Schwabische Auto

A technical question, why is back pressure good at times? Simple logic would seem to indicate that a bigger free-er flowing pipe would be better. however I understand that is not always the case. Thus the limited increase or decrease in performance from many headers available?

ohdavey6286

Having some backpressure allows for low end power. If there is great airflow, you will make good horsepower but higher up the rpm range. You will however lose the low end power used in daily driving. It all depends on what you want the car for.

Jake Raby

Sure. Just give us a call or shoot an email to set up a time for a visit, we work by appointment.

Schwabische Auto

Thanks, but what is the reason that backpressure creates low end power?

Pac996

This link says his way of looking at it.
http://www.uucmotorwerks.com/html_pr...torquemyth.htm

Here the man has a better knowledge of engines instead of talking
http://www.warnertechnology.com/Cars/backpressure.shtml

Then hushpower relates some real factors to the understanding of the total scheme.
http://www.hushpower.com/product/40/...in+Racing.html

Comparitively since bigger tubes give better flow at high rpms then big is good for high end while smaller is good for lower rpms since it keeps the evacuation of the combustion chamber up better at lower rpms. Tappered tubes going large to small give a good compromise.

There was an old write up by some performance engine makers about how some things just didn't figure right with experiments done but the numbers are there that smaller tubes give better torque. It was experimenting around doing a ton of engine mods dynoing all the way. You know, the type of shop that amazes with the way they change cams and cranks like changing radio stations.

nick49

As the intake valve is opening to allow the intake charge to enter the combustion chamber, the exhaust valve is closing allowing spent gases to exit. The period of time that both valves are open is refered to as overlap. Wild, agressive cams have a lot of overlap, conservative cams, less. Anyway during the overlap period, with too little back pressure, part of the intake charge wants to follow the exiting exhaust charge and poor cylinder fill is the result.

The proper amount of back pressure will minimize this. Keep in mind, cams are made so that the intake starts before TDC while the piston is still pushing out exhaust gasses and the exhaust valve is still open after TDC during cylinder fill. The exhaust tubing is sized for uninhibited flow at maximum RPM. The ideal would be a pipe that changes size with RPMs that gives the perfect balance of flow, volume, and back pressure for all engine speeds. Many motorcycles have exotic exhaust systems that incorporate valves that modify the flow characterics and canisters that add volume increasing torque when needed.

Bottom line, bigger is only better at maximum rpm, and a little too big there will decrease velocity which is bad.

Schwabische Auto

Thanks that makes sense.

Ale-Superbike

So you say if I put the collectors of 996 x 51 on 3.6 and the rest of the original exhaust lose hp?

etwd

not quite...........more like gain HP, lose low-end torque.

seanmcr6

So slower launches, faster 60-100 times then?

I'm struggling with deciding if it's worth upgrading the exhaust system. All the manufacturer's claim 10- 20hp from installing complete systems (headers/cats/mufflers)..and their dyno graphs don't show much drop in low end torque (if any) ....so are they all just lying?

I like the fabspeed CAI...although expensive, it looks killer. much nicer than the stock 996 box. Having said that..the 997 stock airbox looks miles better than the 996 one too. Dropping $600 on the intake cause it looks better is unjustifiable (to me) but doing so and getting 10 more HP and a better sound....that's a completely sensible upgrade.

My 04 C4S is 7 years old now and I'd like to give her a boost. I'm not looking for huge HP gains...but 10% gain should be reasonable...even for the 911 no?

Ale-Superbike

Hello I will board the airbox of the 997 and 'filter sports. In fact if you are interested in selling a airbox of the 997.

Jake Raby

Most people don't consider the fact that optimizing exhaust has little to do with "pressures" and more with sound waves and pulses.

seanmcr6

Fair enough Jake, however, I get that.

Unfortunately, I'm assuming THAT is exactly what these exhaust companies are doing when they develop an aftermarket exhaust. So basically, you're saying there is no real R&D gone into these systems, they've just taken a 20yr old premise that larger diameter is better and banged out some systems? Rather depressing thought, but completely plausible.

Is there any system you would recommend?

Izzymends

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SO, TO BE SURE, DO THE STOCK HEADERS FROM A C2S 997 3.8 FIT MY 3.4 996?

158Sierra

See post 37. They will fit, but you will probably lose overall performance.