First a little background, slowly over the preceding 18 months I had been collecting
the necessary parts for the fuel
system upgrade. And I can tell you it was a lot of parts! With the components I eventually installed the
car should have enough fuel for about 500rwhp (~600hp at the flywheel).
Since I'm a cheap bastard I only buy new what is absolutely necessary. Everything else has been
bought used, mostly from eBay. BTW when it comes to AN fittings, eBay is your friend. My fuel
system includes the dual Supra pumps, a
System1 in line fuel filter,
850cc primary injectors and 1300cc (bored ot stock 850’s) secondary injectors, and an
A1000 fuel pressure regulator with a fuel pressure gauge. I used a
single -10 feed line split to a pair of -06 fuel lines to and from the injector rails and the FPR and used
the stock return line (which is about the same as a -06 line). Replacing all the fuel lines has got to
be one of the most expensive propositions I've undertaken...or at least the one with the most pieces.
And certainly a bit of overkill. I successfully tapped a stock primary rail for AN fittings (very
difficult to make them so they will clear the oil injectors), and modified the rail to accept the
850 injectors (so they won't leak). Some have suggested using two o-rings or a spacer to reposition
and seal an 850cc injector in the primary rail but this is asking for trouble, READ: FIRE! I submit the best way
is to simply machine 0.125 inch off the top of the rail to allow the injector to sit lower in the rail.
I had all my injectors cleaned and flow tested (By Witch hunter),
a good practice anytime you
have them out of the engine. The results indicated my “1300cc” secondaries (bored out stock Denso 850's
purchased from SR motorsports which I believe use
RC Engineering for the work) flowed 1424cc’s and 1460cc's respectively.
This is quite a bit more than 1300! More below on the fuel delivery.
With the fuel tank guts taken care of, next is the plumbing from the tank to the injectors.
The first hurdle is the fuel filter. Anyone who has changed the OE fuel filter knows it is royal
PITA. There is nothing like the feeling of cold gasoline running down your arm into your armpit
as you struggle to pull the fuel hose off the fuel filter nozzle…blind. Since I planned to run
all braided SS lines, the OE fuel filter was history. I had picked up a HUGE 11” System1 fuel
filter off eBay (w/-10 fittings, 30 micron filtering, and flowing 28gpm) for a song-and-a-dance.
Now I just had to figure out how to mount it. Some things to consider; 1) it had to be protected
from road debris and crush in a significant rear end collision, 2) it had to be accessible for
service (unlike the OEM filter), 3) it had to be placed such that the large -10 lines could be
routed easily to and from it, and 4) it had to be in a place where it could actually be mounted
properly – I don’t “do” zip ties…PERIOD. I eventually settled on a location just in front of the
fuel tank mounted to the rear subframe. I custom made a bracket to hang the filter and used rubber
to isolate the filter from wear and tear of a rigid mount.
Since I had success with fabricating the braided SS oil cooler lines with AN fittings for my ITS
RX-7 HERE I decided I’d make all the braided
lines myself. This really isn’t a difficult proposition, just rather time consuming. There was
considerable forethought to ensure all the fuel lines were the correct length prior to assembly.
Ensuring the right type of hose end was used also caught me out a few times. For example a line that I had used
a 90degree fitting on after trial fitment ended up needing a 120 degree fitting. But that’s the beauty
of reusable hose ends, simply disassemble and clean up the hose itself and slide on another fitting.
I used a Dremel tool to cut the hose cleanly and used an anaerobic thread sealant on each connection,
even though it may not have been necessary everywhere. AN fittings do not/should not need thread sealant.
The feed line from the fuel tank cover bulkhead fitting (fed by the dual pumps) is a AN-10 line.
Overkill I know but I had an inventory of fittings and hose to use so why not? Routing the short
-10 line from fuel tank to the filter was a piece of cake. The next hose was the long run from the fuel
filter to the splitter on the engine block. It was tricky to route the hose up and over the rear subframe
but there is room to squeeze it through and in a manner that the hose isn’t kinked or would get pinched.
The hose is completely sheathed in ¾” rubber heater hose to avoid any chafing and wear over its entire
length. The rear section is fully hidden/protected by the rear plastic undertray. The hose was run down the
length of the car on the inner side of the chassis frame rail with adel hose clamps every 12inches. It is
well protected by the frame rail and as safe as the OE fuel lines mounted just to the other side of the
frame rail protected by a flimsy plastic cover. Some argue that running the braided SS hose under the car
is very dangerous because it may get snagged on road debris or an object and get ripped from the car with
fuel being pumped out by the still running pumps. I have two responses to the: 1) if there is something
large enough to rip the fuel hose from under my car then the OE return line will go to…as well as the brake
lines. And then I will have much larger concerns since I won’t have any brakes! And 2) there is nothing
that should justify running pressurized fuel lines on the interior of a street car. I cringe whenever I see
surge cans mounted in the hatch area of a street car with no bulkhead separating fuel and driver. The
alternative…run the lines outside the chassis...which is what I did. Let’s move on to the rest of the fuel delivery.
The AN-10 delivery line feeds an aluminum splitter block mounted to the topside of the engine block. The block feeds two short AN-6 lines to each fuel rail – primary and secondary separately. These two short fuel lines were wrapped in a thermal sleeve since they run directly over the hot engine block. A custom made aluminum plate mounts the splitter block. This was possible because the ignition coils were moved and the majority of the hardware used to locate the solenoid rack (and the solenoid rack itself) was cut and removed such that only the UIM support leg remains. This left plenty of room for the lines and fuel hardware. The fuel rails are fed in parallel directly from the splitter.
The OE fuel pulsation dampener was removed. The aftermarket fuel pressure regulator performs this function so no dedicated dampener is necessary. The AN -6 lines feed modified OE fuel rails. Why OE fuel rails? In the state where I live, emissions testing requirements are still in place. To have a truly streetable AND legal car I wanted to build an emissions compliant car that could meet the state emissions requirements and as such felt the easiest approach was to keep the OE air control valve (ACV) in place. Aftermarket secondary fuel rails will not clear the ACV. There are other band-aids to get through emissions but I decided to stick with the tried and true OE air pump and ACV with a stock main cat. Modified stock rails it was. Therefore I VERY carefully drilled and tapped the OE rails to accept AN fitting adapters. This was a very scary and delicate operation. Particularly so for the primary rail where there isn’t much aluminum material to deal with and the holes must be tapped precisely in order for the AN fittings to clear the neighboring oil injectors.
The fuel rails then discharge into a pair of AN -6 lines plumbed to an Aeromotive (A1000 series) adjustable fuel pressure regulator. With two pumps working to move fuel the ability to manage fuel pressure was imperative, hence the need for an adjustable regulator. A fuel pressure gauge was fitted to the FPR for ease of setting fuel pressure at idle even though I have an in-car fuel pressure gauge also. My in-car gauge is fed by a sensor plumbed into the top of the aluminum splitter block upstream of the fuel rails with its own AN -4 SS line. Idle fuel pressure was set to 40psi base pressure at the FPR gauge. The FPR vacuum line is simply plumbed to the UIM. The FPR discharges to a single AN -6 line back to the OE hard line at the firewall. I found a suitable AN to metric hard-line adapter fitting. It required I cut and flare the end of the OE hard line but the fitting fit perfectly and with no leaks. It is a low pressure return line so keeping it leak free was not difficult. Had it been a high pressure feed line connection I would have had second thoughts but this is low pressure so no worries. What I can share is that it took a lot of work and several attempts to figure out the best routing for the SS lines under the manifold. The lengths of each hose had to be perfect and the bend radius of each line maximized as SS hose doesn’t bend like a simple rubber hose does. To make room for the hoses, the coil packs were moved, as mentioned above, to the drivers’ fender, and all the control solenoids were removed or relocated. There is NOTHING under the UIM except fuel lines and electrical wiring.
All this fuel delivery and to feed what? I installed 2 stock 850cc secondary injectors in the primary
rail and a pair of modified 850cc secondary rails bored out to 1300cc. The secondary injectors when tested
were actually flowing about 1450cc/min each! (1424cc/min and 1460cc/min to be exact -
Witchhunter flow tested at 43.5psi) That’s a lot of fuel! Some may argue the benefits
of using 850cc primaries. Hard tuning at idle and poor off idle throttle response are common problems.
Weigh those concerns over tunability during the transition between smaller 550cc OE primary injectors and
larger 1450cc modified Denso (or larger 1600cc Bosch) secondary injectors. I chose the former approach for
better control during moderate driving conditions and decided to forego the idle issues. Yes my idle is set
at about 900-1000 rpms, still exhibits a rough idle, and has a tricky/sensitive off idle response. However,
even with the big primaries, I was still able to pass emissions with flying colors. The larger injector
combination is enough fuel to feed ~600hp. So no more issues of exceeding that 90% fuel injector duty cycle
limit. At 15-16psi of boost and about 420rwhp I still only see about 80% duty cycle on the fuel injectors.
Also there is not so much as a twitch in fuel pressure under full boost.
For more specifcs on the reconstruction of Princess go to the following pages:
OUR "LOCAL7" COMMUNITY | THE WORLD OF THE RX-7
This page last updated March 19, 2009