Another 993 Cat question
01-08-2008, 01:56 PM
#1
Another 993 Cat question
I am about to buy a 100 cell cat and trying to decide between the crossover style (Dach or DW-Technik) or individual cats as Fabspeed or similar. I read a brief note from Colin (Ninemeister) stating that the individual cats give more power, but my research tells me that the crossover or linked type give a better sound and looks like a better engineered solution. Can anyone give me a difinitive reason why individual cats make more power, or even confirm that they do?
I need to decide very soon
GG
I need to decide very soon
GG
01-08-2008, 02:56 PM
#2
Rennlist Member
Even if there is a bench measurable difference, it would be completely negligible - and likely unmeasureable - when driving the car. There are a lot of theories about exhaust pulse characteristics in crossover vs. individual systems, but I think you'll have a hard time finding any dyno or other objective data that supports any advantage of one over the other.
IMO, the main advantage of any of these systems is 1) lighter weight, 2) better sound, 3) replacement for stock cats that can get cooked over time. Hp increases in an otherwise stock motor, if present, are single digit at best.
IMO, the main advantage of any of these systems is 1) lighter weight, 2) better sound, 3) replacement for stock cats that can get cooked over time. Hp increases in an otherwise stock motor, if present, are single digit at best.
01-08-2008, 05:42 PM
#3
Three Wheelin'
I think the basic theory has to do with the turbulence induced by the crossover will reduce flow as the reason for them making slightly less power. As Matt stated, it probably won't be much of a difference and probably near impossible to tell with your butt-ometer. It may be measurable on a dyno, but I would guess that any difference will most likely be in the upper rpm ranges.
01-08-2008, 09:24 PM
#4
Rennlist Member
This topic was recently discussed on the Race and DE forum. I cut-and-pasted this from Bill Verburg (post #45 on THIS thread )
Apparantly you have never disected the oem cat. The 2 banks have a parallel entry into a common mixing chamber w/ lateral exits. The purpose of the mixing chamber is to slow the exhaust gas(increase residence time and temp) before final exhaust phasea nd in original form a place for a single O sensor to reside.
On an even fire engine such as a 911 the goal to obtain max perfomance is to keep the exhaust gas flowing as fast as possible, part of that schema is maintaining max pulse separation, w/ separate banks the pulses are kept a 240 crankshaft degrees apart in a given bank at all times.
On an odd fire engine which does benefit from joining the banks the conection is there solely to relieve overloading inherent in the fireing order where 2 left and 2 right bank cylinders fire consecutively into the same collector. A US 90* V8 w/ a 90* crankshaft is an example of an odd fire engine, ideally the pulses in a given bank are kept 180 crankshaft degrees apart and either need a 180 degree exhaust(ala Ford GT40) which does keep the pulses 180 crankshaft degrees apart or a x - over exhaust which relieves the collector overpressure from the consecutive LL and RR pulses in a given collector (alternately the collectors can be made far larger but this undesireable from a packaging perspective
Ferrari and Indy cars use a an even fire V8 configuration by twisting the crankshaft instead of the exhaust pipes. Their flat 180 degree crank always keeps the exhaust pulses in a bank 180 degrees apart.
On an even fire engine such as a 911 the goal to obtain max perfomance is to keep the exhaust gas flowing as fast as possible, part of that schema is maintaining max pulse separation, w/ separate banks the pulses are kept a 240 crankshaft degrees apart in a given bank at all times.
On an odd fire engine which does benefit from joining the banks the conection is there solely to relieve overloading inherent in the fireing order where 2 left and 2 right bank cylinders fire consecutively into the same collector. A US 90* V8 w/ a 90* crankshaft is an example of an odd fire engine, ideally the pulses in a given bank are kept 180 crankshaft degrees apart and either need a 180 degree exhaust(ala Ford GT40) which does keep the pulses 180 crankshaft degrees apart or a x - over exhaust which relieves the collector overpressure from the consecutive LL and RR pulses in a given collector (alternately the collectors can be made far larger but this undesireable from a packaging perspective
Ferrari and Indy cars use a an even fire V8 configuration by twisting the crankshaft instead of the exhaust pipes. Their flat 180 degree crank always keeps the exhaust pulses in a bank 180 degrees apart.
01-08-2008, 09:48 PM
#5
Racer
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I just have a comment about the sound. I drove my car for a few days (it went back to the garage and we are yet to be reunited) with two 100 cell cats provided by Colin of 9M (I seem to remember that they are from Cargraphic but may be wrong on this). The sound is nothing to write home about. I might even go as far to say that I prefer the stock sound. They certainly are nowhere near as beautifully sonorous as the standard cats with Ruf mufflers, a sound that I miss but would never repeat due to the relatively high noise levels and the detrimental effect on hearing.
01-12-2008, 01:49 PM
#6
Rennlist Member
My experience with crossover sports cats is they are freerer flowing and just result in more noise from whatever silencer you use; i.e. it's the silencer that affects the car's sound. Having said that, RSRs plus crossover sports cats sound great! ;-)
Cheers,
David
Cheers,
David