Re: valve timing and engine speed. general question. - Saab NG900 & OG9-3 Bulletin Board - Saabnet.com
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Re: valve timing and engine speed. general question.
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Posted by gray man [Email] (more from gray man) on Mon, 27 Oct 2003 12:05:50 Share Post by Email
In Reply to: Re: valve timing and engine speed. general question., Dean, Mon, 27 Oct 2003 08:21:53
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I see what you mean that with a turbo the torque has a broader band, and isnt this why they have the wastegate? to bypass the turbine when intake manifold pressures reach a certain value, because you dont need as much boost at low rpm's and low load right? Now in the case of the naturally aspirated engine, the intake runners are larger diameter to reduce fluid friction losses. If I understand this correctly, when the piston is moving from TDC to BDC, the intake valve is open, and a vacuum is created which pulls air through valve into cylinder. when the piston reaches BDC there is still lower pressure in cylinder than in intake manifold, because of the restricted flow right? So even as piston moves back up to TDC air is still coming in until the pressure is equalized, right, so the ideal timing would have the intake valve close after BDC exactly at the point when pressures are equal. Now at higher engine speeds, the pressure differencial is greater, and the optimal time to close the valve is slightly later, and at lower engine speeds the pressure difference is less, and the ideal time to close the valve slightly sooner, If you follow. Correct me if I'm wrong, but the valve timing is set with the camshaft which follows the crank angle, which cannot be changed according to rpm. Is this what honda is trying to overcome wiht VTEC? Now it seems with a turbo, this timing issue is negligable. Now I realize this must be what you were telling me originally, huh. well all in the learning process
thanks a lot
gray man

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