Geo

Ok, agian, it won't increase CR. It may increase cylinder pressure, but that does nothing in itself as it's just filling space with non-combustable gas. As Campeck noted, it will just serve to cool the combustion. That is basically what an EGR valve does.

Campeck

stig. do i have to scan this frikin book again and show you that every muffler one guy tried that had backpressure decreased hp.
i think that this myth came about form ppl putting 3" and 4" pipes on their NA and losing power. so when they put the stock pipe back on and get the power back. they attribute the loss of power to a "lack of backpressure" WRONG!!! with a big pipe gas cvelocity goes down and the pipes get backed up. what you want is a tuned header with good scavenging through exhaust pulses then a moderate sizepipe from headers back and maybe a stepup where the end of the cat would be.

944CS

Everyone is making some valid points, but I'd like to try to bring all the good points together:

COMMON MISCONCEPTION: backpressure is good because it pressurizes the cylinder more, thus increasing power.

The reason this is not the best for optimum power is because the pressure on the cylinder that the residual exhaust gas exherts is little compared to the pressure an equal volume of fresh air and fuel would create if it was allowed to enter the combustion chamber and ignite.

So, you say, why doesn't everyone who wants maximum power run with a straight pipe, no muffler?
Well, on a stock 944, the stock motronic has certain parameters it works with. It will only increase fuel up to a certain point, and if you introduce too much air into the combustion chamber, the computer will not inject enough fuel for a good air/fuel ratio, and the car will run lean. Thus, maybe an exhaust modification will net you a few hp, but this is because you have not went outside the boundaries of the stock fuel map.
For dyno proof, look at mavfan's post awhile back. I forget the exact number, but he lost more than 30 horsepower running an exhaust cut out before the muffler (also for anyone who doesn't think the 944 muffler is restrictive, they have never seen the inside of one)

The fact of the matter is, if you modify the exhaust on the 944, you should modify the fuel map as well, whether you chip it or piggyback the computer. I would not reccomend running a higher fuel pressure regulator.

Ultimately, you will never exceed a certain hp rating with the stock cam. If I were to modify a 944, the first 3 things i'd do is chip, less restrictive muffler, camshaft with some valve overlap

Campeck

actually the chip should be last so you can tune it to the new parameters

944CS

well, speaking in a generic sense.........

how many people with these cars actually burn a unique chip for their car?
yes, theoretically you should change your chip after every modification

TD in DC

I am in a stock racing class, so I can't do anything weird to the engine. I am experiencing mild backfiring, and I don't want to make the problem worse. Actually, I have a small crack in the exhaust pipe about 2-3 feet back from the header. It doesn't seem like that should be close enough to the engine to cause a problem, but then again . . . I believe that mufflers are open under the rules, but I just don't want to shoot myself in the foot by installing one that is worse than the 3-chamber flowmaster that is already on there.

WesM951

I've recommended these before and I think Stig has them on the 928

Dynomax Race Magnum Bullet. Not street legal and sounds badd ***!

TD in DC

In case you guys have never heard a 944 with the whole bottom half of the muffler blown out, check this out: http://video.google.com/videoplay?do...02327219919439

P.S. download it for much better resolution and turn the volume all the way up to experience the proper "acoustics" No matter how smoothly I drive, I just can't stay with a 400hp GT3

Mongo

Hey Wes, I just took it off because my car was deafly loud after putting on a Magnaflow high flow cat, hence the Super 40 flowmaster replacement.


Hey campeck if you really want a chip, I will sell you mine. It's ignition timing is advanced 29 degrees and you can run ANY TYPE OF OCTANE RACE gas on it, and still break every frickin' piston ring in your car. You mention a tuned header with proper scavenging? I suggest you pick up a magazine called 911 & Porsche World for the month of May. They have a very good article on the 944 motor and modifications. They SPECIFICALLY NOTE that the 944's header is of "Equal length design" and "provides optimal exhaust flow and efficiency" for the engine. I can believe them as I have seen virtually no gain with an aftermarket header such as Bursch or MSDS. A turbocharged 944 car WILL only BENEFIT from low restriction exhaust systems and low back pressure.

Mavfan proved that anything wide open at 3" piping diameter would absolutely kill the torque and horsepower on the 944. Why don't some of you understand that no restriction will cause that in the 944? So decreasing back pressure gives you power huh? He had no backpressure. Explain that? 944CS did it quite well up there. The Bosch L Jetronic on the 944 is not tuned for that. So you want to chip it then in order to use some 100 dollar universal straight through muffler? Are you willing to put $1,000+ dollars into dyno time for it? Wow you might as well also ditch the Bosch-L and get TEC-3 and individual throttle bodies. Hell why not just spend $10,000 then into mechanical work to make the engine compatible for a straight through muffler? Reality check. STRAIGHT THROUGH MUFFLERS PROVIDE NO GAIN ON THE 944 NA. PERIOD.

Want to argue more? Go put a straight through 2.5" muffler on your car and dyno it along with a bone stock 944 to compare it with. Enough said.

Manning

Let's see what an exhaust manufacturer has to say about this:

http://www.flowmastermufflers.com/backpressure.html

My experience, no dyno time in this regard. When I my catalytic broke off and I stuck in a section of straight pipe to replace it for a short time, the power band very noticeably moved up in the RPM range. I was able to move it back down by screwing around with how many disks I had in the SuperTrapp. I don't really care how backpressure works or impacts exhaust pulses and scavenging, all I know is that when my power band was shifted up it REALLY SUCKED because my car is street driven.

Travis - sflraver

I am with Manning on this one... Same thing happened with me and that is why I now run a supertrapp. I went beyond just bringing it back though, I sacrificed high end for more low end as the car is primarily used for autox.
With my usual "I don't give a @($&@!" start I can spin them through 1st and half way through 2nd on V710s. With open exhaust I was lucky to get a chirp out of 1st.

Campeck

sheesh. ill bbl to post the page of the book. stig. has anyone trully dynoed a stock car vs a car with same diamter pipe all the way back from headers? no....and if they did, the straight through would win...

WesM951

Loud is good! lol

TD in DC

I love it when I unintentionally start holy wars

Travis - sflraver

How about this Campeck? I will take my car down to the dyno and take all the plates off of the super trap so its open all the way to the headers. Then I will do a run with my plates on. If the low end stays the same or increases, open vs. restricted, then you will have your numbers. If backpressure increases low end torque then I will send you the bill for the dyno runs and a few hours of my time.

Fair enough? aka put your money where your mouth is.

My stance on this is that backpressure tuning moves the HP/Tq curve in the RPM range. A slight increase may be seen with open exhaust but the high range it will fall in will be all but unusable.