Turning Up The P-7100 - Diesel Bombers

94-98 12V Cummins 5.9L P7100 Tech Talk '94-'98 5.9L Cummins 12-Valve P-9100 Tech Articles

Turning Up The P-7100

 

 
 
 
  #1  
Old 09-06-2007, 08:49 AM
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Default Turning Up The P-7100

by Cummins Express

boy where to start?

Let's start with gov arm adjustment. Now maybe I'm just reading too much into it but I set my gov/plate contact with the engine running at idle with the AFC cover off. Why? I got to studying the stock plate and with the engine off, the common knowledge is to set the lever where all the cutaway images show....BELOW the big protrusion on the plate. Then I got to wondering about how that arm is smoothly going to ridse over that protrusion. So I looked at my 100 plate conmtact with the arm at rest (solenoid in run position tho) then fired up the engine and checked it again. The lever won't swing out until you lightly stab the throttle, but I noticed that the arm contacts the plate a lot higher than at rest. We can only attribute this to gov springs acting on the gov arm. So rather than set the plate with the engine off, I run it and set the plate to the arm with the enginre running. The difference in arm height is about 1/4 inch...coincidentally about how much higher the arm would have to be to clear that protrusion on the stock plate. So my belief is that when running, the arm starts on the protrusion and not below it as it would seem. Now not many of us are running stock plates, but the theory applies anyway. I have never had to adjust my gov arm to a plate.....except maybe the "0" I was running for a while. Now I'm not passing this along as gospel, or to shake the foundations everyone swears by, but if you car to dabble, have a go at it and see what I mean. While my engine is off, my arm will swing under the plate, but have never had it bind underneath...this is proof to me that my theory is correct. So if you're setting the bleeding edge, maybe setting the arm on the nose of that pointed plate would be better done while the engine is running.....and hopefully you can get the plate where you want it without adjusting the arm any!!

Next for the AFC.....several adjustments there, but not everyone understands them completely. At this point we know that the AFC is there to limit fuel supply during low, or no boost conditions. That's itsa only function, cut and dried, don't let anyone tell you differently. With a stock spring, after about 6 psi, no more AFC influence on the pump...that simple. If you have low tension springs in it, cut that down to about 2 psi. We know that the AFC lever protrudes down into the pump cavity right next to the fuel plate, and at no or low boost, it catches the gov arm and limits fuel that way. So what are we doing by moving the housing forward? We are simply moving THE ENTIRE ASSEMBLY farther to the front of the engine which allows the gov arm to travel farther which allows more fuel....that simple. So what about the star wheel? That wheel acts on a spring that pre-loads the pressure diaphram which is boost controlled. If we turn the starwheel towards the engine, it moves forward, releasing spring pre-load on the diaphram. As this pre-load goes away, it takes less boost to act on the diaphram and move the AFC lever out of the gov arm travel path. We think of these as the fine and coarse adjustments for the AFC, which is not totally true. The coarse adjustment is indeed moving the housing itself. But the fine adjustment is not the starwheel.....it's the so called smoke screw. You don't see this becasue it is on the back of the AFC housing under a sealed cover with a break off crew. This screw actually bottoms out on the diaphram center...from the boost side of the diaphram. And what happens when we add boost? The diaphram moves the AFC lever out of the way right? So by screwing this stud in, we give the lever a head start getting out of the way...much like moving the housing. That is why it is truly the fine adjustment. So we can set the housing in one position, then fine tune that position with the smoke screw. These adjustments alone will influence off idle smoke....not the star wheel. The star wheel will influence how long it takes to come off pre-boost fuel and into full fuel. So lets tune a pump...

We buy a truck and the previous owner moved the housing forward, cranked the starwheel fwd and didn't even know about the smoke screw...so it's in it's stock position. The thing smokes off the line like a freight train and bogs like crazy...then when 6 or 8 psi comes around, it surges with power and breaks all the tires loose....that darn turbo lag right? Now let's tune the afc.

Start by getting the access to the smoke screw...take the AFC off and remove the break off screw and smoke screw cover. Loosen the smoke screw lock nut and back out the screw several turns until it feels loose. Now screw it back in by hand until you can "JUST FEEL" the screw bottom out on the diaphram. Snug up the lock-nut a little and now set the AFC back to a middle position on the pump. Take out that hex plug on top of the AFC and bring the star wheel back from full forward about 3 full revolutions and put the plug back in. Now we have a starting point. Put her in gear and nail it. Do we have any smoke? No? then adjust that smoke screw a little at a time until it "just hazes" on take off. If you can't get that, loosen the smoke screw about halfway back out and reset the housing .020 farther fwd or so. Now try it again. Still no haze....adjust that screw in to get it. Or did we have too much smoke? back the screw off until it clears a little. You get the idea right? Any more than a slight haze off idle and you're wasting fuel, and maybe even hindering spool-up by quenching the cylinders with excess fuel and "puting the fire out" so to speak. So let's say we got the off idle launch looking good. Now it launches good but it takes too long for the turbo to come to life and the smoke goes away but no boost and it's laggy. That means we need to lessen the diaphram preload with the starwheel. This will hasten the curve out of pre-boost fuel control by allowing the diaphram to move with less boost pressure. Keep adjusting the wheel until there's a slight haze while you gauge shows less than 6 psi or so. If you get too much smoke once your RPM's start to climb, but before 6 psi or so, cranking the starwheel backwards will limit the fuel, cut the smoke, and smooth out the power. It's a balance game between the 3 adjustments....initial fuel delivery vs. building boost without quenching combustion. You'll know when you're right on....it'll launch hard, won't smoke, and boost climb will be smooth and rapid. These adjustments pertain mainly to the tune conscious driver, who wants drivability no matter who climbs behind the wheel, and isn't fueling for a funny car.

Now all of you have spotted my sig and said wait a minute...all this AFC tuning and he took his completley off...what gives? I DRIVE my truck, and my right foot does alot of the tuning as required at the timer of need!! You are not going to remove an AFC circuit and stomp the throttle from a stop and expect anything but a volcanic eruption with nothing to show for it!! You have to roll into the throttle as conditions demand, sometimes more, sometimes less. I like having that control, and not letting the pump decide for me with a one size fits all adjustment. I will tell you that boost response at speed is instant, and off-line punch is tremendous, once you learn how to drive it. Even at speed when our little boost gauges say 2-3 psi....guess who's holding the cards for you? Your AFC....still won't let you into full fuel until boost pressures are realized, and that just may be enough of a lag to give some half a car headstart during a rolling race.....excuse me....I meant to say when you need instant passing power. If you're a step on the loud pedal and go type of driver forget it...you won't like it. But...if you truly DRIVE your truck, give it a try....you just MIGHT like it.

To do this, you don't have to make a block off plate or remove the AFC housing entirely. You just need to remove the arm. This is easy....on the front of the AFC housing is a 10mm bolt that hold what looks like a bellcrank and shaft. Remove this bolt, then with a small screwdriver, pop the bellcrank loose and pull out the shaft. It will have o rings on it to seal the housing. Once this is out, lift out the AFC arm, put the shaft back in and snug up the retaining bolt again. Now it doesn't matter where the AFC housing is, or the smoke screw, or the starwheel....so just bolt it back on. re-attach your boost line....it'll still act oin the diaphram, but the diapohram won't be doing anything so don't worry about it. If you want to remove the AFC signal line go ahead...just remember to replace it with a plug in the manifold!! Now you right foot does all the adjusting for you on the fly, and is completely dynamic to conditions...cool huh? It only takes a few minutes to do, so if you don't like it, put her back in and forget about it.

Wow, this post is way long....sry guys. I have a habit of doing that. I hope I helped, but didn't confuse anyone.....

Later,
Chris
 
  #2  
Old 09-06-2007, 10:23 AM
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Now your digging through the basement Kevin!! How did I spend that much time at the keyboard and still have use of my hands the next day LOL? Some of my views have changed some since then though. After replacing my pump with an aftermarket, I found it to be borderline drivable without being offensive with the smoke. And god forbid the wife tries to drive it. We're talking about leaving the line by just twitching your big toe on the loud pedal. The governor settings on a high output aftermarket pump make it very hard to pedal out, let alone tune out, pre-boost smoke. I re-installed my AFC and went to work. I had to do something about pre-boost smoke without affecting top end power, so I really dissected the AFC and modified it. I found that the amount of tension you place on the star wheel influences the setting of the smoke screw. I needed that AFC arm to reach deep back into the pump as far as possible to try and put a leash on the governor arm to limit fuel. I found that by loosening the smoke screw all the way, the star wheel could be used to flex the diaphram to it's maximum, which was good for about another 1/8" of arm travel. Then once this adjustment was made, the smoke screw could be re-inserted and turned in until it just touched the diaphram with no pre-load. I thought I was out of the woods....except...that by cranking up the starwheel, the total forward travel potential of the AFC arm was severely decreased, to the point where the governor arm wouldn't even contact the fuel plate. This happened because by threading the star wheel out, you also made the AFC spring fully compress alot sooner, limiting travel. So I had to find a longer spring that would still put tension on the diaphram, but allow the star wheel to be turned near max to the out position to regain total travel and get full fuel when wanted. Wouldn't you know that a stock governor spring...the second one....is the perfect diameter and spring rate....but LONGER!! So now I could get full AFC travel, yet provide tension so the gov arm couldn't move the AFC arm without boost signal. AND the AFC arm was tucked as far back into the pump as allowable to limit fuel. I only had one more hurdle to jump. Rack Travel. The plate I made for this pump, along with an extended rack plug, allowed this pump to deliver the maximum amount of fuel that its barrels and plungers would allow. But, the AFC arm needed to be modified to allow for the extra .180" or so of extra rack and gov arm travel. I changed the profile on the leading face of the AFC arm. Normally, the arm face angles upwards and to the back (of the AFC housing) I needed to get extra travel, yet not take material off the arm and counteract my earlier gains in fuel control by letting the arm extend into the pump deeper. So by simply reversing the angle on the arm face, the bottom of the ramp was in the same place as before, but the top of the face ramps towards the front, allowing the governor arm to contact the fuel plate and get full fuel. I also adjusted the plate as well as adjusted the governor arm contact location to help out. The result is a pump that doesn't smoke too badly, yet has a curve that anyone can jump in and drive without having to know how to drive the truck without eclipsing an intersection. It was a lot of hours of tinkering to come up with all of this, and I know it prolly doesn't make sense, but I'll post some drawings of the mods when I can to clear it up.

After driving the truck around quite a bit in the city, in all honesty, I have found that I can drive in valet 90% of the time and still have lots of power....enough even to embarrass quite a few of the wannabe tuners and their fart boxes. So all the work on the AFC was really a moot point, but the theory and practice can be applied to any 12 valve, aftermarket pump or not. It's just another way to tune for absolute control.

Anyway enough for now, I'll post drawings later.
 

Last edited by Cummins Express; 09-06-2007 at 10:25 AM.
  #3  
Old 09-06-2007, 10:59 AM
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Cool.................looking forward to learning more Cris

nothing better than hands on experience
 
 
 
 
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