and pull the DME link.
Why some cat bypasses were made with no O2 probe position beats me !

Colin,

Whoooo . This sounds like a big secret ! Sounds like a few phone calls need to be made !!! I guess the answer is sooo simple that you dont want the idea copied . Now wheres the number for that guy in China !!!!!!!!!!!!!!!!!

All the best

Geoff