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gianttomato
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Re: An actual technical discussion!	Wed, 03 March 2004 21:47

http://tru-442.tripod.com/camselect.htm Mostly pertains to NA motors - ones that are actually modifed. http://www.grapeaperacing.com/GrapeApeRacing/tech/ turbocams.cfm Specific to turbo engines for the handful of people that decide that there is more to "modifying" an engine than a bleed valve and a FMIC. It's aimed at Yank bent 8s, but the principles are valid. Also this: Cam: On naturally aspirated engines that run high RPMs, usually there is a lot of overlap because the air doesn't start moving instantly. By opening both valves at the same time, it gives the intake a head start and helps to flush out the cylinder of the exhaust. The exhaust by now is a column of air that is already headed out the pipe and helps pull in the intake charge. On turbo cars, this valve overlap will allow the increased cylinder and exhaust pressure to flow backward into the intake. (I'm led to believe that this is worsened if the turbo is too small and results in significant back pressure. A well matched [read non restrictive turbo] will have reduced back pressure) Some turbo cam grinds also wait with opening the intake valve until the piston is already headed down. This is because on a boosted engine the exhaust back pressure can be as high as 30 PSI. The intake pressure under boost is only 15 PSI and will be blown backward. By waiting until the piston has started downward, the cylinder pressure drops and the trick is to open the intake valve just as the cylinder pressure is crossing 15 PSI (in this example). Good high performance cams for naturally aspirated engines will have a intake and exhaust duration of say 270, 280, or 288, 298 etc. Notice the exhaust event is longer than the intake. On turbo grinds, it is always shorter, say 270, 260 (intake, exhaust), or 275, 255. So Leigh, get a 272 intake and 264 exhaust. Oh, and that big TA45. [Report message to a moderator]

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