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Old 11-19-2007, 11:56 AM
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Thumbs up 12 Valve AFC Mod's



Guys Ive been wanting to do an AFC Mod write-up and finally got the motivation so here we go. This is a cutaway of a Bosch P-7100 Injection pump found on all 12 Valve Cummins Diesels from 94-98. Theres a lot of finesse to get these to work well and put out power without killing the guy behind you with smoke. You may not get it perfect the first time you mess with it so expect to do a little tuning.






We will start off with #1
It is commonly referred to as the AFC foot. Some people remove it, but if you understand how to get it to work with you it can be your best friend. Modifications you can make to this would be cut approx. 3/8” off the front of the foot (to the left of the #1) The benefit of this is to allow further travel of the foot which will allow more top end fueling. It can be removed by taking the 4 screws off the back of the AFC housing (some are tamperproof) and the small 10mm screw in the front of the afc housing and dropping out the rod that holds it in place.

#2 is the fuel plate. Shown is a lame stock one with a profile geared towards emissions and keeping the engine from killing the weak stock Dodge trans. Examples of modified fuel plates are shown below. You can grind your own or buy one if you like from www.tstproducts.com . A cheap free mod is grind the stock one to the shape of a 100, or 10 and slide the plate towards the front of the truck. The Plate is held in under the AFC housing with 2 flat head screws. As you can see in the pic, the plate has slots in the top for adjustability. Towards the front of the truck gives more power, to the rear, less power. Expect some more smoke, and trans. killing torque.





#3 is the Governor lever which rides on the fuel plate profile. In the pic, the AFC foot is blocking it from contact with the plate, but the AFC foot will move forward out of the way when the engine starts to produce boost. This is to keep full fueling from the engine, which without any boost would create a lot of smoke. When you add a fuel plate and slide it all the way forward sometimes you need to adjust this, as the lever may not contact the plate where it should, or may go under it completely creating smoke and loss of power.
Instructions to adjust this lever can be found here:

http://pdrdiesel.com/TechLeverAdj.htm

#4 is a 7/8 plug nut which allows access to the governor springs. Governor springs is what controls how high your engine can run up to the redline. Most stock trucks in neutral with the pedal all the way down can rev up to 2900 or so, but the power will actually start to fall off around 2300rpm or so. By adding a set of springs, either a 3K or 4K GSK you can run past stock settings and actually have full power all the way up to 3000 rpm’s or 4000 rpm’s. As a safety precaution, if you plan on running up past 3200 often you need to ad a set of 60lb valve springs to keep your valves from floating and contacting your pistons.
Instructions for doing a GSK can be found here:

http://pdrdiesel.com/TechGSK12V.htm

#5 is the adjustment point for the Governor arm, with #5A being the access cover for it.

#6 is the starwheel. Its basic function is to put preload on the AFC spring, which dictates at what boost PSI the governor lever moves out of the way. If you have this set too loose, you will smoke heavy under low boost, if you have it set too tight the lever will not move out of the way under full boost causing you lost HP. The best way IMHO to set this and see how it operates is with a compressor and regulated shop air to mimic boost pressure being applied to it. This will take some tuning for it to be at its optimal smoke/power setting.

#6A is the allen head access plug to adjust the starwheel. Looking in the hole, spinning it twards the engine will loosen tension on the spring, towards the fender will tighten the spring tension. When you remove the plug, be sure not to loose the thin copper sealing washer underneath it.

#7 is the AFC diaphragm. Boost received from the line coming into the top of the AFC housing puts pressure on this, moving the AFC foot (#1) out of the way of the governor lever (#3). Modifications to this include taking the Diaphragm housing off the afc by removing the 4 screws (some tamperproof) and swapping the locations of the 2 washers holding the diaphragm to the rod. One is a cup shape, and will contact the front of the housing holding back the AFC foot (#1) from full forward travel. This is done in conjunction with modifying the AFC foot (#1) to allow further travel of the foot, and achieve higher RPM fueling.

#8 is the AFC foot preload (smokescrew). Picture it as a coarse adjustment on the AFC spring, where the Starwheel (#6) is a fine adjustment. By taking off the 2 screws on the tin cover that conceals the preload screw you can adjust the position of the AFC foot at no boost conditions. This can cause a lot of smoke and loss of performance if you go in too far with it. Usually it dosent need to get messed with unless you need a lot of low end fueling to get a big turbo lit while staging on the line. Too much smoke and yur gonna “put out the fire” in the Cyl’s and not spool the turbo anyways. Once again Fine tuning needed for your application.

Last but not least the whole AFC housing that sits on the pump is adjustable. The 4 holes that the screws go through to hold the AFC to the pump are slotted much like the Fuel plate. Sliding the AFC forward to the front of the truck will (obviously) move the AFC foot location forward since it is all one unit allowing the governor lever more travel at no/low boost before hitting the AFC foot, giving you more low end fuel/TQ

Hope this helps with AFC tuning and operation. Not bad for only 1 cup of coffee I figure! Any questions fire away.
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Last edited by Jim; 03-17-2009 at 06:19 PM.
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Old 11-19-2007, 01:22 PM
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Awesome post, man. Thanks for taking the time to write it up. One question is why would a person want to remove the AFC foot? Another question is, where is this part found on my 24 valve? (Had to throw some sort of BS in, right?)
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Old 11-19-2007, 01:39 PM
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I can only assume the kids do it cause it smokes alot and makes them feel like they have a mighty sledpuller? In reality it hurts low end by giving you full fuel at about 1/4 pedal making alot of smoke with no boost putting out the fire in your cylinders.
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Old 11-19-2007, 02:12 PM
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Bubba seems to smoke more than he should. Then I gotta go wipe the soot off his ass. That ain't no fun.
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Old 11-19-2007, 02:16 PM
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Smarty!!!!
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Old 11-19-2007, 03:12 PM
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$700! There's a certain red 12 valve scavenging all my money at the moment. Hey, whaddya think of that gizmo I PM'ed ya?
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Old 11-19-2007, 03:40 PM
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PM sent...
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Old 11-19-2007, 03:52 PM
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When you remove the plug, be sure not to loose the thin copper sealing washer underneath it.


why is that important??? i lost mine the first day....
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Old 11-19-2007, 03:55 PM
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oh yeah i also lost the afc foot a long time ago too.... is that bad??? hahahahahaha i wish i knew where it was at....
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Old 11-19-2007, 03:58 PM
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Quote:
Originally Posted by Ramdriver15 View Post
When you remove the plug, be sure not to loose the thin copper sealing washer underneath it.


why is that important??? i lost mine the first day....
It wont kill your truck if it goes missing, but i figure if I let you know its there you wont loose it.
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Old 11-19-2007, 04:41 PM
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Quote:
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When you remove the plug, be sure not to loose the thin copper sealing washer underneath it.

I think that is the least of your worries... after seeing the inside of a P-Pump...
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Old 11-19-2007, 07:10 PM
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Great post Jim! this will definatly come in handy!
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Old 11-19-2007, 09:51 PM
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I'm gonna go run out and grind my foot
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Old 11-19-2007, 09:55 PM
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Quote:
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I'm gonna go run out and grind my foot
Been drinking Arts goat buttock herbal cheese?
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Old 11-19-2007, 09:56 PM
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Jim, this is a great post and a real deep dive of your knowledge and experience with these trucks/pumps.

Guys, lets just let the 24-valve drivers have their fun. And, remember, they have what we have, just divided by 2 * 6. Figure that one out.
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Old 11-19-2007, 10:05 PM
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Thanks Jim, I'm gonna try to tune the service truck some. I moved the plate ahead and let off the starwheel, now it goes, but smokes like a pig.
No offense, Tim!
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Old 11-19-2007, 11:38 PM
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Yeah, I grounded the foot thing on my Duramax tonight and now it can do a wheelie! -Must be all them valves! Jimmah, how did you learn about all that crap?:Paully6: Great post, even thou I don't own a Dodge anymore!
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Old 11-19-2007, 11:45 PM
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You learn all this from the p pump in a box thats in the shop? Haha all I learned is how not to dissasemble a pump with a 2 lb sledge and of coarse the best tool ever a big set of channel locks . Good write up I think I moved the starwheel to far forward as it now smokes pretty hard down low and clears up on the top end.
Evan
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Old 11-19-2007, 11:51 PM
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You learn all this from the p pump in a box thats in the shop? Haha all I learned is how not to dissasemble a pump with a 2 lb sledge and of coarse the best tool ever a big set of channel locks . Good write up I think I moved the starwheel to far forward as it now smokes pretty hard down low and clears up on the top end.
Evan
Yep, but the magic 8-ball says (remember as 12v owners we don't believe in computer stuff so...) NEW turbo in your future... me too, hoping for sps62/12 - anyone else here too? maybe we can get a group deal if it's like 3 or more??? lets talk! ! !
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Old 11-20-2007, 12:08 AM
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Quote:
Originally Posted by dieselnomad View Post
Yep, but the magic 8-ball says (remember as 12v owners we don't believe in computer stuff so...) NEW turbo in your future... me too, hoping for sps62/12 - anyone else here too? maybe we can get a group deal if it's like 3 or more??? lets talk! ! !

I was hoping to go with twins but I dont think I will have the money. Need something that will spool quick for towing and stay within the stock head gasket limits. If you can stay within those parameters im up for a group buy.
Evan
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Old 11-20-2007, 01:22 AM
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nice write up after work tomorrow im gonna go "tune" my pump for minimal smoke so that it doesn't smoke like a ford with an injector problem might even slid the fuel plate back a little bit more for some more mpg's for this strugglin 12v'er
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Old 04-02-2008, 12:52 PM
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Good post and good info, I'm with 97dodgectd though I need all the mpg I can get, or atleast until I get a southbend in there
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Old 04-02-2008, 04:29 PM
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i never messed with my plate at all it still just about full forward and its a zero so its supposed to be a lot of fuel, but i tried to do too much on stock tranny and now i have no tranny. but i did turn my starwheel back so my truck doesn't smoke at all down low, and it didn't make any difference in my mpg's. its really how you drive your truck. unless it smokes a crap load all the time no matter how you drive which would be hard to do.
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Old 04-02-2008, 05:44 PM
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Still, more power results from more fuel+air or more efficient burning of the fuel already going in. So my impression is that any messing with the injection pump is going to decrease my mileage....especially with me driving When I put a flowmaster on my old truck the mpgs went down about 20% because I liked how it sounded when it accelerated hard
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Old 02-08-2009, 10:42 AM
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Nice write up.Now I understand a little about how it works.
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Old 09-15-2013, 06:00 PM
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Had to register for the forum just so I could thank Jim for the excellent rundown on modifying the 12 valve AFC. Very informative!
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Old 09-15-2013, 09:36 PM
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No problem sir, glad you found it helpful!
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