Cam thoughts (Was:Help Picking Out a Camshaft)
Cam thoughts (Was:Help Picking Out a Camshaft)
Sy769 said:
I have called Comp twice now and both guys said that for turbo applications the 412 grind leaves some power on the table.
Here is one email from them.
PART NUMBER 09-000-8
ENGINE 4.3 V6 ENGINE
GRIND # CV6 3313S / 3312S HR 114+4
I've been going over some research regarding turbo cams, and what I keep coming up with
is that a small negative overlap isn't far enough for a higher exhaust restriction turbo.
(Like the stocker or a 20 g)
A smaller housing turbo is going to have a TIP (turbine pressure) ratio of 2:1 or larger.
This means for 20 psi of inlet boost, you'll see 40 psi or more on the exhaust side.
The stock 8cm2 housing is probably even higher than that.
So, given that there's 40 psi in the exhaust manifold and ports, at the moment the
exhaust valve closes, there's 40 psi (maybe more) in the chamber.
If the intake opens before or at TDC, you have 20 psi trying to overcome 40 psi.
Not gonna happen.
Doing some fairly ugly math, it would take ~30-38 additional degrees of crank rotation
in order to pull enough suction on the pressurized cyl to get the overall pressure down
to under 20 psi. Opening the intake any earlier than that would be, in the best case
pointless, in the worst, detrimental. Opening earlier *will* cause the hot exhaust to
reverse into the intake, heating the ports/runners and corrupting the clean charge
with exhaust.
Now, this simplifies the system a bunch... there are effects of inertia on the air column,
as well as the the thermodynamics of it all. However, even really good tuned runner
manifold/header systems will struggle to make an effective pressure difference of 3 psi.
(And that's being generous) There's also leakage in the rings, etc.
The thermal effects are a bit harder to dismiss, since it's hard to know how much of
a drop in gas temp will happen in the cyl following the exhaust valve closing. This could
be as much as 4 or 5 psi (? WAG) depending on the amount of temp drop.
Subtracting both factors from the exhaust gets us down to about 32 psi vs. the 20
at the intake. Still too much pressure; air won't flow into this cylinder. It does allow the
intake timing to move back (advance) a few degrees.
This all depends greatly on the size of the turbo and particularly on the size of the
exhaust housing. Larger housings and more efficient header designs can lower that
TIP ratio down to 1.5 or less. The effects of the backpresure in these cases (ie, the
high-end, big dollar builds) diminishes rapidly, and the cam needs to look more like
a standard NA cam.
In the case of the stock system or milder turbos, though, the intake lobe appears to
need to be retarded a bunch. This has the effect of narrowing the duration down too,
and a shorter lobe makes getting full lift a lot harder. (A shorter duration will require
a steeper ramp to get the same lift; bad for valvetrain longevity)
This idea isn't really new; Vizard mentions it in one of his books, and there are references
to it in Haywood and Taylor if you read between the lines.
The later intake opening should allow a cooler, cleaner intake charge, and allow more
boost in the same application. It would also allow running a higher static CR (say,
9.5-9.8:1) without deto on street gas.
Just my thinkin' out loud...
Later,
