Re: 10:1, plus 15#, = 20:1?


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Posted by M Fanoni on Thursday, November 08, 2007 at 15:16:46 :

In Reply to: Re: 10:1, plus 15#, = 20:1? posted by Kaegi on Thursday, November 08, 2007 at 11:58:39 :

Not really. When a turbo charger is rated at 15 lbs of boost, that does not mean that the cylinder is charged to 15psi when the piston is at the bottom of its intake stroke. I am saying that the actual gauge pressure inside the cylinder is always negative on intake strokes, meaning there is a vacuum (this is very evident on not-turboed engines with carburetors). I am not entirely sure what the boost pressure is actually measuring. In theory, as long as we are using the correct theory, we can predict exactly how much air/fuel will be sucked in. So it does "get it all in", but it certainly does not get in 2 ATM of air, as Chris's reasoning suggested. If the piston moved very very slowly, then yes we could double the pressure in the chamber by adding 15 lbs because we give the turbo plenty of time to "fill up" the cylinder. As it is, the 15lb boost from the super or turbo does not have enough time to fill up the cylinder before the piston is on its way back up and the valves are almost shut.

This is easier to visualize on turbos, where appreciable pressure is only built during high RPM situations. Pistons just move too fast and the size of the valve is relatively small compared to the volume of the chamber.

But, the air that is added can make huge differences, as we know. This is because of air density. There is very little air being drawn in by a naturally aspirated engine when the air is very hot. A turbo coupled with an inter cooler can drastically increase the air entering an engine because its air is so much denser.



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