Exhaust: Performance vs Sound
 

TL;DR
Assume you’re doing performance upgrades on the engine and remap (pick anything that puts you over 320HP – nothing crazy but decent). What (if any) exhaust changes would do you to get more power or torque while trying to stay as close as possible to OE volume.

More detail:
Headers, cats, and mufflers (and possible x-pipe vs non x-pipe).
I’m looking for some understanding of the impacts of each of these parts on volume / character-of-sound and performance – given the assumption that cats and muffler cannot be deleted because of the goal of not making it louder.

E.g. Switch to equal-length pipes in the headers, or larger diameter pipes in the headers – any effect on performance or sound? More likely to get drone with matching pipe-lengths or is droning unrelated?

“Sport cats” vs stock, how much will a higher-HP-than-stock engine take advantage of it, and how much will it change the sound? I’ve heard several well-regarded builders here say that at a minimum sport-cats should be used. Has anyone done ONLY a switch to sport cats (100 or 200), and if so, how much did the sound change?

Would keeping stock mufflers restrict the increased air-flow rates of anything changed “upstream” from them, or are those less of a bottle neck? I know Bill V. suggests a switch to 993TT mufflers for airflow while keeping stock sound, so I guess I have my answer for that at least!

(side note: Obviously with a forum as old as this, the topic has been covered in various ways, but I most of what I’ve found from searching on Exhaust Performance is something like; “This set up is loud, this one not so much, nothing improves performance” …or… “what is the coolest sounding exhaust? X or Y?” … or … “on my GT2 at the track this sounds great, but would never pass emissions, and it’s too expense” …or… “I’m getting an awful drone with cat-bypass, how do I get rid of it? “
I’m looking at it through a different lens and so haven’t found my answer from searching. )

Thanks!

Cannot speak for HP increases much ( maybe a couple of HP with the sport cats, seems like it revs better), but sound will be very subjective. What is great for one will be too loud for another.

When I got my car the stock exhaust was too quiet for me. So I read and read on the forums, eventually jumping in on one of the Fabspeed sport cat group buys. Finally had it installed and thought close , but would like a little louder. Leery of going Stage 2 Fister modded mufflers, thinking with the sport cats might be too loud for me. Read on the forums that sport cats plus Stage 2 mufflers are about equivalent to Stage 3 Fister mufflers. I was debating on trying the Stage 1 Fister mufflers, when a set of motor sound mufflers came up for sale. Great , now I can try them , plus still have the stock mufflers to go back to if it ended up being too loud for me.

The sport cats and the motorsound mufflers are perfect for me.

+1. I have stock engine, stock mufflers, but Fabspeed sport cats. Produces a little more growl, but still pretty tame. No clue on any HP gain, and no real "seat of the pants dyno" change either.

Indeed, that's why I was going off of OE as a baseline (and assuming you can't really get any quieter than that!), so let's say "as quiet as possible while maintaining the classic OE flat-six sound".

This is great info that the sport cats were louder, but not too loud - exactly what I'm looking for. The modular nature of the exhaust sure makes it easy to slowly piece something together, it sounds like you took the right approach.
I do not expect ANY power difference from simply changing the exhaust, my understanding is that such a change only comes when the other parts of the engine have been upgraded, and then the exhaust becomes a bottleneck that prevents the engine from pumping out as much air as it wants to.

Still wondering about the effect of aftermarket headers ("heat exchangers") - do they affect sound at all? I know it gets into a whole discussion on back-pressure and cams and typical RPMs, etc, therefore let's use as a benchmark a 3.8 engine with RS cams and valves - would love to know the sound-effect and or usefulness of say 1-3/4 pipes vs 1-5/8 (42mm vs 45mm) and short vs long. Seems the Porsche engineers used the same headers for RS, so maybe not worth changing, but a lot of folks look to change the headers / collectors out on performance builds, so wondering why.

Thanks!

I have both a 3.8Rs and a 3.6 w/ Rs cams and have used both stock and various modifications ion stock and pure after market

here's what i"ve learned wrtt
1) any muffled system almost totally negates the possible tuning effect of properly designed headers
2) even on an open exhaust headers need cam I/E cam overlap to really shine, even RS cams have very little of this type of event
3) stock headers function as well as headers on a muffled system
4 ) best bang for the buck is turbo mufflers, separated cat bypass, stock headers
5) best back for lotsa bucks is good headers, separated cat by pass and turbo mufflers
6) some pf the aftermarket mufflers do sound nice, but they are louder than any of the above and may not pass at noise restricted Parks like LimeRock, I did track side tech for years and got to hear all the variation, Fister 2s were a little louder than stock but still seemed to pass tech, But this is iffy as even miine w/ the turbo mufflers has been black flagged on occasion(Fister track fangs solved that issue)
7) the effect of connecting the sides w/ a cat by pass depends on the implementation, surest effect is to tone the exhaust down a little, a possible effect due to reduced flow if the connection is done ham handedly is reduced performance, see below where if connected the flow from #1 can run into the flow from #6, ideally you want a nice partial vacuum gap kept between the pulses

close up of 993RS top and 911RSR bottom flow, note the overlap in exhaust pulses, which is different from the I/E overlap noted above
pulse overlap is handled in the collectors which ideally smoothly merge the flow from the connected pipes, see below, blue is partial vacuum red is the actual gas pulse,
below is idealized as any flow restriction causes the leading low to slow and the trailing to pile into it, This is why nicely spaced 911 flat 6s have such ideal flow pattern, when kept separate there is a full 240 cs degrees between pu;ses on a side
https://forums.pelicanparts.com/uplo...1670348922.gif
https://forums.pelicanparts.com/uplo...1670348922.gif

This is all great info, Bill - thanks! I've read some of these points in other posts of yours, but this puts it all together very clearly.
Quick question; I assume the turbo mufflers need to be installed upside down and have the pipes modified to fit the stock 993NB connections - yes?

Thanks again.

just mod the inlets to the muffler

Hi Cosmos,
My understanding is the CAT on our cars is well-optimized for engine output and swapping it out with an aftermarket unit has a negligible influence on horsepower. It is very heavy, so for racing applications replacing it may be a reasonable course of action. For a street-driven car, I would leave it intact and modify the components after it if a more sporting sound is desired. Also keeping it intact keeps the car emissions compliant whereas I am not sure any aftermarket modifications are certified as DOT/EPA compliant.
As a note, if you do swap out the stock CAT or other exhaust components keep the original ones on hand. These parts can be very expensive to replace. In the case of the CAT, many used ones are cut up to reclaim the valuable catalyst. Sometime in the future, you may want to revert to stock or when selling the car the prospective buyer will want the parts to restore the car to stock.
Andy

for track use where you don't care about emissions and want to reduce the polar moment this is the way to go
https://cimg8.ibsrv.net/gimg/rennlis...d3b44d1476.jpg

Contact Jake Raby at Flat 6 Innovations. He's done lots of pre and post dyno testing. As I recall, sound and lower weight, not power, are the motivators.

If you want power this is the way to go
https://cimg3.ibsrv.net/gimg/rennlis...98165cdbf1.jpg

And the heat still works!

As does Mike's above, it's just that he had to integrate a gas heater and e -A/C system w/ the stock HVAC, not a trivial task

Just saw this post. Bill, are you aware of a 993 owner who’s installed a gas heater (e.g. Eberspacher)? I’ve been doing a lot of digging into alternative cabin heating options since my current 993 project has race headers. My conclusion is that there isn’t (and likely won’t be) an effective electric option for a 993.

However, I’m surprised that no adventurous soul has pursued the gas option in a 993 hot rod. I saw a post and photos from a guy who did it on an older 911, and there’s certainly precedent for it as the early vw’s and 356’s had gas powered eberspacher heaters. The modern airtronic units (used in vans and rv’s) put out 4000W and are like 5 lbs (dimensions are about 12”x6”x6”). I believe all the 993 heating crap (air distribution tube, ducting, control boxes, weight delta between race headers and stock) is about 80 lbs. So, theoretically someone trying to “add lightness” but wanting cabin heat/defog could install one of these in the front and save 70+ lbs.

Your post got me thinking that maybe there was a 993 owner who actually went rogue and tried it. I’d be interested to see how they ducted the exhaust and channeled the output to the hvac suitcase. I read somewhere that Bruce Canepa may have had one in one of his 993 race cars… but couldn’t find any information or photos.

This is the only photo I was about to find of one in a 911.


https://cimg0.ibsrv.net/gimg/rennlis...960a0bbff8.png

Hi Ninja,,
Could one just fabricate a shroud for the racing headers that act much like the stock heating arrangement or possibly a steel tube wrapped against a section of the header or exhaust with some glass fiber insulation? Another possible alternative would be to add an additional pass-through pipe to one or both of the mufflers that uses the muffler as a heater jacket.
One could also use an uprated aftermarket alternator to power some electric heater coils in the ductwork. I seem to recall the gas heaters were located in the frunk on the old Beetle. I am not sure it is such a good idea for a 993 from a safety perspective.
Andy