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Pros and Cons of 3rd Generation RX-7 Modifications

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The heart of the RX-7

RX-7 modification 101. The stock 3rd gen is fast but it can become blindingly quick with some time proven modifications. Why didn’t Mazda design the RX-7 with these mods in the first place? Mazda had to satisfy a long list of government noise, pollution and safety regulations and satisfy the average sports car buyer’s creature comfort needs when they designed the 3rd gen. By making it a little noisier, a little less reliable, and pollute a little more you can create the razor sharp sports car you’ve always wanted.
You can boost the horsepower output of the engine from it's stock 255 hp at the flywheel to 360 by opening the intake, upgrading the intercooler, adding a fuel computer, replacing the pre-cat with a down pipe, replacing the main catalytic converter with a legal high-flow cat and replacing the very restrictive cat-back exhaust. Road and Track called the stock 3rd gen RX-7 a Trans Am car for the street. At 360 hp and 2900 lbs it is scary fast.
There are pros and cons to almost every mod and some of them are not legal for street use. I’ll try to give both sides of the story when discussing them.
Keep in mind that some of these mods may void your warranty and if you ever decide to sell your modified RX-7 your resale value may be lower than a stock car. Also some dealers will not work on a heavily modified car because they are unfamiliar with the parts. Every time you change your car you take the chance that something will not be quite right or a minor problem may become more prominent. It's good practice to make sure the car is working right before you modify it. It's also a good idea to do one mod at a time so you can tell what is causing your new problem, it makes trouble-shooting easier. Modding your car will also put you into a higher performance category if you decide to race or Autocross. If you hope to sell your car to a collector some day, a stock car will bring a higher price. Keep your stock parts in case a prospective buyer wants to revert to stock.
With that said I highly recommend at least a few mods to improve the car: boost gauge, cat-back and intake are easy and very effective modifications. Also take a look at the Reliability Modifications.

My suggested upgrade order

Boost gauge – Allows you to diagnose performance and boost problems. No real cons here. I just added an AutoMeter 2 5/8 inch (67mm), liquid filled, 0-35 PSI boost only gauge to back up my relatively inaccurate 2 inch vacuum/boost gauge (it’s that important).
Down pipe – Replaces the stock pre-catalytic (pre-cat) converter with a three inch pipe. I believe this will lengthen the life of your engine and turbos and bump up your boost and horsepower. The stock pre-cat tends to clog up and keep excess exhaust heat in the turbos, engine and engine compartment. It also usually trashes the main cat when it fails. The stock pre-cat has a very restrictive inlet at the turbos, a down pipe will allow the turbos to dump the exhaust much easier. Your car will still pass an emissions sniff test with this mod but it may fail a visual inspection of the exhaust system if the inspector notices the pre-cat is missing. I highly recommend you spend the extra money and get a stainless steel down-pipe, mild steel will rust out fairly quickly. A high-flow pre-catalytic converter is available if you want to stay 100% legal but it's expensive and won't give as much of a performance boost as a down pipe.
Cat-back exhaust – Really should be one of your first mods, all the following mods cause more exhaust flow. The car will be louder but it will actually sound like a sports car instead of a vacuum cleaner on steroids. I found myself driving around with the windows open so I could hear the exhaust. There are many different models that vary in loudness and exhaust flow. Stainless steel will last longer and look better but it is more expensive than aluminized mild steel.
Intake – Opening up the intake will help the engine breathe. I used the Racing Beat inlet duct and it worked great with a K&N filter in the stock air box but an article by an SAE engineer about the 99 RX-7 convinced me that the RB inlet duct would suck hot air backward through the intercooler. I upgraded to a GReddy intake (one large exposed basket) and fabricated an aluminum heat shield. The GReddy intake rubbed the hood a little and I've always been a K&N fan so I installed two cylindrical K&N filters. It’s not that difficult to reinstall the stock air box if you want to go back to stock. You will hear the whooshing of the charge relief and air bypass valves (and the air pump) without the stock air box’s baffles. If your are concerned with noise try to ride with someone with an after-market intake so you can hear what you will have to live with. The engine will breath hot engine air unless you purchase/fabricate a heat shield. The air pump may be damaged by using a hot air intake too. The "cold air induction" kits include a heat shield or cold air ducting to solve this problem. Replacing the stock air box will also make more room for a larger intercooler. I think the best intake mod is to Open Up the Stock Air Box and add a stock size K&N air filter. You simply cut some holes in the bottom of the stock box and run hoses to the opening to the right of the radiator. You get all the cold air you can use, solve the reverse airflow through the intercooler problem, keep cold air going to the air pump and don't get the air pump and pre-control noise of an open intake.
Air/Fuel meter – About $55 with a mounting cup. When you open up the exhaust with a cat back, down pipe and intake mod you may start to run lean, with an A/F meter you’ll know it. I recommend the AutoMeter  A/F meter, it has 20 LEDs indicating .05 volt increments. If you are running lean you will need some sort of fuel computer – a used one runs from $250 - $1200. Running your engine lean can lead to premature failure. See the Other How To's for installation instructions.
Fuel computer – A modded engine gets much more air pushed through it and the stock ECU (Electronic Control Unit) will not compensate, so the engine can run lean and hot and the air/fuel mixture may begin to detonate (very bad for the rotary's apex seals). An after-market fuel computer fools the ECU into spraying more fuel into the intake. The added fuel will save the engine and add power. No real down side to this mod other than the cost. You also have the option of having the ECU modified (re-programmed) to accomplish the same thing and the price is comparable.
Intercooler – Another mod that can actually help performance and reliability. The stock I/C is woefully inadequate and needs to be upgraded for safe, serious power. Lowers the temperature of the intake air after it’s been compressed and heated by the turbos. Larger I/Cs may require removal of the stock air box, air pump and relocation of the battery.
Vacuum hoses – Replacing the rubber hoses with silicone is a major, all day (usually multiple day) affair but it is needed as your car racks up the miles (heat cooks the old lines and makes them brittle) and as your boost climbs the likelihood of a hose blowing off goes up. You should be able to get the hoses and tie-wraps for about $50. See the Vacuum hose replacement and Manifold removal pages for instructions.
Boost controller - Some people have had problems with boost spikes with the above mods. The fix for the spikes can be an expensive electronic wastegate controller (HKS Electronic Valve Controller, etc.) or inexpensive manual bleed valves. The PFS Powertrain Management Computer (PMC purple computer) controls fuel, ignition timing and boost. Most of the above mods will increase your boost above stock levels because more exhaust will be flowing through the turbos, this is good as long as you’re injecting enough fuel to keep the engine happy. See the Boost Controller page for installation instructions for a manual boost and boost spike controller.
High-Flow Main Catalytic Converter - The stock main cat doesn't flow very well so a high-flow replacement will boost exhaust flow and aid turbo spool-up. It will also make the car a little louder than with the stock cat but not as loud as a mid-pipe. They start at $310 from N-Tech, and go much higher. Make sure you get all stainless steel.
Wheels and Tires - Stickier, lower profile tires can make a great difference in handling. The car will turn in quicker and you'll have less tire squirm. You'll also have a harsher ride because you have less sidewall to absorb bumps. The consensus on the RX-7 mail list is that 17 inch wheels are the best compromise between wheel weight (larger diameter weighs more, adds unsprung weight) and lower profile (larger wheel, lower profile). Larger diameter wheels and tires will also be more expensive. Personally I liked the looks of 18 inch wheels. There are also fewer tire choices and almost no racing tires for 18 inch wheels. See the Tires and Wheel Offset Diagram pages for more info.
Springs – After-market springs usually lower the car about 1 to 1.5 inches. They are usually much firmer than the stock springs. The stock springs are linear, 270 pounds-per-inch front and 195 rear. The car will handle better because of the lower center of gravity (CG) (less weight transfer), the coefficient of drag (CD) will be lower because less air will be moved by the lower car and less air will flow under the car, and body lean will be significantly reduced during cornering. But the car will ride harsher than stock. Interior rattles will be much more prevalent. Eibach progressive springs are the mail list favorite for the street and Eibach Race (non-progressive, straight rate) are preferred for the track. See the Shocks & Springs Installation how-to.
Shocks – There are adjustable and non-adjustable shocks available. Most of them are firmer than the stock shocks and will make the car ride harsher. I've been told that the adjustable Tokikos are softer than the stock shocks at the softer settings but can be adjusted to much firmer than stock. Coil-overs are the ultimate shock/spring setup and they allow you to control ride height but are very expensive. See the Shocks & Springs Installation how-to.
Antiroll Bars (also called Anti-Sway, Sway or Stabilizer bars) - If you feel the body roll in the corners is too much you can upgrade the anti-roll bars. In reality the R1 suspension is a very good street/race course compromise. Most alterations will make it ride even harsher. Some people have suffered anti-roll bar mount failures during extended track use. There are beefed up after-market mounts available from Racing Beat, Mostly Mazda, and Crookedwillow Racing.
Big Brakes - See the Big Brake Install How To. The stock 3rd gen RX-7's brakes work fine for everyday sports car driving but on the race track when exposed to continual high speed deceleration the 11.5 inch front brakes are shown to be a weak link. The stock brake rotors are too small and retain too much heat. The heat is transferred to the brake calipers which in turn heats the brake fluid beyond its boiling point. When the brake fluid boils the gas bubbles in the brake lines compress and absorb the pressure placed on it from the brake master cylinder. The brake pedal gets spongy and you experience brake fade. Hot brakes can warp your rotors and Sandy Linthicum fried his front wheel bearings after 40 hours of track time because the stock rotors got and stayed too hot. The small brake cooling ducts in the R1 chin spoiler don't help much and a more direct form of brake cooling would be beneficial but nothing beats more "swept" area for the brake rotors. Your car's brakes work by converting kinetic energy to heat and the larger the rotor (the larger the swept area) the more rotor you have to dissipate heat. Converting the front brakes to a larger rotor (usually 12 to 13 inches in diameter) is the best way to prevent brake fade and damage caused by overheated brake rotors. Note: A car with upgraded brakes will not stop any shorter than stock until brake fade is encountered. As long as the stock brakes can lock-up and activate the Anti-skid Braking System (ABS) the stopping distances will remain virtually the same as a car with upgraded brakes. But stickier tires will shorten braking distance because more tire grip will allow more deceleration before skidding occurs. The bottom line is a big brake upgrade is a benefit experienced only during track style driving, although there is always the aesthetic benefit of large rotors behind the spokes of your wheels. A potential downside of a brake upgrade is increased brake pedal travel and a change in brake bias. If the big brake kit has larger (or more) brake caliper pistons than stock, more brake fluid will have to be pushed through the front brake system to move the pistons. Some people have upgraded the brake master cylinder to one from the 93 Mazda 929 to solve the pedal travel problem but it doesn't fix the brake bias change. See the 929 Master Cylinder Upgrade How To. The brake bias may also shift to more front brake bias because a larger front rotor gives the caliper more "leverage" to stop the wheel (this may be balanced by a caliper that requires more brake fluid flow). The ABS system will mask the bias problem by activating on the front brakes until enough brake pressure is applied to the rear brakes to cause them to activate the ABS too. You can install a brake proportioning valve to tweak the bias back to the point where the front brakes lock-up just before the rears. See the Brake Proportioning Valve Install How To.
Mid-pipe - Replaces the stock main catalytic converter with a three inch pipe. The car will be very loud with a down-pipe, mid-pipe and cat-back. If you don't have one yet you'll definitely need a fuel computer. A mid-pipe is normally an off road (race) only modification, your car will not pass an emission sniff test or visual inspection. If you add a mid-pipe you would normally want to remove the air pump too. With an open intake and complete exhaust on a well running car, boost creep will occur. Boost creep happens when the stock wastegate is overwhelmed and can't dump enough exhaust to keep the boost level from creeping too high. I had to replace my mid-pipe with a high-flow catalytic converter so I could control my boost. Others have had their waste gate enlarged but this requires removal of the turbos and exhaust manifold (very serious work). I only recommend this mod for those serious racers that want the extra power and are willing to modify their wastegate to handle the extra exhaust flow. If you try to run an open intake and exhaust without an upgraded wastegate you're asking for over-boost and engine damage.
Ready to modify your car? See the How To section.
 

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