Power Modifications:
2004-2008 Power Modifications subforum: https://www.rx8club.com/series-i-aftermarket-performance-modifications-23/
2009+ Power Modifications subforum: https://www.rx8club.com/series-ii-aftermarket-performance-modifications-162/

From the factory, the 8 is VERY well tuned (mechanically). There are only 2 things you can do for easy power:
- AccessPORT custom tune to remove some of the "safe" parts of the tune
- Removing the naturally restrictive catalytic converter.

Doing the rest of the common upgrades piston cars get (intake, exhaust, pulleys, etc...) nets you MAYBE 5hp. MAYBE. Depending on the brand, alot of them are inferior to the OEM stock system, and plenty cause you to lose power.

If you want power, you have to go forced induction, turbo or supercharger. And you need a starting amount of about $8,000 to $10,000 for a complete set up. If you find what you think is a "deal", please don't get fooled. If you spent $1,500 on your turbo system, you will still spend $8,000 to $10,000 in modifications, errors, swapping parts, getting stuff to finally work right, and probably an engine replacement along the way. If you dont have an AccessPORT, you have very little chance in making it work.

If you don't go that route, then the most you can expect is about 200 whp without a custom engine rebuild, which can run you $5,000+ once you factor in the labor of pulling and reinstalling the engine and the shipping. The advancements in porting work have been slow coming, but they are still moving the needle upwards.
BDC has taken the new top N/A dyno (non-race team funded) posted on 5/16/2011 here: https://www.rx8club.com/series-i-major-horsepower-upgrades-93/bdc-street-port-vs-stock-port-dyno-comparison-216762/ 223whp:
Please note that "heavy" has not been defined as of this edit. The gains are from more than 1 source. Likely full 3 inch headers to tips on the exhaust, entirely unknown what the intake is. The porting work above was mostly on the intake ports rather than the exhaust ports. There was also likely significant improvement in the seal tolerances over factory specs.

Eric Meyer posted his 233hp dyno sheet here: https://www.rx8club.com/series-i-major-horsepower-upgrades-93/233-rwhp-dyno-sheet-218322/
But keep in mind that it's likely his team invested $100,000+ (just a guess) into the R+D of the engine and drivetrain to get it there (including special treatments of the transmission and diff to reduce drivetrain loss)

Don't get fooled by any "chips", ebay or otherwise. There is no "chip" for the RX-8 that will do anything beneficial. The best you can hope for is nothing at all...a costly metallic sticker on your ECU. You can easily fry your ECU with one though, and that is $1,600+. The only flawless method of improving the ECU is through an AccessPORT. Hymee's ProTuner software and brick is also capable, but there are still bugs at the time of this post, hard to come by, and it is possible to wipe your ECU accidently. The piggybacks such as Interceptor-X and GReddy's E-Manager are terrible for the RX-8. You can get it to run, kinda. But you won't be making more power than stock.

If you think you want to go turbocharging or supercharging your engine, start here: https://www.rx8club.com/series-i-major-horsepower-upgrades-93/ and keep reading thread after thread. Get to know the issues people have had, read their responses, their solutions, their frustrations.

What about other engines? Don't even think about posting a question about an engine swap. Please. Do yourself a favor and just avoid clicking the "new thread" button. 20B (3 rotor engines) have been done, but they cost $20,000+ just for the swap itself. 4-rotor swaps have been completed I think twice. They run $100,000+. There have been a few swaps with the older 13Bs from RX-7s. They are cheaper, but still alot of work.
There have been a few other successful swaps, including a couple of LSx engines and at least one 2jz engine. More have been started than have been completed, as they usually run out of money, out of patience, or out of desire. Does it mean these other swaps can't be done? Not at all. It just means that you will have all of the pain, frustration, lack of answers, and lack of support.. And if successful, you would have the glory from a few, the hatred of some, and the lack of notice or caring from the rest.

Power Modifications

Putting power at the top, because that is what everyone goes for first.

Guideline: If you can't find a dyno to prove the gains, assume that claims of power gains are 'Willful misrepresentation of the truth in order to bilk gulible customers out of their money'. Also known as 'lying'. The sole exception to this is if it's an actual race team making the claim. They typically don't sell their developed products, and won't share their exact results. But then, they aren't trying to scam you either at that point. Remember that unless the dyno comparison is done on the same car at the same dyno in the same environmental conditions, comparing the numbers is meaningly. Comparing PEAK numbers is also largely meaningless, as it's worthless to gain 10hp in a 200rpm band by dropping 10hp across the rest of our 5-6,000 rpm band.

Guideline: The relative "high power" limitations to the Renesis are largely due to the exhaust ports. They can not be increased in size since they are run through the side plates, between the oil and coolant passages, and the studs, etc... Prior RX-7s had the entire end of the rotor housing to work with for exhaust dumping. Adding exhaust ports to the housing is viable, though imperfect and makes the engine no longer a Renesis, rendering this point moot. Any high power goals will have to take this fact into account with some serious consideration.

Guideline: The more power you are making, the more heat you are pushing through the exhaust ports. Consistently high EGTs (exhaust gas temperature) have been shown to cause problems with the side seals and side seal springs. There are resolutions to this, if you are having the engine rebuilt. High power cars with a reliable and stable tune and fueling will likely have their side seals fail before the apex seals, however when a side seal fails, it's because of the spring deforming, pushing the seal out until it clips the exhaust port, shattering it and the seal fragments then destroy the apex seals, so most people might assume apex seal failure when it's actually the side seals that went first. High power with an unstable tune or fueling moves the likely failure point to a detonation that destroys the apex seals first. The results are the same between these.