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Subject: "RE: Running on Crower 2s!!"

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DR1665	Jul-23-09 11:14 PM
Donating 2GNT member 14692 posts,

#126759, "RE: Running on Crower 2s!!"
In response to In response to 5

Originally posted by 740 turbo brick
Really? I figured since they are drawing more air and the sound my intake was making that the vacuum would be higher.

It's overlap. The combination of lift and duration means that your exhaust valves aren't completely closed before the intake valves begin to open. As the piston drops in the cylinder, it draws in the next charge. While the exhaust valves are open, even slightly, there amount of suction created is split between both intake and exhaust, thereby causing the reduced vacuum in the intake manifold.

This is why it's been said that C3s or other very aggressive NA cams aren't the best idea for boosted applications. As you know, boost is a function of the compressor pressurizing the intake manifold and cylinders, right? If these more aggressive cams have even more overlap than you've got right now, it means that the turbo is trying to pressurize cylinders with what could be considered a leak. (Meaning it requires a greater volume of air - CFM - to produce the same amount of pressure - boost.) Since the gains associated with the C3s are due to BOTH lift AND duration (thus, overlap), dialing the overlap out to improve boost response can effectively reduce the potency of the cam profile.

There's more to it than that, but this should give you an idea of why there is reduced vacuum with those cams.

As an aside, the reduced vacuum is also the reason why the car might seem anemic below, say, 3000rpm. The low end is where you feel your torque. (Torque gets you moving - horsepower keeps you moving.) On an NA motor, the velocity of the intake charge contributes to torque down low. Since there's less air flowing, it needs to be accelerated by the design of the intake runners. As you can imagine, more vacuum in the intake manifold means a faster intake charge velocity. Since the overlap reduces vac down low, it also results in reduced charge velocity, which hampers low end torque.

There. I feel better. After you've read up some more on overlap, consider looking into the differences between street and race porting on your head. Don't forget to consider throttle body sizing as well. Finally, remember that ignition timing advance, as provided by the AFX/R ECU or MSnS plays into this equation.

As is the case with exhaust piping size, careful consideration of the airflow into the engine is critical on an all motor engine.

And (jesus, I'm running over on this one), I'd suggest you sell that SAFC and get yourself some MSnS goodness. Timing is going to make the most of those cams. With the AFX/R, a Fierro race head, oversized valves and C2s, Daisy hauled all kinds of ass. I was easily doing 100mph in 3rd getting onto the highway at 8000rpm. Once I sold the AFX/R ECU (Casbon, right?), she was limp below 3000rpm, starting to wake up by 4500rpm, and just making noise above 5000rpm. You're not going to be able to maximize your investment with an SAFC.

Drive without limits.

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