clubsport1

BillV interesting that you mention in post'9 that; "there was also an RS version for Austria which was quieter,".

Could I ask where you found that information, always keen to learn.

I have an Austrian RS,M003 with standard RS exhaust & G50/32, it sounds the same as German market RS cars.

I am aware that Switzerland had stricter noise reduction rules which resulted in them fitting the longer ratio G50/33 gearbox over the regular 993 RS M002 fiitment of G60/31. If any country had a quieter exhaust on production 993 RS, I would have expected that to be Switzerland?

Bill Verburg

I wasn't thinking as clearly as I could, It was the Australian 993RS catalytic converters that was unique to that market.

Bill Verburg

not at all the intake is tailored to the output, but the better the outflow the more inflow and as a result better power/torque can be obtained
The Porsche911 flat t 6 has an almost ideal pattern to the 4 parts of the cycle for generating power

Here is an idealized model showing the 4 cycles for each cylinder and below the exhaust separated out


the exhaust gases leave the exhaust port @ ~120m/s and gradually cool, expand and slow as they move through the system, momentum or inertia of the gasses is the product of the mass and speed

when kept separated each pulse in a pipe or in the collector is separated by a fair vacuum this helps to suck the succeeding pulse onward'

bends contribute to sloweing

at each change in cross sectional shape or area there is a separate acoustic wave reflected back up the pipes

the larger the delta of cross sectional area the stronger the reflected signal, the largest occurs when there is an open unmuffled exhaust. To use this reflected acoustic signal we want it too be at the exhaust port @ TDC compression, when both Intake and exhaust valves are off of their seats, This is called the overlap period, the more overlap the more effect the reflected signal can have. The bigger the cross section delta the stronger the reflected signal.

We can specuate the the factory was trying to utilize that reflected acoustic signal by using the open ended pipe in the n/a mufflers, where there is a fairly large change in area

however a closer look at a stock 993 exhaust flow shown here


shows very little overlap w/ stock cams, the actual overlap spec for stock is ~7°

RS cams a re a little better @ ~ 13°

Cups even better @~21°

back in the day S cams had ~72°

In any event w/ stock cams on a muffled engine preserving flow velocity is by far more important than trying to utilize exhaust acoustics to enhance power

anything to reduce pumping losses helps, on either side of the combustion chamber

orangecurry

So just to summarize - once we are into the mufflers, on a stock na engine with stock cams, there's (probably) no advantage to having the open ended pipe, yet there is an advantage to having a better flow velocity - which is delivered by the constant diameter and connected design inside the turbo muffler?

With your actual experience with this, does it free up a few more BHP after a rolling-road tune I presume?

Thanks Bill.

Bill Verburg

Yes w/ a muffled exhaust and stock cams the overlap effect is negligible

As to difference it's not going to be huge, going to a 285/30 tire from a 265/35 makes more of a difference

I've run both my 3.6 and 3.8 both w/ RS cams w/ multiple header & muffler setups including stock Robin Sun's, Flowmaster's 2 different Fabspeeds There is honestly not a huge difference in feel from any one to any other, Most of the 3.6 Dyno runs were w/ various headers and Fabspeed or Flowmaster muffler, most of the 3.8 were w/ stock headers and various modified stock muffler cams but no back to back, because chips and air cleaners were also changed.

My biggest concern wrt the exhaust was always passing LRP noise where I had to add Fister's track fangs

biggest difference in dyno tests was getting rid of the cats

here';s a back to back w/ and w/o cats and separated vs merged pipes

clubsport1

Thanks for the clarification, that makes sense.

orangecurry

This is why I'm interested - I think all of my exhaust is stock, including cat, but I only just get under 105dB noise testing.

Just to be clear - is the above the difference between the stock cat, and an x-pipe with no cat at all?

Bill Verburg

There are 3 runs shown
only difference is , stock cat(orange line lowest of the 3), separated cat by pass(yellow line) and merged cat bypass(grey line)


the 2 by pass runs are roughly equal, what I don't like is the resonances in the merged system

both cat replacements are clearly superior power wise to the cat equipped system

one ast thing to add to the LRP noise issue, I also had to tape up the side perforations in the Motorsport air box cover

Fister track fangs