Cummins Power Curves Confidential?

I have just over 3,500 hours on a pair of year-2000 Cummins 270B diesels. I believe Cummins builds a great engine but I'm disappointed by their customer-support priorities. They appear to spend considerably on customer support and have many very nice people to answer the phones, but corporate policy is getting in the way. I recently requested the spec sheet for my engines and was told that they were classified Cummins Internal Use Only (Cummins response: PowermasterAtCummins.htm. That Cummins refuses to release this data to customers is dooming many to expensive repairs that could have easily been avoided. I don't think that Cummins MerCruiser Diesel ( is intentionally trying to force customer into making mistakes that will lead to premature engine failure, but their information disclosure policies will have exactly that effect. What's somewhat ironic is that Cummins actually has posted the performance curves for many of their current engines to their public web site but insists that performance curves are Cummins Confidential and can't be released. This article explains the importance of this data and how to use it, and requests you urge Cummins to make this data available to their customer.

Update: This data is now reported to be available. Apparently the Cummins folks I spoke with at 1-800-diesels were incorrect in saying the power curves were Cummins Internal Use Only and should have released them.  They directed me to who sent this letter (referenced above) explaining why customers don't need the data and that they were unable to release it due to corporate policy.  Apparently they were incorrect as well. Tony Athens and Etienne Grobler both followed up with Cummins and both were told the folks at PowerMaster and 1-800-diesels were incorrect.  Etienne has approved me posting the letter they sent to him explaining the error: WaveMasterAtCummins.htm which offers more detail.  The good news is we can get the data we need (thanks for following up with Cummins Tony and Etienne). The bad news is there appears to e a surprisingly large number of folks in Cummins customer support willing to take a firm position with insufficient data. Nonetheless, I'm glad to see the data  available to all.  The remainder of the note explains why I find this data useful.

Diesel engine overload (Diesel Engine Overload) is among the most common causes of early failure in recreational marine engines, yet most manufacturers do nothing to help customers understand and avoid it. Boat manufacturers prop new boats for the maximum speed possible in a lightly-loaded vessel. But once a customer buys the boat, rigs it, fills the fuel and water tanks, and possibly fills the bait tank, the boat gains considerable weight. Boats typically continue to gain weight as they spend years in service. As an example, our boat is advertised by Bayliner as a 24,000 lb vessel and when last lifted out of the water it weighed in at over 29,000. In addition to gaining weight, the bottom condition will deteriorate and, as even minor amounts of marine life accumulate on the bottom, engine loads at a given speed can go up dramatically. To avoid overloading a diesel engine, prop pitch must be reduced as the boat gets heavier and bottom conditions deteriorate from "as new" condition. Excess prop pitch will require more HP at a given RPM. The conventional wisdom for safe operations is to run "200 RPM off the top" but this might actually be asking for full rated RPM or even beyond and put the engines in a dangerous overload situation. Most customers simply don't know this and think they are treating their engines exactly as recommended by the manufacturer.

An over-propped engine runs much hotter and has a greater tendency to soot the transom.  If the overload is particularly severe, the engine may emit black clouds (although absence of smoke is not a guarantee of safety). This is a Bayliner 4788 at cruise returning from the San Juan Islands. The boat is almost certainly seriously over-propped and its owner likely has no idea that he is dramatically shortening the life of engines and is at risk of catastrophic failure.

The best way to avoid overload is to be aware of the problem and have good information. Unfortunately the industry continues to not discuss overload broadly and to not teach customers how to avoid it. It appears that economic factors will cause this to continue, but good educational sources are now publically available on the web. The second factor, information, comes in two forms. First, you need good gauges. Many boats come equipped with low quality analog tachometers that can be off by surprisingly large margins. I replaced my analog tachometers with Aetna ( digital units. I've checked them for accuracy using an optical test instrument and found them to be accurate to +/-1 RPM. The installation is described at: Digital Tachometer. Continuing with gauge package improvements, I installed pyrometers and boost gauges. Pyrometers became the instrument of choice for the over-the-road truckers crossing the Rockies many years ago for exactly the same reason they help in a marine application: pyrometers are remarkably sensitive to engine load and make it crystal clear when the engines are overloaded. The second ingredient of information needed is the manufacturer performance curves. This is where Cummins corporate policy shows no respect for customers. They supply these to dealers and distributors, but won't allow customers to have them.

Although Cummins refuses to release these data to customers, many dealers, distributors, and resellers understand that customers need this data. I'm lucky that some distributors care more about their customers than they do about silly corporate policies that leave customers at risk, or I wouldn't have this data at all. This curve sheet isn't marked Cummins confidential, so I'll use it as an example here: 6BTA_270_JWA_CCPL2205.pdf.

These curves show rated HP at all RPM. There is a trick to computing HP using fuel burn introduced to me by Tony Athens ( All high-speed diesels produce 18 to 20 HP per gallon per hour of fuel burn. This number is remarkably consistent across all manufacturers. Older less-efficient engines are towards the bottom of the range and modern full-authority electronic engines are near the top. To figure out where yours are, take the max HP and divide by max fuel burn. My engines are 270 HP metric (marketing), or 260 HP (real), and they burn 14.4 gallons/hour at rated RPM. This means I get 18.05 HP/gal/hour. The actual fuel consumption will change with load over the RPM range and Tony tells me that these engines are closer to 19 HP/gal/hour under normal operating conditions but we use the data we have as a calibration point. With this number, I know if cruising at 2000 RPM and burning 15 gallons per hour, I'm producing about 270 HP or about 135 HP per engine. This is roughly 52% of the maximum 260-HP load. But there is still risk even at this seemingly very safe output level. In the prop curves above, the max-rated RPM at 1,100 RPM is well under 100 HP. Asking for 135 HP at 1,100 RPM would be seriously overloaded and yet asking for 135 HP at 2,000 RPM is only lightly loaded. From the curves above, we can see that the engines can produce well over 200 HP at 2,000 RPM confirming we're operating well within the safe zone. Without the manufacturer-supplied curves, Cummins is asking us to operate blind. 

Other data available from the curves I find useful is: idle speed, high idle speed, and exhaust gas temperature (EGT) after turbine. The EGT is what the pyrometers are reading. The maximum EGT for my engines at full rated RPM and load is 802F. I want my engines to last well, so I operate conservatively and chose to never run EGT of beyond 800F. It's not uncommon for some engines to actually run higher EGT under some load and RPM conditions, but I just play it safe and choose to keep it under 800F.

As I write this, relaxing on buoy off Saltwater Marine State Park just south of Seattle Washington, Dirona's engines have 3,538 hours and they continue to run well. 3,500 hours doesn't seem high for diesels, but I'm amazed by the number of failures I've read about at well under 1,000 hours. Diesel engines can fail quickly if operated under the wrong conditions, so a bit of knowledge and good data is an excellent investment. The fact you're reading this indicates you care as well.


To get long engine life, you need accurate tachometers and pyrometers, and I find having boost pressure gauges also useful. You also need the curve data from the engine manufacturer. The good news is that the performance cuves are available from Cummins.  The bad news is that you may have to call around a bit to find someone who knows they can make it available.  From the small sample we have, it appears that your most likely to get what you need in the marine world by contacting first.  Good luck with your engines and let's keep helping each other.

 Last changed by James Hamilton 2007.12.02

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Copyright 2012 Jennifer and James Hamilton