Every big power build hits the same wall. You need enough injector to feed the engine at full noise, and the injector that does that is too big to idle properly.
The usual answer is to accept it. Fit the 2000cc units, put up with a lumpy idle and a car that hunts in traffic, and tell yourself it’s the price of the power.
It isn’t. Here’s what I did instead, and the arithmetic behind it.
Configuration
Status: Verified
- 2JZ-GTE VVTi, 3,353 cc, 1,114 whp on E85
- Plazmaman intake manifold, twelve injector provision
- Twelve CP1000 injectors, rated 1,000 cc at 3 bar
- Aeromotive regulator, 4 bar static, 1:1 rising rate
- Three Walbro 485 pumps, -10 feed lines
- Emtron KV12, twelve dedicated injection channels, flex fuel
The problem is minimum pulse width, not flow
An injector is a solenoid valve. It takes a finite time to open and a finite time to close, and during those transitions the flow through it is neither zero nor fully open. It is somewhere in between, and it is not linear.
Above a certain pulse width, the opening and closing transitions are a small fraction of the total and the injector behaves predictably. Below it, the transitions dominate. Two commands a fraction of a millisecond apart can deliver very different amounts of fuel, and the relationship stops being something you can characterise reliably.
That’s the minimum controllable pulse width, and it’s roughly a property of the injector rather than of how much fuel you’re asking for. A big injector doesn’t take proportionally longer to open. It just moves more fuel while it does.
Which is why big injectors idle badly. At idle you need a very small mass of fuel. With a big injector, that mass corresponds to a pulse width down in the region where the injector is unpredictable. The ECU asks for the same thing twice and gets two different answers. That’s your lumpy idle, and no amount of tuning fixes it, because the hardware isn’t repeatable down there.
Twelve small ones instead of six big ones
The Plazmaman manifold takes twelve injectors, two per cylinder. Six primaries and six secondaries, each addressed individually on its own channel by the KV12.
At idle, around town, and at any low load, only the primaries fire. Six injectors, each a 1000cc unit, sharing the same small fuel demand a set of six would normally handle. Pulse widths sit in the region where a 1000cc injector is well behaved.
When load arrives, the secondaries stage in and the full twelve are working.
So you get the idle characteristics of a 1000cc injector and the flow of something more than twice the size. That’s the whole idea, and it costs you nothing except a manifold that takes twelve injectors and an ECU with twelve output channels.
The arithmetic, including the bit people get wrong
The injectors are rated 1,000 cc. They are not flowing 1,000 cc.
Injector flow scales with the square root of the pressure differential across it. Rated at 3 bar, run at 4 bar:
flow = rated_flow × √(actual_pressure / rated_pressure)
= 1000 × √(4 / 3)
= 1000 × 1.1547
= 1,155 ccTwelve of those is roughly 13,900 cc/min installed, which is the equivalent of six 2,300 cc injectors.
That square root matters more than it looks. It cuts both ways: raising fuel pressure buys you less flow than you’d expect, and dropping it costs you less than you’d fear. A 33 percent pressure increase gave 15 percent more flow.
It also means anyone sizing injectors off the rated figure without checking their base pressure is working from the wrong number.
Why the regulator has to be rising rate
What actually drives flow is the pressure differential across the injector: rail pressure minus manifold pressure.
With a fixed 4 bar rail and 2.8 bar of boost in the manifold, that differential collapses to 1.2 bar at peak. Flow would fall to roughly 55 percent of what the ECU thinks it’s getting, at exactly the moment the engine needs the most fuel.
A 1:1 rising rate regulator references manifold pressure and raises rail pressure to match, so the differential stays at 4 bar whatever the boost is doing. At 2.8 bar of boost that means roughly 6.8 bar of absolute rail pressure.
Which is what the three pumps are for. Walbro 485s at 6.8 bar are working considerably harder than the same pumps at 3 bar, and their flow falls off as pressure rises. Fuel pressure has been confirmed to hold through full load E85 pulls, and the third pump exists for exactly that margin.
A headroom check
Status: Working conclusion
Rough demand at 1,114 whp on E85, assuming a brake specific fuel consumption around 0.66 lb/hp/hr and a nominal drivetrain loss, lands somewhere near 8,400 cc/min.
Against 13,900 cc/min installed, that’s roughly 60 percent duty at peak power.
Both of those assumptions are mine rather than measured, so treat it as a sanity check rather than a specification. But the shape of the answer is right: there is real headroom, which is what you want on a car that has to survive being driven rather than just surviving a dyno pull.
What this costs
It isn’t free.
Twelve injectors means twelve to buy, twelve to wire, and twelve output channels on the ECU. Not every standalone has them, and a manifold with secondary injector provision is a specific purchase rather than a generic one.
There is also a calibration cost. The handover between primary only and both sets firing has to be blended properly, or you get a step in fuelling at the transition. That is real work and it is work a single stage setup doesn’t need.
Whether it’s worth it depends entirely on what the car is for. If it lives on a dyno and does quarter miles, fit six big ones and don’t think about it. If it has to sit in traffic on the way to Scotland and then do something violent when it gets there, the staging is the difference between a car you drive and a car you tolerate.
This fuel system is what makes 2.8 bar on E85 possible in the first place. What that boost is actually worth, and how much of it is the fuel rather than the pressure, is covered in What E85 Actually Buys. Full specification is on the Supra project page, and the build itself is in The 3.4 Stroker Build.
If you’ve run a staged setup and measured where the handover actually lands in your fuel trims, I’d be interested. That transition is the part I know least about and it’s where I’d expect the problems to live.
