Injector Options by ECU Type
Ideally you want to be able to run larger injectors at a lower fuel pressure, however, without additional fuel management this is not possible. The stock ECU has some wiggle room to run a larger injector; the 1.6 ECU should be able to idle up to 305cc injectors. However, it will require tweaking the AFM idle-mixture settings and spring tension. Any +94 miata should have the wiggle room built in to idle +10/15% larger injectors. There's nothing you can really fool with to trick the ECU to properly run anything larger.
Stock Miata Injectors:
|90-93 (Blue) 230cc - #195500-1970||94-97 (tan) 265cc - #195500-2180||99-00 (red) 260cc - #195500-3310||01-05 (purple) 265cc - #195500-4060|
If you notice the body shape of the 99+ injector they are a more desirable injector due to the pintle design; they atomize fuel better.
If you are using a “piggyback” fuel management unit such as the BEGI or Vortech FMU, the popular injector to use is a 305cc injector from the Supra.
The best option for running larger fuel injectors is to use a standalone ECU, such as Megasquirt. See the Aftermarket ECU section for more info on that. If you are running a standalone ECU there are two main options to consider for larger injectors: scrap yard injectors and EV14s
Scrap yard injectors
Plug and play injectors from other cars are necessarily going to be from the same era as the miata, since modern cars aren't built with the same type of denso injectors. That means they are a few generations older, and you will sacrifice spray pattern (atomization) and idle quality. This is the cheapest option though, so if budget is your number one consideration then you can go this route. The most common scrap yard injectors are 420cc yellow tops from an RX8, followed by 460cc or 550cc injectors from an RX7 turbo. The 460s should be able to support around 250rwhp, the 550s around 300rwhp at stock fuel pressure levels.
The Miata uses high ohm injectors. Also know as saturated injectors.
Complete list of Miata plug and play replacement injectors from other vehicles (“F” connector):
|87-88||Toyota MR2||4AGE NA||213cc – Beige Top||#23250-16080|
|89-91||B2220 Truck||224cc - Yellow Top?||#23250-74040?|
|99-00||Toyota||4AGE||250cc – Green Top|
|99-00||Toyota||4AGE||250cc – Violet Top|
|94-97||Mazda Miata||254cc - Tan Top||#195500-2180|
|99-00||Mazda Miata||260cc - Thin Red Body||#195500-3310|
|01-05||Mazda Miata||NA & TURBO||265cc - Thin Lt. Prple||#195500-4060|
|01-05||Toyota||3SGE||295cc – Green Top|
|89-92||Toyota Supra||7MGE NA||305cc – Light Green Top||#23250-70080|
|93-95||Toyota Supra||3.0L||312cc - Maroon Top||#23250-46030|
|93-95||Toyota||3SGE||315cc – Pink Top|
|90-92||MX6/626/Probe||Turbo||326cc - Gray Top||#195500-2150|
|89-91||B2600 Truck||326cc - Gray Top||#195500-2150|
|89-91||Mazda 323 GTX||Turbo||360cc - Black Top||#195500-2130|
|89-91||Celica/MR2||3SGE NA||370cc - Green Top||#23250-74160|
|03-12||Mazda RX8||420cc - Yellow Body||#195500-4450|
|89-92||Mazda RX-7||NA||440cc - Blue Top||#195500-5740|
|89-91||RX-7||NA||460cc - Red Top||#195500-2010|
|89-91||RX-7||Turbo||550cc - purple top||#195500-2020|
The same connector shape can be found on low ohm injectors, but they require wiring in resistors to increase the resistance and filing down the tabs on the sides of the injectors.
Direct fit LOW OHM injectors (“E” connector):
|89-92||Toyota Supra||(7MGE) NA||295cc – Yellow Top||#23250-70040|
|88-91||Toyota Collora GT-S||Turbo (4AGZE)||365cc – Red-Orange Top|
|92-95||Toyota Collora GT-S||TURBO (4AGZE)||365cc – Red-Orange Top|
|86-92||Toyota Supra||(7MGTE) TURBO||430cc – Black Top|
|Toyota MR2||(3SGTE) TURBO||430cc – Black Top|
High Ohm vs. Low Ohm tabs
As you can see, the location of the tab on the connector are in different locations. Simple filing them down allows the direct plug in of these types of injectors. Some aftermarket ECUs have the ability to control Low Ohm injectors. Otherwise, it's easier to stick with the High Ohm lot.
Bosch EV14 injectors are the most modern port injection injectors available today, and are used by most OEMs. Major advancements in fuel injection technology has been made in the past 20 years, and the result is well worth it. They have better atomization and will idles better than stock when well tuned.
EV14 injectors are not plug and play out of the box:
- They typically have 14mm diameter tops rather than 11mm for the miata
- They have EV6/EV14 plugs rather than the denso plugs on the miata
- They have a different seat
To make run them in a miata, you need several adapter pieces. Prices range from $250 if you go DIY to $299 for a plug and play and flow matched 610cc set to >$500 for 1000cc+ units from Injector Dynamics.
To build a DIY set, you need:
- injectors - 48mm standard length such as the 550cc or 610cc OEM injectors from the Shelby GT500
- top hats - 12mm (or 12.5mm) long 14mm to 11mm diameter adapters
- o-rings - usually come with the top hats
- Denso bottom cushions - you can reuse the ones from your stock injectors or buy these new
- spacers - used to raise the stock fuel rail by ~2mm. They should fit around the stock fuel rail isolator. You can either find a washer that fits, make your own, or buy a special purpose spacer
- connectors - must be EV6/EV14 female to Denso male, and be the type with wires between the ends. The solid type connector doesn't fit under the stock fuel rail.
Plug and Play
Choosing a Fuel Injector
In most cases a turbocharged engine will have a B.S.F.C of .60. B.S.F.C is brake specific fuel consumption; How much fuel you are using per horsepower per hour. This means that the engine will use .60 lbs. of fuel per hour for each horsepower it produces. Using .55 for the Miata seems to result in more accurate calculations.
Most injectors will max out at 80-85% duty cycle and this is the accepted industry standard.
Using these numbers as a guideline, you can select the approximate injector size in the following formula for your Miata to reach an estimated 200HP (engine)
Est. Horsepower x B.S.F.C / # of injectors x duty cycle = lb/hr per injector
cc = lb/hr x 10.5
200 x .55 / 4 x .8 = 110 / 3.2 = 34.4 lb/hr x 10.5 = 360cc
Then determine the amount of fuel pressure you will have @ the rail to determine what injector will reach this desired cc level. Since injectors are typically rated at 43.5psi, this calculation will determine what they will flow at a higher pressure level:
FUEL PRESSURE / 43.5 = New Flow Rate
Square Root of NFR x Old Flow Rate = New Flow Rate of Injector
At 6psi with a 8:1 ratio FMU we will have about 96psi of fuel @ the rail. We will use that to find an injector that will flow 360cc at 96psi.
96 / 43.5 = 2.206
sqrt of 2.206 = 1.485
1.485 x 265cc = 393cc
That means at 96psi a 265cc injector will supply the needed 360cc (and a little extra breathing room) to reach 200 HP (165rwhp).
So the 1.8 injectors will be less than 80% duty cycle at 6psi of boost with 96psi of fuel pressure to reach 160rwhp. But what if we add more fuel pressure. Remember that calculation was with a 8:1 ratio. The most aggressive ratio is 12:1. Let's see how that would effect a 1.6L injector:
12 x 6 + 48 = 120psi of fuel
120 / 43.5 = 2.75
sqrt of 2.75 = 1.65
1.65 x 203cc = 334cc
200 x .55 / 4 x .8 = 110 / 3.2 = 34.4 lb/hr x 10.5 = 361cc
So the 1.6 injectors will be short of our target to supply enough fuel for 200BHP even at 120psi of fuel at the rail.
If you want to take the easy way out the same calculations can be plugged in here: http://www.rceng.com/technical.htm
Link to Toyota production injector applications