Honda Injector Guide
Choosing the correct size injector for you build is a vital part of the process. There's a lot to consider, including, size and type. There are two basic types of injectors that work on Honda and Acura cars and every size for every power need. Let's dive in.
Peak / Hold VS Saturated Injectors
Peak and Hold:
Peak/hold injectors are low impedance, usually in the 1-4 ohm range. When the ecu calls for fuel to be injected, it sends out voltage via the wire clips until a certain current level is reached (the peak part) (varies by injector size, company). For the duration of 1 pulse width, that current is slightly reduced and held (the hold part).
Advantages of this design:
- minimizes the injectors "on" time, resulting in faster response
- increases coil heat, which can lead to failure over time
- On most obd1+ Honda's, this type requires a resistor box, which can mean wiring changes.
Saturated:
Saturated injectors are generally higher impedance than peak and hold, running in about the 10-15 ohms range. Unlike peak/hold, a saturated injector remains "on" for the entire pulse width.
Advantages of this design:
- reduces heat
- slower response time
- can't handle large CC or lb/hr styles due to limitations in its speed.
Injector Type Decision
There are a ton of factors involved in this.
- First, power level. If you're going big, there simply won't be a saturated injector that will support your power level. The largest common saturated injectors on the market are 550cc, with 440cc being a common "cap" in saturated size. Most boosted people run peak/hold setups for not only their availability, but also their ease of upgrading later on if so desired. simply swap the injectors. no need to re-wire again.
- How is your car wired? most obd0 civics and some preludes are wired for peak/hold and have a resistor box already. No point in changing to a saturated. Obd1+2 civics, and most other Honda/Acura cars after 1992 are all saturated and are not wired with resistor boxes. Again, if there is an injector available in your power range for flow of your car's design type, there's little sense to change.
- Price. DSM eclipse/talon 450cc peak and hold injectors fit Honda rails and, with just a minor modification, plug right in. Best of all, these can be had for about $50-75 a set of 4 instead of $350+ for a set of aftermarket injectors. Add a $25 resistor box from an obd0 civic/crx, wire it up, and you're good to go in you're saturated car.
Amazon product Amazon product Amazon product
Search for RC Injectors on eBay
What size injectors do I need for my build?
Picking the correct size injector is a critical part of your build. Too small and you'll not have enough fuel to make the desired power or run lean. Too big and you'll have streetability issues such as requiring a high idle and poor part throttle smoothness.. Larger injectors will pour in more fuel for any duty cycle length and it may simply be too much leading to cylinder wash out.
Luckily, there's a formula to help you choose!
injector size in lbs./hr. = (Max. HP x BSFC) / (number of injectors x duty cycle)
- Max Horsepower. Understand your realistic horsepower goals. If you just put a header on, no, your goal is not 300whp. Be Real. This figure will help determine the proper size.
- Target your BSFC, Brake Specific Fuel Consumption factor. Generally, turbo/super charged cars target richer a/f mix. Some tuners will target a 10:1 ratio while all motor builds may use a leaner 11.5:1 ratio. This multiplier helps account for this difference.
- All motor BFSC: .45 - .55
- Turbocharged BFSC: .55 - .65 (lower for lower hp, higher for higher hp)
- Safe Duty Cycle. As a general guide, you want to target no more than 80% duty cycle at peak RPM. Some race cars break this but the streetability factor is greatly reduced.
lbs/hr = (300 * .60) / (4 * .80)
lbs/hr = (180) / (3.2)
lbs/hr = 56.25
I used .60 as the BFSC factor as 300 whp is a 'medium', if not small, build factor that won't need the extra fuel multiplier.
At this point, due to most import injectors using cc/min instead of lbs/hr, we need to do one final conversion.
There's a simple formula for converting lbs/hr to cc/min:
cc/min = lb/hr * 10.5
If you ever need to go back, the reverse is also true:
lb/hr = cc/min ÷ 10.5
Back to our example and we have 56.25 lbs to convert to cc.
cc/min = 56.25 x 10.5
cc/min = 590
So, we should be targeting at least a 590cc injector (but that's an odd size), and even going to a 650cc is a good idea.
Adding Fuel Rail Pressure
If you already bought 550cc injectors for your 590cc need, there may be hope! If you come up a little short due to making more power than expected, you can add a little rail pressure to help push a little more fuel in. This should be used only as a 'fine tuning' objective and the proper size injector should be used wherever possible.
Most Honda's run 42 psi rail pressure from the factory. You can increase this with a pressure regulator. Say you go to a 50psi rail pressure setting, you can add on some flow.
Once again, we have a formula for that!
[Square root of (new pressure/old pressure)] * old flow rating in lbs/hr = adjusted flow rating at pressure (AFR@p)
[Sqrt(50/42)] * 56.25 = AFR@p
[Sqrt(1.19)] * 56.25 = AFR@p
1.09 * 56.25 = AFR@p
61.36 = AFR@p
By adding 8 psi of rail pressure, we bumped our flow rate over 9lbs/hr or, almost 100cc!
That puts our 550s at 50psi of rail pressure comfortably in range to run the 300whp goal. Again, caution on going too high on pressure - 10-20% gain is as high as you want to go in most cases.
Of course, remember to adjust your tuning software to match the injector size. It is critical to a successful tune.
Related content:
Honda Injector Flow Rates
All obd 1/2 B series are 240cc's saturated All obd 1/2 D series are 240-cc's saturated 1992-96 Prelude non-vtec Peak and hold 240cc 1992-96 Prelude VTEC Peak and hold 345cc 1997-up Prelude VTEC Saturated 280cc
DPFI to MPFI Conversion
How to Convert DPFI TO MPFI: A Wiring Guide SUPPLIES: 1 black 0BD0 ecu connector w/ pins 1 0BD0 MPFI engine harness Distributor plug w/ wires 1 0BD0 MPFI ECU 1 Injector resistor box (w/ enough wire to solder to) Various wires in colors close to factory colors as possible heat shrink tubing...
what does a resistor box do?
hey i have a 92 jdm h22a and my mechanic tells me im supposed to have a resistor box. all i know is that its for the injectors. does it regulate the voltage or something? what is it and what exactly does it do. i dont have one and my car ran great.
Get help in our tuning forum ECU's, Electronics, and Tuning
Last edited by a moderator:
