Quote:
Originally Posted by Draco-REX
Don't forget that the FA20 has variable cam timing.
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Yea I've written about it several times, the way volumetric efficiency is "modified " shall we say, by a cam phasing system is by changing the point at which the cam closes. They can make it close earlier to prevent air from being pushed out the cylinder, but you can only advance the cam a certain amount before you're getting a lot of hot exhaust gas stuck in the cylinder (and possibly pushed into the intake tract at some point), and this creates a knock (remember fresh exhaust gas is at like 700C or higher under load!) and efficiency problem.
According to the 2GR-FSE paper that's been referred to a lot in the Engine subforum, the "internal EGR limit" is about "20%" (whatever that means). Since the cam is optimized for a good 1000rpm or so higher than on a lazy high displacement engine like the 2GR which makes max torque much earlier, the cam duration is going to be rather long, and because the limit to how much exhaust gas you can trap is still about the same, you inevitably lose VE across a greater range of engine speed. As you can see on the dyno chart, at 1000rpm or so the engine has abysmal torque, about 1/2 of its peak torque rating! That means the cam is pushing something like half the air back out even on maximum allowable advance.
This is basically why having 2 cams like VTEC is so nice, because the cam intended for running best at 7000-8000rpm cannot be advanced to get any sort of reasonable low end torque because it just has way too much duration. Of course it's much much better if you can adjust duration continuously but a fully variable system like that either needs more moving parts or is much more expensive.