ronniebiggs

Hi Im trying to turbo my 1991 miata 1.6 and have most the parts except the injectors. I have read the post about the injectors and flow rates and fuel pressure etc but im still not sure how to work things out.

Could someone tell me would the 550/560cc (Not sure which it is) injectors off the RX7 be suitable for my car.
Im running (Or will be!) a garrett T28, 3" exhaust all the way through with megasquirt and im expecting BHP of approx 230-250.

Any advice is most welcome.

Thanks
Andy

sicklyscott

iTrader: (1)

Yes

ronniebiggs

They wouldnt be to big?

astroboy

iTrader: (18)

550's will support the hp level you are going for and you won't need a rpfpr.

ronniebiggs

forgive me what is a rpfpr?

Something fuel pressure regulator?

Many Thanks
Andy

Joe Perez

iTrader: (8)

It's a typo.

RRFPR = Rising Rate Fuel Pressure Regulator. Increases the fuel pressure in response to boost, by some non-linear ratio such as 6:1 (6 pounds of fuel per pound of manifold pressure.) It's an old-school method of increasing the flowrate of a fuel system beyond stock.

luigi

iTrader: (1)

RRFPR=rising rate fuel pump regulator.
Best of luck with your goals.
Luigi.

ronniebiggs

Thanks for all the replies.

Im just trying to establish with the seller if the injectors are 460cc or 550cc. Either way I know they are off an RX7 so would think they will suit my build.

gospeed81

iTrader: (51)

Even if they are 460s you will still be fine as long as you don't get over 250whp (which not only requires bigger injectors, but forged internals and a 6speed as well).

This is with 80% duty cycles, and assuming a BSFC of .55.

ronniebiggs

Thanks I have paid for them today and so expect them to be delivered by friday. I will let you know what they are/colour etc.

ronniebiggs

Just checked the codes and they are the RX7 550cc injectors so hopefully they will be good enough

stranges12712

iTrader: (1)

I went 550cc and they werent enough for my power goals and i wanted MOAR BOOST so i went with 660cc who knows i will probably go even bigger once i find out that they are maxed out at the dyno...

ronniebiggs

WOW, thats an impressive setup what power are you running?
Im aiming for a reliable 230-250 BHP if it means sacrifising power for reliability I may have to do it as I want the car to last without having to do expensive rebuilds.

I would think the 550cc should be enough to get me to the bhp. You must be above 300bhp or there abouts are you?

NateTheShake

iTrader: (3)

Forgive my ignorance, but is this required if the user is going to be using something like the MSPNP?

samnavy

iTrader: (7)

^No.

To make things very simple, there are 2 ways to get more fuel into the engine... mechanical and electrical.

Mechanically, you increase the size of the fuel injector or increase the fuel pressure. You can increase fuel pressure using a high pressure fuel pump and an RRFPR(Rising Rate Fuel Pressure Regulator) or AFPR(Adjustable Fuel Pressure Regulator). Those 2 terms are used interchangeably, but can be different devices. An RRFPR doesn't necessarily have to be adjustable, and can have a fixed rate of rise. The Vortech unit that comes with Miata Greddy Turbo Kits is technically adjustable by dismantling the device and swapping a different sized internal disc. The most popular AFPR around here is extremely adjustable with an allen key and can vary it's rate of rise within a huge range... called the BEGi 2025.

Don't forget that increasing your fuel mechanically is invisible to the ECU. It is still going to behave like it has stock injectors and pressure... and will provide pulses to the injectors accordingly. This is why huge injectors will not idle on the stock ECU. I will not discuss open/closed loop fueling here, but basically, if you have a 265cc injector in an idling engine, the stock ECU will give exactly enough injector pulse(fuel) to idle at 900rpm or whatever. If you swap in 550cc injectors and start the car up, the stock ECU will still open the injectors at the same rate, and since the injectors are twice as big, you get double the amount of fuel going in... and flood the engine.

So the main limitation in using mechanical devices is in the stock ECU's ability to change the injector pulse... it runs off a fixed table that won't change... much. Adding more air(boost) will not add more fuel because the stock ECU has no ability to sense boost. So you have to mechanically add fuel when the boost comes up, and the stock ECU doesn't know anything is different, and is happy (mostly).

The second way, electrically, is done by an aftermarket ECU that can see boost. As boost comes up, the ECU opens the injectors longer and gives the right amount of fuel. Of course, the stock injectors can only stay open so long before the intake valve closes and spark plug ignites. This is why you go to a larger injector. Using stock fuel pressures and doubling the size of the injector and using an ECU that can leave those injectors open longer is like getting a fuel system 4x larger than stock on the stock pressure. So at a certain point, you're adding more air than the stock injectors can provide fuel for because they can only stay open so long... and you need to get larger injectors.

Conventional wisdom is that 550cc injectors on stock fuel pressure with a fully controllable ECU will get close to 300whp in the 80-85% duty cycle... which is as high as you ever want to go.

Aftermarket ECU's:
A standalone aftermarket ECU completely replaces the stock ECU, receives all inputs from the sensors and controls the engine fully.

A piggyback can work 3 ways...
#1: In most cars, the stock ECU does more than control fuel/spark. It also works the alternator, air conditioning, idle control, idiot lights, etc... It's possible to wire in an full control aftermarket ECU and only wire it to control fuel/spark, allowing the stock ECU to control the rest of the functions.

#2: Wiring the piggyback in between the stock ECU and the injectors/ignitors. The stock ECU will do everything normally not knowing there is boost... but a vaccum line is hooked up to the piggyback, and it sees boost. As the signals to fire the injectors/spark plugs are sent out, the piggyack intercepts/modified/lengthens/shortens/changes those signals.

#3: Wiring the piggyback between the stock sensors and the stock ECU... changing what the stock ECU sees... artificially telling the stock ECU there's more air. I think this is how Xede works.

ronniebiggs

Thankyou for that explanation.
I have read the FAQ section several times and I couldnt fully grasp the injector section. That was very informative and cleared up the topic for me.

Many Thanks
Andy

JMLTOY

CAN WE ADD THIS TO THE FAQS?? VERY well put Sam

To make things very simple, there are 2 ways to get more fuel into the engine... mechanical and electrical.

Mechanically, you increase the size of the fuel injector or increase the fuel pressure. You can increase fuel pressure using a high pressure fuel pump and an RRFPR(Rising Rate Fuel Pressure Regulator) or AFPR(Adjustable Fuel Pressure Regulator). Those 2 terms are used interchangeably, but can be different devices. An RRFPR doesn't necessarily have to be adjustable, and can have a fixed rate of rise. The Vortech unit that comes with Miata Greddy Turbo Kits is technically adjustable by dismantling the device and swapping a different sized internal disc. The most popular AFPR around here is extremely adjustable with an allen key and can vary it's rate of rise within a huge range... called the BEGi 2025.

Don't forget that increasing your fuel mechanically is invisible to the ECU. It is still going to behave like it has stock injectors and pressure... and will provide pulses to the injectors accordingly. This is why huge injectors will not idle on the stock ECU. I will not discuss open/closed loop fueling here, but basically, if you have a 265cc injector in an idling engine, the stock ECU will give exactly enough injector pulse(fuel) to idle at 900rpm or whatever. If you swap in 550cc injectors and start the car up, the stock ECU will still open the injectors at the same rate, and since the injectors are twice as big, you get double the amount of fuel going in... and flood the engine.

So the main limitation in using mechanical devices is in the stock ECU's ability to change the injector pulse... it runs off a fixed table that won't change... much. Adding more air(boost) will not add more fuel because the stock ECU has no ability to sense boost. So you have to mechanically add fuel when the boost comes up, and the stock ECU doesn't know anything is different, and is happy (mostly).

The second way, electrically, is done by an aftermarket ECU that can see boost. As boost comes up, the ECU opens the injectors longer and gives the right amount of fuel. Of course, the stock injectors can only stay open so long before the intake valve closes and spark plug ignites. This is why you go to a larger injector. Using stock fuel pressures and doubling the size of the injector and using an ECU that can leave those injectors open longer is like getting a fuel system 4x larger than stock on the stock pressure. So at a certain point, you're adding more air than the stock injectors can provide fuel for because they can only stay open so long... and you need to get larger injectors.

Conventional wisdom is that 550cc injectors on stock fuel pressure with a fully controllable ECU will get close to 300whp in the 80-85% duty cycle... which is as high as you ever want to go.

Aftermarket ECU's:
A standalone aftermarket ECU completely replaces the stock ECU, receives all inputs from the sensors and controls the engine fully.

A piggyback can work 3 ways...
#1: In most cars, the stock ECU does more than control fuel/spark. It also works the alternator, air conditioning, idle control, idiot lights, etc... It's possible to wire in an full control aftermarket ECU and only wire it to control fuel/spark, allowing the stock ECU to control the rest of the functions.

#2: Wiring the piggyback in between the stock ECU and the injectors/ignitors. The stock ECU will do everything normally not knowing there is boost... but a vaccum line is hooked up to the piggyback, and it sees boost. As the signals to fire the injectors/spark plugs are sent out, the piggyack intercepts/modified/lengthens/shortens/changes those signals.

#3: Wiring the piggyback between the stock sensors and the stock ECU... changing what the stock ECU sees... artificially telling the stock ECU there's more air. I think this is how Xede works.
__________________
Miata projects put on hold due to life...
'05 4Runner SR5 stock
'06 Mazdaspeed6, TMIC/SRI/Forge

j555pre

+1 on the great explanation. Thanks

I've gone for Yellow injectors from a RX8, I'm hoping they will be fine for my build.

I'm up the North East every now and again, it would be good to see someone elses build and talk mx5's.