Toymods Car Club Inc: Tech & Conversions => Port & Polishing Turbo motor heads?

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oldcorollas
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Kita-Ku, Sapporo, Japan
Registered:
January 2003

Re: Port & Polishing Turbo motor heads?	Wed, 18 June 2003 05:59

in an ideal flow situation along a straight tube, that's totally correct (ignoring wall friction differences in higher pressure) i take back what i said about the 4AG heads.. i think i was referring to the big ports extra potential when it comes to boosting this is where we get into the art of handwaving cos it's so damn difficult to quantitatively or even qualitatively model what's happening. when the valve opens, and the piston goes down, there is a pressure difference between the cylinder and the plenum, which causes the air to rush into the cylinder (wether boosted or not). a higher pressure differential will cause more air to go into the cylinder, but will be slowed down by the friction on the port walls, friction past the valve head and seat, and also friction from any restrictions in the port. as boost increases, the friction from all of these components increases (i can't remember if the relationship is linear or not). you can think of a port having an effective diameter, which is the diameter of the port minus the effective restrictions. as you increase the friction, you decrease the effective size of the port, and at some (high) boost level, increasing the pressure differential will not cause any more air to go into the cylinder as the port is restricting any more flow. what gets even trickier when analysing this stuff is that the pressure measured at the plenum of an unboosted motor is a measure of the air that _didn't_ get into the cylinder. so just because a motor has more boost, doesn't mean it has higher volumtric efficiency and is getting more air into the cylinder. i think the idea of having bigger ports for a boosted motor comes from increasing the total diameter so that the effective size when boost is present doesn't decrease as much. there will be an ideal size that nets the most gain for certain levls of boost, but i think it is beyond anyone to predict that size accurately without extensive real world testing (like with single cylinder testing they do for formula 1 motors) i apologise for hijacking the thread a bit, just wanted to share what my thoughts are on the subject, and have you guys pick holes in my knowledge , only way to learn. oh yeah, there will be a difference in velocity for the same size port and different boost levels. because the boost is a pressure differential, there is a pressure gradient acros the entire manifold/port/valveseat/cylinder, and the velocity at any point is a product of the acceleration of the air across this differential. the way to solve this is to quantify the pressure differential (at a given moment in time), and then do finite difference calculations to determine the velocity at each point.... and then do this for each moment in time across the whole pot event a bit beyond my knowledge atm.... anyhoo, i'll shut up now [Report message to a moderator]

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