TUNING/       FUEL

Fuel is really important to get right as it is the key to dealing with the tremendous amount of turbo'd rotary Combustion Chamber Pressures and Heat.


The most powerful 2018 Supercars generate less CCH and CCP than most  single turbo RX7s. 




Rear Wheel Horsepower per Cubic Inch Displacement

2018 Corvette ZR1                    1.7

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                                      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 rwhp         1.36

1993 Mazda RX7 350 rwhp                2.19

1993 Mazda RX7 400 rwhp                2.5

1993 Mazda RX7 450 rwhp               2.81

1993 Mazd a RX7 500 rwhp              3.13

1993 Mazda RX7 550 rwhp               3.44

1993 Mazda RX7 600 rwhp                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).

A 350 rwhp RX7 has almost the same CCP/CCH as the highest level 2018 SuperCar! We consider a 350 rwhp RX7 to be a very low stress setup.

                                                                                                                              WRONG


It is with this understanding that we approach the fuel decision. If the fuel isn’t up to the job we can generate knock/detonation and possibly break the motor.


Proper fuel is the fix and improper fuel creates a broken motor.

.  

Please do carefully review the Knock Section as it discusses timing and response.

I recently did some road tuning and found that what had been a good timing map a number of years ago generated knock around the 470 rwhp level.

I discussed this with my go to tuner that I have used since 2003. The culprit was a combination of lower quality "winter" gas and the fact that i had bought the gas from a lower volume Mobil station. Gasoline loses at least one octane a month from the time it is distilled.

Had I not been logging knock I would not have even noticed as power was within 10 rwhp and there was no notice from the driver’s seat.

I reduced ignition lead about two degrees and knock disappeared.

It is all about fuel and the only way to both capture the power and maintain powerplant integrity is with logged knock.

Unless you are buying racegas I suggest you consider the 93 or 91 octane gas you are using to be a moving target as to antiknock properties.

As you raise boost, burn-speed rises. That’s why we run around 25 degrees advance at zero boost and less than half that number at 20 psi.

Boost of course is just an expression of resistance to flow. Since it correlates to flow we use it as a proxy. What we are really addressing is FLOW. As I previously have stated there may be no motor that flows like a properly ported rotary.

So we have a ton of flow (even at 350 rwhp) going into a 2.6 liter motor that does not benefit from an every other cooling cycle like a 4 cycle.

                                                                                   Turbo rotary equals HUGE CCP/CCH.


Clearly detonation avoidance is job one. Fuel is the first decision and there are many options.

“EngineLabs” recently released the fourth of a five part series entitled: “The Big Fuel Test.” Bob Morreale, a highly accomplished tuner, authored the series which compares numerous fuels powering a 2016 Corvette with a Vortech V3 intercooled supercharger and a few other mods making 574 rwhp at 8 psi on 93 octane pumpgas.

Both the timing and AFRs were adjusted using knock onset as the limit. (of course).


Here’s the findings…

Here's the pump:

All of the above fuels are gasoline as a base. All were tested at 8 psi. The addition of methanol as AI delivered the most power both peak and average at 7.3% and 8.3% respectively. Notice the runner up was a 50/50 mix of water meth and it was a close second at plus 5.9%  peak and 7.1% average. The three VP racegas options were approx 5% peak and 4% average. Intake air temps were reduced 40 F from 100% meth AI and 25 F from the 50/50.

Timing was interesting:

93  15 degrees

93 plus Boostane 18

93 plus 100% meth 24.5

93 plus water meth 24.5

VP C9 15

VP MS103 22.5

VP MS 109 21

VP C 16 24

Air Fuel was interesting:

93  11.5

93 plus Boostane  11.63

93 pls water meth  12.75

93 plus meth  12.75

VP C9  11.2

VP MS 103  11.3

VP MS 109  11.7

VP C16  12

While the 93 plus 100% meth made the most power the leaner mixture (12.75) which was a reduction in the amount of base fuel (gasoline) offset the additional BTUs added from the methanol.

I was somewhat surprised with the 93 plus water meth as it produced almost the same (435 V 440) average power as 93 and straight meth. Other advantages are that it isn’t flammable and is a bit less corrosive.

Of course as boost and power are raised we may have a different result.

E85 as a base fuel on the surface looks interesting and has a significant usage within the turbo rotary community. Ethanol burns cooler and certainly delivers better detonation protection than pump gas especially over the 600 rwhp mark.

After running 93 pump and methanol for a number of years I switched to E85 to run the October 2013 Texas Mile as 17 seconds flat out in fifth gear around 575 rwhp to 200 mph seemed to be quite the challenge for the motor..

In preparation for the Texas Mile I did run to 205 mph on the dyno. Here’s the log showing 330 KPH/205 MPH. While running on the rollers isn’t the same as on the asphalt we did run to 8650 in fifth.

The second benefit is that the manifold offers FOUR secondary injector ports which when combined with excellent Full Function Engineering fuel rails makes a world beating package. Thanks for the picture Pineapple.

                                                                                                                                                                                FUEL PUMP


You can select the right fuel injectors, size the injectors correctly, run the right base fuel with AI, have the correct injector ontime (tune)  and still go lean and break your motor if you don't have the correct fuel flow rate or pressure. The heart of delivery is the fuel pump. The truth detector is the ESSENTIAL digital fuel pressure sensor... ( why essential covered in the Tuning Section/Hardware.)


If your pressure is set at 43.5 static and you are at 20 psi boost your fuel pressure better read 63.5. Since the rotary requires approximately 30% more air and fuel versus a piston engine the pump needs to have some hair on it's chest. DC voltage pumps respond to voltage so the first thing that needs to happen is to remove and bypass the plastic OE fuel pump wire connector near the tank and hard wire the pump. The pump will step up from around 11.6 V to 13.5, a big step forward. Just as almost everything has made significant leaps forward, fuel pumps have also taken a big jump. i do have a significant treatment as to fuel pumps which is in the sticky thread part of the Single Turbo Section of the RX7club if you want an A to Z rundown so i will keep it short here...

i recommend the Walbro 90000274. OE build quality and the highest flowing bolt in pump on the market at a friendly price. If run at 13.5 Volts the pump can produce 5678 CC per minute at 30 PSI boost. Since 4736 CC are needed for 600 the pump, if hard wired running at 13.5 Volts or more, the pump will get it done.

Assuming 13.5 Volts to the pump, the redlines show approximately 90 gallons per hour ( 5678 cc per minute ) drawing a modest 18 Amps.  Here's the chart:

Maybe there is a magic bullet and I am always open to potential fixes but since I have no interest in anything much above 600 rwhp and gas with AI will get it done without internal carnage I am back in the gas camp. I expect to run the October 2019 Texas Mile and will probably run VP Q16 racegas and a healthy (1000 to 1500 CC) dose of methanol as AI.


My firm view is that if you are running to 600 or anything less your base fuel is pumpgas as it delivers the BTUs (energy) AND much needed lubricity with premix. In addition to gas as base fuel i believe that every FD should run some form of (AI) Auxiliary Injection. For many, 300/400 CC of water will work to clean engine internals and eliminate knock. Please see the Auxiliary Injection Section for more info.

                                                                                                    "how much injector capacity do i need?"


The answer to this question starts with the determination of MAXIMUM power output... which is determined by maximum turbocharger air output in pounds per minute.


You will either need to consult a turbo compressor map (Garrett and Borg Warner) or in the case of Precision which offers some great options take the power estimate and divide it by 1.3 to get to rotary power. (for instance 900(piston) /1.3 would  produce 692 rotary rwhp)

350 rotary rwhp requires 46.4 pounds per minute of air

400 rotary rwhp requires 53.0 pounds per minute of air

450 rotary rwhp requires 59.6 pounds per minute of air

500 rotary rwhp requires 66.2 pounds per minute of air

550 rotary rwhp requires 72.8 pounds per minute of air

600 rotary rwhp requires 79.4 pounds per minute of air


Here is one of my favorite compressor maps. The Borg Warner S300 SX-E offers cutting edge engineering, is inexpensive (contact me for any Borg Warner pricing), and can deliver 550 rotary rwhp. Since it can make 77 pounds per minute in an overboost situation you will want to size your injectors accordingly. 

Once max power output is determined, injector capacity can be settled.


Overkill on fuel capacity is good and underkill is a broken motor. 


Since fuel relates to airflow the answer to the question primarily relates to the turbocharger max flow capability. Do not make the mistake of buying an 80 pound per minute turbo thinking you will run it at 60 pounds and not need fuel for 80. Overboost of 10 additional PSI can happen in less than one second by virtue of a sticking wastegate and you not only need an ignition cut but fuel to maintain a proper Air Fuel Ratio. If you choose an 80 pound per minute turbo it is essential to have net fuel into the motor to deliver a 10.0 Air Fuel Ratio.

Assumptions are based on a static 3 BAR (43.5 PSI)  static fuel pressure setting, a maximum 85% duty cycle and lag at 20%.





350 rotary rwhp requires 2763 CC/Min into the motor after 85% Injector Duty Cycle and 20% Lag. In order to make up for 85% Max Duty Cycle and 20% Lag you need 4061 GROSS/Nominal Injector Capacity.


400 rotary rwhp requires 3158 CC/Min into the motor after 85% Injector Duty Cycle and 20% Lag.  In order to make up for 85% Max Duty Cycle and 20% Lag you need 4641 GROSS/Nominal Injector capacity. 

450 rotary rwhp requires 3552 CC/Min into the motor after 85% Injector Duty Cycle and 20% Lag. In order to make up for 85% Max Duty Cycle and 20% Lag you need 5221 GROSS/Nominal Injector capacity. 


500 rotary rwhp requires 3946 CC/Min into the motor after 85% Injector Duty Cycle and 20% Lag.  In order to make up for 85% Max Duty Cycle and 20% Lag you need 5801 GROSS/Nominal Injector capacity. 


550 rotary rwhp requires 4341 CC/Min into the motor after 85% Injector Duty Cycle and 20% Lag.  In order to make up for 85% Max Duty Cycle and 20% Lag you need 6381 GROSS/Nominal Injector capacity. 


600 rotary rwhp requires 4736 CC/Min into the motor after 85% Injector Duty Cycle and 20% Lag. In order to make up for 85% Max Duty Cycle and 20% Lag you need 6961 GROSS/Nominal Injector capacity. 





Bosch has the primary patents on fuel injectors and the newer 21st century Bosch EV14 fuel injectors should be powering your motor. The dynamic element that controls flow weighs one fifth of the older FD style injectors. Better low speed flow for improved idle, better materials, better atomization. Various vendors have sought to put their twist on them in order to take them from commodity status to becoming a proprietary item. i am quite happy to just run the EV14 as is from Bosch and enjoy the friendly price. i have dealt with Fuel Injector Connection in Georgia. Other excellent vendors are Injector Dynamics, and Fuel Injector Clinic.


While we are on the fuel hardware subject i love my Ground Zero/Xcessive lower intake manifold for a couple of reasons. The OE FD LIM is a nightmare flow-wise due to the EGR influence. Symmetry is essential for equal service to both rotors. A picture says it all courtesy of Tony at the RX7club. 

While this was going on I had three customers who were running E85 during the summer of 2015. One at around 615 rwhp, one 550 and the other at 500. At the end of the summer all 3 motors arrived so that they could have studs installed. All were running perfectly at the time of disassembly.

All looked exactly like my motor. Severely degraded rotor housing contact surfaces and lots of goo in the ports and on the rotors.

on my engine...

All three customer motors and my motor had good compression as the apex seal and rotor housing had conformed with each other but the chrome on the housing was wearing significantly causing a short usable life.

No Thanks.

My view is that if you are looking on the other side of 600 hp E85 probably is  the ticket as it shines with very very high CCP. However, if properly fixtured and tuned, good quality gasoline along with AI will get the job done at 600 and under.

No goo build up, no under-lubed rotor housings and apex seals.

This is my 500 hp four year rotor… compare it to the E85 rotors after 4 months of living on alcohol.

At the Texas Mile I was running over 160 between the ¼ and 3/8 mile marker when my fuel pump fuse decided to give up but prior to that the motor was fine on E85. The lean condition flattened the corner seal springs and when I took the motor apart I realized I had a larger problem than corner seal springs.

At that time I had probably opened and rebuilt 75+ 13B-REWs. My excellent condition rotor housings prior to the Texas Mile looked like they had been run with absolutely no lubrication as did the apex seals.

In addition I had a tar like black goo in my intake port and on the faces and sides of my rotors.

I had spent lots of time picking what I thought was the best E85 premix and was running 1 ½ oz per gallon for the Mile.

A couple of months later at the 2013 Performance Racing Industry tradeshow I talked with almost all the oil companies re what they thought would deliver the best defense against the “dryness” of alcohol. At that time I was just figuring I had the wrong premix and a better premix would solve the problem.

I had lengthy discussions with Klotz, Redline, Driven, Amsoil, and Ashland/Valvoline. I was impressed with Scott Wieland, Senior Staff Scientist, CLS. Scott mixed a special 5 gallon batch of premix and… same result.

The part number on the chart is for the pump only, you want the pump and installation kit 90000274. Walbro introduced a gas pump also but it flows a bit less and the E85 pump is gas friendly.


Dual pump setups perhaps were a viable option back in the day before the very high flowing single Walbros were introduced. Dual pumps for 600 rwhp on gas should be extinct going forward.

Two different excellent tuners

Three different apex seal brands

Three different premixes

Two common problems.


Lack of proper lubrication between the apex seal and rotor housing surface and… goo.

Goo all over the rotor (both face and sides) and the intake ports.