[Alan Roehrich]

I have not tried mufflers on a stocker. I have tried them on various race engines. MOST any muffler that actually works as a muffler will present at least some restriction.

It depends on how much of a restriction the mufflers are compared to your open headers. The more of a restriction they (the mufflers) are, the leaner you'd need to be, compared to the open exhaust. Of course, that all depends on how close you were to ideal to begin with. It also depends on how well your headers were tuned to your combination to begin with. I'm not sure on timing. However, if the mufflers present any real restriction, they MAY contribute to left over exhaust contaminating the intake charge. That MIGHT require more timing. Again, it depends on how close to ideal you were to begin with.