[Durzel]

Apologies if this has been covered before.

When buying injectors, does it matter what cc they are?

Before everyone shouts "duh!", what I mean is - if you uprate the standard injectors (444cc?) to 700cc, as opposed to "the next step" - 550cc - would that cause problems? I would've presumed the cubic capacity was only relevant when you're pushing them to the limits, ie. if the injector duty on a set of 550cc is, say, 80% for whatever power you're running, having 700cc (for example) ones would only make the duty less - and allow for future mods.

Would there be any issues with fuel pressure with using injectors with a larger than required cc on a stock fuel pump/regulator?

 

[hipogtr]

As you're probably aware the ECU is the most important part. At a given fuel pressure the larger injectors will flow more with each pulse. You need to compensate for that otherwise you'll run into increasing problems with idle & overfuelling as the injector cc increases.

Also, IMHO you're better off upgrading the fuel pump, regulator and injectors in that order. Obviously the ECU should be included after the regulator if you haven't taken care of that yet. Just some thoughts.

Cya O!

 

[Durzel]

Well, the ECU (PowerFC) is being mapped at the moment. Problem is Im hitting 100% injector duty, and is struggling to get above 400bhp. I have been advised to get bigger injectors, but I cant realistically afford bigger injectors, uprated fuel pump, uprated fuel rail and regulator right now.

So I need a stop gap solution really.

What confuses me more is I know of someone running 468bhp with stage 1 cams, stock injectors/pump/regulator - and he has mentioned that he's running in the region of 85% duty. So what gives?

Ultimately my question boils down to (on the injector side of things) does it make any real-world difference in my current situation whether I get 700cc or 550cc (or even 680cc) injectors. Surely the magic is in the map and the cc is irrelevant? Or am I wrong?

 

[Paul.GTR34]

OK

You got a deal Durzel . A couple of squid either way will not break the bank !

They'll be out next week .Probably on tuesday . Do ya wonna collect ?

 

[Durzel]

Paul,

Gotta sort a couple of things out over the next couple of days, but I'll give you a shout as soon as possible

Daz

 

[Frosty]

Big Injectors

The problem you would face by fitting OVERSIZED injectors is this:

Injectors are sent a pulse when they are to deliver fuel. The length of this pulse (pulse width) determines how much fuel is delivered.

Say a 444cc injector is held open for 5 milliseconds and delivers x amount of fuel to give a correct AFR.

Now say an 888cc injector (for simplicity) is held open for 5 milliseconds. The amount of fuel delivered is (2 * x).

To map the 888cc injector to deliver the correct amount of fuel means the pulse width must be half of five milliseconds, but MOST ECU's will not handle floating point numbers (ie those with decimal places), so your new injectors will have to stay open for 3 ms (3ms for rich rather than 2 ms for lean lean).

This is just an example, but the problem outlined can cause rough idling and poor throttle-response etc.

Hope this helps

 

[Durzel]

You are a star.

That's the clearest explanation I've heard thus far!

Given my injectors are apparently maxed out (444cc) at 100% duty, which (assuming the same cost) would be my best bet in your opinion - 700cc or 550cc. Car is being mapped using Apexi PowerFC.

Car is likely to have stage 1 cams, but not definite yet. All other fuel components currently stock (fuel pump, regulator and rail) as far as I am aware.

 

[Frosty]

I think the best thing to do would be to give one of the tuners a call and see if they have any second hand 550cc injectors off a car which has just moved up to bigger once. That way you can ask about any precautions while you're there.

This will bring the duty down to a much safer level as you said while still giving a fair amount of control.

Then if/when you need 700cc injectors after a load of work at a later date you haven't spent a fortune.

What ever you decide to do - good luck.

 

[somberg]

My 2 Eurocent worth: sounds to me like you have a fuel- or an air delivery problem but not an injector size problem right now. It should be able to get more ps. Why don't you consult one of the tuners to discuss your complete engine configuration in order to pinpoint the real problem before deciding what to do next ?

Good luck, André.

 

[hipogtr]

Given that you've got the Power FC and are encountering the duty cycle you mention, I agree with Somberg. You need to check out the maps on the Power FC and the AFMs. With the factory injectors you should be able to fuel 450ps at 95% duty cycle with A/Fs of 11:1 max.

As for your original question, you can use the Power FC to accomodate any of the 700cc, 680cc or 550cc injectors. You'll obviously need to bear in mind the fuel rail mount size, fuel pump flow, and fuel rail capacity as you increase your injector size but, as you mention, the cc is less relevant than the ability to control the injector pulse width (which you can well manage through the Power FC). So, get the bigger injectors and plan for the future.

Cya O!

 

[Hugh Keir]

AFR

As said above, the problem most likely lies in the fact that your air fuel ratio is not set - up properly.

You need to make sure that you gain acess to a 5 wire lambda AFR meter. A four or three wire ( standard ) lambda will tell you lies at anything richer than stochiometric 14.7 : 1.

I know that several of the tuners have them as they are considered necessary tools to do the job properly.

You might try reading the archives, unfortunately you are not allowed to search on a string smaller than four letters like AFR ( why Joss? )

If you can find the threads you will read Mario reported on the downunder site that maximum power comes at 12.66 :1, but watch you don't go any leaner as you will be on the edge of melting your pistons.

 

[Durzel]

I've removed my previous comments until the relevant parties have had chance to present their side of the argument to me in person