Project [Rotary] FD RX-7: Part 2 - Fuel System (Engine Side)

by M-P Spierer

There are three key ingredients for combustion in any gasoline engine: air, fuel, and spark. Parts 2 and 3 of this project will focus on the fuel component of this recipe. Rest assured, this won't be your typical run of the mill RX-7 fuel system.

The RX-7's factory fuel system is somewhat unique because it utilizes staged injection. It consists of two 550 cc/min side feed "primary" injectors that run the engine when little fuel is required (idle, cruise, etc) until they reach a programmed threshold. When this happens, two 850 cc/min side feed "secondary" injectors kick in to provide more fuel. At this point both the primary and secondary injectors run together to supply the required fuel.

The FD RX-7's factory fuel system consists of four side feed injectors. The short rail houses the two 550cc primary injectors and the long rail houses the two 850cc secondary injectors.

Most aftermarket fuel systems utilize the factory primary rail populated with either the factory 550cc or 850cc injectors, while the secondary injectors and rail are replaced with an aftermarket rail and a pair of high flow top feed injectors. For a long time the high flow secondary injector of choice was the 1680 cc/min Bosch EV1 (160 lb/hr for those of you who are SAE inclined). One complication with using these injectors is that they are low impedance which most aftermarket ECU's for the RX-7 can't handle natively. There were two ways to address this, wire in an injector driver box designed for low impedance injectors or the much cheaper, easier, and more common way of wiring in a resistor to increase the impedance of the fuel injector circuit.

Again we will fast forward to the present where we don't have to do silly things like wire in resistors because technology has improved and we have significantly better options for high flow injectors. In this case we're talking about the modern Bosch EV14 injector; a compact, high impedance, high flow injector with a great spray pattern that is much easier to control than the old EV1 injectors (especially at low duty cycles). We can then take it one step further and use dynamically flow matched pairs of Bosch EV14 injectors from Injector Dynamics.

Pictured above are two pairs of flow matched Injector Dynamics fuel injectors. The injectors on the right are ID850 and the left are ID2000 injectors. The names of these injectors can be a bit misleading because nominally at 43.5psi they actually flow 885 cc/min and 2225 cc/min respectively. The naming convention probably has something to do with the fact that it is easier to remember ID2000 than it would be to remember ID2225.

This injector combination (885 cc/min primaries and 2225 cc/min secondaries) yields a total injector flow capacity 6220 cc/min, which is immense when you think about gasoline fed piston engines. However, because rotaries require more air and fuel than piston engines to yield the same power (read higher BSFC to the engineers out there), this means the power potential of this injector combo isn't quite as impressive as the flow rate makes it sound. With some assumptions and a maximum injector duty cycle of 85%, this injector combination should realistically supply enough gasoline for somewhere in the low to mid 600 wheel horsepower range.

The next step in the process is to mount these injectors. In the factory fuel system, the primary injectors spray fuel directly into the center iron housing of the engine, while the secondary injectors spray fuel into the outer runners of the lower intake manifold (entering the engine at the front and rear iron housings). Since the primary and secondary injectors are in different locations, separate fuel rails are required, which is a driving reason for many people to only upgrade the secondary injectors and rail. As stated earlier, we are doing things a bit differently for this build. In Part 1 of Project FD RX-7 we talked about the Xcessive/GZ oil pan and its benefits, this time we are introducing their lower intake manifold.

Xcessive Manufacturing's cast aluminum lower intake manifold. We had it thermal barrier ceramic coated to help keep radiant heat from the turbo and engine out of the intake air. 

This intake manifold incorporates several benefits that can be taken advantage of. The most important advantage is the fact that this manifold eliminates the non-symmetric intake path of the OEM manifold and increases the airflow rate up to 13%. Another less significant advantage is that this manifold permanently removes several emissions mounting features. This could also be considered a disadvantage if you are trying to meet specific emissions requirements.

Comparison shot between the OEM and Xcessive lower intake manifold. Notice the symmetric intake path between the front and rear rotors on the Xcessive manifold. Also, the Xcessive unit eliminates the need for block-off plates for the deleted emissions items.

The other major advantage of this intake manifold is that it features four injector bungs (one per runner), which increases the possible fuel system configurations. One example is E85 users that opt to both utilize the primary injector ports on the center iron housing with "normal" sized injectors and populate the four ports in the lower intake manifold with equally sized high flow injectors, giving them the ability to flow the volume of fuel that E85 necessitates.

In the case of this project, since E85 is not in the plans, the primary injector ports won't be used at all and the four ports in the intake manifold will house both the primary and secondary injectors. This type of configuration requires a four port fuel rail and a solution for blocking off the primary injector ports.


Page 1 of 4 Next Page
Bookmark and Share
Monday, November 25, 2013 7:15 AM
Thank you for using tubing on the fuel system. I think it looks much cleaner and professional than hoses everywhere. I'm sure you're already flowing fuel through them, but a quick shot of black or other colored paint goes a long way to make it even cleaner. And please thank Aeromotive and your fitting supplier for the black finish on their parts. I've always hated the blue and red color scheme. Fantastic job on the injector plugs. Again, super clean execution.
Monday, November 25, 2013 7:51 AM
I was under the impression that RRFPR's were "bandaids" to make up for a lack of proper engine management. Now, I heard that from forums so it's probably wrong, but I have seen quite a few high HP builds that didn't use RRFPR's. Is it a rotary thing?
Monday, November 25, 2013 8:07 AM
jeffball610: Thanks!

cbgoding: RRFPR's are only bandaids if they are used to directly tune the car (i.e. install one to increase the pressure/flow instead of buying a proper ecu to control the injector duty). Is a RRFPR required? No, but I like using them because you always have a constant pressure differential across the injector and know how much it is flowing at a given duty cycle. Even with a RRFPR, a proper engine management system is needed.
Monday, November 25, 2013 8:16 AM
Nice work on the plugs! And the new lower intake manifold is purty.... Have you thought about getting the manifolds extrude honed? Considering the length of the runners, I bet there's significant improvements to be had with an extrude hone; well, a few percent more power.
Monday, November 25, 2013 8:35 AM
So I'm guessing that as long as you have a pump that can keep up with the increased fuel pressure demands (maybe a fuel cell running high flow pumps and a surge tank with a high pressure pump?), and tune accordingly, the RRFPR can be used?
Monday, November 25, 2013 8:41 AM
I don't know what the industry terminology is, but I consider a rising rate reglator to be one that has a ratio of more than 1:1, i.e. 1:1.5 or 1:2.

1:1 regualtors are the norm for 90's fuel injected cars. The aftermarket 1:1 regulators just have a higher flow capability compared to a factory regulator. Factory regs. may not be able to bypass enough fuel back to the tank at low injector duty cycles when using upgraded pumps, leading to an increase in fuel pressure relative to manifold pressure.
Monday, November 25, 2013 10:18 AM
It just sounds like some terminology confusion we have going on here. I've always considered any FPR that increases fuel pressure with boost pressure a "rising-rate" regulator (1:1 or otherwise).

Either way, I edited the article to remove any confusion. Thanks.
Tuesday, November 26, 2013 6:56 AM
Okay, I get it now. I was wondering why the hell my little car doesn't have one if they're common, and what the benefit was. I feel like an idiot for not realizing that this is what makes a return fuel system possible. And the benefit of these aeromotive units is the ability to keep up with much higher pressure and flow pumps than the stock FPR's.

So, hijack alert, how would anyone convert from returnless to return? Buy a fuel rail that supports it, right? What if there isn't one? Pay a shitload for a one-off rail, or is there some alternative?
Tuesday, November 26, 2013 7:42 AM
cbgoding: Expensive way: but a custom fuel rail. Cheap way: drill into the closed end of the fuel rail (thinking cast here) tap threads and put a connector in. The fun part is running a return fuel line under subframes back to the fuel tank and modifying the (stock) fuel tank to accept a return line.

If you go with a fuel cell then you're most likely covered.
Tuesday, November 26, 2013 8:34 AM
I changed the stock returnless system on my old 350Z to a full return system and like widelyunknown said...the fun part is doing the return line and modifying pump hanger in the tank.

What do you drive? Are there no off-the-shelf aftermarket fuel rails?
Tuesday, November 26, 2013 11:59 AM
It's a GE8 honda Fit...I've been told repeatedly to just sell it and buy a cheap performance car. Honestly that makes me want to build it even more. There aren't exactly a bunch of success stories though, unless you count bisimoto's CR-Z that uses a similar engine.
Tuesday, November 26, 2013 1:32 PM
My buddy has a manual Fit and it's a fun little car to drive (and ridiculously utilitarian). If you're looking to enjoy yourself then throw tires and brakes at it. Maybe suspension. Then learn to drive it fast and enjoy having a sweet car that you could put a damn couch into.

If you want to compete, it's not the right car. The engine is tiny and aside from a cold air intake, you're looking at spending massive $$$ for minimum hp gains. IMO Honda doesn't make engines that you can modify easily. Start with a factory turbo car and you can get to 300, 400 hp easily enough (experience being a factor).

But ultimately, what do you want to get out of it. Do you like having a nice car that starts every morning? Do you like leaky vacuum lines, forgetting to replace pinched injector o-rings and having a car that never idles right, has a bad tune, runs lean and melts something? :-)

That being said, a wastegate that vents to atmosphere is an addicting sound...
Tuesday, November 26, 2013 1:58 PM
Yup...already threw on 205 width summer tires (up from 175 width all season), rear sway bar, stiffer springs, and CAI. Everything but the CAI had a noticeable performance improvement. The CAI is just loud. Still feels twitchy on the highway, but its a very fun, economical, utilitarian car in the city and that's where I spend the vast majority of my time.

But yeah, before (and if) I go turbo, I think the best route may be getting a weekend beater (miata?), and attempting NA-T on that first, so I know what I'm getting into and don't ruin my daily.

Sorry this got so off topic.
Anonymous User
Anonymous Userlink
Monday, December 02, 2013 9:20 PM
Post Comment Login or register to post a comment.

MotoIQ Proudly Presents Our Partners:

© 2018 MotoIQ.com