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Tonger Tonger is offline
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Join Date: Sep 2013
Location: Atlanta
Posts: 221
Originally Posted by Tippy View Post
30 year old technology.....
Exhaust pulse collision is reduced by the twin scroll exhaust plenum effectively halving the chances of exhaust pulse overlap (each side of the motor is now isolated). This should help mitigate the effect of the unequal primary tube length. To take advantage of this, a twin scroll turbo should be used. Shorter and straighter primaries and secondaries in unequal length headers also reduce the volume of gas that needs to pressurize (and the resultant thermal energy loss) which reduces turbo lag.

Equal length headers are great for NA motors but cause more of a packaging compromise for turbo motors. By definition all primary tubes are constrained to the length of the longest primary tube (cylinder furthest from the collector) which increases the contained exhaust volume and likelihood of boost lag.

The final thing to consider is that primary tube length is just one parameter. The velocity of the exhaust pulse is a function of the pressure drop along the primary and secondary tubes which is directly proportional to the resistance encountered. The curvature and diameter of the tube play a direct role in the backpressure/resistance encountered by the pulse. Equal length headers require more curves to equalize the length which can result in more resistance to flow.

Therefore header design is a compromise between primary length/straightness, exhaust pulse overlap, pressurized volume, and thermal energy loss. It would be overly simplistic to just zero in on one parameter (like primary length) and declare the value of the entire design. It would be like using the size of the valve covers as the sole measure of a motor's worth. Ultimately, it's up to the dyno to speak the truth.

To the OP: The dual wastegates are required to preserve separation between the left and right exhaust banks. Note the divider in the T4 header flange that keeps both sides separate. If there were only one wastegate for both banks, that would allow an exhaust pulse to cross over from one side to another increasing the likelihood of increased exhaust back pressure, and creating more turbo lag.

Last edited by Tonger; 03-20-2014 at 06:12 PM..
Old 03-20-2014, 06:04 PM
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