That depends on a couple things- mostly what type of turbo you've got now, and how extensively you modify. You've got an 89 or earlier turbo which has a larger turbine and therefore a greater propensity to lag.

On my '89 780, I removed the lower portion of the stock airbox and replaced it with a piece of chimney flashing (flat piece of plastic w/ a rubber surrounded 4" hole where the chimney pipe goes thru). I kept the upper part completely stock and epoxied the chimney flashing to the little strip around the lower part that's got all the clips. Needless to say, this was a non-recoverable (destructive) method. I then piped 4" landscape drainage piping (do you really need 4"? No, but it was cheep!) straight down (had to move the washer fluid reservoir and cut a hole in the sheet metal) and around to the air dam area where I made two grille intakes. This mod increased HP a little, and MPG on the highway 2-4mpg, but the biggest difference was in the time it took the turbo to spool up- noticeably reduced. Total cost (all parts bought at Lowes), around $50, but I already had a K&N.

On my '91, I didn't want to go quite so all out b/c the '91 turbo spools up quite quickly. I decided to forego the ram air thing, and go for a semi-cool air induction method. I removed the whole airbox, detaching the AMM meter from the top and saving the 90 degree plastic circle-to-oval pipe b/c I figured it'd come in handy (it did). Where the airbox usually plugs in, I put in the AMM. From there, I used the 90 bend to point the piping down towards the hole where the airbox was and the washer reservoir still was. I knocked out that 3" cutout below and behind the headlight, just to allow a little more flow into the area. Then, I used a 3" straight length of tubing w/ a K&N cone filter on the end of it to extend it down into that recess and away from the heat of the engine. Total cost, around $40 b/c I had all the parts I needed already, but had to buy the K&N filter. This mod did very little except increase highway MPG by 1-2mpg, and maybe a couple more HP.

In my opinion, this is a good mod to knock out when you're bored on a weekend and feel like dicking around w/ your car, but you don't have a lot of cash to do it with.

Good luck,

-Jon