Engine/Transmission (1994 - 1998) Larger injector theory

[MatthewPark]

I have put together and tuned quite a few turbo gasoline engines and I am very familiar with how injectors work on them. The size of the injector, the time the injector is open (injector pulse width), and fuel pressure affect how much fuel is delivered at a given time. Your car would run horribly if you just swapped in injectors 2x the size of your original ones, and had no way of controlling them. Gas would likely drip out your tailpipe.



Now, I understand that adding large injectors to diesels works without any kind of controls. You just bolt the injectors in and bam, you start hauling ass with no more effort than putting the pedal down. I just don't understand how it works. I've searched quite extensively for the answer, but I haven't found it. If someone could inform me in a couple of paragraphs, that would be great.

 

[Blakers]

Air/fuel ratio is very critical in a gasoline engine. The ratio must be kept at about 14. 7:1. Therefore the injector size must correspond with the available air for combustion.



A diesel (almost) always runs with excess air and the air/fuel ratio varies constantly and has little importance. When a larger injector is installed, chances are there is already enough excess air to support the combustion of the fuel added by the larger injector.



Blake

 

[MatthewPark]

So technically by adding more fuel at a given time, more of the air is being consumed and more fuel is being burned? From all the reading I have done, it seems that most people's fuel mileage under normal driving does not change with larger injectors. How can both of these situations be true?

 

[Blakers]

Because for a given load you are not putting more fuel in. Just because you have bigger injectors does not mean you are always putting more fuel in - it means you have the potential to put more fuel in when you want to = more throttle travel.

 

[Turbo Thom]

But at the same time, you can and do run out of air. Only so much fuel can be injected before more air is needed. Anytime you see black smoke, unburned fuel is present. On a drag truck when the smoke turns into a gray haze, the ratio is right. But just on no 2, black is almost always present. I've just about gotten to the point where the installation of a single turbo is a waste of money. Twins feed so much more air and are so streetable, not to install is a waste of money, if air is your quest. 680 HP and little, black smoke, is a sign of a good burn and proper injection selection. A good set of twins too.



But remember that diesel combusts with heat and pressure. NO spark here. No control on the fuel with a jet. The injector and the bowl size on the piston is your jet here. I will say this though, I've seen a number of gas hottroders, leaving the gas field. Diesel really has them in a fog. How, they say? How does it go so fast?



. . Preston. .

 

[Forrest Nearing]

with a gas motor, you need ~12. 5-13:1 air:fuel for peak horsepower production...



a diesel engine just needs FUEL!!!! a diesel will continue to make power BEYOND the point that black smoke becomes visible in the tail pipe emissions... the ONLY thing you need air for in a diesel is to keep the EGT's down and keep the tailpipe emissions down...



with the right turbo and higher boost, you introduce more air to burn the excess fuel more cleanly... this reduces EGT's, thus allowing you to USE the horsepower you're making.



there also comes a point where you're flat out running rich, and the truck will stumble/etc. there DOES need to be oxygen present for proper combustion of diesel fuel, but the point of "running rich" is well beyond the point that smoke becomes visible in the tailpipe emissions.



hth,



Forrest

 

[fox]

A diesel injection pump will only send enough fuel to the injectors to run the rpm that your foot is asking it too.

Also, if an injector is very small, it is posible to reduce part throttle fuel use be going to a bigger injector.

This is why 275 injectors( for the 24valve motors ) ussually show improved mileage over stock. IMO, it is because the larger injector can deliver the requested fuel shot quicker( while the piston is still high on its power stroke), thus maximising efficiency.