Excess capacity of typical fuel injection system 1

[ivymike]

Does anybody have a feel for how much excess capacity is built into a modern automotive fuel injection system, for a spark-ignition engine?

How much extra air would your engine need to be taking in before your injectors could no longer deliver enough fuel?

What part of the system is typically the most restrictive? Fuel pump capacity? Injector capacity?

Thanks for any info.

 

[NickB]

It all depends on the system, some will hit limits in one area before others.

Example #1: Turbo Eclipse/Talons, mass airflow system with karmann vortex MAF: The early cars will hit "fuel cut" when the computer tries to go above 100% duty cycle on the injectors, after which point it will shut down. Typically this happens at a little under 300 hp i believe, stock levels are 195-205 depending on year. Change the injectors and trick the computer with one of the air fuel adjustment computers, and the next problem you'll run into is the flow limits of the mass airflow sensor. Aftewr that, the fuel pump.

Example #2, Chrysler turbo 2.2/2.5 engines: Speed density system based on a 2 bar MAP sensor, so anything above 14.7 psi, and the computer doesn't see it. Remap the cmputer for the 3-bar map and you'll run out of injector very fast. You'll run out of fuel pump at about 290 hp.

Generally, there's enough injector there to get at least some more power out of it. I'm running 18 psi on a bone stock chrysler 2.2 fuel system, but i've tricked the computer so it will never see an overboost condition on the MAP sensor, a remapped computre is inthe works. Depending ont he type of system the car runs, there's different strategies requried to get the computer to flow the extra fuel, and or compensate for larger injectors so it will idle.

If you have a more specific question regarding this, i may be able to help out more.

 

[evelrod]

Isaac---I don't pretend to know much about the electronics, I leave that to others. Having said that, and in answer to your question---In the early 80's we did a TurboCoupe to about 300hp by simply using an aftermarket intercooler, larger injectors and, a Bosch adjustable pressure regulator. At about 18psi to 20psi boost(some other mods to basic engine and exhaust were done) we seemed to run out of fuel. The pump, probably, the EEC4, maybe. But it did go 155mph on the lakes.


Rod

 

[GregLocock]

Fuel injector sizing on a production car is limited by (1) the maximum fuel flow you expect to see and (2) the metering accuracy of the injector at idle. They only come in a certain sequence of sizes (sorry, I don't know what), but they have a fairly broad tolerance band, so you tend to pick a size larger than you 'need', but no more.

These two factors are directly related, unfortunately in opposition.

The reason we need good fuel control at idle is that the emissions cycle has a fair bit of idle in it, and also that stumbles or rough running at idle are a real warranty/customer satisfaction headache. The simplest solution would be to up the idle speed, but that is a nasty fuel economy and emissions hit. If all things carry on as they are I'd guess we might see cylinders being switched off as a way of increasing the load and improving the idle quality.

So that's why your 200 hp engine comes with 300 hp worth of injectors, they are as big as they need to be, but no bigger. Cheers

Greg Locock

 

[Whittey]

Some vehicles (The 300 Ford inline six) injectors are strangling from the factory. Instead of 19lb injectors like used in the 302, they used 12lb injectors (and later 14lb) with higher pressures. The problem in this case is that as they get slightly clogged and/or the fuel pump wears out slightly (there were two versions of the pumping system, the earlier models used low-pressure pumps in-tank and a high-pressure frame rail mounted pump while the later used two high pressure pumps in each tank. i'm not sure about single-tank applications though) you just plain cannot feed enough fuel. The computer programming on the 300 was also pretty bad so even running at less than 100% duty cycle, it would still run lean.


-=Whittey=-