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Old 11-27-2010, 11:06 PM
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Default Dodge Cummins P7100 Timing

P7100 Timing
The P7100 has static timing unlike other Cummins injection systems (VE, VP44, etc.) This means that the timing is in one spot all the time and this spot can be the wrong spot or the perfect spot depending on how you drive, what you have done to your truck, or even the climate.

A stock P7100 on the dodge has the timing degree set at 12.5-13.5 Degrees (depending on HP rating) BTDC (Before Top Dead Center). This means that 12.5-13.5 degrees before the piston reaches the power stroke, the injector starts to fire. You want the injector to have time to fire and "get the kindling going" before the power stroke so that it is producing power for the entire length of the power stroke.

Timing Adjustments
The timing can be set through various methods. There are charts on the net showing where the plunger needs to be at when the engine is at TDC #1 during the compression/power stroke transition. You use the #1 plunger to set the timing by sticking a dial indicator in it and measuring how far the plunger has moved (plunger lift). By getting it to a specified lift, you can set the timing to whatever you want.

To get the timing to stock values (12.5-13.5*), there is a pin inside the P7100 that lines up that you are able to "lock". When the engine is at TDC, the pin should be lined up (if it is stock). If you are reverting back, lock the P7100, loosen the nut on the pump gear and pull the gear off the shaft, then rotate the engine to TDC backwards going past it a ways, then coming back and stopping it at TDC using the timing pin under the injection pump (next to the vacuum pump). Then tighten the gear/nut back on the injection pump and unlock both pins. They are very easy to break since they are plastic so you must be very careful not to move the engine or p7100. Even the slightest movement will break it right off.

Spill Port Timing Thread started about Spill Port Timing.

Timing Details Explained
Changing the timing has several side effects "for better or for worse". To start this explanation, think of what happens when the engine fires the injection. It fires and blows up and hopefully gives the power stroke an entire length of explosion. So what if the timing was during the power stroke, as in something like 5* AFTD.. Then the explosion would be way too late and you would only get partial power out of it, and probably black/white smoke since it didn't have time to burn it all. Now go the opposite direct and set it at something like 30* BTDC. Now it fires too soon and will explode on a piston that is still trying to come up, which can cause damage to the connecting rod/crankshaft/etc.

I stated earlier that it takes time for the injection to ignite. Since they are stock at 12.5-13.5* BTDC, it means that it takes roughly that much time in crankshaft degrees to get the injection to explode and provide efficient power for the duration of the power stroke. However, this is a tradeoff since engine RPM requires different timing to be efficient. The timing needs to be retarded enough (close enough to TDC) to start in the cold (get to that later), and advanced enough to run efficient at higher RPMs (later). To do this, engineers had to pick a spot in the middle so you would have the startability and efficient high RPM operation of the engine. The earlier cummins' had lower power but at lower RPM, which means the average of the HP RPM and Torque RPM set the timing lower, at 12.5*. As the power rose over the years to 215HP, RPM ratings were also, so to match, the timing was bumped to 13.5*.

Cold Starting
As for timing and startability, the concern is during the cold winter months. As the piston comes up compressing the air, it heats the air with it. While you are starting it, the heat is sparse and the cylinder walls and piston readily steal the heat from the compressed air. The piston compresses and heats all the way to the top, so to get the most heat out of it to start (since diesels rely on the heat of compression), you would want to inject the fuel right when there is the most heat. Retarded timing is therefore the answer.

The tradeoff is once it is started, the engine has to rev, you are going to be driving at 1500-2500RPM in most cases. Just to give you an idea, at 1500RPM the engine has 75 power strokes a second. This means there is not much time for fuel to inject and ignite. To counter this, we inject it sooner to get it to start burning by the time it reaches TDC. If you think about it, you realize that as the engine turns faster, there is less and less time so you need more and more advancement.

Timing Preferences
Many people advance the timing since the trucks usually start easily in the winter, they have some room to advance it and have it still start in frigid temperatures. Advancing it to 15 or 16* is very common and means it is more efficient higher in the RPM, thus giving you more power. It does shift it though, so there is less power in the lower RPM's. The sweet spot is at around 15*, providing good efficiency all around and good startability in the cold.

Some More Side Effects
Advancing the timing does, however, increase cylinder pressure. Running a stock head gasket with high boost pressures (over 40-45psi) can begin to push your luck depending on how far you have advanced the timing. Up to 16* will generally keep you at a safe spot for up to that 40-45psi, but remember that the HX35 on all of the P7100 trucks is only good up to 35psi, after that it is out of it's map and becomes inefficient and generates too much heat, raising EGT's.

If I think of more I will add more to this.
1997-12V-NV4500-Dana 80/3.54/2wd-350k---21-27mpg
Removed AFC--No Plate--13.5* timing--BHAF--5" Exhaust--Valair Clutch
1994-Jeep Cherokee-5spd-3.07-4wd-202k---16-19mpg

Last edited by stodg73; 03-13-2012 at 04:50 PM. Reason: Added information from ISX
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Old 11-27-2010, 11:12 PM
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Great post! Thank you!!
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Old 03-04-2011, 06:43 AM
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ISX ISX is offline
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I can't edit my post for some reason. I found out some more on spill port timing.

What you do is take the delivery valve out so that you don't have to overcome the valve with a ton of fuel pressure. You do still have to pressurize the system a little, so you have to use and electric fuel pump or hand prime it while you do it. You bar the engine over the normal rotation and the fuel will come out of the line and you keep going until the fuel amount decreases to a constant drip. You want to stop right at that constant drip (dripping every 5 seconds or so). That is where the plunger filling stops and injection pressure building happens, so the start of injection. You would stop there and then loosen the timing gear, crank the engine to where you want timing, then tighten it back up.
1997-12V-NV4500-Dana 80/3.54/2wd-350k---21-27mpg
Removed AFC--No Plate--13.5* timing--BHAF--5" Exhaust--Valair Clutch
1994-Jeep Cherokee-5spd-3.07-4wd-202k---16-19mpg
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Old 02-12-2012, 12:47 PM
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rdsutton rdsutton is offline
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Location: NE of Chattanooga, Georgetown TN.
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Default spill port timing

This is to compliment ISX's overview of p7100 timing. View the following link on spill port timing. Thanks Dave. Utube search spill port timing & click on brotherhood sleeve valve engine timing. Goolge spill port timing & click on dieselpowermag article on p-pump timing. Hope this helps.
98 3500 4X CC with a flatbed, A work in progress, BHAF, Banks Big horn, 4" MBRP, 4GSK, AFC, #10, Draw straw I with 1/2 supply to LP, 3/8 return, 16* time. Soon $$ Injectors & Turbo.
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cummins p7100, p7100 cummins timing, p7100 cummins tuning, p7100 timing

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