Duramax Diesel Truck Balance Rates
If you’re big into diesel engines, especially the Duramax diesel engines, like we are, you have probably heard of balance rates. You are likely even familiar with checking them so that you can determine the health of the injectors in your Duramax. But do you know what they really mean, and how they work? Did you know that the simple numbers can be misleading without looking a little further? Balance rates can even be used to identify and help fix other mechanical problems, too.
To start, let’s briefly touch on what a balance rate is. A Duramax truck with stock injectors and tuning, at operating temperature, should idle with a calculated fuel rate (CFR) of 7-9 mm³ – and that is the baseline, i.e. balancing point, from which each of the individual cylinders gets its rate from. So, for example, if a truck is running at an 8 mm³ CFR, and there are cylinders at +3.4, -2, etc., it means that those cylinders are 8 + 3.4 (11.4), 8 - 2 (6), and so on. This means that even though those balance rate numbers are within the acceptable industry standards of +/-4 mm³, there is a variation of almost 100% in the actual flow rate of fuel between the two!
Now, the variation itself isn’t the biggest issue we see with balance rates. With aging diesel engines, we often see them belching smoke under load, suffering a lack of top end performance, or throwing out a haze under idle conditions - despite acceptable balance rate numbers. Why is this?
A lot of the time, the answer is just plain old worn parts. As the injectors wear out, the high fuel pressure (in excess of 26,000 psi) causes fuel to leak past the worn areas, and more fuel reaches the cylinder than the computer is commanding. So to compensate for the extra fuel, the ECM dials the CFR back to say, 2 mm³ instead of 8 mm³, meaning that there’s an actual discrepancy of 6 mm³ in the reported amount needed to maintain a 700 RPM idle. These trucks will start fine, and run fairly smoothly…but then the problems begin, when more power is applied or they idle for a while.
Say you have a truck running at 83° C with a 2 mm³ CFR, a tolerance of +/- 2 mm³, and your balance rates are shown as:
On the surface, these are all within the industry specs, but when you factor in the low CFR (i.e., adding in the missing 6 mm³ that should be present in the balance rates), it becomes clear how these “good” balance rates are actually skewed very heavily:
Out of those, how many are within two mm³ of the CFR? Only Cylinder 5, at 4 mm³, is even close – which means that 7 out of 8 of these would be considered beyond saving, and we wouldn’t even do just 7. That last one would be replaced to match the rest, because we want it done right the first time, ensuring you don’t have to come back in a month to do it again.
This lays out why a truck with good balance rates can still be in need of all new injectors in their Duramax. But that’s just one area – the worn injectors! The balance rates can also indicate mechanical issues with the engine itself, because they are a function of cylinder contribution – meaning that you can use the numbers to find out if a specific cylinder is low on compression due to valve issues, a bent connecting rod, a cocked piston, broken rings, and so on.
Let’s take a look at the engine stats below, for the same engine at 83° C with an 11 mm³ CFR:
We can see that Cylinder 6 has a huge compression loss issue, from something mechanical going on, and the truck is running that cylinder at 25.5 mm³ (11 + 14.5), while the others are around 4-5 mm³. Even the high CFR itself, at 11 mm³, is often a sign of something going wrong – it’s common on engines with compression differences of this size, and may be mistaken for a bad injector. Further tests may be required to really get to the root of the problem.
At Schultz Diesel Sports, we strive to diagnose the engines we see, properly and quickly. We hope that this gives you some insight to the calculations behind balance rates, and how they work and what they mean – so that if you encounter trouble of this type, you know the background of the problem and how to identify it, too. That way, it can get fixed faster, and you’ll be back on the road enjoying your Duramax in no time.
A product that we recommend is Edge Insight. It shows you your balance numbers and your CFR in one easy-to-use display, which is why we sell them off the shelves and put them in our own trucks. Of course, there’s still a lot more to learn about diesel engines and all the science that goes along with keeping them running at their best – so if you ever have any questions about your truck, don’t hesitate to reach out to Schultz Diesel Sports!