How do you know what size injectors to run?

[L18M]

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.

thats what i have in mine. serial fuel rail, not gone parallel as costly and not much point unless you going clost to 500bhp mark.
braided fuel lines in engine bay also.

[dthebuk]

thats what i have in mine. serial fuel rail, not gone parallel as costly and not much point unless you going clost to 500bhp mark.
braided fuel lines in engine bay also.

if you want to go parrallel i may have something that\'ll interest you !

[L18M]

if you want to go parrallel i may have something that\'ll interest you !

a parallel fuel rail kit..? lol
not me. m1tch prob will though.
need a couple of Y bits, parallel fpr.. might as well get full braided fuel lines from tank to front, better still.. twin pumps :p

[mukumu kurubushi]

Well, i\'ve spoken to a few people who all agree with the advice Pete has been given - keep the primaries stock and run 1680cc secondaries.

Still don\'t fully understand why there\'s such a large difference in size, and I always thought it was recommended to keep injectors as small as possible (to stop lag or running too rich when the secondaries stage - maybe the PowerFC has some way of overcoming this).

The consensus seems to be that the duty on 800s is too dangerous, and its better to go too large, than too small as dthebuk says.

I\'ve also looked into the high/low impedence issue and found these products on offer. You might also be interested in these, Pete.

http://www.statracing.com/buy/aem/aem-peak-and-hold-injector-driver.asp

http://www.fjoracing.com/products/injdrvr/

[pdtaylor18]

Also been speaking to a really clever chap on the RX7club forum and suggest 6 ohm resistors, he is the one that has rewritten my map for me and knows his stuff on the PFC and Datalogit.
http://gb.mouser.com/search/ProductDetail.aspx?R=RH0106R000FC02virtualkey61300000virtualkey71-RH10-6.0
That way the injectors will work fine.

[Prof]

It\'s worth keeping in mind that those running the stock injectors on FD\'s are in fact running 550 and 850\'s respectively. I think as long as you keep an eye on duties then you\'d be fine, but if your goal is 400 or over then go bigger
 
Running 1680\'s isn\'t a problem if you\'re ECU has good resolution so can adjust the duty in fine steps.  It also helps if it has Peak and Hold capability as it will be able to more quickly switch on turn off the injector.
 
In any case the finer control is required on the smaller injectors anyway, as they will handle most of the normal driving conditions.  The big ones are only on when you trogg it :cheers

[mukumu kurubushi]

Here\'s an interesting explanation from rx7club, most of this has been posted before in some form or another, but i thought it was a good breakdown of why my original attempt at the formulas went wrong:

Explanation of that formula:
350 hp * 0.60 lb/(hp*hr) Brake Specific Fuel Consumption (BSFC) = 210 lb/hr

Why your number is too low:
1) You specified 350-400hp in your thread, but only used 350hp in the formula. Always use your highest planned horsepower number as a target, which would be 400hp.
2) BSFC is fuel efficiency as defined as Fuel Consumption / Power. This number depends a lot on the type of engine and how it is tuned. As you may know, rotary engines suck at fuel efficiency. Also, if you are the typical tightwad RX-7 owner who refuses to pay a professional to tune the engine, you plan on running a lot of boost, retarding the timing, running a silly-rich mixture while chasing a wideband AFR meter reading, you buy cheap Chinese Ebay parts, etc., then that also hurts the fuel efficiency. I would plan on 0.70 BSFC unless you have a pro tune the engine, in which case 0.62 is probably more accurate. If you manage to get .60 or better, then that\'s great. If you plan on 0.60 and run out of fuel, then you will need to buy bigger injectors and/or upgrade other components of your fuel system, which is not fun.
3) You did not compensate for the fuel injector duty cycle. The fuel injectors will burn up if you run them full-open. The rule of thumb is to give them at least 20% rest time, which results in a 80% duty cycle.
4) You did not compensate for fuel pressure. Most (not all) aftermarket fuel injectors are flow rated at 3bar (43.5psi) pressure. The stock RX-7 only runs about 34-40psi pressure. As you may imagine, a fuel injector will not flow as much at 34-40psi as it will at 43.5psi. If you plan on using an aftermarket fuel pressure regulator and increasing the pressure to 43.5psi, then you do not need to compensate for fuel pressure in your planning. A higher pressure will inject more fuel than the injector is rated, and you can push most fuel injectors up to about 4bar (58psi) before they hit a brick wall. The pressure conversion equation is: Revised Pressure cc/min = Original cc/min * sqrt(Revised Pressure / Original Pressure).
5) There are many other factors, but the above are the most significant.

Assuming that you will run 43.5psi fuel rail pressure, I think the following is a little more realistic. I wrote out the equation so you can recalculate with different numbers as needed.

(400hp * 0.70 BSFC) / 80% duty cycle = 280/0.80 = 350 lb/hr * 10.5 = 3675cc/min

Therefore, 720/1200 or 4x1000 should work fine, or you could go with 720/1000 if you tune better, run a higher fuel rail pressure, and/or shoot closer to the 350hp number as opposed to the 400hp number.

[pdtaylor18]

Good stuff that. I have now ordered some injectors from Clive (1680cc) so that and the 550 standard ones will be just fine. Although they are low impedance they will work only too well with the PFC without modification. A FPR and uprated pump though goes without saying.

[mukumu kurubushi]

Ah really? so no need to solder in resistors to the injector circuits? thats one less thing to worry about! :cheers

[Prof]

Clives 1680\'s are about 4-6 ohms from what I remember - so should be ok on the PFC - particulary given that they won\'t be on for very long (low duties because they are bigger).
 
That\'s an excellent post from the RX7club by the way
 
I am using higher fuel rail pressure - so I guess that\'s why I get away with it.  I am also aiming for 300 rear wheels - which is about 360 ish on the fly.  At the moment I\'m finding I need to cut quite a bit of fuel in the emanage - not add any (apart from the obvious change to 800 secondaries).  I\'m still seeing low afr\'s of mid 9\'s under full boost.
 
I\'m not hitting 80% on the secondaries even at 1 bar boost.  This is based on my logging, so actual figures from the engine running - not calculations.  I think there is a tendancy to over egg fuelling on rotaries because people are ..... scared of running lean and popping engines, which is perhaps no bad thing.

[pdtaylor18]

Over rich is good but then you lose some power because of it but that side of caution is never a bad thing. Innovate reckon 9\'s should be the lowest and aim for 12\'s under boost. I had some readings down as low as 7.5!!  until I messed with the map. I have ordered some resistors as a just in case and I think standard Mazda were 12 or 13 ohms but having talked to Carl today and by email with Clive I think running those 1680\'s with the PFC will do no harm. Made a good thread this as so many of us are in the same position.

[pdtaylor18]

When you want to have a good session with the Datalogit let me know and when we get the chance I can pop down and look at the map in the PFC for you. Also gives me the chance to copy some map files for you to your laptop.

[TiddlesRX7]

I can see you becoming pretty popular with that datalogit Pete;)

[mukumu kurubushi]

When you want to have a good session with the Datalogit let me know and when we get the chance I can pop down and look at the map in the PFC for you. Also gives me the chance to copy some map files for you to your laptop.

Thanks Pete, really good of you to offer, appreciate that a lot! Think i\'ll definitely be taking you up on that fairly soon. I do have the hand commander, but i\'ve heard it would take a lifetime attempting to do any tuning on it!

Had a quick look around the FC Edit software, don\'t understand what most of the values mean, but I can see if you know what you are doing it would be much easier to use.

Its quite useful our setups being so similar, I want to learn more about tuning so seeing what you\'ve done with the maps would give me a good starting platform:cheers

[pdtaylor18]

1st thing is to make sure that you have an FD air inlet temp sensor, FD map sensor or 3 bar one and that should be enough to get the car going. I actually used my standard S5 boost sensor and that worked fine but its not ideal. I do use my wideband gauge alot to check AFR\'s and thats crucial in getting any map close to what is ideal in all conditions.
The datalogit is very good and I have explored using several maps but none have really suited the car, all engines are different so will need fine tuning along the way. The difference with the Datalogit is that you see the whole map and not just a small portion of it.