Information and Recommendations

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The Rotary Engine:

There are some terms specific to the rotary engine that may help you understand its operation, or that you may want to refer to when viewing the table below.

Rotor:
A rotor is a somewhat triangular shaped engine component. It is roughly equivalent to the piston of a conventional engine, except that it has a total of three combustion surfaces (located between each apex) to the piston's one (the top or face of the piston).

Apex:
Each rotor has three apexes, which are the points of the triangular shape of the rotor.

Eccentric Shaft:
The rotors drive the eccentric shaft, which is the equivalent of the crankshaft in a piston engine.

Rotor Housing:
A rotary engine consists of a sandwich with several layers. The rotor housing is one such layer that is the same width as, and contains a rotor. The inner shape of a rotor housing, which the rotor's apexes follow, is called a peritrochoid curve. These housings contain the exhaust ports*.

Side Housing:
A side housing is another layer of a rotary engine sandwich that is much like the bread of a regular sandwich. Every rotary engine has exactly two of these as they are the layers that cap each end. These housings generally contain intake ports.

Intermediate Housing:
The intermediate housing is found between two rotor housings. Because the rotary engines found in RX-7s have two rotors, they have only one intermediate housing. Intermediate housings also contain intake ports*.

*Note: There are some rotary engines, called 'peripheral port' engines, that have their intake ports in the rotor housings and none in the side/intermediate housings. Mazda has reportedly developed a rotary with all side ports, including the exhaust ports, for use in the RX-01.

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This photo is of a TKT Banzai 3 rotor engine prior to assembly. The front row, from the left, is the intake plenum, the two turbochargers, a side housing, a rotor housing, an intermediate housing specific to three rotor engines, another rotor housing, the intermediate housing common to two and three rotor engines, the last rotor housing, the other side housing, and the three rotors.
Notice the exhaust ports in the rotor housings.

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How the Rotary Engine Works:

Piston:	Description:	Rotary:
	Intake:
	An intake event occurs when there is a chamber with an expanding volume open to the intake system. On a piston engine, this is when the intake valve is open and the piston is moving down. Intake on a rotary takes place when the intake ports are uncovered by the rotor, at which time the chamber open to the port will be increasing in volume.
	Compression:
	A closed chamber decreasing in volume describes the compression cycle. A piston engine is in compression when the valves are closed and the piston is moving upward. Compression is achieved on a rotary as a result of the rotor moving in its housing such that the volume of its closed chamber is decreased.
	Power/Expansion:
	The power or expansion cycle begins as the compressed intake charge is ignited by a spark. The gas expands as it is heated by the burning fuel. In a conventional engine, this force pushes the piston downward which works to turn the crankshaft. In a rotary engine, this force pushes rotor in a direction that expands the chamber which rotates the eccentric shaft.
	Exhaust:
	The exhaust cycle clears the contents of the chamber preparing it for another full cycle. This is achieved in a conventional engine by opening the exhaust valve as momentum carries the piston upward. In a rotary, the leading apex of the combusting chamber uncovers the exhaust port in the rotor housing through which the spent charge is exhausted.

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Other Comparisons to Piston Engines:

Displacement:
Rotary engine displacements seem small when compared to piston engines of similar power. In fact, both displacements are measured the same way. Displacement is the sum total of positive combustion chamber volume increases for one complete revolution of the main shaft (crank or eccentric). In a piston engine, this means the total amount of space swept by its pistons. In a rotary, it is easiest to think about the difference between the maximum and minimum volumes for a single chamber multiplied by the number of rotors (where each rotor has 3 chambers). Remember that the rotor actually revolves at one third the speed of the eccentric shaft, which is the reason only one chamber's displacement is used in the calculation. The difference in power is due to the fact that the rotary uses its full displacement to produce power for each revolution of the eccentric shaft while only half the displacement of the piston engine is producing power for each revolution of the crankshaft. Other differences also play a role; rotaries do not have the losses of reciprocating motion and there is no valve train to power.

Combustion Frequency and Power Stroke Duration:
When you consider the facts above, you will see that on a rotary, each rotor fires once per eccentric shaft revolution. In a piston engine, only half of the combustion chambers fire for a given revolution. This means that a 2-rotor engine fires as often as a 4-cylinder engine. However, the power stroke duration in a rotary is 50% longer, it being 3/4 of a main shaft revolution to the piston engine's 1/2. This makes a 2-rotor engine similar to a 6-cylinder.

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Where does the turbo fit in?

Turbocharging is the exhaust-driven form of supercharging, wherein air is forced into the combustion chamber. When more air is available, more fuel may be burned, producing more power.

Mechanical supercharging involves a belt- (or sometimes gear-) driven air pump of one of two types, Roots or centrifugal.

The Roots type supercharger typically sits on top of a large V-8 engine and pumps air down into the intake manifold by the intermeshing of worm gears.

The centrifugal supercharger spins a fan-like blade to pump air through a pipe to the intake manifold. It's placement options are more flexible so this type is more widely used.

The centrifugal supercharger may be driven by a belt (as described above), an electric motor (new technology), or by an exhaust driven turbine. This last form is turbocharging.

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The Turbocharged Rotary Engine:

In the above diagram you can see a series of turbine blades being propelled by the force of the exhaust gasses rushing out of the engine. These blades are connected, via a shaft, to a compressor which forces air into the intake via an intercooler. The intercooler is an air-to-air heat exchanger designed to cool the air which has been heated during the compression process. Cool air is denser than hot air and dense air is the goal of turbocharging.

Notice that the behavior of the intake airflow arrow differs in the turbocharged engine diagram from the normally aspirated engine previously shown. The size of the intake (and exhaust) airflow arrows signifies flow in volume and speed. In the normally aspirated engine, this is dependent on the vacuum created by the change in volume of the combustion chamber. Near the end of the intake "stroke" of the rotary engine, the volume of the combustion chamber nearly stops expanding, dramatically slowing the draw of air. In the turbochanged engine, the force of the turbo continues to ram air into the still open intake port, pressurizing the chamber with air, unlike the slight vacuum in the normally aspirated combustion chamber. With more air to mix with, more fuel may be added, and more power produced.

Prior to the turbocharger is the wastegate, a vacuum or spring held trap door which leads to a shortcut around the turbine half of the turbo. When turbo boost reaches a preset level, this door is gradually opened to bleed off the exhaust pressure, avoiding overboost. The diagram shows this wastegate in an open position.

When less power is needed, the turbine naturally ceases pressurizing the air and the combustion chamber's vaccuum draws air in. Thus a turbocharged car can produce more power on demand without using more fuel under less demand.

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How are the turbos configured?

The 3rd generation Mazda RX-7 has the world's first production twin sequential turbocharged engine. The key word here is sequential. In every other automotive twin turbo setup, the turbos provide boost simultaneously. Each of the turbochargers in this type of application is generally smaller than the one turbo used in a single turbo setup. A small turbo accelerates quicker, suffering less from "turbo lag" than its larger counterpart and, as a result, produces less power and torque but sooner and at lower rpm. Fitting twin turbochargers in sequence produces better results as the first turbocharger receives the full force of all the exhaust gasses (instead of sharing with the other small turbo) and gains speed much quicker, which enhances throttle response and increases low speed torque. At a predetermined speed, the second turbocharger is called upon to add more boost. With the twin turbos in full operation, exhaust gas flow resistance is greatly reduced, contributing to higher power output.

Assuring a smooth transition from single to twin-turbo operation as been an inherent problem with the implementation of a sequential turbo system. If the secondary turbo is not spinning at a high enough speed when it is brought in, the whole system "staggers", temporarily failing to produce enough torque for a smooth change-over.

Mazda's rotary engineers attacked this problem with a vengeance and perfected a solution to this technical challenge. In the primary boost stage, when only he primary turbocharger is operating, a portion of the exhaust gas is led to the secondary turbocharger, spinning it into a "pre-operation" mode. The boosted air from the secondary turbo is not required at this stage, so it circulates in an essentially closed intake chamber. Left in this condition, the turbo would eventually go into what is called "surge". This phenomenon is accompanied by a rapid temperature rise at the entry and exit of the compressor, which would harm the turbocharger if prolonged. In order to preclude this surging condition, a bypass valve is opened to form a loop in which the air circulates.

The secondary turbo maintains a pre-operation speed of around 100,000 rpm. However, this is still not high enough to effect a smooth transition to twin-turbo operation. The secondary turbocharger must accelerate faster. This is achieved by deliberately inducing surging by closing the bypass valve and letting the compressor spin within a closed chamber. This sends the secondary turbo's speed to as high as 140,000 rpm. When this speed is attained, the secondary turbocharger receives its full share of exhaust gas, and, at the same time, a control valve opens, allowing the secondary turbocharger to start supplying boosted air, adding to the primary turbocharger's. As previously stated, surging is harmful if prolonged, but in this transition state, it only lasts a few seconds, and therefore has not detrimental effect on the engine's durability and reliability.

The RX-7's 13B engine used twin Hitachi HT12 turbos with a 51 mm, 9 blade turbine and a 57 mm, 10 blade compressor. The turbine and compressor blades are a curved "high-flow" type that offers less resistance to air and gas flow. This results in faster turbine and compressor spin-up at high rpm.

The twin turbos are mounted on a cast iron exhaust manifold which has been named "Dynamic-Pressure" manifold by Mazda's rotary engineers. This manifold is elaborately shaped to minimize the distance between the exhaust ports and the turbochargers' entry paths, improving low speed boost by as much as 25 percent.

A special blueprinted, balanced, and contoured version of this same unit is used on our race cars. These units are capable of producing higher boost levels for extended periods.

Always remember to properly warm up and cool down your turbo and they will reward you with trouble-free operation.

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What about handling?

Good balance is the key to great handling. Although the 3rd generation RX-7 is admittedly a good handling car, it is still a compromise. At Pettit, we seek perfection, and for this reason, we have developed a complete line of suspension components that allow you to fine tune your car's suspension to your driving style. However, some tuning can be accomplished by changing alignment settings and tire pressures.

Alignment is critical on any car, especially on a performance car like the RX-7. We've seen new cars that are slightly out of specification, so it is a good idea to to have your alignment checked prior to high speed maneuvers or track events. The following recommendation come from our own testing, as well as from conversations with customers who compete in all different types of events. Remember, this is only a guide.

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RECOMMENDED ALIGNMENT SETTINGS:

Front Wheel:	Toe	Camber			Caster
Wheel Diameter:		16"	17"	18"
Street:	1/16" in	-1.2	-0.9	-0.8	+6.0
Long Track Event:	1/16" in	-1.5	-0.9	-0.8	+6.0
Short Track Event:	0	-1.8	-1.1	-1.0	+6.0
Autocross:	1/16" out	-1.8	-1.3	-1.1	+6.0

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Rear Wheel:	Toe	Camber			KEEP REAR THRUST ANGLE SET TO ZERO
Wheel Diameter:		16"	17"	18"
Street:	0	-1.1	-0.3	-0.0
Long Track Event:	0	-1.3	-0.5	-0.2
Short Track Event:	0	-1.5	-0.5	-0.2
Autocross:	0	-1.5	-0.5	-0.4

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HANDLING ADJUSTMENTS:

Adjustment :	To Increase Under Steer	To Increase Over Steer
Front Tire Pressure:	Decrease	Increase
Rear Tire Pressure:	Increase	Decrease
Front Wheel Camber:	More Positive	More Negative
Rear Wheel Camber:	More Negative	More Positive
Front Springs:	Stiffer	Softer
Rear Springs:	Softer	Stiffer
Front Sway Bar	Larger (Stiffer)	Smaller
Rear Sway Bar:	Smaller (Softer)	Larger

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Whenever making adjustments or changes in chassis setup make only one change at a time and be sure the change makes an improvement. This way you can see the effect of each change on the car. It is also a good idea to record these changes.

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How should I care for my RX-7?

The following was posted to the RX-7 list by Jeff Witzer. It was titled "Lessons Learned"...

"In the struggle to solve problems with my car, I have learned a few things that I'd like to pass on. This is prompted by last weekend's fix of some engine performance problems that had taken much of the fun out of driving.

There are several people on the list with much more intimate knowledge of our cars, but this might be a good collection of advice for the novice who whats to take good care of their 3rd gen. I've got well over 80,000 miles on my car and it still drives hard and produces a solid 14.5 pounds of boost with no complaints. Cam Worth at Pettit has remarked several times that he can't believe how well it still runs. He asked me to pass these on...

Warm the car up before driving hard
Start the car and immediately poke the throttle to prompt the kick-down. (Pettit actually recommends turning it off for a couple of seconds immediately after it catches to allow freshly pumped oil to seep into the bearings while they're loose, then restarting.) A lot of wear occurs during that 30 seconds or so at 3,000 RPM. It does this to warm the cat to operating temp sooner, but at the expense of your bearings. Within a minute, start driving. Warm up the car under light load, not sitting idling in your garage. Wait until the temp gauge shows normal operating temp before going above 4,000 RPM or above 5 lbs boost (see boost gauge below).

Let the car cool down after driving
Allow at least two minutes of cool down at idle after normal highway driving or after short bursts of full throttle. Allow up to 5 minutes after extended use of high boost. This allows cooling oil and water to reach the hot bearings in the turbo. Neglecting this will cause the oil there to coke into solids, accelerating wear. Always allow at least 30 seconds of cool down after using boost. I use and recommend a turbo timer which lets the car run for a preset time after the key is turned off and removed. I've heard rumors of list members getting in trouble for leaving their car running while unattended, so YMMV.

Oil and Filter changes every 3,000 miles (max)
This should be obvious. General consensus (and my practice) dictates 20W50 and OEM filters (new crush ring each time). This is for summer driving (which is all we get in Tampa). Some recommend synthetics, which can be run in rotaries, but may leave deposits as oil is routinely burned. For non-racing applications, stick to dino juice. I also use Pettit's Protek-R fuel lubricant, but some on the list have argued against it's claims... again, YMMV.

Rotate tires every 6,000 miles
You wouldn't believe how much this extends tire life. Of course this only applies if you've got the same tire sizes all around. I also get my alignment checked at this interval, but normal driving probably doesn't demand this.

Spark plug and fuel filter changes every 15,000 miles
You wouldn't believe the crud that the fuel filter grabs. Remember, the 3rd gen uses Miata filters, but flows twice as much fuel. New plugs have been the fix to most of my hesitation problems. The manual says 30,000 miles but I haven't found anyone who has gotten that much out of stock plugs. For normal applications, stock plugs are best.

Replace oxygen sensor around 60,000 miles
This is what has been biting me over the past few months. Major 3000 RPM hesitation, stumbling over 5000 RPM, loss of power if held at constant RPM with light throttle, and loss of fuel economy. Thanks to Cam at Pettit for this cheap fix. I was convinced my fuel injectors had clogged or finally given out (major $$, major effort).

Use synthetics in the gearbox and differential
After the car is broken in, replace these fluids with synthetics to improve shift feel, quite gearbox whine, and reduce friction and wear.

Install a boost gauge
This will be the best diagnostic tool you'll have. It's best to determine if that loss of power can be blamed on a loss of boost (due to the common splitting, cracking, or loosening of vacuum lines) before going through the expense of tracing fuel and electrical problems.

Be wary of dealer service departments
Since most dealerships service few 3rd gens, they tend to botch most non-routine procedures. Oil changes, spark plugs, fuel filters, etc they seem to do fine, but recalls (especially the fuel line recall) seem to be impossible for them. There are caveats, of course. If you don't see a third gen in a bay or two in the garage, I'd worry. Find a specialist you can trust.

Take upgrades slowly
Upgrade your car one step at a time. This way, if one of the mods is faulty or the car isn't prepared for it, you'll know which mod is to blame. Usually it's best to follow this order: cat-back exhaust, intake system, intercooler, ECU, main cat replacement, pre-cat replacement. Make sure your ECU can handle the increase in boost that the following cat replacements will generate. Talk to specialists before removing cats to ensure your engine is ready.

Check tire pressure and wear every car wash (weekly)
The 3rd gen is quite touchy when it comes to tires. Don't overlook these. Detailing your car is another subject...

All other normal car checks apply
Follow normal procedures for everything else. Take note of new noises or changes in performance and handling seriously.

Good luck and have fun!"

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Before you upgrade...

There are several important considerations that should be made prior to modifying your 3rd generation RX-7. These considerations, when dealt with in the proper manner, will greatly improve your chances for "driving a fun, top performing, long term reliable 3rd generation RX-7".

First, always keep up with routine maintenance schedules. Then you will begin adding modifications to a vehicle that is 100% (i.e. fresh, clean fluids and filters, both turbos working properly, brakes and suspension operational, etc). Obviously, if your vehicle is not running properly, upgrading could cause more harm. From our experience, we have found that most premature engine failures are caused by a dirty fuel filter. Poor fuel quality also contributes. Make sure to run 93 octane, national brand fuel. Run a race gas mix if you consistently run more than 12 psi boost.

Always upgrade in logical stages. This allows you to evaluate the changes in vehicle dynamics made by each modification. Then, if any problems arise, you only have to go back ones step to find the cause.

Now, if you are sure about your car's condition, you are ready to upgrade. If you are not sure, it is cheap insurance to have a full maintenance service performed. Along with this, we highly recommend installing a turbo boost gauge to verify proper turbo operation. This is the only way to be sure that both turbos are working properly.

The whole concept behind upgrading is to have fun and to enjoy this one-of-a-kind vehicle. When you rush things, or do too much at one time, it is much more likely to create problems. Take some time and do only one upgrade at a time.

©2003 Pettit Racing. All rights reserved.

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