This story originally appeared in SportsCar magazine's August 2013 issue. SportsCar is the official publication of the Sports Car Club of America. www.scca.com.
Most of us don't give fuel injectors a second thought – that is, unless something goes wrong. However, these simple little devices help create so much power it's not a topic that should be overlooked. Perhaps there is even an upgrade in your car's future.
Typically, you are going to find one of two types of gasoline fuel injectors under the hood of your racecar, a “peak-hold” or “saturated driver” injector. These two types differ by impedance value, and how they are activated. While both may appear identical on the outside, it's very important not to mix the two.
According to Chris Mills of DeatschWerks, a peak-hold injector will have impedance between 1.5-4ohms, while a saturated driver injector has higher impedance, closer to 11-15ohms.
The first step is figuring out what type of injector you need; using an injector with the wrong impedance can cause tuning issues and possibly even an ECU meltdown.
“Some ECU manufacturers require a low ohm [injector], some require high ohm,” explains Tim Marren of Marren Fuel Injection. “There are two distinctly different electrical types of injectors – this is the problem in the industry where people get caught with their pants down all the time. It's Ohm's law: If the ECU requires a low ohm injector, you've got to use a low ohm injector. If you start mixing and matching the injector driver, the ECU will cut the injector open time. If you request 10ms of open time at 6,500rpm you might only fire for 6ms because of the delay factor; you will be running lean. If an ECU requires a high ohm injector and you put in a low ohm injector you will blow the fuse in that ECU – or hope you do – because if you don't you will burn out that ECU.”
Next up is calculating the capacity of the injectors required for your engine. There's a mathematical formula you can use to calculate this, but fortunately you don't have to, as companies like Marren Fuel Injection, DeatschWerks, and RC Engineering offer fuel injector calculators or worksheets online.
“It starts off by punching in numbers based on [target] horsepower,” says Marren. The next factor is Break Specific Fuel Consumption (BSFC), which is a factor based on rate of fuel consumption and induction type. A naturally aspirated piston engine will have a BSFC around 0.5, while a rotary powered car or one with forced induction will have a slightly higher value – fortunately, this information is typically provided on the Websites for you, and some simply let you select induction type on a worksheet. The final steps include plugging in the number of injectors you plan to use and the duty cycle you want to run them at.
“We are doing an SCCA GT car right now, and its estimated horsepower is 325hp,” says Marren. “If we punch that in, and it is naturally aspirated, we put a BSFC factor of 0.5. Then we look at how many cylinders you have, which is basically how many injectors you are going to use; in this particular case it is a six cylinder. Then what you want to do is [input an] injector duty cycle of 0.8 – what that means is the injector duty cycle should be 80 percent or less. If you hit calculate you will see we need about a 33lb/hr injector.”
Don't start shopping for injectors just yet – you will also need to be mindful of how injectors physically fit your application. “The other thing you've got to worry about is O-ring to O-ring; physically fitting in the intake manifold and fuel rail,” Marren says.
“Typically, domestic [and most European] injectors are 14mm upper and lower, but [Japanese] imports are typically 11mm on the upside and use a grommet on the lower side. We end up making special injectors for the import market because some of the OE applications have limited by size availability and flow characteristics, so we will take a Bosch or Siemens injector and machine it to fit an import.”
If you have any doubts as to whether or not you are selecting the right injector for your application, reach out to any one of the many manufacturers or vendors that specialize in this market. Perhaps one of the advertisers on pg. 56 will have exactly what you need.
Search for fuel injectors here:
Injector Dynamics Precision Turbo Engine DeatschWerks
www.injectordynamics.com www.precisionturbo.net www.deatschwerks.com
(214) 607-9022 (219) 996-7832 (800) 419-6023
Fuel Injector Clinic RC Engineering Marren Fuel Injection
www.fuelinjectorclinic.com www.rceng.com www.injector.com
(561) 427-0082 (310) 320-2277 (203) 267-FUEL