What are you looking to do with this car?

There aren't many short-term solutions. The ways to effectively increase power in these cars come from more permanent solutions, the most common being the turbo.

Drop in a B21FT turbo engine from a similar-year 240 and tweak it as needed. Compared to some cars, it's relatively easy to break 200 or even 300 hp if you know what you're doing.

There's someone on TurboBricks who has supercharged his Volvo. I'm not sure if he has the original Volvo engine or if he's done a swap, but I understand it's a fast car (it actually might be for sale at the moment). I don't think this job is an easy one, but it's been done.

You'd be wasting money with the nitrous--how much extra power do you expect to get out of a stock engine that only puts out around 110 hp? Installing a turbo would be a much better investment.
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'89 244 GL -- 111,005 miles (see profile for info on car)