Turbo GA cam questions, expert advice wanted :)

PatScottAKA99XE · Jul 27th, 2002, 10:53 AM

Ok from what I have learned about forced induction, Cams with relitively low overlap are best. What about duration and lift???
In my mind I see high lift, low durations being best, is this true? One nice thing about the GA is that the overlap is adjustible, so-to-speak through the use of the cam phasing. Does this mean that the JWT ecu runs the cams for shorter overlap on a turbo application as opposed to a NA app? If so this means we dont have to worry about overlap induced problems with the turbo GA, right? How do the JWT cams "stack-up" as far a being a turbo friendly cam(lift adn duration)?

Jul 27th, 2002, 09:27 PM

Turbo cars like a lot of valve lift.

The amount of overlap they can tolerate is dependant on the engine pressure differential. This is the difference between intake manifold pressure, and exhaust manifold pressure. You can measure exhaust manifold pressure by hooking up a boost gauge to your exhaust manifold. Make sure you put an expansion chamber, like an old fuel filter, in the line, or the exhaust pulses will break the gauge. Turbo cars that run a positive pressure differential (where intake manifold pressure is higher than exhaust manifold pressure) will like a high overlap cam. If you are heavily backpressuring the engine, you will want a low overlap or even a zero overlap cam.

Turbo cars tend to like a more advanced exhaust cam. This helps the turbo spool up with a stronger blowdown pulse. Turbo cars also tend to like a more retarded intake cam. This gives them more 'inertial filling' of the cylinder, which they can use due to the increased charge density you get from turbocharging.