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.
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.
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.
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.
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.