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How To: Fuel Injector Sizing/ GASOLINE AND E85

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How To: Fuel Injector Sizing/ GASOLINE AND E85

Old 02-02-14, 08:52 AM
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How To: Fuel Injector Sizing/ GASOLINE AND E85

Much of this post can be found scattered around this board but i thought it would be a good idea to put it all together and add in alternative fuels such as E85, ethanol, methanol and that non-fuel water..

The rotary is unique. It would be a big mistake to use piston metrics when planning setup.

How unique?

Let's talk airflow:

A piston engine can generally make ten hp for every pound per minute of air, so a 70 pound per minute turbo can make 700 piston rwhp. As we navigate the web looking at turbos we often see a hp tag... for instance a 73 pound per minute TO4Z/GT500/PT67 is often rated at 730 hp. If you are looking for 730 you'd best look elsewhere because they are talking PISTON lingo.

Should you wish to translate to rotary you'd take the pounds per minute, or the estimated HP number, and divide by 1.3.
73 pounds per minute of air is 561 ROTARY rwhp.

BTW, that doesn't mean the rotary takes a back seat to the piston engine. While it does take a discount due to lower volumetric efficiency (VE), it more than makes up by flowing more air versus displacement.

O K, let's get into it starting with airflow and hp.

FDs are all about airflow from turbos.

Most turbos have a compressor map which sets forth maximum airflow in pounds per minute and other metrics such as efficiency as well as airflow at differing boost levels etc. (see Sean's “How To Read a Compressor Map” thread sticky in the Single Turbo Section).

This thread is about properly sizing the fuel system(s) so let's just focus on the maximum air as set forth on a commonly used GT3582R compressor map.
Airflow is represented in pounds per minute. The standard GT3582r makes approx 62 pounds per minute max.

Rotaries require 1.92 CFM (cubic feet per minute) to make one rear wheel hp.
62 pounds per minute times 14.471 equals 897 cubic feet per minute.

Now that we have CFM we divide by 1.92 to get rear wheel rotary hp.
897/1.92 = 467 max rotary rwhp.

TO4Z, GT500, PT67, Borg Warner S300 63, the newer GTX3582r turbos make approx 73 pounds per minute.
73 X 14.471 = 1056 CFM/1.92 = 550 max rotary rwhp

moving to the 80 pound+ turbos…

GT4094r makes 80 pounds, 1157 CFM, 603 rw rotary SAE hp

BW S300 64 FMW makes 80 pounds, 1157 CFM 603 hp

GTX4088r makes around 84 pounds, 1215 CFM, 633 hp

GT4294 puts out 85 pounds, 1230 CFM, 641 hp

Now that we have an understanding how to get CFM from pounds per minute and then rw rotary hp let's move on to solving for fuel requirements from air generated.


Air Fuel Ratio... 10 to 1, 11 to 1, 12 to 1. These ratios are for air and fuel measured in POUNDS.

11 to 1 is 11 pounds of air and one pound of fuel.

Back to the standard GT3582r. Let’s calculate fuel injector sizing.

We need some "givens."

"Given" we want to have enough fuel to run 10.0 to 1 AFR if necessary. A small amount of overkill in the fuel department is prudent. We won’t be tuning 10.0.
"Given" we want to run our fuel injectors at no more than 85% capacity (expressed as duty cycle). Injectors run at more than 85% become unhappy.

Assuming 62 pounds per minute of air, at a 10 to 1 ratio we need 6.2 pounds per minute of gasoline.
62 pounds air
6.2 pounds gasoline

The 6.2 pounds is NET into the engine.

When sizing injectors there are two deducts to get to NET that need to be added back to get to GROSS which is the common number used when referring to injectors by size.

The two deducts are duty cycle and lag.

We wish to limit our fuel injectors to 85% duty so we divide 1 by .85
1/.85 = 1.1764

another adjuster that needs to be recognized and added to the calculation.... LAG.

Fuel injectors are sold based on max flow output such as 850 or 1600… this is measured when the injector is open continuously.. in the real world, our world, injectors open and close.

It takes time to open fully. During this time and the duration of the closing time the injector is not flowing as much as if it were wide open. This is called “lag” or “deadtime” and it costs flow. The amount is somewhat variable based on voltage and other variables.

The delivery cost of lag varies and I will use 13%.

My method of injector sizing starts w a known, the airflow in pounds and the AFR, which gives us fuel in pounds. We now can convert pounds to cc per minute and we know the NET Fuel Needs.

In order to select the correct injectors we need to calculate the Gross Fuel Needs.

Which is now easy…

Back to the 62 pound GT3582r

Adjusting for 85% duty and 13% lag we find we need to multiply our 6.2 pounds by 1.352 to get to GROSS Injector requirements.

6.2 X 1.352 = 8.382 gross pounds of fuel.

The next step is to convert the pounds of gasoline to gallons. Gasoline weighs 6.35 pounds per gallon.

8.382 pounds / 6.35 = 1.320 gallons per minute

Google now does conversion so Google “1.32 gallons per minute to cc per minute” and you get 4996 call it 5000 cc per minute.

5000 GROSS injector capacity will properly service 62 pounds per minute or 467 rotary rwhp..

Given the above calculations we can now easily calculate Gross Fuel Injector delivery requirements for other turbos based on max air output.

TO4Z, GT500, PT67, BW S300 63, the newer GTX3582r turbos make approx 73 pounds per minute.

550 max rotary rwhp/467 = 1.177 X 5000 = 5888 Gross Injector

moving to the 80 pound+ turbos…

GT4094r 603/467 = 1.291 X 5000 = 6456 CC Gross Injector

BW S300 64 FMW 603 = 6456 CC

GTX4088r makes around 84 pounds, 633/467 = 1.355 X 5000 = 6777 CC

GT4294 641/467 = 1.372 X 5000 = 6863 CC

Yes, you can make the above power w slightly less injector because I am wanting you to have enough to run 10.0 if need be. Further, you can up the rail pressure from 43.5 psi for more net flow. The newer Bosch EV14 injectors can run at 8 BAR (116 psi). Just remember that you get the square root of the pressure increase.

As example, let’s say you raise static pressure 26.4% from 43.5 to 55 psi.

You would get a 12.4% flow increase.

of course all of the above is for gasoline. there is a growing trend towards E85 which changes injector requirements. actual rotary power output per volume of E85 is not as settled as gasoline however the BTU relationship between the two fuels is settled.

Gasoline (racing or pump) has 116,090 BTUs of power per gallon.

E85 has 82,300. (i am using 82% ethanol, 15% gasoline and 3% non-fuel)

116,090/82,300 = 1.41

IF the fuels relate on a BTU basis in the rotary you will need 41% more fuel injector capacity.

notice the word "IF"

it may be possible that other factors besides BTU content influence power output. i have found in my limited E85 experience that my actual E85 consumption is somewhat less than the BTU relationship and this could be entirely possible. i will soon be back on the dyno for a lengthy evalutation of my BW EFR9180 and will have better info at that time. if, however, you go w the 41% increase w E85 you won't be underfueled.

more later on E85.


Last edited by Howard Coleman CPR; 04-21-14 at 06:58 AM.
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Old 07-31-14, 09:39 AM
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This is AWESOME info!

Side Note: I didnt know CPR was in WI! I will be having you guys build my motor when she finally lets go! I am right in Madison!
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Old 08-06-14, 08:54 PM
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Thanks for the awesome info!!
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Old 09-11-14, 07:36 PM
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Thank you so much for this info Howard. Couldn't come at a better time. Absolutely enjoy reading your posts, they have influenced my build so much.
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Old 05-23-15, 12:49 PM
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Great post man, very informative. Broken down enough for even ME to understand and assist in choosing what size injectors I need for my 72mm full bp build. I'll be using e85 so I may contact you further for some assistance though. Again just an excellent post.
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Old 03-14-18, 03:46 AM
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awesome information, very good explanation and breakdown
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