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lucittm lucittm is offline
Manassas, VA
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Join Date: Apr 2007
Posts: 1,211
I thought I would chime in just for fun. Brian, please correct me if I am wrong.

The header pipes being equal is not as important as the length if they are correctly "tuned". By tuned, I mean that they are at such a length that when the engine is at peak design HP RPM, the exhaust gas pulse or wave is just starting to exit the pipe as the next exhaust valve is opening to create another pulse. Because as the pulse exits the pipe it "pulls" some of the remaining exhaust from inside the pipe due to the rapid expansion of the gases as they leave the end of the pipe. Think top fuel dragster exhaust pipes. The header pipes can be combined into a single pipe as long as this single pipe is shorter than the individual cylinder pipes.

Now, if you can properly tune the overall header pipe length (this number should be known for a stock 911 engine) to within about 1/8" of optimum and then make all of the header pipe lengths equal, you will have accomplished something that will enhance the performance of the engine.

In order for this to work, the pipe diameter must fall within a certain range, generally agreed to be about 1 5/8" to 2". If the pipes are too small, the gasses cannot be pulled efficiently. If the pipes are too big, the effect is diminished. A collector cone or megaphone on the end can help too.

So, tuned header pipes, with equal lengths, about 1 5/8" in diameter is most desirous.

Now enter the turbo... The tuning length is not as critical because there is not a much larger area for the gases to rapidly expand at the end of the header pipes. The equal length is good because this keeps the time between pulses consistent. The pipe diameter is still pretty important because you don't want to create restrictions; rather you want to keep the ID as consistent as possible to keep the flow going to the turbo without changing the velocity (which happens when the pipe ID changes at a constant flow of exhaust).

I think for us turbo drivers having the shortest equal length headers possible will help direct the most exhaust heat and pressure energy to the turbo. After the turbo, the length is not too critical because the pulses have been "chopped" by the turbine blade and just need to get out of the way. This is facilitated by a large diameter exit pipe with minimum bends, say around 4". Of course most jurisdictions require a muffler, and maybe even a catalytic converter. Clearly these impede the efficient exhaust system, but what are you gonna do?

1991 964 Polar Silver Metallic Turbo Coupe
Old 01-09-2010, 05:32 PM
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