About a full second in stock form, and once you start to mod them, the 86-87 gets more return for your $ than the 84-85s. Drop a $1000 into a good 86-87 and your running mid to high 12s pretty easy. Drop $1000 into the 84-85 and you may hit the 13s. Its a world of difference and people know that the 86-87 bring more money. That is why you can pick up the non intercooled versions for fairly cheap.
I bought mine for $5400 and it had a brand new engine w/ forged pistons, new crank, ported and polished head, stainless valves, brand new tranny. This was 4 years ago, and I haven't had to open up either one so far. It is a high mile car though.