Have alluded to the fuel injectors on the FI models in previous threads. I'm presently trying to make sense regarding how the injectors are sized and what can replace them. The TPS is an issue as well and may be an active accomplice in this issue, but will be for another thread.
The basis for this quest is that the injectors on my '85 were cleaned and flow tested last year, and I have small stumble, ever so slight, on acceleration, but the main issue is the fuel smell at start. Have this smell with the OEM low impedance injectors as well as the newer high impedance injectors. Can this be the TPS alone, probably not, but as mentioned, a willing accomplice in this issue.
The specs for the fuel system are:
Fuel Flow - 630 cc/min minimum
Injector resistance - 1.5 to 1.8 ohms
When I had the injectors cleaned and flow tested, the flow at 40 PSI (for the fuel injected models - 36 PSI static; 28 to 34 PSI dynamic) was 65/66 ml/min (cc/min). I expect this to be a maximum flow rate of the OEM injector.. The resistance was checked and recorded at 2.5 ohms. Should use 14 volts as reference because this is what the electrical system voltage is to be maintained at; however, all the sites I have visited regarding injectors and mention current use 12 VDC - so I shall as well.
New OEM low impedance injectors at 1.5 to 1.8 ohms relate to a current requirement of 6 to 9 amps. At 2.5 ohms the current requirement is 4.8 amps. Low impedance injectors are a peak and hold injector meaning that a high current is used to trigger the injector, and once open a lower amperage to keep it open. With the lower amperage because of the increased resistance of the injector, the injector performance will be compromised. It is reducing to that required by a high impedance injector that requires about 1 amp to operate. Another issue to keep in mind is that high impedance injectors do not require a resistor pack to operate, then again neither does a low impedance injector, but the low impedance injector will quickly overheat and performance again suffers until the injector fails.
There is also a resistor pack installed in the injector circuit. This limits the voltage to the injectors so that the injectors will not overheat. If the design is for a peak current of 6 amps, I estimate the resistor pack is designed to reduce the injector voltage to 9 VDC - can't say for certain, but not a bad estimation, giving a constant 5-6 amps to the injectors. Using 9 VDC with a resistance of 2.5 ohms, the current falls to 3.6 amps, and the injector(s) performance is degraded. Honda appears to have close tolerances when it comes to the injectors, as well as other aspects of the system.
Low impedance injectors are also generally related to high fuel flows.
High impedance injectors can operate adequately in a low impedance circuit because of the lower amperage rating for operation, typically about 1 amp.
With a minimum fuel flow of 630 cc/min, and splitting this between 4 cylinders, I estimate that each cylinder should be using approximately 158 cc/min. This equates to a fuel flow of 15 lbs/hr. If I use the figures from the cleaning of 65/66 ml/min, and with four injectors, this would give me a fuel flow of approximately 264 cc/min that should allow for adequate idle and no fuel smell at start because of the lower fuel flow, but it does not.
Another thought I have is that because of the lower amperage required because of the increased resistance of the injector(s), the ECU is increasing the injector fuel duration to get the engine up to an RPM for fast idle, and once the engine has warmed up to operating temp, the injector fuel duration is reduced to accommodate. Using this analogy, on acceleration, since the injector has a reduced fuel flow and the amperage is not at the required design limit for peak and hold limit, the injector fuel duration is not allowing adequate fuel into the cylinders for smooth operation.
This web site is very informative and also has a video that illustrates what happens when low impedance injectors are used without using a resistor pack. The temperature graphs are illuminating as well. The site is: https://www.hondata.com/tech-low-high-i ... -injectors
You may ask where I got the 15 lbs/hr fuel flow requirement. I used the hp rating of 94 hp at the crank as the basis, plugged it into several on line calculators. Using the fuel pump fuel flow of 630 cc/min gave me a range between 17 and 19 lbs/hr injector fuel flow depending on the HP rating that was used.
Injector technology has advanced and Honda apparently no longer uses low impedance injectors. High impedance injectors sacrifice response time, but generally has a longer component life. Low impedance injectors require a higher amperage, approximately 5 amps, to open, but prolonged operation at this level will overheat and damage the circuitry. There is probably a switching circuit in the system that reduces the current to a more acceptable, lower amperage level once the injector is open, hence the peak and hold analogy.
In most peak and hold circuits, one driver generally operates two injectors giving the circuit a peak of 2 to 2.5 amps per injector with a hold of approximately 0.5 amps. Using this information and having low impedance injectors with a higher resistance of 2.5 ohms, the amps of 3.6 using a resistor bank reducing the system voltage to say 9 VDC, the amperage in the system is becoming inadequate to open the injector fully.
Here is a site that has considerable information regarding FI systems and components: http://performancefuelsystems.com/tech.htm
Not trying to over think this issue, just trying to understand it. All input gratefully appreciated. Also corresponding with a company in the US regarding new injectors.