The symptom everyone knows
The steady-state tune looks great. Every cell is on target, idle is smooth and part-throttle cruise is clean. Then you snap the throttle open and the engine hesitates, bogs or pops before it pulls. When steady running is good but quick throttle changes aren't, the culprit is nearly always transient throttle enrichment.
One feature, lots of names
Carburetor people call it the accelerator pump. Others say throttle pump, tip-in, or fuel tip-in. Haltech calls it Transient Throttle Enrichment. They're all the same thing: extra fuel added because the throttle is moving, on top of what the fuel map already asks for.
Why a carburetor needs a big shot
To tune it well, it helps to know why the problem exists in the first place, and carbs make it obvious.
A carburetor mixes air and fuel at one spot, right at the throttle blades. When you floor it, the blades open and a huge gulp of air heads down the runner. Liquid fuel is roughly 600 times denser than air, so it simply can't accelerate as quickly. The air reaches the cylinder first, the intake valve closes on a mixture that's far too lean, and you get compression and spark with little or no combustion. That repeats for a few cycles until the fuel arrives.
That's why every carb has an accelerator pump circuit, a small arrangement of springs, a piston and check valves that squirts fuel whenever the throttle moves, regardless of airflow. It gives the heavy fuel a head start. If that circuit clogs, you know exactly what happens: gentle driving is fine, but a quick stab gives a big lean stumble, misfires and sometimes a backfire through the intake.
Even a well-sorted carb shows a pattern on a wideband: a lean dip the moment the throttle opens, then a long rich tail as the pump shot and the normal fuel delivery overlap.
Why EFI needs much less
Port injection takes away most of that problem, for three reasons:
- Location. The injector sits close to the intake valve, not up at the throttle body. The fuel starts the race most of the way to the finish line.
- Pressure. Fuel leaves the injector under roughly 40 psi (about 275 kPa) of pressure, so it's already moving quickly. The race becomes a rolling start instead of a standing one.
- Atomization. A modern injector breaks fuel into a fine mist, and small droplets speed up and evaporate far faster than a stream.
The result is that a port-injected engine usually needs far less transient enrichment than a carb. Many tuners coming from carbs over-do it and end up chasing a rich bog instead of a lean one.
TBI puts the injector back where the carburetor was, at the throttle. It has the same air-versus-fuel race a carb has, so expect to need much more transient enrichment, delivered early.
Injection timing plays a part too
With sequential injection, the ECU can choose when in the cycle each injector fires, not just how long it stays open. Where that window sits relative to the intake valve affects how much fuel is sitting on the port walls, how much reaches the cylinder straight away, and so how the engine responds to a sudden throttle change. It's a big subject on its own, but the takeaway is simple: injection timing affects drivability, including transient response.
How we approach tuning it
Here's the order we follow on the dyno and on the road:
- Finish steady state first. Get the main fuel table, dead times and the injection angle map right. Transient enrichment added on top of a bad base tune just hides the real problem.
- Log fast. Log throttle position, lambda, RPM and the transient enrichment channel at a high sample rate. The event lasts a fraction of a second.
- Test real throttle movements. Try small blips, medium stabs and full snaps from a few different RPM and load points.
- Read the lambda trace. A lean dip after the throttle moves means not enough enrichment. A rich spike or a lazy, rich tail means too much, or it's decaying too slowly.
- Change a little at a time. Adjust, repeat the same test, and compare logs side by side.
Be aware that a wideband sensor responds with a small lag, so line the lambda trace up against throttle movement with that in mind rather than expecting a perfect match.
Troubleshooting
| Symptom | Usually |
|---|---|
| Cough or stumble on a quick throttle stab | Not enough transient enrichment, or it arrives too late |
| Bog with black smoke, lambda goes rich | Too much enrichment, a common mistake coming from carbs |
| Good response when warm, stumbles cold | Cold engines need more. Look at the coolant temperature correction for transients |
| Response changes after an injection timing edit | Normal. Retest transients whenever injection angle changes |
| Nothing seems to fix it | Go back and check steady-state fuel and dead times first |
Where to set it up
For the menu-by-menu settings, see Transient Throttle for Elite ECUs and Fuel Transient Throttle for Nexus. If you're on an Elite with the enhanced strategy, read Transient Throttle Enhance, and for cold starts see transient coolant temp correction.
FAQ
Is transient enrichment the same as an accelerator pump?
Yes, the same job done electronically. The ECU watches how fast the throttle moves and adds fuel for a short time.
Why does my EFI engine need so little compared to my old carb?
The injector is close to the valve, under pressure and atomizing well, so fuel arrives almost as quickly as the air.
Should I tune transients before or after the main fuel table?
After. Steady-state fueling, dead time and injection angle come first. Our base map basics article is a good starting point if you're building a tune from scratch.
