Chevyman,
You are not the first to question the approach I am planning. But nothing ventured, nothing gained. I have done a lot of reading on Miller cycle cams and talked with a couple of guys who have experience with this approach - when I called Iskendarian cams, they knew just what I was talking about and reported that they had done Miller cycle cam grinds for a number of different engines.
The reading I have done has been specifically about the Mazda Millenia and work done by Jaguar, Toyota and Nissan in recent years. The argument is that when the compression of the air/fuel mixture is equally shared between an efficient supercharger (aka Whipple design) and the subsequent compression of the charge by the piston, that maximum efficiency and performance can be had.
Low speed idle is reported to be smooth and bottom end torque is actually up to 30% higher than a NA motor. This is all documented by a variety of sources - in print (Corky Bell's book "Supercharging") and on the web in numerous automotive engineering "white papers".
I know it is not conventional - and that is why I went with the 9.5:1 compression ratio in the first place. It is ideal for NA application if my Miller experiment doesn't work, and optimum for the Miller approach as well.
The pressure balance point during the intake cycle where the intake charge stops entering the cylinder is actually between 25 and 35 degrees past BDC. If you run a standard grind, you are slamming the intake valve shut well before the cylinder has reached the same pressure as that of the supercharger induced pressure in the intake plenum. By delaying the closing of the intake valve, the cylinder pressure reaches equilibrium with the plenum pressure without any heating due to mechanical compression. Then, as the piston compresses the charge for the remaining 65 - 70 degrees of crank rotation, you have much less heating than if you had a full 85 or 90 degrees of compression stroke.
Does this make sense to you now or do you think I should save my money and put a 390 cfm four bbl on it?
Here is one
link on the miller cycle engine.
