10-4 Magazine - For Today's Trucker

July 2006 - Performance Zone

LOW RPM’S?
By Performance Specialist Bruce C. Mallinson

Is low RPM really better for fuel mileage than moderate RPM? After all, everyone is saying you have to run low RPM in order to get good fuel mileage. I say it’s a bunch of HOGWASH! It’s not the RPM that determines your fuel mileage; it’s the amount of turbo boost you have to use to pull the load.
Think about this: the fuel injection system will only inject enough fuel to maintain the speed you, the driver, have chosen. If you are running downhill and the load is pushing you and the tachometer says your engine is at 1700 RPM and your foot is not on the throttle, there is just enough fuel being injected to keep the engine idling (some fuel injection engineers say there is no fuel being injected when the load is pushing the truck down the mountain). So if this is the case, then it doesn’t matter what the RPM is, there is no fuel being injected when your foot is off the throttle and you are going down the hill. To me, no fuel means no fuel - regardless of engine RPM.

Imagine that you are driving along the level interstate on a perfect day. The humidity is low, the ambient air temperature is moderate and your engine is running at 1300 RPM, however, to run this low RPM the engine is developing 15 psi turbo boost. If it’s a 500 horsepower engine, it develops about 16.6 horsepower per pound of boost. Now this means the truck is using about 249 horses to move across the highway. But what happens if you bring the RPM up to 1,450 or even 1,550 and the turbo boost drops to 8 psi or even 10 psi? Do the math: at 16.6 horses per pound, the engine is now producing between 132 and 166 horsepower to move the load across the highway. That means the truck will use 100 less horsepower at 250 RPM higher.

So what is this telling us? We need to have rotating mass to move a given amount of weight across the highway. Sure, the lighter the load, the lower the RPM could be, so long as the turbo boost is low also. However, if your foot is into the throttle and the boost is much above 10 psi, then there is no fuel mileage. In my opinion, you are better to use the shifter and split down a gear to keep the RPM slightly higher and the turbo boost lower to obtain better fuel mileage.

This is why a larger turbo on the DD4 Detroits (along with straight through quiet mufflers) helps to improve the fuel mileage. If your exhaust is restricted, the truck will not run at highway speeds at low turbo boost. The 550 Cats also have a turbo that is too small which restricts the exhaust. The Acert Cat engines with twin turbos lose 60 horsepower through their muffler system. Think about it - a 60 horsepower loss just because of the mufflers - incredible! That poor engine has to work its heart out just to get rid of the exhaust.

Do you know why we put headers on racecars? To get rid of the back pressure. Your truck is the same as a racecar - it works its heart out up every hill and into every headwind. Restrictive exhaust will kill an engine in half the time because the soot level will be higher and so will the parts per million of iron particles. If the exhaust can’t get out, how can the fresh air get in? And where is the soot going?

Let’s think about your body and your engine - I say they are both the same - both use oxygen, take in fuel (food), burn it, and then have to get rid of the waste. Thank God our bodies only have to get rid of its waste once a day unless the fuel (food) is bad, however our engine has to get rid of its waste every second! So think about this: if you restrict one of the major elements of life (oxygen, fuel or waste), what happens? Limit your oxygen and you can’t breathe, let alone run up a mountain; restrict the fuel (food) and you have no power; restrict the waste, now you are really in trouble.

Something else I want you to think about is the weather. When it’s uncomfortable for your body, it’s uncomfortable for your engine. Hot, humid days are killers to the human body and to diesel engines that are expected to work hard. What about high altitude? Have you ever tried to run up the mountain when you’re parked on Vail Pass at 10,400 feet? Because there are half the molecules of oxygen at 10,000 feet, you will NOT be able to run very far up the mountain. However, you expect your truck to prance over the same mountain like a thoroughbred when you yourself can’t do it. Remember, your engine needs the same elements of life as your body does, and the weather and altitude will affect both of them!

Now that I got way off the track of fuel mileage and RPM, let’s go back to that subject. Bill Fillman of Avon, Illinois is 62 years old. He has been an owner-operator his entire life, and his truck is one of our test trucks. He owns a 1997 Western Star powered by a 550 Caterpillar engine, 3:70 rears, low-pro 24.5 tires and an 18-speed transmission. Bill’s truck has the Pittsburgh Power Performance Computer, Pittsburgh Timing Box, a larger-than-stock turbo, straight through quiet mufflers, and a FASS Fuel System. If we think a product will make better fuel mileage, it’s been put on this truck.
Bill pulls a step-deck or drop-deck trailer and is usually 12’ wide and high. Pulling normal loads, he has seen 7.1 mpg and usually runs about 68 mph, which is about 1,550 to 1,560 RPM. Last week he left Illinois loaded for California and ran the entire trip at 56 to 57 mph at 1,450 RPM, and he kept the turbo boost at 7 to 10 psi. His load was grossing 79,000 pounds. His average fuel mileage for the entire trip was only 5.28 mpg. This was pitiful mileage for running so slow.

On the way back to Illinois, with a load that was only about 1,000 pounds lighter, he ran his usual 1,550 RPM at 68 mph and got 7.31 mpg. By allowing his truck to find its “sweet spot” of power and speed, he gained over 2 mpg. How do you find the sweet spot? The next time that you’re driving along the interstate on a perfect day (with no wind), stop paying attention to your tach, speedometer or boost gauge and just allow the truck to find its own comfortable speed. That is usually the engine’s sweet spot, and it’s where the engine wants to be. It’s running free and it’s very comfortable there.

You probably know where the “sweet spot” of your engine is, but everyone keeps telling you to keep the RPM down, even though your engine is fighting you to run slightly higher. Let her run where she wants, and see if the fuel mileage improves. If you have any questions, visit us at www.dieselinjection.net, call 724-274-4080 or e-mail me at bruce@pittsburghpower.com.