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the turbo has been used, the exhaust gases need to be removed as quickly as possible. In theory the best possible turbo set up is ot have virtually NO exhaust pipe after the turbo and waste gate -- the energy has been captured, the rest is potential back pressure, thus a loss in efficiency. The only *real* reason it's there is for the safety of the driver and passengers.
Granted the volume of the gases will drop as it cools, but I'm not so sure that it'll cool that quickly especially when the engine is under full boost.
The old stroker 92's and 93's used the entire exhaust system to help scavenge the cylinders as best as they possibly could. Essentially from manifold to exhaust tip, it is/was a very long header system. More aptly termed a "tuned pipe". We use the same basic technology in high performance model airplane engines. (11,000 to 24,000 rpm 2-stroke engines, pushing 10 to 20 or more lb. "models" to 200+ mph!). These pipes can be somewhat tempermental. If you have too short or too long you lose efficiency. The pulse of the preceeding stroke pulls the next succeeding one.
The turbo itself somewhat negates the tuned pipe, as the pulses are "smoothed out" by the turbo to a certain extent, but the effect is still there. I gotta believe that much in the same way that having too short or too long a pipe in the example model airplane engine can lose efficiency, having too small (restricting) or too large (lack of scavenging efficiency)could affect our cars as well. Though I haven't tested the theory, from what I can see/tell here in the BB, 3" from the turbo to the tip looks to have the best all around efficiency for our turbo 2.0 - 2.3 liter engines. Would love to bench test or dyno various exhaust systems and see what is really happening.
I can tell you that when breaking in a new airplane engine, different propeller sizes and tuned pipes affect the total engine performance in much the same way. View the propeller, if you will, as a flywheel, the tuned pipe as the exhaust system. The same pipe with a different propeller, or vice versa, same propeller with a different pipe will have wildly different results in performance. Same theories apply for all engines, whether 2.4 cu. in. or 2 liters.
On a side note, what really kills me is to see these rice-racers with 4" pipes. Talk about loosing scavenging efficiency! Sounds "cool" (matter of taste, I suppose) but do they realize that they've probably damaged what engine improvements they have by putting too large a pipe for the application they have?
Ramblin' once again. 'Nuff said...
Jim M.
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