Choosing a Turbo for Your Power Goal, and the Fuel System to Feed It

Two mistakes sink more turbo builds than any others. One is picking a turbo off a forum because it made big numbers on someone else's engine. The other is sorting the turbo, then treating the fuel system as an afterthought and running out of fuel before the turbo's even working. Both come from the same place: not starting with a clear power goal and sizing everything backwards from it.

So let's do it the right way round. Pick a number, size the turbo to it, then build a fuel system that can actually feed that number.

How to choose turbo size in kW

The honest starting point is a realistic power goal in kW, not a turbo model. Once you know what you're chasing, say 400kW (about 535hp) at the wheels, you're really asking how much air the engine needs to make that power, and which turbo moves that much air efficiently on your displacement.

A few ideas do the heavy lifting here:

• Size the compressor for the airflow you need. More power means more air. The compressor side of the turbo has to move enough air for your target without being pushed off the edge of its efficiency, where it just makes heat instead of boost.
• Turbine housing A/R trades response against top end. A smaller A/R spools faster and feels punchier down low but can choke the top end. A larger A/R frees up the top but makes you wait for it. There's no free lunch, just the balance that suits how you drive.
• Frame size sets the ceiling and the lag. A bigger frame can support more power but takes more exhaust energy to get moving, so it comes in later.

The classic mistake is going too big. A turbo two sizes past your goal looks great on paper and drives terribly, spooling so late that the car's useless on the street and barely better at the track. On a JZ that lives on the road, a turbo that's right for your number will almost always be more fun than the biggest one you can afford. Match the turbo to the goal, not your ego.

Sizing the fuel system to match

A turbo can only make the power the fuel system lets it. This is the half of the job that gets rushed, and it's where a lean cylinder and a melted piston come from. Two parts matter: the injectors and the pump.

Injectors: flow scaled to power and fuel

Injectors are rated by how much fuel they flow. The bigger your power target, the more flow each injector needs to deliver in the time it has. Size them so they're not maxed out at your goal, because an injector running at 100 percent duty has nothing left in reserve and leaves no margin for error.

Fuel type changes the maths. E85 makes great power and runs cooler, but it carries less energy per unit, so it needs roughly 30 percent more flow than pump petrol to make the same power. If there's any chance you'll run E85 later, size the injectors for it now rather than buying twice.

Pump: flow at the pressure the engine actually sees

Here's the part people read wrong off a spec sheet. A fuel pump's flow rating is only meaningful at the pressure it's working against. A pump that flows a big number at low pressure flows a lot less once it's pushing against base fuel pressure plus boost. Under boost, the pressure the pump fights climbs, and its real flow drops with it.

So size the pump on its flow at the actual pressure your engine sees at full boost, not the headline figure on the box. Build in sensible headroom on top, because a pump running flat out has no margin and runs hot. As with the injectors, if E85 is on the cards, factor in the extra volume it needs.

Getting the combination right

The turbo, the injectors and the pump are one system aimed at a single number. Get any one of them undersized and it becomes the ceiling for the whole build, no matter how good the other two are. That's why we'd rather spec the lot together than sell you a turbo and let you guess at the fuel side.

Have a look through our Garrett G-Series turbos, fuel pumps and Injector Dynamics injectors to see what's in stock, or call 0800 NZ PERFORMANCE with your power goal and we'll help you size the whole package so the turbo and the fuel system arrive at the same number together.