Injectors - sizes

[Menguin]

Hi all,

I have a 1993 MX5 1.6 with a T25 turbo and 330cc injectors. It is currently running at just over 190bhp. If I wanted to increase this, how much power would the injectors be able to cope with? I've had a search online, but can only find calculations that only serve to confuse my mathematically challenged brain!

Cheers!

[rob0971]

They will be good for around 200hp. Not much headroom left!

[Menguin]

Thanks - I'd kinda come to that conculsion using the calculators, but didn't know if I was way off or not! Looks like some 550cc may be needed

[youngsyr]

Slightly off topic, but still related - I keep reading about people upgrading injectors, but rarely read about upgrading any of the other parts of the fuel system, such as pumps, fuel lines, FPRs and fuel rails.

Can anyone shed any light on what the standard Mk 1 and Mk 2 fuel systems can support in terms of fuel flow (however it's best measured)?

I'm considering adding two additional sub-injectors to the inlet manifold of a Mk 2 and am wondering how much I'll be able to flow through these before maxing out the standard fuel system.

[MX-5 Lazza]

I wouldn't bother adding additional injectors, you are better off just fitting bigger injectors. With extra injectors you can't guarantee that all cylinders are getting the correct fuelling!

I can't really add much else to this thread other than to say the Mk2.5 (and possibly Mk2) has a higher pressure fuel system - 60psi as opposed to 48psi on Mk1 and that is supplying my 550cc injectors and giving 243bhp with no apparent problem at all (I haven't checked fuel rail pressure at max DC yet).

[snotrag]

I think most high power applications tend to use an auxilliary fuel pressure regulator.

Interestingly I know that Pat off Nutz has been using an AFPR his cheap tuning project, and it turns out that hes getting better results with a fuel pressure at the rail lower than the stock item was giving.

I guess its all related to careful speccing and tuning of your system as a whole - making sure your within spec for your injectors etc. That would go for the whole process of modifying anyway I guess - well thought, carefully chosen products - buy the right thing once only, rather than chopping and changing random odd bits till you get to something that works.

[youngsyr]

MX-5 Lazza said:

I wouldn't bother adding additional injectors, you are better off just fitting bigger injectors. With extra injectors you can't guarantee that all cylinders are getting the correct fuelling!

I can't really add much else to this thread other than to say the Mk2.5 (and possibly Mk2) has a higher pressure fuel system - 60psi as opposed to 48psi on Mk1 and that is supplying my 550cc injectors and giving 243bhp with no apparent problem at all (I haven't checked fuel rail pressure at max DC yet).

Thanks - the additional injectors aren't primarily for adding fuel (at least at the planned boost level), the standard injectors can flow enough. They're there to provide evaporative cooling so that a higher boost level can be run, apparently they can drop the inlet charge temp by 22* celsius.

Getting even fueling across the cylinders has proven tricky with the system (FFS E-cooling), but apparently they've now cracked it by using two different sized injectors placed in the manifold at specific points.

Thanks for the numbers, a simple comparison shows that you're supplying a total of 2200cc worth of injectors with fuel at the redline and 240 bhp - I'm looking to supply injectors totalling 2,350cc, but they shouldn't be worked as hard as the engine should only reach around 200 bhp.

[MX-5 Lazza]

I still don't like the idea of using fuel to cool the intake charge. It's impossible to know from the AFR at the exhaust that you have the correct fuel distribution and you definitely don't want a cylinder to be running lean - that will lead to a dead engine very quickly!
Why not just use water injection or an IC?

[youngsyr]

MX-5 Lazza said:

I still don't like the idea of using fuel to cool the intake charge. It's impossible to know from the AFR at the exhaust that you have the correct fuel distribution and you definitely don't want a cylinder to be running lean - that will lead to a dead engine very quickly!
Why not just use water injection or an IC?

Water or alcohol injection is not an option at this stage, it adds additional complication and potential for catastrophic failure that I don't want or need. Further down the line I may look into running a 1:10 meth/petrol mix in the tank for further cooling and knock suppresion, this will work very well with the E-cool system (assuming the standard parts can handle a 10% meth mix) and will mean that I can run a partial alcohol injection system through the standard injectors and the additional injectors. That would mean that I cannot ever run out of meth(/water) without knowing about it - as I'd also be out of fuel - there's no additional pump to fail and any one injector failure shouldn't kill my engine.

An air/air intercooler isn't an option with a coldside supercharger.

The uneven fuel distribution has been tested and cracked, it was tested using individual EGT sensors in the exhaust header runners. The latest testing shows that they're almost identical to standard EGTs despite running 10 psi boost without an intercooler.

[MX-5 Lazza]

youngsyr said:

...and any one injector failure shouldn't kill my engine...

So what do you think will happen if one of those additional injectors fails?

[youngsyr]

MX-5 Lazza said:

youngsyr said:

...and any one injector failure shouldn't kill my engine...

So what do you think will happen if one of those additional injectors fails?

I would imagine a small amount of knock (which would hopefully be picked up and indicated by a knocklite) as the additional injectors are only providing a small amount of cooling while the standard injectors will be referencing the car's narrowband O2 sensor to provide fueling and maintain a safe AFR regardless of what the additional injectors are doing.

If it were a separate water/meth kit injector and it blocked on the otherhand, it could well lead to serious knock and engine failure if the car is tuned to take advantage of the extra knock suppression of the water/meth mix. If the car's timing isn't advanced to take advantage of it, then there's little point in running it over the E-Cool system - it just adds a lot of hassle, additional parts and cost for a small amount of extra of cooling, which isn't needed at the power level I'm looking to run.

I haven't ruled out running W/A injection and a power hike in the future though and the kit obviously already has the ports for the injectors, it's just that I don't want the hassle or cost of it at the moment.

[MX-5 Lazza]

The ecu will only reference the an O2 sensor at cruise and idle and low revs/throttle openings. During WOT and any boost it will be running from a programmed map. That means, if an additional injector fails it will appear normal at low speeds and idle but as soon as you give it some it will run lean AND hot. Knock and heat together is a very bad thing.
That is unless you are running some sort of fancy ecu I'm not aware of that is doing something very clever...
Also, the NB O2 sensor only sees an AFR of 14.7 and gives an output to say it's higher or lower than that. An ecu can't use it at all for anything other than idle & cruise.

[youngsyr]

MX-5 Lazza said:

The ecu will only reference the an O2 sensor at cruise and idle and low revs/throttle openings. During WOT and any boost it will be running from a programmed map. That means, if an additional injector fails it will appear normal at low speeds and idle but as soon as you give it some it will run lean AND hot. Knock and heat together is a very bad thing.
That is unless you are running some sort of fancy ecu I'm not aware of that is doing something very clever...
Also, the NB O2 sensor only sees an AFR of 14.7 and gives an output to say it's higher or lower than that. An ecu can't use it at all for anything other than idle & cruise.

I am going to be running a box of tricks that is very clever, in conjunction with the Emanage Blue - it's an electronic kit put together by one of the mods on the miataturbo.net forum called Autotune.

It replaces the TPS signal in to the EMB with the narrowband O2 signal. The whole box of tricks is activated by a pressure switch, so it only interferes with the stock ECU's fueling when it senses boost. It runs altered fuel maps so that instead of fueling based on the TPS or boost signal, it fuels based on the narrowband O2 signal.

Apparently it can reliably fuel to 12 AFR on boost and I've seen a third party video of it doing so. I still don't understand how it can accurately fuel to that low an AFR using the standard narrowband, but by all accounts it does.

So, if the system works as intended, Autotune will automatically tune the fuel maps to a specific AFR using the standard injectors whilst referencing the narrowband O2 signal and the additional injectors will be activated independently on a boost/rpm basis, providing evaporative cooling (and some additional fuel, which Autotune will adjust for as required to maintain the target AFR).

I'll then pull timing in the boost zone using the EMB referencing rpm and the aftermarket MAP as required. Autotune also includes an O2 clamp, so it allows you to alter the timing and fueling even when the ECU is in closed loop.

It should be quite a neat little set up if it all works properly.

[MX-5 Lazza]

Sounds like a nightmare to set up!

Fair play if it really works but it still leaves me with too many questions & doubts to be happy with it.
For example, what happens if one of the additional injectors fails? It sounds like it will see the AFR go lean and add more duty cycle to the additional injectors until the AFR is correct. In that case it will mean at least one cylinder being lean (and probably not cooled either).
At the very least I'd fit a WBO2 and gauge so i could monitor what it was doing and log it through the emb as the emb can't use the NB for anything.

[youngsyr]

MX-5 Lazza said:

Sounds like a nightmare to set up!

Fair play if it really works but it still leaves me with too many questions & doubts to be happy with it.
For example, what happens if one of the additional injectors fails? It sounds like it will see the AFR go lean and add more duty cycle to the additional injectors until the AFR is correct. In that case it will mean at least one cylinder being lean (and probably not cooled either).
At the very least I'd fit a WBO2 and gauge so i could monitor what it was doing and log it through the emb as the emb can't use the NB for anything.

I think the initial installation will be easier than installing a WAI kit as there's no additional pump, tank, level sensor or controller to worry about. The benefit is though that it should be very low maintenance once fitted and set up as the EMB should sort its own fueling out and it uses the same fuel and fuel system as the standard injectors (apart from the very last part to the additional injectors, obviously).

As for an additional injector failure, that is pretty much the worst case scenario, in terms of hidden problems, as far as I can see - if both fail then the AFRs should be even over the cylinders and the det should be obvious, but not catastrophic, from the lack of cooling. If one fails then it could lead to slightly uneven fueling and cooling, as Autotune should provide some (but possibly not enough) additional fueling to the lean cylinder(s). This is where the knocklite comes in though.