I understand why it's necessary fo a tune when venting only one valve cover, but why is it so when venting both? I was told tuning would not be necessary when venting both. Is it because the blowby gases aren't recirculated and they are in part meterd.

There is no longer any vacuum source when both are vented, so the potential to suck in unmeterd air is no longer there as in venting only one side.

 

That's really too detailed to get into properly here, but just very
briefly.............

Venting only one cam cover or both is not so much a factor as other factors - the "vacuum" effect that allows the engine to take in unmetered air is basically present at idle & cruise (no-load & very light-load conditions). At heavier throttle, the pressure inside the crankcase is going to be significantly higher than normal atmospheric pressure, thus no unmetered air can enter the motor via a breather cap. Venting both cam covers (or valve covers) does not basically "negate" engine vacuum, as can quickly be seen with a vacuum gauge.

And this varies from model year & model - for example - on 1999-2002 Lightnings, we *do* need to compensate the tuning for the use of a breather cap. But for 2003 Lightnings, that is not the case - we R&D'd that specifically when we worked with Superchips on the 2003 Lightning's tuning R&D project, and found *no* difference in A/F's with a wideband O2 in place either with or without the breather cap, at idle, at cruise or @ WOT. This can vary in other models as well, so you're generally going to want to consult with a knowledgeable tuner about your specific vehicle and all it's mods, etc.

I hope that brief info helps a bit, feel free to call us if you'd like to discuss your specific application & setup, and good luck with your truck!