THE "WHY" OF ENGINE SYSTEMS AND DESIGN
Before we get into systems and setup it is important (again) 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
2018 Porsche Turbo S 607 hp 232 cubic inches
2018 McLaren 570S 562 hp 232 cubic inches
2018 Merc Benz AMG GT C 550 hp 244 cubic inches
2018 Accura NSX 500 hp 213 cubic inches
Nissan GTR NISMO 600 hp 232 cubic inches
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
2018 Porsche Turbo S
2018 McLaren 570S
2018 Merc Benz AMG GT C
2018 Accura NSX
Nissan GTR NISMO
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.
1993 Mazda RX7 OE 217 hp 160 cubic inches
1993 Mazda RX7 350 hp 160 cubic inches
1993 Mazda RX7 400 hp 160 cubic inches
1993 Mazda RX7 450 hp 160 cubic inches
1993 Mazda RX7 500 hp 160 cubic inches
1993 Mazda RX7 550 hp 160 cubic inches
1993 Mazda RX7 600 hp 160 cubic inches
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 they 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, your 400 rwhp RX7 is way more stressed than you think.
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.
Power is easy.... mitigation of Combustion Chamber Pressure (CCP) and Combustion Chamber Heat (CCH) is the challenge.
The Normally Aspirated (NA) world is a very different from Turbo-Land
NA motors must make do with nothing but 14.7 psi of atmospheric pressure. No boost knobs in sight. The engineering effort is totally focused on getting every CFM available into
the engine. It is all about oxygen flow/mass into the motor just like a turbocharged app but with a much much smaller driver behind the flow.
Flow benches are the Normally Aspirated tool.
Flow benches are one of the weapons of choice in the unending search for every CFM.
So what does all this mega engineering deliver? Prostock motors are 500 cubic inches and make around 1300 rwhp on mandated Sunoco SR18 fuel. SR18 is a race gas with no alcohol and an octane rating of 118.
NHRA features another class, Pro Mod, that I find interesting. There are three iterations, Nitrous Oxide, Supercharged and Turbocharged. The turbocharged motors are 540 cubic inches, two 88 mm inducer turbos and run on methanol. They make as much as 3200 hp.
Since flowbenches measure static flow, computer aided modeling of the array of dynamic factors is now at the cutting edge leading to volumetric efficiencies (VE) of 20% more than atmospheric pressure. Every detail of every
port, intake runner volume, length, shape and cross sectional area is scrutinized.. Add in valve timing, temperature management, fuel and of course highly sophisticated computer engine management.
Nowhere (other than OE) is this better expressed as in NHRA Pro Stock. State of the art NA.
The primary defining factor for the rotary 13B-REW is its’ ability to flow HUGE amounts of air. Air of course is POWER and when combined with the vastly uprated 21st century turbos really big power is easily generated.
What we are talking about is the fact that it is easy with an appropriate turbo to generate very high Combustion Chamber Pressure (CCP) and Combustion Chamber Heat (CCH)..
Easy to the point where we would be talking, comparatively, to a 1300 rwhp Corvette. BUT Chevrolet stopped around 640 with the top of the line 2018 $100,000+ ZR1.
Our objectives are making big power with reliability.
Big power is easy as slap on a big turbo.
Big Turbo = Big Hammer
Of course Big Hammers can Break Things
Especially motors that have almost infinite flow capability.
We create stratospheric CCP and CCH.
Optimized FD setup should combine NHRA ProStock NA flow technology with NHRA ProMod CCP and CCH management. (sure, both NASCAR and F1 would also work)
Structure in all things has a great deal to do with outcomes and structure certainly has a lot to do with turbo rotary performance.
Get it right and you are a happy guy, get it wrong and you will be moving on to a Camaro or whatever.
I come from the NA world having, among other things, raced for 16 seasons in SCCA’s GT3 class in the very competitive Central Division. During the first ten seasons I built all my own 10,000 RPM single overhead cam dual Weber DCOE45 sidedraft carbs. They made 205 flywheel hp. I switched to Mazda for the last 6 seasons. (see details in Suspension Section).
All NA, no turbos. Every year we were racing for inches. Every detail counted. When I purchased my FD (1999) I started acquiring file pictures of engine bays, manifolds, intercoolers and coming from an NA background I was totally shocked.
It appeared, often, that PACKAGING trumps everything. The tail is wagging the dog. The dog being performance. Let's now move on to system design.
Type your paragraph here.
Pro Stock 2.6 hp per cubic inch
Pro Mod Turbo 5.92 hp per cubic inch
1993 Mazda RX7 @600 rwhp 3.75 hp per cubic inch
Most of us do not have either the time, resources or interest to engineer our cars to this level.
We do have access to a resource not available to Pro Stock that will enable us to easily make more power per displacement:
Note the multiple spacers between the top and body of this plenum. The spacers change plenum volume. Plenum volume too small can starve a motor… too large will make it less responsive.
How's this for an air intake?