WonPutt

I need help in deciding between a catbypass/free flow muffler/chip or SSI's and a free flow muffler on my 85 Targa. I have been told by Fabspeed that their bypass method would give me as much, or more HP increase than the SSI route at a cheaper price.I am interested in people who have these systems & what muffler they have and how much HP increase they have since they installed their system. I want a throater sound, but don't want a system that is too loud. I also need to decide between a single out or dual out system. Any suggestions, brands, cost associated, etc? Thanks!

aquabat911

the big advantage with the SSI's is that they are equal-length headers, while your stock exchangers are not. If Fabspeeds system isn't using equal-length headers you are not going to get the HP of the SSI's. The advantage of using a bypass pipe is that you can put your cat back in once a year for inspection without too much of a headache. Most aftermarket exhausts I have heard on 911s are louder than stock, mine is WAY louder, but I love it. Dual outlets seem to be a little louder and resonate a little more than singles. Good luck

WonPutt

The SSI route would be a header system, no cat, directly into a muffler. The Fabspeed approach is leaving the stock HE's (unequal lengths) removing the cat for the bypass tube into a muffler. Advantage is most likely passing smog, or an easy swap if it doesn't and a lower price. But since I'm doing this for HP, I want to get what they are stating. I've heard pros and cons about Fabspeed & their exaggerated HP claims.

Mark Kiwior

I have a B&B header system with the single out muffler and Autothority chip tuned for the headers on my '86 911 and I love it. The sound is deeper than stock but isn't too loud in my opinion. Also the car definitely feels stronger with the headers. I just never liked the idea of both heat exchangers merging into one pipe/cat. From other posts I’ve read the cat bypass offers no increase in horsepower. Just my opinion. Mark

[ 06-27-2001: Message edited by: Mark Kiwior ]

Bill Gregory

As several have said, with the equal length headers of the SSI's, you're simply not going to get that same benefit leaving the stock heat exchangers on. The benefit comes from the equal length pipes, not from taking the cat out of the circuit. Others have said they liked the Fabspeed mufflers - maybe you get the SSI's and one of their mufflers. If I were looking today, I'd probably go with a dual output Monty or a Fabspeed.

Steve Weiner-Rennsport Systems

Wonputt:

If you are contemplating doing this upgrade for a power increase, then the SSI/Sport muffler setup wins; hands down. This will give you about a 15-18 HP increase over the OEM HE/cat/muffler combination. We use a dual input/single outlet muffler to keep the noise level reasonable and not boomy. OEM mufflers (74) are quieter than aftermarket ones.

Bypassing the cat will only make more noise, not power unless the existing cat is partially or competely plugged up.

I would encourage you to use liberal amounts of copper-based antiseize on all of the exhaust studs and hardware to make it easy to swap back for any smog compliance tests.

Enjoy,

Martin S.

Bruce Anderson's 911 Performance Handbook
Get a copy of Bruce's book. It is very specific on exhausts, chips, etc...what works and what doesn't. Bruce is the National Technical Director (I believe) for Porsche Club of America, as well as the Technical Editor for Exellence Magazine. He has an excellent reputation.

Also, I suggest you listen to what Steve Weiner has to say above. And be advised he sells solutions, not "snake oil".

On my 80SC, I installed the SSI's, the requisite oil lines and a Burch muffler. The sound is fantastic and you can count on 10 to 15 HP out of the deal. Good luck with the project.


[ 06-27-2001: Message edited by: schacht993 ]

WonPutt

On your Bursch muffler, did you go with single or double out?

While I understand the SSI header system makes your exhaust balanced & flow better, why wouldn't a catbypass provide "some" benefits by having a significantly more free flowing exhaust? Then by adding a free flow exhaust on top of that give you maybe 50-75% of what a header system will give you?

Clark Griswald

Wonputt

The issue is not really back pressure, since the stock system is already "free flowing".

Although SSI (headers) MAY reduce back pressure, that is not their primary advantage. Their value is that they are "tuned" pipes of equal length. "tuned" means they are of the proper diameter and length to perform optimally for a given engine and RPM range.

Let me 'splain.

Exhaust gases don't flow continuously, they flow in pulses from a given cylinder, with a new pulse coming each time the exhaust valve opens. If those pulses are exiting through a pipe, the pipe will have points of high and low pressure along its length. Highest pressure in the middle of a pulse, lowest pressure between pulses.

Each pulse of gas has mass, and as it moves down the pipe it develops inertia. If you suddenly try to stop the flow of that pulse - that is stop its forward inertia - it will attempt to resist this and try to keep moving (a body set in motion whishes to remain in motion blah blah blah).

The result is that something of a vacuum is created behind the pulse. And if the exhaust valve is still open, that vacuum sucks the residual exhaust gases out of the chamber, improving evacuation. This is called "scavenging" and is one of the key benefits of a header system.

And if the cam has a bit of overlap, the intake charge is sucked into the cylinder.
You get a dense, uncontaminated intake charge.

The header is pipe, and the exhaust is a fluid flowing through that pipe. Its behavior is not unlike water flowing through a pipe. Ever hear water pipes in a house bang when the water turns off? That is because the water has inertia when flowing and when the valve closes and stops that flow, the water wants to continue. The result is the banging sound. Headers exploit this property to great advantage.

The size of the header tube is critical. For a given size engine, a smaller tube will cause the exhaust gases to flow at a faster rate down the tube, thus increasing the inertia and the "scavenging" effect. But too small a tube and the back pressure increases. So an optimal size must be found for a given engine. The right size depends on how much exhaust a given engine flows and depends on factors like displacement and RPM and lots more.

This is why a header will perform optimally at a given RPM range. Above that range too much exhaust is flowing and too much back pressure is created, below that range and insufficient inertia and scavenging is developed. Hence race engines - big pipes, small street engines - small pipes.

The other factor is equal length. Remember the pulses? Each cylinder fires at a different time - 120 degrees of crankshaft rotation apart - and since the pulses from different cylinders travel down their respectve header pipes at the same speed and the pipes are equal length, each will arrive at the Collector (the point where the header pipes merge into one) at different times. Hence the high pressure pulse from one cylinder does not collide with the high pressure pulse of a different cylinder in the collector, but instead arrives in the low pressure zone between those pulses. Thus reducing overall back pressure of the system.

If the pipes where not equal length, this would not be the case and pulses would collide in the collector.

So properly tuned pipes on an exhaust port actually produce more power than an exhaust port venting directly into the atmosphere with no back pressure or exhaust system at all. That is why ALL race cars have headers.

FYI, intake runners have a similar pipe effect. This is why engine components need to be balanced - exhaust, cam, intake, RPM range, displacement, all have to work together.

------------------
Chuck Moreland '86 Cab - "Sparky", '77 Targa - "Sweet Pea"

Bill Gregory

Wonputt,

Just noticed you have a 3.2 engine. You should consider that SSI's, while providing a benefit on 3.2 engines, doesn't provide as much benefit as it would on earlier engines. As I recall the size of the pipes would have to be a bit larger to provide the "full" benefit on a 3.2L engine. Bruce Anderson, in his "911 Performance Handbook", quotes a benchmark on a 3.2 engine, which with SSI's, an early-style muffler, and a chip matched to the exhaust, gained +13 hp.

WonPutt

From Clark's very detailed and highly appreciated reply is sounds like a header system is the way to go. That being said, besides the SSI system which I guess to be the most expensive, what other header systems can this board recommend? I am also still wanting recommendations on a good sounding muffler. I'm thinking of sticking with a single out to eliminate too much noise.

Bill Gregory

Is heat/defroster action a requirement? If yes, besides the SSI's, B&B makes heat exchangers, and I believe GHL also does. If you're willing to give up heat/defroster action, then you have quite a few choices for pure headers.

In terms of muffler sound, you really need to check this out yourself, especially because you indicate you don't want something too loud. Go to a Porsche event somewhere, maybe a drivers education session at a nearby track. Listen and talk to people. Everyone has different ideas on what works. For example, show me a muffler that puts out less than 89 db when under full throttle near redline on Lime Rock's front straight that gets me past the noise monitor. Life can be simple, really

WonPutt

Yes, I still want the heat! I just joined PCA & see that they have a few AX events that I want to check out in my area. Hopefully I'll find some headers with different mufflers at this event. I want to get this installed before I go to the Russell high performance driving school at Sears Point in October.

carnut169

Thanks Clark! Your answer is the type that keeps me reading this board almost everyday. You get 5 stars from me!

Periokid

Pardon my ignorance.. What does SSI stand for?