The true improvement is when the headers are open.
Go get a coupla timeslips with the exhaust system hooked up, then go uncap the headers and make a coupla runs, - be prepared for a real difference . .
You will definitely feel a big difference and the timeslip will prove it.
If you have a set of collector reducers, as used connecting the exhaust pipes, - put them on, and see how much you slow down, no Dyno required, just your *** ! !
