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, Sat, 16 Jul 2005 08:48:09
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The concept is used with naturally asperated tuned length exhaust systems. With a turbo, the lengths involved are way to short to do any such thing. Equal length pipes in a turbo header will help to distribute the exhaust gas pulses to to the turbine in a uniform fashion. But the tube length travel time is so short compared to the exhaust stroke time that this might not be a big issue. Certainly not critical for most all tuning situations. And the biggest advantage of a tubular manifold is low flow resistance. As always, one does not want to port match the exhaust and the runners should be of a smaller local profile than the port on the head... to decrease reversion flows. One can go to extremes to get to equal length, but this does not mean equal flow resistance. Some runners can have one varying bend and others can be very contorted with bend reversals. Expecting these to be equal in anything other than length might be unrealistic, but again, the differences might be trivial. I will never design or make one of these, but I would lean to simplification of the bends and make rigorours equal length less of an objective. There is also a need to make the transitions from the bolting flange smooth from an angle point of view. And smoothness is probably what counts the most. Flow turbulance and separation is not sensitive to the radius, but sensitive to the rate of change of the radius.
Note that at 15 PSI of boost, one expects to see 30 PSI or so of turbine back pressure in the exhaust manifold, and up from there at higher boost. So the gas densities are higher that one might expect which does reduce velocities from what one might have otherwise expected. The gas is not as pulsing as a NA engine, but the piston is pumping out the exhaust gases against the turbine back pressure, thus delivering power to the turbine via the exhaust gas flow. So after the intial presssure and flow pulse from the exhaust valve opening, the piston exhaust stroke creates a time extended flow profile. So the turbine sees a pulse then a 1/2 sinewave flow of longer duration. So life for the turbine is never very smooth. The turbine to compressor torque pulses might be creater from the inital flow pulse than from anything else. So equal length tubes might be smoothing the lesser of the pulsing factors. Knowing such things for sure would be very difficult to work out from an R&D point of view, but one can be still make a better manifold and optimise one thing or the other and know that the result is going to be very nearly optimal. The only way probably is to make two variations and do engine dyno studies.
Another thing that concerns me is how much extended surface one gets vs the stock manifolds. There is a huge increase in surface area which increases convective and radiation heat transfer. And the increased bulk of the whole affair brings the manifold much closer to other components, many of which are now rubber or plastic. Stainless or carbon steel will scale and will develop high emissivities. The ceramic coatings may help a little bit, but are quite ineffective at best. So one needs to consider the heat damage implications.
Bottom line: increased power and decrease exhaust temps, with greater effect in high output applications which otherwise suffer the most from restrictions. Critical issue might be one of valves surviving or not. And increased temps can also lead to octane issues and total engine loss.
, Sat, 16 Jul 2005 08:48:09, Sun, 17 Jul 2005 14:46:01
, Sun, 17 Jul 2005 03:55:39
, Sat, 16 Jul 2005 16:27:20
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