AFC stuff

[Dual K20s]

I finally got a chance to rewrite this thing. Its different than the previous thread, but we'll build on it with what we want.

I’m about to attempt to explain the basics of how an AFC works, and how to modify it to allow maximum rack travel. This may be a little long so bare with me or skip through sections. I’ll start with the basic operating principle behind how it works. Since the AFC limits the maximum amount of rack travel allowed dependant on the amount of boost pressure the engine is operating under. For the AFC or fuel plate to come into effect you need to be under significant throttle input. I'll try to go through the main pump factors from a no boost situation to a high boost situation.

So assuming you are pedal to the floor: the limiting factor at no to low boost is the "pre-boost" screw, aka the smoke screw as others call it. The adjustment of the screw is also relative to the position of the AFC housing. People often forget to adjust the screw when moving the housing. Others adjust this to gain more smoke. It has no effect on power so I personally run mine backed out as far as possible. The adjustment is on the very back of the AFC under a small cover held on by two bolts. Under the cover the screw is locked in place with a jamb nut. Turning the screw clockwise into the housing will increase the potential fueling levels before boost is built. Counter clockwise does the opposite. Make sure to tighten the jamb nut when you’re done so it doesn’t move. The pre-boost setting usually doesn’t limit power but it might make the truck hesitate just slightly before starting to pull. Backing the screw all the way out and sliding the housing towards the radiator is a decent place to start and will allow you to adjust the housing if the truck fuels to heavily with minimal disadvantages.

Once some boost has been built the starwheel and the AFC spring comes into play. Tightening (counter-clockwise as facing the same direction as the truck or turning the teeth away from the motor) the starwheel increases the tension on the spring which rides on the AFC diaphragm which will require more boost to be built before the AFC arm will move and how much boost is required to allow full fuel. Loosening again does the opposite, and removing all tension would effectively be like sliding the housing forward or turning the pre-boost screw in. Starwheel setting is somewhat relative to personal taste. A tight starwheel will limit smoke but too tight and it will cut into midrange power also. Too loose and the truck will smoke heavily and power may also be reduced because of excess fuel. Changing the starwheel setting is a fine adjustment (maybe +/- 5psi to full travel) for large changes using different springs can be very effective. I try to set it just tight enough that a slight haze is the max smoke level seen.

Once significant boost is built the AFC allows full fuel and the governor arm is free to ride on the plate. Moving the plate forward or using a different one would allow the governor arm to travel farther and increase rack travel effectively increasing the duration that the fuel is injected. Because boost is already built and most likely the rpms are high enough, (hopefully your setup doesn’t allow full fuel before the engine is ready) the aggressiveness of the plate usually has little effect on smoke levels especially with stock injectors and turbo. This is where max power happens though so an aggressive plate is usually desirable. Leaving the pre-boost screw backed out and sliding the housing to the desired pre-boost fueling is a good way to compromise the max travel without giving up unwanted low-end smoke.


The stock AFC doesn’t always incorporate enough travel to obtain full potential rack travel. Modifications can be made to allow full travel but some may give up a little ability to limit smoke. Careful choice is required. Switching the washers that hold the diaphragm allows additional travel but will not allow the arm to return to its home position and will be like turning the pre-boost screw in a few turns. I prefer to use a modified washer, to replace the cupped one, which allows both. Another common practice is to grind the tip of the AFC foot flat (only take off the angle do not grind any more than you have to, I ground way too much on mine and the smoke control suffers). Some instead like to add a small washer between the back of the diaphragm washer and the rod that controls the arms position, and grind the forward part of the arms barrel.


Once you get the AFC off the pump, it’s very easy to dis-assemble starting from the rear. I replaced all of my mounting screws with M6x1.0x what ever depth is required for the situation from ACE Hardware.




Stock governor spring, stock 215 spring, and #27 wire

This picture shows why grinding the foot or adding a washer and grinding the barrel is needed


Since I've got a lot of pictures and more still on the camera I'll give you a link to my photobucket to save space http://s258.photobucket.com/albums/h...Modifications/

I'll post more about setup when I get a chance but for now I'll let you know that I run a stock governor spring from a 215 pump with the starwheel almost bottomed to avoid any potential coil bind, and overly modified foot, two flat washers, no preload, with the housing full rearward. And my plate is slid a few mm back to avoid the "retard notches"

What do you think?

[StrokeThis]

even with your afc backed out all the way, and no tension on the spring, are you still putting out the fire if you dont roll into the throttle?

Im running the same setup(stock governer spring, afc backward, ground foot, etc), but i dont have shop air so its all a guessing game right now.

[JakeDiesel]

Thats a great write up thankx

[blackcloudCTD95]

So then basically with the housing FF..... and the star wheel all the way up you just cancled them out... I noticed NOTHING with the wheel all the way up or back or middle.....

[Farmboy's Diesel]

If You have the wheel all the way forward you should notice a bunch of fuel right off the line. Its also a remarkable difference if you run the aneroid screw all the way forward.

[StrokeThis]

Originally Posted by Farmboy's Diesel

If You have the wheel all the way forward you should notice a bunch of fuel right off the line. Its also a remarkable difference if you run the aneroid screw all the way forward.

Yeah, with stock injectors, but right now if i did that, i'd be dead pedal driving since i would be putting out the fire for so long until boost hits.

Stock Injectors- Full Forward Preboost screw and starwheel is fun.
Bigger Injectors-FF Screw makes lots of smoke but no pedal response.

[Dual K20s]

The stock springs are too short and develop "slack" when you loosen the starwheel too much. Longer springs (like aftermarket springs from TST) are long enough to maintain pressure even with a bottomed wheel.

Originally Posted by StrokeThis

even with your afc backed out all the way, and no tension on the spring, are you still putting out the fire if you dont roll into the throttle?

Im running the same setup(stock governer spring, afc backward, ground foot, etc), but i dont have shop air so its all a guessing game right now.

If you roll into it slow enough the engine should be able to keep up, but it depends on how overfueled you are and how quick your roll is.

I just use the tester as a reference. The deciding factor is how it runs in the truck, because that's what really matters. Its just convenient to be able to get into the ball park before I run it.

Originally Posted by blackcloudCTD95

So then basically with the housing FF..... and the star wheel all the way up you just cancled them out... I noticed NOTHING with the wheel all the way up or back or middle.....

The wheel makes such small changes, I can't really notice much in the truck either.

The starwheel doesn't have any effect on no-boost fueling. The pre-boost screw has no effect on in-boost fueling. And the housing position changes, well it shifts the whole curve and changes everything.

[Farmboy's Diesel]

Actually with the starwheel completely forward the spring is loose and takes a very minimal amount of boost to give full fueling. I like having everything forward because you can control the fuel by the pedal, its always nice to be able to black out an intersection.

[later]

Good to see you back, why did you go back to Dual K20?

[Grnkawi]

im happy with the way i have mine setup... i wont even be changing it when i get the heavier spring from Later