Transient Throttle (Throttle Pump) Explained

Transient Throttle (Throttle Pump) Explained

In this article we are unravelling one of the mysteries of tuning; the often talked about but rarely understood dark art of tuning the Throttle Pump, or as we call it here at Haltech the Transient Throttle Enrichment function.

Transient Throttle

If you have even a passing interest in engine tuning you have probably come across a vehicle or engine the runs fairly well under steady-state conditions at all load and RPM areas, but when you mash the throttle quickly the engine coughs a splutters before eventually taking off again. The most likely explanation for that happening is the incorrect setup of the transient throttle enrichment.


Don’t be too worried if you are unfamiliar with the term “Transient Throttle Enrichment” this feature goes by a few different names. If you come from the carburetor world you probably know this term as the Accelerator Pump or Accelerator Enrichment Valve. It’s also known as the Throttle Pump, or the Fuel Tip In, or Throttle Tip In.

These terms all refer to the same thing, and in the Haltech software we call it the Transient Throttle Enrichment function.

Let’s start by explaining what transient throttle enrichment is and why we need it – because this will set us up for a good understanding of how to approach the tuning process.

The Carburetor Conundrum

If we cast our minds back to the yee ole days of carburettors you will notice that in every carb there is a fuel circuit that is activated purely by moving the throttle. The way it works is irrespective of the amount of air moving through the carburettor, simply when you move the throttle you get an extra spray of fuel. The accelerator enrichment circuit in a carb is an intricate balance of springs, pistons, and valves. 

If you have ever had the acceleration enrichment circuit clog up and stop working on your carb you will know what happens next. The engine starts and runs just fine and so long as you are real gentle on the throttle, everything is good – but stand on the throttle quickly and the engine has a massive lean spot, it hesitates it often misfires and occasionally backfires through the intake.
What we’re experiencing there is a big lean spot, the engine is running fine until you mash the throttle, and then it goes lean. Why? It’s all about physics – in a carburettor the air and the fuel are both injected into the intake manifold at the same spot – the carburettor. 

What happens when you mash your right foot to the floor and the throttle blades slam open is you get a big gulp of air rushing into the intake manifold. However, and here comes the physics, air is approximately 600 times lighter than fuel, so when you get a rush of air into the intake – the air makes its way to the intake valve and combustion chamber 600 times faster than the same mass of fuel.

So the next time the valve opens, the air rushes in, the fuel hasn’t yet caught up, the valve closes, we get compression and spark but no bang. This happens for a couple of engine cycles until the fuel finally decides to show up to the party and we start making fire again.

The throttle pump circuit on your carb is designed purely to try and give that fat, lazy, heavy fuel a head start in the race to the intake valve in the hope that some of it makes it there in time for the intake valve opening.

If you put an air-fuel ratio meter on a carburettor-controlled engine, you will notice that even the most finely tuned carb will always have a lean spike followed by a long rich tail on acceleration. You get the rich tail because the regular fuel delivery eventually catches up with the additional fuel in the acceleration enrichment event and the engine runs rich.

What about EFI systems?

EFI systems remove one of the limitations found on a carburettor that cause this phenomenon – the fact that air and fuel are both coming into the intake at the same place. Typically we find EFI fuel injectors located much closer to the intake valve than the throttle body – so effectively the fuel has a massive head start in the race to the valve.

The second advantage we have with EFI is the pressure under which we are injecting the fuel. With an additional 40PSI of pressure pushing the fuel toward the valve, we are roll racing fuel and air rather than drag racing them as in the carbureted system. Finally, most modern injectors do a really good job of atomising the fuel into very fine droplets, so they can accelerate very quickly.

What all that means is that in most cases we need a lot less additional fuel to be added to the intake manifold when the throttle is snapped.

We say “in most cases” because there are always exceptions. If you are running a throttle body injection setup you tend to find you still need to provide a lot of addition fuel in the transient throttle enrichment because you are dealing with the same limitation that a carburettor is. The fuel is still much heavier than the air and because it’s a throttle body injection setup the fuel and air are being injected at the same spot so we need to throw much more fuel at the race as early as possible to try and prevent the engine from misfiring.

Another advantage of an EFI system is the ability to use sequential fuel injection and being able to time fuel delivery event with the valve opening. We won’t go too deep into this here because it’s a topic that requires its own article but we the basic premise here is injection timing – that is when the fuel is injected (rather than how much fuel is injected), often plays a big part in the overall vehicle driveability which includes the transient throttle enrichment.

Our suggestion is that you don’t put too much effort into tuning the transient throttle enrichment until after you are happy with the tune-up on all of your steady-state fuel maps, which includes the injection angle map.

For a step-by-step Transient Throttle Enrichment setup in the Haltech tuning software watch the video at the top of this page.

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