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What would be some good mods to start building power on a 1988 RX-7 GXL?

Depends on what you mean by "power." If you want to go fast, turbo swapping it is the best option. What are your goals?

-Perform a complete service and make sure all the stock systems are performing as they should.
-Now leave it alone and appreciate what you have.

Even a stock FC turbo can hardly be considered a "fast" car by any modern standards and upgrading them to be so requires more skill and money than most are willing to invest.
You have a fairly reliable and comfy daily driver/highway cruiser...learn to love it for what it is.

If this answer leaves you unsatisfied, the fastest and ultimately cheapest, path to more power is a V-8 swap.

Yup. This is probably the best advice for anybody on the forum, new or old. There will be plenty of gremlins for you to work on, but once everything works like it should, you'll be a happy camper. Make sure the car is making its full original potential before adding on.

+2.

believe it or not the stock engine is a pretty good combination, and there are some incremental gains to be had, but nothing big.

the big gains are to look at the whole car. add a few hp, and remove some weight, and play with the suspension, and it makes a huge difference. think of it like a stock class race car.

weight reduction: find an Aluminum hood. next up is to find a cat back, the stock cat back is HEAVY. AL hood, and a cat back is an 80lbs savings. its more if you have a steel spare tire. if its still there the stock stereo (the whole thing, deck, amps, speakers) weighs a lot, any modern thing is much lighter. you can of course go further, but then you're pulling out comfort stuff, and there is no need to do that.

for the suspension: start with the tires, find something short, 205/50/15's make a huge difference. align the car, i like to run as much negative camber as you can get, its about -0.5, but that is good on the street. zero toe in the front. in the rear the stock rear toe in spec, but forget the spec, and make it dead nuts on.

for power, i would run the stock exhaust manifold, this pipe Down Pipe for 86-91 RX-7 - Racing Beat into a stock main cat. its a longer post, but you get 90% of the gains of a full exhaust, but get to keep mufflers, and the emissions stuff, so its still civil. the intake is fine, just make sure all the valves are working. paper air filter is best. i find the S4's like 1-2 degrees more timing. the next step would be an ECU.

and for just being a nice car, buy a tube of this, and lube everything. its magic stuff.

I did most of these suggestions and I absolutely love it. The tires mean almost everything!