UNDERSTANDING THE TURBO’D ROTARY
Before we get into a build, systems and setup it is important to take a step back and fully consider the turbocharged rotary engine.
The 13BREW, when correctly ported, probably outflows any engine on the planet.
Flow is potential power.
Don’t believe me?
Let’s look at a number of the absolute cutting edge 21st century supercomputer designed boosted 2018 engines in comparison. Horsepower per cubic inch…
2018 Corvette ZR1 LT5 755 hp 376 cubic inches 2.00 hp/cu inch
2018 Corvette ZO6 LT4 650 hp 376 cubic inches 1.73
2018 Porsche Turbo S 607 hp 232 cubic inches 2.62
2018 McLaren 570S 562 hp 232 cubic inches 2.42
2018 Merc Benz AMG GT C 550 hp 244 cubic inches 2.25
2018 Accura NSX 500 hp 213 cubic inches 2.35
Nissan GTR NISMO 600 hp 232 cubic inches 2.59
All of the above are Flywheel hp. Let’s remove 15% to get to the type of power we are more accustomed to since we rate our power on chassis dynos…
Rear Wheel Power
2018 Corvette ZR1 1.7 hp/ cubic inch
2018 Corvette ZO6 1.47
2018 Porsche Turbo S 2.23
2018 McLaren 570S 2.06
2018 Merc Benz AMG GT C 1.91
2018 Accura NSX 2.0
Nissan GTR NISMO 2.2
Average Rear Wheel Power per Cubic Inch 1.94
Now that we have a frame of reference let’s take a look at our turbo rotary:
Rear Wheel Power
(Mazda lists displacement at 80 cubic inches. There are a couple of legitimate ways to derive displacement. In order to be conservative I am going to use 160 cubic inches so as to better compare apples to apples.)
1993 Mazda RX7 OE 217 hp 160 cubic inches 1.36
1993 Mazda RX7 350 hp 160 cubic inches 2.19
1993 Mazda RX7 400 hp 160 cubic inches 2.5
1993 Mazda RX7 450 hp 160 cubic inches 2.81
1993 Mazda RX7 500 hp 160 cubic inches 3.13
1993 Mazda RX7 550 hp 160 cubic inches 3.44
1993 Mazda RX7 600 hp 160 cubic inches 3.75
This is profound. A “lowly” 350 rwhp RX7 is within spitting distance of the highest output boosted motors offered for 2018!.......
2.19 V 2.23
The second profundity is that unlike the above four cycle motors the rotary does not enjoy a cooling cycle after a power cycle. The rotary is a two cycle motor and as such it makes power every time the rotor passes the spark plug!
Power output per displacement is all about Combustion Chamber Pressure (CCP) and attendant Combustion Chamber Heat (CCH).
Get it wrong and you have detonation and perhaps a broken motor. Is it any wonder the turbocharged rotary has a checkered past as to reliability?
Just imagine the internal stress!
Let’s look at it another way:
Let’s assume we take the 376 cubic inch Corvette and pump it up to the same output per displacement as a 550 hp RX7…
376 X 3.44 = 1293 rwhp! Would you consider that motor might be stressed? Chevrolet already added a dry sump oil system, titanium rods to the motor so it might live at “just” 642 rwhp.
In spite of these features Chevrolet elected to STOP at 642… our 550 rotary is similar to running the same motor at 1300rwhp.
I think I have made my point.
Simply put, at any power level, your single turbo RX7 is way more stressed than you think. Great respect must be offered as to tuning and setup.
Setup and systems are all about two things… power and reliability.
A careful read of the above may have your knees knocking but proper engineering can create, IMO, a 500 reliable dual purpose powerplant with occasional blasts to 600. To the extent you operate below these numbers you will, of course, extend life.
Let’s briefly expand the focus a bit to something a bit more fun…
Beating all of the above $100,000++ cars with our RX7 rocketships.
Again let’s go back to good old math:
Rear Wheel Power To Weight Ratios
2018 Corvette ZL1 LT5 3750/ 642 = 5.84
2018 Corvette ZO6 LT4 3619/ 553 = 6.54
2018 Porsche Turbo S 3550/ 516 = 6.88
2018 McLaren 570S 3300/ 478 = 6.90
2018 Merc Benz AMG GT C 3804/ 468 = 8.13
2018 Accura NSX 3868/ 425 = 9.1
Nisson GTR NISMO 3920/ 510 = 7.69
1993 Mazda RX7 2862/ 215 = 13.31
1993 Mazda RX7 2862/ 350 = 8.18
1993 Mazda RX7 2862/ 400 = 7.15
1993 Mazda RX7 2862/ 450 = 6.36
1993 Mazda RX7 2862/ 500 = 5.72
1993 Mazda RX7 2862/ 550 = 5.2
1993 Mazda RX7 2862/ 600 = 4.77
A 500 rwhp RX7 has a better power to weight ratio than all of the above cars! Of course if the motor can't stay together all this potential fun is a fiction. Keeping it together starts with the understanding that the motor is highly stressed even in lowish output levels.
The primary objective of this site to properly deal with this challenge.