I know back about 90-91ish I had a stock system & had a dump pipe hooked up. Back then it ran
12.71-12.72 every pass. With the dump I'm not 100% sure but I remember about a tenth maybe 1.5 but it wasn't a lot. Altho a high 12 sec car & a low 10 sec car might be a different animal? Now I like it quiet & have a RJC dp with a 2 3/4" system from a US vendor.