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Custom Fuel Setup Questions, Injector Port Offset, 14mm O-Ring, etc

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Custom Fuel Setup Questions, Injector Port Offset, 14mm O-Ring, etc

Old 02-16-18, 06:41 PM
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Custom Fuel Setup Questions, Injector Port Offset, 14mm O-Ring, etc

First off, I have to thank deatschwerks and diyautotune for answering my questions.

This is most likely a one time crazy person attempt, doesn't deter me from trying though.

Back-story: The 465 Holley setup on my streetport 12a leaves a lot to be desired. Decided to design my own fuel injection manifold, rail, intake manifold etc. This will use an FC ignition system and modified FC throttle body. Fuel and spark will be taken care of with an MS3. I'll be reusing the Holley intake manifold, spacing it out from the engine with the aluminum chunk below. It is the shape it is for ease of machining since I'll be doing it all ton a manual mill. Connecting the existing holley square bore flange to the FC throttle body I can do a couple things. The easiest would be to use this:

and build an adapter plate to hold the throttle linkage, omp linkage and the body itself. That one would act more like a plenum and air speed through the tubes would only really be independent once it entered the holley casting.

I could also build my own adapter between the flange and throttle body. While I am at this, I could extend the throttle body over towards the driver’s side around 20” or so. That would give me an intake runner length of nearly 30-35”. This one would also be individual aluminum tubes from the port all the way up to the throttle body. My goal is to have mid rpm range power by sacrificing higher rpm (+7000) breathing. I am now trying to sort out if it would make sense to offset the injector port through holes on the primaries so they are off of the center line of the port. I have enough space to machine the primary ports either way. The secondaries, due to packaging constraints, force the use of an offset. This got me thinking, that offset might end up helping with flow by inducing slight vortices in the air stream due to the injector pulses into the air stream. That vortex (not much of one) should help in getting the air fuel mix around the corner into the chamber a little.

Here’s what I am talking about:


Turns out the offset primaries end up with a spacing to the secondaries similar to the 2006 miata fuel rails. I could cut one of those in half, weld it back together in the middle and get something that could work for little money. But, the purple injectors run at 60 psi and only give around 280-290cc/min. I am thinking I want more head room available for tuning. I also plan on reducing the engine side port size of the secondaries to help with reversion. It might not matter at higher rpms but I want the secondaries to be clean even at lower rpms when they are opened.

Some background per PeeJay

Power goals of this are mild. With the 465 setup, I made 141 whp and 117 or so torque. I have that somewhere on another computer. I would most likely be able to broaden out that torque profile and gain more low end torque with the intake design and the timing control.

Estimating 175 hp at the crank, a BSFC of 0.45, 4 injectors and a duty cycle of 0.85 gives an injector flow rate of around 23 lb/hr or 240 cc/min.

If those numbers are wonky for a rotary, please let me know. Using all sorts of disparate sources and sometimes the info has to be made up until I know better.

After talking with anyone that would respond; deatschwerks and diyautotune have been the most helpful. DW were able to send along the 2d drawing of a 14mm injector for me to design around. The one in these pictures is a placeholder that I got from grabcad.



That injector is from a 2014 MAZDA MX-5 MIATA 4Cyl 2.0L and the 18U-00-0350-4 injector set would work well (I think) since they are flow matched and could be used with kurt staging if I am having trouble keeping up with fueling. Those also have nice linearity down to small dwell times. I plan to run staged injection with only the primaries being used for low load and cruise. I plan to start with Alpha N control and see how that goes before even looking at speed density. If I go with individual runners, that makes things tricky and the vacuum signal on my streetport is not the best either.

The fuel system is also getting upgraded with one of these and FC fuel tank parts… another thread at a later date.
Walbro 255 LPH Fuel Pump Kit - 86- 88 Mazda RX-7 [GSS341-400850] - $109.98 : Himni Racing, Turbocharger, Turbo, Garrett, Turbo Kit, GReddy, Mazda RX-7, HKS, Apexi, TiAL, TurboXS, TurboSmart, Flange, Turbonetics, Exhaust, Intercooler, ACT, Intake

Back to chunks of aluminum. Here’s a few detail shots of this thing so far.

Rummaging through sites with any mention of drilling your own injector port seats got me to those last dimensions. Checking with my machinist handbook today though gave me a whole other set of numbers. Those are dealing with pressures of 100psi through 5000psi though. Completely different world than 43-60 psi. Side note: Viton seems to be the preferred o-ring of choice for fuel injectors.

So, all that to get to the part where I’m asking you for help…

- Are the injector cushions always used? On a 14mm oring setup, can I get away with just having the injector body held down in direct contact with the aluminum? (assuming I cut the port counterbore correctly)

- From various sources, it seems 11mm and 14mm orings are interchangeable?! What the? Is this injector specific or are they all the same gland size and the 14mm o-rings just have a higher durometer and more gap to deal with?

- 14mm is about 0.551”, if I cut the bore at 0.531” (17/32) is that considered too much, enough, not enough squeeze? I can drop down to the next 64th size and get 0.515” (33/64) and end up much closer to the handbook’s recommended tolerance with the injector body… that seems too tight though. Either way, hand polishing those bores to get around a 16-32 micro inch finish will probably be done, partly for sizing and partly just to get the best sealing surface and to smooth out the transition from the lead in chamfer to the bore.

- Does anyone have a 14mm port tool they wouldn’t mind loaning for a short time?

- Does anyone know if a 14mm o-ring port is governed by any SAE, ANSI, ISO spec? If so, I could just get something from here.

This fuel block would be able to turn any 12a engine into EFI. As long as the intake manifold doesn’t use the coolant passages any longer, it looks like it will work… just a whole lot of details to sort through before I start making chips.

Let me know what you think, thank you.

Last edited by swbtm; 02-17-18 at 07:56 AM.
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