Written By: L.T. Tolman
If you spend any time on the web, at truck shows, or talking diesel with your buddies, the topic of horsepower will surely come up sooner or later. Anyone can claim their truck makes a thousand horsepower, but the only real way to verify gains and to declare a winner of bragging rights amongst friends is to measure the output on a chassis dyno.
What exactly is a chassis dyno?
In its simplest form, a chassis dyno is a large metal roller on which the drive wheels of a vehicle are placed. The vehicle will use its full engine power to accelerate the roller throughout its RPM range, and based on how quickly the roller accelerates, a torque and horsepower number are calculated. It’s important to note here, a chassis dyno will give power numbers AT THE WHEELS, not at the flywheel, which is where the O.E. manufacturers rate their engines. So, if you take your bone stock L5P to a dyno day, don’t expect to make the rated 445hp on a chassis dyno. Power is lost through the automatic trans, transfer case, rear differential, and wheels and tires. A general rule of thumb is to add 10% to 20% to the numbers at the wheels to get a rough estimate at the flywheel.
In the automotive aftermarket, there are two main types; an inertia dyno which only has the weight of the roller to give resistance against the engine, and a loaded dyno, which typically have an eddy brake which can apply varying amounts of resistance using electrical current. This is particularly helpful when it comes to helping diesel trucks spool the ‘chargers up on the dyno, making similar boost numbers at lower RPM that you’d see driving on the street. This is why generally a loaded dyno can give the most accurate horsepower figures on a diesel truck.
How does it measure horsepower
First of all, a chassis dyno MEASURES torque and CALCULATES horsepower. Knowing the given mass of a dyno roller, the speed of the roller, and the engine RPM, its easy to calculate how many lb.ft. of torque an engine is making at the drive wheels. Then, its simple math to determine horsepower. (HP = Torque X RPM ÷ 5,252).
How to dyno your truck
For usually less than 80 bucks, you can get 2-3 pulls and find out just how much (or little) power your rig puts down. With the recent rise in popularity of dyno competitions, it’s helpful to know how to conduct a dyno, since most dyno operators prefer owners to run their truck. First of all, make sure the truck is up to operating temp, you never want to flog a cold truck. This goes for the engine and transmission. As you drive your truck onto the dyno, you’ll be guided into the correct position, and the truck will be safely strapped onto the dyno for you, usually with four straps on the rear axle, in a cross pattern to hold the vehicle from moving forward and side to side, and two straps on the front to keep it from rolling backwards.
Once the truck is secure, you’ll have to turn off traction control (if equipped) and gently accelerate, shifting up to 4th or 5th gear. Ideally, the test will be conducted in whichever gear your transmission is at a 1:1 ratio, but depending on the dyno, how much resistance the eddy brake can apply, and how quickly your turbos will spool, the dyno operator may advise you to run a higher gear. Then, you want to bring the truck up to a good cruising RPM and wait for the converter to lock if you have an automatic. Personally, I like to start most tests at around 1,900 to 2,000 RPM.
If you are on an inertia only (un loaded) dyno, all you do it step on the go pedal, and let ‘er rip. On the other hand, if you’re on loaded dyno, things aren’t so easy. Once you indicate to the operator you are ready, he will start the test and resistance will be applied to the roller, which can suddenly drop the speed of the truck. You have to anticipate this, and feed in the throttle to keep the RPM from dropping, but not so quickly as to force a down shift. If you’ve done everything right, the RPMs will stabilize, the turbos will start to spool, and boost will start climbing. Now, just put your foot to the floor and let the engine pull to redline (or whatever RPM you are comfortable spinning your diesel to). Once you’ve completed the pull, just take your foot off the gas, but don’t hit your brake pedal. The dyno will slow itself down, and come to a stop, and you will hopefully be excited about your results. If it all goes well, a properly executed dyno run should look something like this…
If you dyno a truck at sea level on a cold 20° day, the air will be dense; conditions are perfect for a good strong number. However, if you test same truck at 5,000 feet of elevation on a 90°, the thinner air will kill horsepower. To compensate for this, a correction factor is applied to most dyno results to make them comparable regardless of where the test was conducted and what the weather was that day. There’s one thing to keep in mind however… commonly used correction factors work great for naturally aspirated engines, but don’t take forced induction into account. At my last shop in Idaho, the 4,800 feet of elevation gave a 20% correction factor on warm days, which was not entirely accurate. So, to keep things fair I would always give customers both corrected and uncorrected results.
Play it safe
Before you strap your truck onto the rollers, keep in mind the general condition of drivetrain components, because you will be putting a ton of stress on the parts, and a dyno test could cause a failure. If the U joints on the driveshaft are questionable or your tires have some dry rot, think twice before testing. Also, if the check engine light is on, or there is some other mechanical issue, you probably want to get that taken care of before doing a full power test. For example, if you notice a significant drop in fuel rail pressure on the hot tune, you may want to consider adding a second injection pump to get the most out of your dyno time.
As a spectator, dyno events are a blast to watch, because its not your truck on the line. But safety is still an important thing to keep in mind. Number one, don’t stand directly behind the truck under any circumstances. Small rocks can be caught in the tread of the tires and when the wheel speed comes up, rocks will fly out at a high rate of speed. Also, avoid areas directly to the side of the engine, transmission, and driveshaft. I’ve seen first hand pistons, connecting rods, and even turbocharger pieces go airborne. Stay at least 15-20 feet away and be smart. And if you ever see someone standing with a nitrous bottle over their shoulder near the engine, you are about to witness the “ghetto fog” and should back way up and get your camera out. Chances are, some destruction is about to happen.
Now go out and build a monster
If you want to really pump up the power of your truck and you’re not sure where to start, be sure to check out the recipes Diesel Power Products has developed for your truck. You’ll find everything you need, and next time you go to a dyno event, you’ll knock the socks off the competition.