The Truck:
1995 2500 4X4, auto, 68,000 on the clock, 4.10’s, 285/75/16’s, 1968 CPL, 160 hp rating

The Current Mods:
marine 370’s, 191 dv’s, 15* BTDC, stage 2 snow 625/375, 100 plate, gutted AFC, 4” turbo back exhaust, HTT stage 2.5 compressor/wheel, ventilated 12cm^2 turbine, boost controller, K&N in a cold air box, DTT Billet Auto, upgraded trans lines .003” tight on the exhaust valves, 3K GSK, boost, pyro, trans temp gauges

So what’s the problem?? A popular build right? You bet….and its QUICK. 0-60 in 5.8, standing quarter in mid 13’s, more power than anyone should be allowed to have….yeah right! This is my boss’ truck. He bought it after watching me for a few years with mine. He wanted to skip right to the good stuff and asked me how. First order was the gauges. Then a short call to Bill at DTT and we had the truck profiled and a trans being custom built for him with a little billet for good measure. Now we were ready to start bombing. In went the injectors, DV’s, the turbo upgrades, the water/meth, ah hell…everything else up in the mods list. It all happened in less a month…..from tame low on power rattle trap to rip snorting inferno. But what’s wrong? Yep, you guessed it, really hot, very smoky, and not driveable by anyone that doesn’t know how to finesse a Cummins. That’s how he wanted it at first, so I left it alone. It was within it’s safety limits as long as he didn’t push the top end too much. He was educated, and new when to quit digging in the coal bucket.

Now the shine is worn off of the smoke, the adrenaline doesn’t quite pump as hard, and he’s ready to let me tune this mill into a machine that is driveable and legal. Little does he know, we’re going to extract a few more ponies out of her while we’re at it!!

Tuning a 12 valve without the aid of a dyno is purely a seat of the pants affair. But it doesn’t have to be. We can use the gauges that monitor safety, to actually tell us what the engine is doing and how to make it use its fuel and air supply to its highest efficiency. The first thing we will do is test and tune the fuel plate position. We want peak EGT’s to be in a target area of around 1250 - 1300. This is a safe level for stock internals, as well as the Cummins’ sweet spot for peak thermal efficiency. His 100 plate is a fuelin’ mother, so we’ll clip its wings a little to curb those 370’s down just a tad, as they have the potential to overfuel that little HX35 no matter what we do. So I set it back to a “stock” location to baseline it. We’re not going to concern ourselves with the AFC just yet…it’s gutted, but doesn’t affect EGT’s at all, and that’s our first goal. We’ll deal with the AFC once top end fueling is matched to the limits of the turbo. Speaking of turbo, what about it? This upgraded turbo still has the same performance ceiling as the stocker….35 psi. The upgrade doesn’t open up any more pressure, just some more MASS (the weight of the air). So for the same given pressure, we will be moving cooler air, and cooler air is denser, or heavier, air that contains more oxygen for combustion. We already have the Wastegate set for 35 psi, and that’s where we want it to be, so we’ll leave it alone.





We have the plate initially a little forward from center here at our starting point. Incidentally, there is no need to remove the intake or injection lines to remove the AFC housing. All you need to do is replace the slotted screw on the inside forward bolt hole for the AFC with a hex head bolt. With a standard bolt there, a universal joint, 10 mm socket, extension and ratchet can be used to go underneath the lines to remove the bolt. It is handy to do this instead of removing the intake and loosening the lines every time you want to adjust a plate. During this session we adjusted the plate some 20 or so times!!

Okay, at this setting we had good power, but temps would exceed 1600 degrees in high gear full throttle. While the low and midrange didn’t produce high temps , the smoke was obnoxious, and since he has full exhaust, this is somewhat offensive to other motorists!! Just to get an idea what a full back setting would do, we moved the plate all the way to the firewall and went for a drive.



Same thing, high end temps and a lot of smoke. So we can’t physically pull enough fuel to curb temps or smoke with this 100 plate. While this is not always the case with all trucks, in our case with a very tight converter and well built trans, engine loads are high and torque throughput is highly efficient. A higher stall converter, or trans that allows some power loss, might not load the engine as hard and let RPM’s climb higher to keep temps down. Okay, so now we need to do some plate work. I took his 100 plate, covered it with marking compound and put it back in. We went for a drive but didn’t let the engine run to redline. Instead I wanted to see full throttle, but at what point on the plate we were at around 2400 rpm, and not redline. After curbing RPM to 2400 we pulled the plate and noted where the arm was at on the plate in relationship to that 2400 RPM. Why? Because this is the point temps star to climb, and where we need to start thinking about defueling the pump. We again replaced the plate and continued to do another drive at full throttle, but this allow the engine to run to redline. Back to the shop again to remove the plate and take another measurement. Now I want to know how far up the plate the gov arm travels at full gov speed. Now we have the two measurements that are going to dictate how I design the plate. You can see from the picture below what I ended up with. We leave a hook on the bottom to limit off idle fuel just a tad because the DV’s and 370’s can put out the fire pretty easy, even with the AFC tuned. From here, you’ll notice a very sharp ramp into full fuel, the curve is short and brings full fuel very quickly because the turbo spools very quickly as well and we want to take advantage of that. Now at the deepest part of the plate we start the defuel ramp. I chose this point because of our 2400 rpm measurement. The defuel point on the plate is exactly where the gov arm is at 2400 rpm. From there I designed a linear defuel rate, because with rpm comes more heat at the same constant fuel rate. This is due to heat cycle times becoming tighter with the onset of higher rpm. So we want to defuel a little more as the RPM rises to keep us manageable. So working on up the plate profile, we continue to defuel, until at a point we peak. It is at this peak where redline rpm occurs FOR THIS ENGINE and pump. Everyone getting the idea? This plate has been custom designed to take in account the spring rates in the governor for this pump and brand of aftermarket springs, as well as total governor travel in relation to the plate, and a known midrange RPM point. Does it get any better? Welcome to custom plate design. Now we just have to install the plate and tune it.

I placed the plate in the full forward position and went for a drive. I wanted to see a baseline for a full forward position to make sure this plate would overfuel…..I was not disappointed, high temps, lots of smoke etc. The next step was to move the plate full back, to make sure we could limit the fuel to the point that the engine was down on power and the turbo could not receive enough energy to even make max boost. Again, no disappointments. Max temps were 1000 degrees and I couldn’t build more than 20 psi of boost. Mission accomplished. I now know we have the range in this plate for any fuel rate we want. These may seem like un-needed steps, but I consider them necessary to test the profile of an unknown, custom plate. Next step is to place the plate in a middle of the road position in the pump and start the tuning process. The first run was to prove the defuel portion of the plate, and make sure it didn’t need to be modified. Power was good and boost achieved maximum, but as rpm increased, the temps climbed at a faster rate, they climbed fast and could still pin the pyro. Out comes the plate. We needed to add some material to the defuel ramp to pull a little more fuel, but have the peak defuel occur at the same place on the plate profile. So a little welding and .020 was added. We refinished the plate surface and blended the ramp back into the fuel full fuel area. Why didn’t we just slide the plate back a little? Because low and midrange power was good, and smoke was under control. It was just at the threshold of high load/rpm that the plate became hot. If we slid the plate back to answer the temps, we would pull fuel away from the area that was in check and making good power. After refinishing the plate it was placed in the same position as earlier and taken for a test drive. We hit it right on the head….the extra de-fueling kept temps under control, and consequently smoke. We did, however, lose our ability to achieve maximum boost. But before changing the profile on the plate again, I decided to move the plate forward a little, since our low end and midrange were clean and cool, maybe the extra fuel gained by moving the plate forward for the top end would not throw the bottom end and midrange into overfuel. We moved it .020 forward, and upon the test drive, gained our boost, and just a slight hazing with temps in the 1250 range. Low end and midrange was clean and powerful…we were on the right track. BUT, we had lost some obvious power up top, even though we had full boost. So we had a choice to make, and this is a perfect example of 370’s combined with a non-marine piston, do we want to allow some smoke and temps in order to achieve the power level we are looking for, or do we live with the power loss in favor of a clean engine and low temps? Duh, well that’s obvious…let her smoke a little. So .010 at a time we moved the plate back and forth to find that happy medium between power and efficiency. We ended up at a setting where temps peaked at 1400.. A little high, but still well within the safety zone for intermittent periods, and considering that these operating conditions would only occur once in a while, I felt it was a good trade-off…the midrange, low-end, and all but the highest of the top end ere relatively clean and cool. Since this truck has methanol, we could still bring those temps down to target 1200-1300 anyway.

here's the roughed out final plate profile. All that has to be done now is some face polishing, face hardening and final installation



So there you have it, an in depth approach to tuning a plate in a specific pump and truck. Making plates may not be everyone’s cup of tea, but if you have the ability, some dynamite plate profiles can be had by trial and error. If you’re using out of the box plates, the theory still applies in how to tune one. The one area I didn’t touch on was adjusting the governor arm to a plate. I had the luxury of not needing to do this, and making a plate to work with the current lever setting. To keep this thread no longer than it already is, I’ll leave gov arm adjusting to the many tech links already devoted to it.

We still have to put the guts back into the AFC on this truck and tune the pre-boost fuel, and I’ll continue this discussion with more photos and tech when we get around to doing that part of it. Because of my recent KDP issue, tabbing his is highest on the project list for now, so we’ll do that before any more tuning!!

For now…keep bombing….and don’t be afraid to dig around in your pumps. They’re the heart and sole of Cummins power, and everyone should intimately know the theory and operations in their pumps to really consider themselves a bomber.