How to Prepare for Dyno Day

Updated on October 27, 2019
chriscamaro profile image

Chriscamaro loves playing sports, modifying cars, and playing video games.

What's a dyno and why do I need it?

Simply put, a dyno (dynamometer) is an instrument used to measure the power of your car. You drive onto a large drum (usually), strap the car down so it won't move and then hit the gas. The tires spin the drum and the drum measures the power being fed into it. At a minimum you'll get peak horsepower and torque as well as a graph, showing these values at every RPM in the range recorded during the run.

It's important to get your car dyno'd if you are into aftermarket modifications or if you care at all about your car's performance. It lets you know where you were, where you're at and where you're going. If you upgrade your cam, for example, and the internet says it'll add 50HP to your car, you have only their word to go on unless you measure the difference. Not that they're lying to you, but sometimes a mod won't deliver unless you install it properly or do an electronic tune or some other homework that helps the car properly adjust to the change. Dynos allow you to establish a baseline profile of your car's performance before and after making a change. In this way you can see visually and numerically how the change you made affected performance. If it's not what you expected, then you can go about finding the root-cause responsible for the disappointing numbers.

Choosing the Type of Dyno

There are several types of dyno available. Some are better than others. Some offer a trade-off of benefits. Let's have a look at them so you can decide which one is right for you.

Inertial Dyno

This is a broad category of dyno and it just means that the power of your car is determined by calculating how easily the car can spin up a fixed mass (the drum of the dyno). Since the mass is fixed, a powerful car will spin the dyno up faster than a weak one and this can be directly measured and converted into torque and HP. It's cheap, simple in construction and reliable. However it cannot apply a backwards load to the car so it can't be used for steady-state tuning.

Load-Cell Dyno

This category compliments the former in that it is not of the inertial type. It has a load cell attached to the shaft, which acts as a brake and sinks energy from your car, to load the car down and prevent it from going faster. This allows you to hold the car at a particular speed and HP level so that you can tune that load point specifically. You can move the load point up or down so that you can very accurately tune various load points in your car. This is extremely useful, as doing this on the road is sometimes impossible. For example, if you're on the highway, you're at 1 load point. If you wanted to maintain your current speed but increase the airflow (the load), you can't really do it because the highway is flat and not providing any resistance. This is where a dyno comes in handy, since it can change the resistance to your car electro-mechanically. These dynos are more expensive, more complex and require careful calibration to be accurate however so plan accordingly.

Rolling Road Dyno

Another broad category, this simply means that the dyno is full-sized and meant to have your car drive over it and sit on large rollers. It will have either 1 or 2 drums (rollers), depending on whether it's AWD capable or not (sometimes many thin drums are used under a single axle) and may be installed in a pit in the ground, in which case the dyno is flush with the ground, or it may be elevated above the ground, like a mechanic's lift and you have to drive up ramps and onto it. Roller dynos are a more accurate representation of real-world road conditions, as most of the car is interacting with the dyno, as opposed to a hub dyno, described below.

Hub Dyno

Hub dynos resemble small modules that are independent, self-contained and sit next to your car's wheels. The wheels actually have to be removed and the hubs bolted directly to the dyno's input shaft via an adapter. A load cell inside allows for tuning and there are no traction issues, as there can be with a rolling dyno. You can also do 2WD or AWD configurations easily. However this is less than a real-world simulation because the wheels are actually missing and the dyno modules are mechanically de-coupled so some error will be inherent to the figures and the car sensors may freak out a little, depending on how sensitive they are. This is usually a cheaper alternative to a rolling dyno because less garage infrastructure is required so it is less costly of an asset to own for the mechanic.

2WD vs AWD Dynos

2WD dynos are simpler and only have to measure 1 axle but if you have a 4WD/AWD car, you will need to measure all wheels simultaneously. This means the dyno has to have 2 drums, 1 for each axle of your car. The problem for the dyno designer is finding a way to couple the drums together so that they spin at the same speed AND accommodating cars of different wheelbase (distance from front to rear tires). Remember the road is 1 solid piece of material and the car expects this but sometimes AWD dynos allow the front and rear drums to move independently, allowing for differential movement that can confuse the car and make it think it's slipping. This can cause physical damage to differentials but even if it doesn't, it can confuse the ABS, traction control or other control systems in the car, meant to monitor and control traction to the road. Therefore be sure to ask about this and see how the dyno deals with these problems if you have an AWD car.

How to Get the Most Value From Dyno Day

Sometimes you can actually purchase a tuning package together with your dyno run. If this is the case, a mechanic/tuner will actually do the work for you. They'll not only dyno the car but calibrate your car's computer based on the results, so that you get more power. If you're just measuring your car stock, the way it is, they can try to get you a few extra HP just from optimizing the tune, based on the dyno curve. If you've already got your baseline and just made a change (like a cat-back exhaust or something), then they can modify your tune to account for the new hardware. More often than not, making a physical change to the car, REQUIRES making a tuning adjustment to the computer, to realize the full gains of the mod. However you will never know what that adjustment needs to be until you actually measure the car's performance first. Some manufacturers or internet gurus may try to tell you what to change, like it's a hard and fast rule, but without real measured data, you can never be sure, since every car behaves so differently.

If you'll be tuning your own car, make sure to bring your laptop and tuning module to the dyno center so that you are prepared to make changes between dyno pulls. Otherwise you are basically wasting your time. You should be prepared to have a quick look at the graph after each pull to see where the depressions are in the torque curve. You should pay attention to the AFR readings from the tailpipe sniffer to see if there are any lean or rich spots. Then you should make a quick ad-hoc change to your tune before the next pull to see if you can fix the problem. Particularly for those who just installed a supercharger or turbo, their fuel and spark tables will be a mess right after the installation and won't work very efficiently so you'll need to make extensive changes to dial in the new boost hardware. Make sure to road scan your car while it's being dyno'd so that you not only have the dyno graph but also your own diagnostics to go by.

Have Fun and Make Power!

Sure, there are a lot of responsibilities to be undertaken on dyno day if you wish for it to be anything more than an excursion from the banality of your normal life. But...

It's dyno day!!! :D

Have fun and enjoy it! Nothing is more thrilling to a bunch of dudes than standing around a car with its hood open while it does a quarter mile at full throttle without actually going anywhere. Add boost or nitrous to the mix and there's always a chance of a nice explosion at the customer's expense... an added bonus for the bystanders!

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


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    • chriscamaro profile imageAUTHOR


      9 months ago from Ontario, Canada

      And if you fall asleep at your desk, an intake backfire may just wake you up and save your job!

    • Larry Slawson profile image

      Larry Slawson 

      9 months ago from North Carolina

      At my old job, there was a dyno shop across the street from my office. Best sound in the world hearing those engines revving all day haha.


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