AFC housing adjustments - what does each do?

[JGK]

I've read virtually every post in this forum for the past 16 months that talk about torque plates, the AFC housing, smoke control, etc. but I'm still left with a few questions about the AFC.



As I see it, there are 4 basic adjustments you can do with the AFC.



1. Move it forward or backward.

2. Change the spring to a lighter (or heavier?) one.

3. Rotate the star wheel.

4. Adjust the preload screw on the back.



Any others?



My question is, what do changes to each of these do if all other things are left alone. That is, what does each adjustment affect and in what way? I know the wheel will help with low-end smoke control and response, and I think the preload spring will help with fueling at start-up and just off idle.



I have a #11 plate, the medium spring, wheel backed off all the way (almost), and the preload screw in just about a half or 3/4 turn.



I am going to do some adjustments to fine tune the low end and I'm looking for better understanding of each of these before I mess things up!



Thanks, all.

Jay

 

[Deezul1]

They all do the same thing in one way or the other which is alter the way the pump responds to boost. like how much fuel will flow and what pressure it does it at.

I am sure there are guys out there that can give a much more detailed explanation but thats the basics.

 

[Don M]

1. Move it forward or backward. = (moving the AFC housing forward lets the gov lever move further forward or it makes sure the AFC wont hold it back from forward travel.



2. Change the spring to a lighter (or heavier?) one. =(the spring tension is what determines how much boost must be built before the AFC begins to move forward) Therefor the lighter spring moves the AFC slide with less boost. This equals more low end fueling. I have found the medium spring wont fully open until about 28PSI. The heavy 215 stock spring is closer to 34psi.

I installed the light spring last night and on the bench it was about 30% open at 6 PSI and fully at 25. This is not a constant, some spring preload from the starwheel can change this.

I dont mind smoke and I want full fueling so my star wheel is all the way flush with its seat, or turned all the way in. Pushing the starwheel away from you towards the center of the engine is way it is accomplished. )



3. Rotate the star wheel. ( the star wheel is a sorta smoke control for low boost, the closer the seat is to its bottom, the less effort or boost is required to move the AFC slide forward. Rotating the wheel forward towards the center of the engine takes preload out of the AFC spring. Rearward rotation gives the opposing effect and adds preload to the spring.



4. Adjust the preload screw on the back. =(this allows the diaphram to fully seat against the inside of the housing cover. If it is too tight the diapram cannot fully slide rearward to lean the low end fueling, I believe some guys turn it in to add more low end fueling. )



Further, if the AFC diaphram 10mm nut that holds the diaphram to the shaft is too tight, the slide travel can be reduced and the top end fueling will be reduced.



On your 215 pump there is washer between the diaphram cup plate and the shaft to provide proper spacing. This little booger must be re-installed if you change springs.



More low end could come from forward rotation of the starwheel at the expense of smoke.

I dont know the elevation of Boise, but in Pocatello(4500ASL) I had to tighten the starwheel to avoid too much smoke at low speed.



Don~

 

[Deezul1]

Yea Don, Thats what I meant. Good explanation

 

[Cooker]

Excellent post Don!

 

[JGK]

Excellent, Don

Thanks, Don. What Cooker said! I'm printing this off for reference.



Elevation in Spud-town is about 2700', by the way.



I have the medium spring in and the wheel backed nearly all the way off (if not completely). Very little smoke for having the #11 plate and the 370 Diamond Bs, but I have the plate position set a little conservative to save the clutch.



The problem is, it's a dog in 4th or 5th until the boost starts to hit at about 14 - 1500 rpm. Then whoosh! I'm going to try to smooth that out a bit without blinding everyone with smoke.



Also, I have a Piers custom 35 on the way. We'll put that on first and then do the AFC adjustments.



Thanks again,

Jay



P. S. Don, I've been following your posts. Sounds like when you finally get yours together and 'set', it'll be a screamer. Look me up if you ever spend any time in town.

 

[C Schomer]

Jay, mine runs like that too - I think that's normal. I haven't figured out all the tricks with the plates and afc yet but I think main fueling starts at about 1500 with the 11(quote from Joe D). Even with the afc loose and forward, it's still there. The bottom ramp of the plate does that! I haven't seen a 10 but I hear the bottom is nearly gone - giving maximus fueling at the bottom end.

 

[JohnE]

Piers?

Jay;



You're dealing with one of the best 12 valve techs around.

Have you picked his brain for fine tuning your big P7100?



-John

 

[SlyBones]

Wanted to add a little seat of the pants observation.



My understanding. WOT fueling is controlled by the cam plate, and its curve. Before that the AFC housing is controlling the fueling. Boost from the manifold pushes the rack forward, via the diaphram in the back of the housing. The AFC spring controls the rate of the movement. Adjustments are like Don says. Not trying to change that.



Moving the housing forwards or backwards does like Don says, but I think also changes the starting place for the rack. So you have more initial NO-AIR fueling, and more smoke. Also the boost has less spring to push against before it hits the plate. You get much quicker response. What I have noticed playing with sliding the housing back and forth is how quick or slow the boost builds in the 2#-15# range, and smoke.



I think this is where the light spring comes in handy. Allows you to keep the housing slid back for less initial fuel and lower smoke, and then push out of the way faster, for quick response, and gets you on the camplate faster.



Another observations is low rpm response ( 1500 rpm ) , versus freeway response ( 2000 rpm ). If I adjust the response so the right foot can have some amount of control over the boost on the freeway, then low speed response suffers. If I set it up so that low speed response it adequate. Response on the freeway is too touchy. Your at cruise boost 4-6-8 psi or I am at 25-30+. No middle ground. Adjust for control and bottom end suffers a little. I dont think there is much we can do about this, pick one or the other.

 

[JGK]

John,



Yes, I've spoken a little to Piers about this, but my last conversation with him was about the turbo. Once I get that on and I've driven it a bit, then I'll start fiddling with the AFC. I'll probably call him again. I just hate to take much of his time but he is such an easy guy to talk to. Truly a gem. As usual, the more info you get the broader picture you begin to form. That's the reason for asking here.



Slybones,



Thank you for the info on sliding the housing. That was the one adjustment Don was a little vague on and I still had questions about. Piers said the AFC controls the off-boost fueling. Once boost comes on, the AFC is out of the picture. I expected that adjustments in one area would have consequences on others, like response vs. smoke. There's always a trade off. The trick here is figuring out which adjustment to fiddle with to get a little more poop between idle and about 1400 without trying to kill every bug for a square mile behind me every time I pulled away from a stop.



Jay

 

[Turbo Thom]

JGK:

You may be right. My wife drives a 98 with the Banks kit and makes more smoke than me, but has a very strong bottom end. I'm speaking of the truck. She has the twin air ram, 14 turbo housing and fuel plate. I run the 370 B injectors and a stronger P 7100. I miss the bottom end which I need for towing. I really don't know how to get it out.



Still looking.



Preston

 

[SlyBones]

Jay and Preston,



Talk to Piers about 191 high flow delivery valves.