How do you know what size injectors to run?

[mukumu kurubushi]

Good to know our findings match Prof :Thumbs-up

I will probably be going for 800 secondaries, although the thing is my FC is an s5 and has high resistance injectors. I\'ve read that I either need to run high impedence injectors or use low resistance ones with resistors soldered into the ECU harness. (Or i\'ll fry the injector drivers on my PowerFC??)

By coincidence the largest high resistance injectors i\'ve found so far are 800cc (SARD).

I\'ve also read that high impedence injectors aren\'t very good / don\'t work well above 550cc, so are low impedence injectors the ones to go for? whats the compatibility like with an s5?

[Prof]

I\'m running the SARD 800cc high resistance ones and they work fine. They are a straight swap as well - no custom fuel rail or anything required, although I have a customised rail with a FEED pressure regulator on it.
 
The other good thing is that the pulsation damper works fine with this size injector as it\'s not massively bigger.
 
I think the comments about the injectors not working as well may be to do with the fact that most of the newer injectors are peak and hold and are lower resistance.  To use these properly you need an ECU that drives an initially higher peak current and then a lower holding current that helps prevent the injector overheating.  You can use this type with a series resistor to limit the current - but then you lose the benefit of the peak/hold (slightly more rapid turn on with peak drive).
 
Any differences in fuel amounts injected are easily trimmed in the maps.

[pdtaylor18]

Glad this was all discussed as I was going to post on the same subject. With my almost complete setup:
TO4E turbo, HKS wastegate,FMIC, exhaust and intake etc etc mine is managed by a FC specific Apexi PFC. I have also got the FC datalogit and LC-1 wideband. I was told by Banzai racing that my intended fuelling of 550/850 would not be enough unless I was going to be running 0.7 bar and no higher.  You have to remember that injectors are not that accurate and it has been known for 1600 injectors to flow as low as 1400!! With this in mind then best to go bigger and allow for expansion,wear, inaccuracy etc in the injectors.
I have a set of 850\'s waiting to go in but may well change my mind and go bigger to at least 1000\'s for secondaries, I may even use the 850\'s as primaries. Like you i just need good advice and to buy the right combination.
I am now a member of PFC-DL-Tuning which is a forum for FC datalogit and PFC users( invited members only)I will use this forum to get as much as I can map wise and then I shall post back on MRC with maps,good info etc.

[m1tch]

I might have some spare stock 550s when I change mine, perhaps you could run a tertuary rail so you can run 6 550s

[L18M]

I might have some spare stock 550s when I change mine, perhaps you could run a tertuary rail so you can run 6 550s

i\'ll have 2 spare also.. means u can run a Quaternary rail :Giggle

[pdtaylor18]

Well just found out my injectors are low resistance and not sure if the PFC will allow for this on just the two. There are some excellent injectors on the US Ebay site, cheap and seem to have all the good flow qualities we need. I have emailed a few to make sure they are high resistance. I think I may well go 550 and then maybe 1200 if they come back in the right specs. Might even add additional injectors, i have spares but need to make sure the PFC can control them.

[mukumu kurubushi]

That\'s interesting that you\'ve been given different advice, Pete. Your set up is surprisingly close to mine! I wonder what reasons they have for saying that, they didn\'t happen to mention them did they? Or it just down to the inaccuracies in injector flow rate?

I\'m guessing you are aiming for a similar power figure as me then, around 350?

I\'m keen to see what you find out about those US injectors. I\'ve fired off an email to a couple of UK tuners, looking forward to seeing what they say on the subject...

:cheers

[pdtaylor18]

Just been told by Carl to run 550 primaries, 1680 secondaries and supported with a FPR and uprated pump if this helps as you will have similar mods to mine. With the 1680\'s I am aiming for the 400 mark.....well now I am.

[mukumu kurubushi]

wow... really?? :2eek

1680 seem kinda large to me! But I guess if anyone would know, Carl should!

according to the forumlas on the website Prof found, at 80% duty that set up would flow 340lb/hr of fuel :guinness and and 566hp!

Im even more confused now than when i started haha! There must be some bit of information we\'re missing...

[dthebuk]

or maybe 1680 secondaries is just a safe bet? Surely they cant cost THAT much more (i have no idea of the cost). So the extra safety and the ability to up the power if need be, is pretty much the best option to go for

[pdtaylor18]

I think the key to the formula is quality, buy quality and then being over cautious need not happen ie going bigger.
I have found a set on ebay:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=370239843055&viewitem=&sspagename=STRK%3AMEWAX%3AIT

but again low resistance so as we are both running the Apexi it might be worth seeing if the Apexi can manage low resistance injectors or is there something else on the market somewhere that suits our needs. I wonder where Liam got his from?

Dont forget my engine has been ported as well so that will make a difference to my fuelling.

[Prof]

It\'s worth bearing in mind that the stock 550 Primaries work for most of the dynamic range - certainly after logging with emanage, the secondaries don\'t kick in at all in the non boost part of the map, and only after 3800 ish rpm\'s and about 4 psi boost, so most of the time driving around you are only on 2 injectors anyway.
 
I\'m not convinced that the injectors are that bad or non linear over their working range.
 
If you look at a typical injector waveform, there is a delay whilst the injector turns on - which is voltage dependant so gets called battery offset or lag - the higher the voltage the quicker the turn on time. This is where the injector needle moves to an open positon. It\'s generally in the range 0.2-1.6ms and is often larger for bigger injectors, and lower for low impedance injectors.
Then once it\'s open, the flow rate is reasonably linear with respect to the length of time or duty that you apply to the injector.
 
At the far end of the scale, if you are trying to rapidly turn on and off the injector, it can get itself stuck in a half open/closed state - and will suddenly only flow about half full rated capacity, - this generally happens at higher rpm\'s and very high (>90%) duty, where the solenoid in the injector simply can\'t keep up. Then as you get to 100% duty the injector will go full open. This is very dangerous as basically you can end up with the fuelling dropping to half right when you are on boost/load and it\'s bye bye engine.
 
One thing you need to be aware of is whether you have enough time to inject the required amount of fuel at higher rpm\'s, as you have to factor in this lag time before you are on full flow, and the window during which you can inject fuel in the intake stroke becomes progressively shorter as revs increase. Porting the engine allows slightly longer induction times
 
That\'s why you want to rate your injectors for 80% duty and no more - and if you need to squeeze a little more out, you need to do one of 2 things:
 
1) Go for bigger injectors
 
2) Up the fuel pressure
 
The injectors are rated at a certain pressure - generally around 3 bar, and the fuel pressure regulator recieves a manifold reference so that the fuel rail pressure increases with boost to keep the pressure the injector sees constant. You can get rising rate regulators that have higher than 1:1 ratio - these increase the fuel pressure more at higher boost - but you then have to map for the increased fuel that\'s going in. You can also just adjust the base pressure up, and leave the 1:1 ratio intact and adjust the maps to suit.
 
There are equations out there to work out the effective size of the injector with the increased fuel pressure.
 
Obviously large increases over stock ratings are not a good idea, but a little can buy you some headroom.

[m1tch]

or maybe 1680 secondaries is just a safe bet? Surely they cant cost THAT much more (i have no idea of the cost). So the extra safety and the ability to up the power if need be, is pretty much the best option to go for

1680ccs cost about £75 each

[pdtaylor18]

Where from!! I need two

[L18M]

mazdarotaryparts
mine are low impedance also.

how am i gonna get them to work with pfc? i\'ll let james figure that out hehe
u know im nt that techi about cars lol
though though / was planning on just using resistors as i think prof mentioned before.