Just bought 99 S70 AWD
 

hello I just purchased a 99 S70 AWD with 130,000 Miles. If I want to start adding muscle to it where is the best place to start? I was thinking cold air induction and exhaust. When would be the appropriate time to update the ECU? Thanks!

is yours a turbo or the naturally aspirated (NA) engine? If its the NA, don't waste your money on trying to add HP. the factory intake is a good design. You may get 5-10 HP by going to a louder exhaust which you wouldn't even notice (outside of the noise) and I doubt anyone markets a reliable ECU tune for the NA. I'd just invest in making it run better - do a full stage 0 tune up, go to synthetic oil, flush the transmission etc. If its a turbo, an ECU will add 40+ HP and if you match to a downpipe you can get another 20 HP easy without adding any stress to other systems.

It’s turbo

Sounds like its going to be a real beast with all the mods you've planned. Enjoy your ride and keep us posted!

For turbos, if you want to think in terms of HP per dollar, start with the ECU. There's quite a few to choose from. Second would be the downpipe which helps get the air out of the turbo for better spooling and heat management. After that I'd go to bigger injectors. This will get you about +60HP. Next would be a bigger turbo (ie 18T). This will get you another 30-40HP (but you may need to get a stage 2 tune). The bigger turbo would want to be paired with a larger intercooler and better boost hoses. The factory air intake is pretty good so I'd leave that out or do with the bigger turbo. At this point you'd be in the 300-320 HP range and anything more will stress out the transmission so a larger transmission cooler would be warranted. As you can see it becomes a factor of cost per HP.