mapping and injector size possible issues

following on from my thread regarding injectors and sizing and having a few probs lately identifying certain sized injectors we have left in stock it suddenly occurred to me that this could actually cause a few probs with regard to mapping the car.

it seems that the injector size is only a guide and the main factor is the pressure in your rail which determines how much fuel is injected into the cylinders so if you don't know you could well be buying a lets say set of 650cc injectors which may be determined as opening at say 3 bar pressure constant then that's how much fuel they will inject at that given pressure.
but if your using a stock gtir fuel pressure regulator they are based on rising pressure and can alter dependant upon there age some read just over 2 bar at base idle others read 2.7bar as ive tested a few to see comparisons on the same car.
same goes for when on boost some are reading just under 3 bar yet others are hitting 3.8bar  . so in my mind this could be a huge problem when mapping a car due to unstable fuel pressures which would in turn mean constantly changing afr's and knock levels resulting in det and possible borewash dependant on whether the car is used mainly around town on no or little boost or mainly at wot on a track.

I did away with my uprated fpr on my car in favour of the original set up but im now beginning to wonder if this was such a clever idea so am gonna go back to an aftermarket one as hopefully things should be a little more stable with that over what could be a 23year old + stock fpr.

also when you take into account when a car is mapped the mapper will normally install a preset map from the mods stated to him and turbo size etc then the map will be tweaked slightly on the rollers then its job done normally within 2 hours but is that enough to determine the graduated map and any possible fuelling issues allowing for varying readings on fpr's

certainly something to think about guys, il try get daves opinion and comments on this as I want to run big power on the car il be building up and don't want it going pop through fuelling issues.
never really thought about it before in this way and maybe its just me being overly cautious this time round but theres a lot of people out there spent a lot of money on their engines so worth having a topic about I reckon.

http://www.rceng.com/technical.aspx did you see this in the other thread ?

Bob, this is a very important point you have raised. As you have pointed out, the standard GTiR FPR uses a 1:1 rising rate, which means that at 1 bar of boost the 'rail' fuel pressure should also be 1 bar higher than when the engine is running with no boost. However, the problem is that at idle the engine typically sees vacuum (i.e. negative boost), which means that the actual 'rail' fuel pressure will have more than a 1 bar difference between it and when driving at 1 bar boost. The base GTiR fuel pressure is typically set by:

1 - Remove the vacuum/boost hose going to the FPR.
2 - Measure the fuel pressure.
3 - Adjust to 2.991 bar, or  3.05 kg/cm^2, or 43.4 PSI (depending on your local measurement units!).
4 - Pop back on the FPR hose and drive as normal.

This works great providing your FPR is working in 100% condition. As Bob said above, if its not then you might not be flowing enough fuel (i.e. not having enough fuel pressure) at higher boost values, which will affect your A/F ratios. Remember, that when dealing with fuel pressures, there are actually 2 types of pressure to consider. Injector Dynamics wrote a good article on this, and went as follows (modified slightly to make it applicable to the GTiR):

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There are two pressures that people need to consider: rail pressure and effective (or differential) pressure. Rail pressure is self-explanatory; it is the pressure inside the rail. When you stick a fuel pressure sensor on the end of a rail, it is reading the pressure inside of the rail. This is why with a rising-rate FPR you will see the rail pressure increase with increasing boost!

Then we have the 'effective pressure', which is the actual applied pressure to the injector, and is what injector flow rate is ultimately based off of. When an engine is idling, there is a vacuum in the intake manifold. This vacuum essentially pulls fuel out of the injectors, and increases the effective pressure across the injector to a pressure higher than the rail pressure itself. When a supercharged or turbocharged vehicle is in boost, the pressure inside the manifold is trying to push fuel back into injector, resisting the flow and decreases the effective fuel pressure below that of the rail pressure.

This concept is important because it changes how the fuel system needs to be set up. Return style systems will bleed excess fuel back to the tank through the regulator (FPR). Return style systems hold a big advantage in that with a vacuum/boost referenced fuel pressure regulator, the system can maintain a CONSTANT effective fuel pressure, which can extend the range of fuel injectors and help them function at lower fuel demands as well.

With a return system, the base pressure is set with the engine off, but the pump running (i.e. see the steps above). For a GTiR, this pressure is usually set to 43 PSI (factory fuel pressure in the rail). The vacuum/boost referenced regulator will help to change the pressure in the rail based on the pressure in the manifold. When an engine is idling, it may be pulling 20 inHg of vacuum, which translates to roughly 10 PSI. The reference to the regulator will allow it to adjust and lower the pressure in the rail to 33 PSI, resulting in 43 PSI effective pressure, which is the same as the base pressure. When the engine is making 10 PSI boost, the regulator will adjust and increase rail pressure to 53 PSI, again resulting in 43 PSI of effective pressure. The regulator will constantly bleed off pressure inside of the rail to maintain the same effective pressure at all operating conditions. This helps to prevent a loss of effective pressure during wide open throttle, and also helps to prevent injectors from having to run extremely low pulse widths to fuel at idle. A downfall of return systems is the fact that they circulate fuel through a very hot engine bay, ultimately carrying that heat back into your fuel tank.
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Some race cars often do away with the rising-rate FPR, and instead stick to a static fuel pressure that matches the desired effective fuel pressure at full boost. This could work something like this:

1 - Remove the vacuum/boost hose going to the FPR (or better still get a non-rising rate static-pressure FPR).
2 - Calculate your maximum boost pressure (say 1.2 bar).
3 - Calculate your maximum desired rail fuel pressure (2.991 + 1.2 = 4.2 bar).
4 - Set the static fuel pressure to 4.2 bar.
5 - Drive car.

This will mean that at low boost and/or idle conditions, the fuel pressure will be far too high (perhaps even something like a 6 bar effective pressure - i.e. remove the 1 bar of boost and add another 1 bar of vacuum!). If the effective fuel pressure is much higher than that at idle you can tailer the fuel map to inject slightly less fuel at these load-low and low-RPM areas by (e.g.) reducing the pulsewidth. However, since the car will be used mostly on track, you'll be running 90+% of the time at full boost and so you'll not have to worry about the FPR being unhealthy and not adjusting fuel pressure properly at high boost levels. Perhaps a better approach if you intend to dedicate your car to track day driving.

Hope this helps,
Dave

cheers Leighton I just had a look and that's very handy guide seems il be needing a minimum of 800cc injectors at 3 bar which there advertised at

dave interesting write up thanks.  
so in your opinion when mapping a car for road is it still better to use a stock fpr or an aftermarket adjustable one as there seems to be to many variants in the stock ones regarding pressures through wear unless of course you have a brand new one fitted.

also if you overkilled on injector sizing then I take it the stock fpr would be of no use as there would be no way to further lower the pressure to say 2bar at idle as youll be chucking shed loads of fuel in until you come on boost and even then the pressure will just rise to say 1.1 bar at the given boost setting which will still mean its overfuelling, so if anything its better to get a slightly under sized injector as opposed to one too big if that makes sense : /

another very interesting read - thanks for the info guys - great for less mechanically minded/educated members (such as myself) to learn such stuff !

Reading what you have both said - would it be recommended to replace the standard FPR if running over stock boost ?
Asking this question as more of a general improvement rather than a performance modification !
I usually roll with the - if it aint broke dont fix it rule BUT would replacing the old (assuming original) FPR gain any benifits or peace of mind ?
The rising rate option sounds like a fair change to make maybe ?

Cheers - AL.

Bob,

In my opinion for a road car you'd be better keeping a rising rate FPR. Not a high-rate version like the FSE Power Boost Valve which are designed to increase rail fuel pressure at a rate of 1.7:1 instead of the standard 1:1. This would mean that at 1 bar of boost the rail pressure would be (1*1.7 + 2.991) 4.7 bar instead of the OE 4 bar. This would mean that the standard already-too-rich maps would be even more flooded with fuel. A remap with one of these would definitely be advised, which is why I wouldn't bother, and simply stick to a standard 1:1 FPR (either aftermarket or OE). Obviously I'm not here to advertise other parts, but I've never had any trouble with my Fuelab FPR, which can be seen here:

Fuelab FPR

This FPR has a 1/8NPT port built in so you can attach a fuel pressure gauge to monitor things inside the car, as well as AN6 ports for easy fuel-line plumbing. This is also a 1:1 rising rate FPR (like the OE one) so will not adjust any rail pressures beyond what a fully-working OE FPR would do.

If you overkill the injectors (i.e. fit some 1200cc jobbies!), then you are correct that even lowering the base rail pressure (to say 2 bar) will result in far too much fuel being sprayed into the engine. In this case it probably won't even start. The only way around this is to adjust the injector pulsewidths when doing the ECU mapping. The good news is that adjusting the pulsewidths applies to ALL sections of the fuel map (since you're using the big injectors for all driving scenarios), so a simple tweak (basically adjusting the Nissan 'k' constant) will result in the 1200cc injectors being compatible with the standard fuel map  However, the only downside is that if you choose too big an injector (say something ridiculous like 2000cc injectors), you will physically not be able to set a pulsewidth small enough to spray the correct amount of fuel to maintain idle. At idle the car will require the least amount of fuel to keep it ticking over. Each injector has a minimum value for the amount of time it can open for, and even using this minimum value on (e.g.) 2000cc injectors will result in too much fuel, causing the engine to flood and not maintain idle. The real benefit of using bigger injectors is that you can reduce their duty cycle at higher engine loads. If you look at the articles on ECU tuning (on this site) you'll see that the stock 440cc injectors are near 90% of their maximum duty at around full load. If you increased the injector size then they would only need to open for a fraction of a second to spray the same quantity of fuel into the engine, thus helping efficiency and longevity of the injectors.

In a nutshell, I'd always choose:

1 - The biggest injectors you can afford which satisfy the fuelling duties at maximum engine load, and
2 - The biggest injectors you can afford which can still maintain idle (i.e. not overfuelling) at their minimum pulsewidth.

Hope this helps,
Dave

Welling_AL wrote:would it be recommended to replace the standard FPR if running over stock boost ?

No, I would only replace the standard FPR if you think it is not functioning properly, or you want to change the fuel system (i.e. to braided lines, or add a fuel pressure gauge) to such an extent that it becomes silly buying adaptors for the stock FPR.

Welling_AL wrote:
I usually roll with the - if it aint broke dont fix it rule BUT would replacing the old (assuming original) FPR gain any benifits or peace of mind ?

There is always a trade-off in terms of price/peace-of-mind. I would no-doubt be happier if I had a shiny new aftermarket FPR controlling my fuel pressures, but then again would that £100+ be better spent on something else on the car that is already failing? As I said above, I've never had a blip from my Fuelab FPR in 3 years of hard use, but then again I never had any troubles (that I know of) from the OE FPR when I replaced it. My only peace-of-mind now comes from the fuel pressure gauge I have installed in the car so I can see that at full boost I'm getting over 4 bar rail pressure.

Welling_AL wrote:
The rising rate option sounds like a fair change to make maybe ?

The standard FPRs are rising rate, where rail pressure is increased 1 bar for every 1 bar of boost (i.e. a 1:1 rise). Some aftermarket FPRs have faster rising rates, like the FSE Power Boost Valve which rises at 1.7:1. I'd stay away from these as our cars naturally run rich, so increasing the rail pressure above-and-beyond 1:1 will result in even richer A/F ratios while on boost.

Cheers,
Dave

Many thanks Dave - appreciate the information !

Bob I'm only running 750 or 850 injector can't quiet remember off the top of my head with 4 bar and I think I'm only 50% duty and you have seen my power graph and the power I'm running when I up the boost I will throw in a set of 1000cc I don't like mine to run 100% duty

cheers dave just wanted to check before buying some and based on the info you and others have given and the power il be running il go with a set of 800cc injectors they should be big enough I reckon
thanks for your help on this and enlightening my Essex brain lol


lenny..........yep noted fella
don't suppose you wanna flog me your injectors that you got now do you if your going for the 100cc jobbies or I may just aswell buy a set of 1000cc ones if not?

Bob don't do what iv done just go for the 1000cc this way if you want to up the power you can  

See i would disagree dont run 100% but on the other hand running way  bigger than what you need at say 50% duty surly gives a shit spray pattern 

I am no expert but just what i thort

johnny gtir wrote:See i would disagree dont run 100% but on the other hand running way  bigger than what you need at say 50% duty surly gives a shit spray pattern 

I am no expert but just what i thort

You are correct that a very short pulsewidth will give a crap spray pattern, particularly if the injector is barely opening. I didn't mean to suggest that you should use 1000cc injectors if you only plan on running 300 BHP, but leaving some overhead is always a good idea so long as the spray pattern and minimum pulsewidths are sufficient to maintain a steady idle and mid-range.

Cheers,
Dave

Yes i see your point there agree with that. I dont explain things properly thats was my point exactully when people fit way over the top like injectors cams but dont realise what there doin

 Wrong thread