Injector Theory

[Erni902]

Right guys, I have posted here as its not specifically VR-4 related. I have just a couple of questions regarding injectors in general.

Is there a difference in the way injectors work in N/A applications and forced induction applications. I know the results are different in N/A and FI applications but do the injectors themselves work differently? For example if a supercharged application is running 475bhp and a set of 570cc injectors are running at 80% duty cycle then will they still run at 80% duty cycle in a N/A application also running 475bhp?

Next question is what details would I need to know in order to calculate the estimated duty cycle of those same 570cc injectors in a 320bhp application?

Last question off the back of the previous one, off the top of your head would you say 570cc injectors would be running at too low a duty cycle in a 320bhp application considering its at 80% @ 475bhp.

Any input much appreciated.

[Nick Mann]

The little bit that I have picked up.

In a six cylinder application, the CC's of the injector are approximately the max you want to consider squeezing in horsepower. So with a 390cc injector in a VR4, 390hp is around the max you can get. This could be too much though, depending on fuel pressures and actual duty cycles.

Modern injectors are much better at controlling fuel flow at lower duty cycles than older injectors. So a modern 1000cc injector will most likely be better than an old school 500cc injector in a 300hp application when pootling around.

Other than that, I can't help!

[Erni902]

Aha yes I forgot to mention all applications in the above examples are V6's and the injectors are actually stock R35 GTR injectors although the supercharged 475bhp was on a 370Z. Not sure of the exact bhp and duty cycle the GTR was running so went with the 370Z figures.

Good info there Nick as I hadn't looked at it like that before. The 570cc injectors may be suitable for my application then.

Also posted on the Zed forum but get answers like 'this is pointless just buy a set of 440cc injectors for £680!' but I can get R35 injectors for £100 and the rebuild kit for £50 so a saving of £530 is the reason why haha.

Thanks again Nick.

[paulg23]

My 2 penneth for what its worth. My understanding is... To make power we need to burn fuel. The more we burn the more power we can make (assuming similar efficiency). So a big engine can burn more fuel and make more power than a small engine. All obvious stuff I guess. A fuel injector is just a device used to get fuel into your engine (at a controlled flow). The type of engine it is feeding shouldn't make a lot of difference assuming similar efficiency. The only issue I can see is fuel pressure as on a turbo engine you may be feeding a manifold that is at 20psi where on a NA engine it may be vacuum. I think most fuel pressure regulators have a pressure feed from the inlet manifold so they can compensate for this.

[fassi1]

Injectors themselfs wont differ but fuel pressure will, especially in forced induction.
Fuel pressure has to be greater than (overcome) plenum air pressure, otherwise it would never pass through the air towards intake valves.
From the experiance wth lpg aplications, gas pressure has to be around 1bar greater than plenum pressure, so for that reason the actual gas preassure under high load on forced induction has to be much bigger than on NA engines. With given gas (petrol) pressure, injectors are just controlled by ECU with calculated duty cycle.
I bet it works just the same with petrol. just don't know what is the pressure difference target for most petrol injection aplications.

[fassi1]

My 2 penneth for what its worth. My understanding is... To make power we need to burn fuel. The more we burn the more power we can make (assuming similar efficiency). So a big engine can burn more fuel and make more power than a small engine. All obvious stuff I guess. A fuel injector is just a device used to get fuel into your engine (at a controlled flow). The type of engine it is feeding shouldn't make a lot of difference assuming similar efficiency. The only issue I can see is fuel pressure as on a turbo engine you may be feeding a manifold that is at 20psi where on a NA engine it may be vacuum. I think most fuel pressure regulators have a pressure feed from the inlet manifold so they can compensate for this.

Almost there. It has to be air fuel mixture, more of it means more power. Increasing the amount of fuel only, wont make more power.
Tht's probably what you mean, I just like to be more precise.

[paulg23]

Almost there. It has to be air fuel mixture, more of it means more power. Increasing the amount of fuel only, wont make more power.
Tht's probably what you mean, I just like to be more precise.

Now i've re read it I see what you mean... yes air/fuel mixture.

[Nick Mann]

Certainly in our cars the fuel pressure changes with plenum pressure. The FPR has a plenum air feed for exactly that reason. I don;t know enough about it to know if it is true in all applications. Also, an adjustable FPR means you can change the pressure scale, so you can force the injectors to have a higher pressure than they would normally have across the whole range.
Before ECU flashing, this was the easiest way to offset fuel cut - use an adjustable FPR and a piggy back ECU, so you can force more fuel in and use the piggy back to reduce the fuel requirement on the stock ECU. So at fuel cut boost the stock ecu was seeing less air flow/fuel usage than normal, thus offsetting fuel cut.


Back towards the original question, would it not be more sensible to work out your current injector size, your current duty cycle and then ratio it out to the new injectors? So if you are moving from a 250cc injector to a 500cc injector, duty is currently at 80% then it will drop to 40%. I guess it won't be quite that simple - there will be opening and closing times to allow for too - but it should be a better rule of thumb than comparing to a different application?

[foxdie]

Just to chime in here (with a drunk post);

It's not worth jumping up huge injector sizes. Scale them for your application. Don't think "okay, I'm close to the 80% duty cycle, I need new injectors now, lets put some big ones in to be safe".

ECUs control injectors with steps (aka the duty cycle), there are only a set number of steps, the bigger the injector the larger the gap between the steps. So the bigger you go, the less accurate your fuelling will be.

This next statement is more theory than reality; Lets say you have 390 cc injectors and 100 possible fuelling steps (0-100% duty cycle), that means each step is 3.9 cc of fuel. With 1000 cc injectors that's more than double the difference for each step (10 cc of fuel). Multiply this by the number of cylinders and the difference can become quite significant.

To be safe, you'd probably step up to the next step too, erring on the side of safety by opting for richer rather than leaner running, which means a drop in fuel economy.

[Erni902]

Cheers for the posts guys. The only reason I brought the subject up was because I have a suspect faulty injector (on the Zed) and would like to change all 6 due to them all being the same age and probably the rest will go soon enough. Looking at buying a single stock injector is circa £50 so £300 for a full set. A guy on the forum is selling a full set of low mileage R35 GTR injectors for £100 and Ive now found a rebuild kit for £14.95.

The GTR injectors are plug and play and as such will be a huge saving. If the car would idle fine and run fine with no adverse effects then it seems a no brainer. In the near future I plan cams and longtube headers and in the long term Im looking forced induction so if I can get hold of a set now that will do me for my current setup and also help in the future then it wont be a waste of money, as I could spend £300 on a stock set that wont see me past cams and headers.

Obviously whatever set I do end up getting I will be remapping the ECU afterwards for anyway.

Totally agree with you Nick and Im trying to find out but even the companies selling the stock injectors dont seem to know what they are I will try Nissan themselves on Monday but hold out no hope there haha. IDC and timings were among the things I want to check once I get my hands on some datalogging capabilities for the Zed, this should hopefully give me a better idea of how the injectors are running at the moment.