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View Full Version : How piggy backs control Fuel and Timing. - By Exhaust Depot

11-02-2019, 07:33 PM
Many of us have been throwing the word piggy back (PB) and clamp around for some time with out really know what it does. I’m going to do my best to lay this down Roper Room style so we can get a grasp on this.

PB computers (AFCII, E-manage, SMT-6 etc) They all control fuel by allowing the ECU see what ever you allow it to see. The PB go in between the MAP sensor and the ECU.

The question that lies ahead of us is how do we add (remove) fuel to the engine (http://www.srtforums.com/forums/showthread.php?t=51794#)?

There are basically 3 ways to add fuel to the engine (not counting a NOS wet system or additional injectors).

1. The first is via the MAP sensor. What is a MAP sensor you ask? It stands for Manifold Absolute (no not the drink) Pressure. It turns pressure into an electrical output. As pressure increases, the MAP output voltage increases. The higher the voltage the ECU sees, the longer it turns “on” the injectors.

2. Up the pressure. By installing a fuel pressure (http://www.srtforums.com/forums/showthread.php?t=51794#) regulator, you can up the factory 58 PSI to what ever you want. This will cause more fuel to dump into the fuel system.

3. Install bigger injectors. Bigger injectors have a larger orifice (Uh, he said orifice) so you get more fuel that dumps out.

What is a CLAMP?

First, the word clamp from an electrical circuit which utilizes diodes. With diodes you’re able to setup a clipper or clamper circuit. A clamper allows you to regulate how much voltage an item is able to see. For example, on our MAP sensor, the max output voltage is about 5V which is introduced into the ECU. If you clamp the MAP sensor say at 4.5V, the ECU will never see more than 4.5V, it can see less, but not more. We use piggy backs as a clamp. We allow the ECU to see what ever we want it to see. The MAP sensor works off a 0v-5V output. So say at .9v your at 1PSI of boost and at 18PSI (max boost MAP sees) your at 5V.

How a piggy back can lean or richen your fuel:

Let’s take into consideration a car running S1 with no modifications to the wga and no boost controllers etc. As most of us know, S1 runs pig rich. Many have seen 11/1 even 10/1 AFR. You can get a good 5-10+ hp extra by leaning it out. S1 runs about 15 psi or so. Now, let’s make up some numbers and say that instead of getting 15psi of fuel, the computer would only give you 14psi worth of fuel. This will lean out your fuel to give you say a nice ideal 12/1 or 12.5/1 AFR. The piggy back sees the actual true voltage coming out of the MAP sensor but shows the ECU a lesser voltage so that the ECU keeps the injectors opened for a shorter time. Now you can do the opposite. If your lean, the PB can show the ECU a higher voltage to give you more fuel. The PG also monitors RPM points so its able to add or remove voltage at different RPM settings. This is also how a PB is able to lean out your larger injectors. If you use larger injectors, you want to keep them closed longer than the smaller injectors.

I personally like the E-manage ECU over the AFC unit because it’s able to clamp and also add fuel with out upping the fuel pressure etc. The higher voltage the ECU sees, the more timing it will pull. The more advance your timing, the more HP you will make. An ideal timing is as much as you can go with out detonation. To much timing will cause you to detonate.

I have gotten a good 15-20 HP from just clamping the MAP signal. Others here such as Neon Parts and Aaron have also seen good results from clamping there map sensor. I like to clamp my sensor to 4.2 -4.5V. 4.5V is about 15psi of fuel. By showing the ECU 15PSI, the car runs about 22 Deg of timing at 18psi. With out the clamp, at 18psi, you have about 10-12 or so. I don’t recall the exact numbers but it was a big difference.

So say you now take your S1 car and install a boost controller and are now running 18psi. If you clamp your MAP sensor to 15PSI, you will make more HP! BUT here is the problem with the AFC unit. How do you add fuel? Remember, at 18psi, your only getting 15psi worth of fuel. How do you make up for the extra fuel? You have to make up for the fuel by doing what was stated above. You install a return system, or you install larger injectors with a bigger pump. I personally will go with the Return VS the bigger injector and bigger pump, because you can get more fuel that route (assuming you don’t put a Godzilla pump with Godzilla injectors etc.)

This is where the E-manage shines vs the AFC. The emanage has an additional injector harness. This harness splices into your injectors. So when the ECU tells the injectors to turn off and give you only 15psi worth of fuel at 18psi, the E-manage keeps the ground on the injector and holds it longer to make up for the extra 3PSI of boost. This is the only piggy back that I know of that can do this thus far and this is why I used the emanage vs the AFC and other units out there.

You also save fuel with the e-manage. You don’t have to run the higher fuel pressures and don’t have to buy the bigger injectors. If you do plan on going over 300hp or so, you WILL need a return system. You can only keep the injectors open for so long and get only so much out of it. Also its not good to saturate your injector.

So as you can see, using an AFC will allow you to run bigger injectors and a fuel pump, but you will not be able to get a nice consistent clamp. Using a bigger fuel pump and a 630CC injector, I was able to get 295 with a 98% duty cycle which pretty much maxed out the fuel system. This was using an upgraded wheel on the stock turbo.

The next upgrade for this car is a return line.

On higher HP applications where your running alot of boost and have plenty of fuel, you might get detonation. This will require you to retard timing a little. This is simple to do, just raise your clamp voltage to say 4.6 or 4.7V which is maybe about 16psi worth of fuel. So as you can see, you can also advance and retard your timing based on where you set your clamp.

07-10-2019, 07:36 PM
i think i just learned more from from reading this than i have from anything else... very nice.. i can actually picture in my head how this stuff works...