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Porting the exhaust side will slow down the exhaust flow, since the same volume flow rate is moving through a larger cross-sectioned exhaust port.
Having less bends and direction changes in the exhaust (particularly in the exhaust manifold) will decrease losses.
However, part of the benefit of having an equal length header in a turbo car is that the exhaust pulses will arrive at the turbine inlet timed equally, and so help spool the turbo. Keeping the ports and primary tubes of the header from getting too large also increases the velocity and helps spool. The benefit of equal length primary tubes for naturally aspirated cars is for a resonance effect which has less of an effect for us.
The characteristic angry burble in the C900 exhaust note goes away with the addition of near-equal length headers, because the burble actually comes from the exhaust pulses interfering with each other from the layout of the log style manifold. Part of why the 9000's and NG cars which have very similar head and cam designs sound so different, among other things.
The header primary tubes should also be as short as is practical (without doing anything terrible to the resonance frequency) so that the exhaust gas stays as hot as possible, more heat in the gas is more energy, which makes the turbine more efficient. That is the methodology behind heat reflective ceramic coatings like Jethot, and behind turbo insulating bags, glass-fibre header wrap tape, etc.
I believe that when Dave K and Lesco modify Saab turbo heads they pretty much leave the exhaust ports alone, aside from removing any imperfections, and polishing the surfaces if desired. Did I get that right, Dave?
Best,
Drew
posted by 165.124.166...
, Sat, 1 Dec 2007 09:02:29, Sat, 17 Nov 2007 12:54:56, Sun, 18 Nov 2007 18:26:02, Mon, 19 Nov 2007 01:10:19
, Wed, 14 Nov 2007 11:59:47, Wed, 28 Nov 2007 18:26:24
, Wed, 14 Nov 2007 18:43:20, Wed, 14 Nov 2007 18:55:19
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