"I've just had it on the dyno, bro." The subtext to this statement is often that the speaker has a) more money than sense, and b) thinks that just sitting his bike on a dynamometer is enough to make it go faster.
It’s just possible, of course, that he understands the role of a dynamometer in tuning and has spent the money to get the maximum value from his new Two Brothers exhaust system.
By themselves, dynos don’t ‘fix’ anything – they just provide information a mechanic/tuner can use to diagnose problems and they measure the results of any changes made.
Their major benefit for workshops is that real road conditions can be recreated on site without staff having to risk their lives and licences on actual roads.
If you’re a technician with access to a dyno, it no longer fills you with terror when a punter arrives and tells you, "It’s got a flat spot and a bit of a miss at 220km/h". Those conditions can be recreated safely without the bike leaving the workshop.
HOW IT WORKS
Your bike is rolled onto the dyno and a moveable clamp is attached to the front wheel to secure the bike with its rear wheel sitting on a rolling drum. The bike is then strapped down. When the bike’s engine is started and it’s put in gear, the moving rear wheel rotates the drum. Modern dynos are linked to a computer and information from the engine is recorded and interpreted. This information can include the power and torque produced by the bike’s engine at any given engine speed (rpm).
When manufacturers release a new bike, its specifications usually include an engine power figure – for example, Triumph’s new Daytona 675R claims 92.5kW (124hp) at 12,600rpm.
This is power measured with an engine dyno at the crankshaft. Interestingly, in dyno world the metric units are a bit thin on the ground. The dynos and the blokes who run them mostly talk horsepower and seldom mention kilowatts.
The dynos we’re most familiar with are chassis dynos which measure power at the back wheel. This will always be less power than that measured at the crankshaft because of losses through the gearbox, drivetrain (chain and sprockets) and tyre. It’s pretty typical to lose about 15 per cent of the engine’s flywheel power by the time it arrives at the dyno drum. Anyway, chassis dynos give real-world figures – it’s the power that’s actually available for us to use.
There are two common types of motorcycle dynos: the inertia type and the braked type. The difference relates to the how the drum or roller operates.
With an inertia-type dyno, the roller is free to accelerate as the rear wheel on the bike turns. The attached computer plots power against road speed or rpm.
Inertia-type dynos are good for reliable power figures and testing changes you make to the bike. They’re sensitive enough, for example, to record the difference in power output if you changes from 10W40 mineral oil to 5W30 synthetic oil.
Braked dynos are arguably more useful for tuning in that by controlling the rotation of the roller (usually a magnetic eddy-current retarder but let’s not go into that), you can hold an engine under load at various points in its rev range and see exactly what’s happening. Real road conditions can be recreated in this way to deal with situations like: "It seems to lose power climbing steep hills".
The latest motorcycle dynos, including the Dyno Dynamics 450DS, offer the best of both these worlds so the distinction between the two types is becoming less important.
DOES IT HURT?
The first time you see your bike on a dyno can be a bit confronting. You stand there watching your stationary bike at maximum revs in fourth gear wondering when it’s going to explode. It actually sounds like that when you’re at max revs in fourth gear on the road – it’s just that you’re leaving most of the sound behind you on the road and it tends to be masked to some extent by wind noise.
Dynamometers are usually housed in a dedicated room within the workshop and include some sound-proofing and the ability for fans to push cooling air through the radiator (recreating the conditions the engine normally experiences in real-world riding).
A well-designed dyno room doesn’t really put any more stress on your bike than it experiences in day-to-day use except that if, for example, you’re retuning for a new exhaust system, your daily ride probably doesn’t have quite as much maximum-rev action or for anywhere near as long. As you’ll read in the Spannerman column this month, some aftermarket can manufacturers advise against dyno-tuning as the heat generated can damage any carbonfibre used.
If any of my own bikes spend long periods on the dyno, I usually do an oil change afterwards. Oh, and never stand behind your bike in a dyno room – centrifugal force can shoot nails and accumulated road grit out from the tyre at handgun speeds. That can really hurt!
FRIENDS WITH BENEFITS
Modern braked dynos allow the easy use of an exhaust-gas analyser (EGA) which feeds into the computer to provide arguably the most useful information a dyno can help provide: air/fuel ratios at given revs. This information appears on the computer screen alongside the lines for power and torque production.
EGAs typically look like long, stick-like probes which are inserted into your bike’s muffler.
It’s not uncommon for manufacturers to design bikes with built-in engine flat spots – points along the rev range where the air/fuel ratio is leaned out (less fuel, more air). Typically, this will be at the speed used by governments to check ride-by noise levels so that bikes meet their environmental responsibilities. An EGA can be used to locate the flat spots and they can be tuned out, resulting in a smoother engine with better throttle response.
Bikes vary in their capacity to be retuned accurately. If your bike has carburettors, air/fuel ratios can be changed by changing pilot and main jets, needle position and even slide cutaways. Compared with electronic fuel injection (EFI), though, the carburettor is a blunt object. Minor, incremental changes needed to get the air/fuel ratio across the rev range perfect are more difficult to achieve. With carbs, close enough often has to be good enough. For most engines the rule-of-thumb air/fuel ratio to aim for across the rev range is 13:1. What you’re wanting to see is as straight a line as possible across the chart.
Put yourself in the position of a manufacturer. The bikes you produce have to pass various environmental standards in a variety of world markets. They also have to run reliably for an acceptable service life (perhaps 200,000km). The last thing they want is amateurs playing with the stock air/fuel ratios and the ignition timing. Mistakes could lead to catastrophic engine failures.
Early EFI bikes had no provision for easy adjustment. The demand for it led to aftermarket products (most prominent probably being the Power Commander) which allowed for remapping. These units come with connector plugs that match the bike’s standard wiring harness connectors. This enables them to be plugged into the bike’s wiring without any cutting or soldering (it also allows them to be easily unplugged and removed).
Manufacturers started responding by offering on-board mapping options (sport, economy, rain modes etc) but, with a couple of notable exceptions, haven’t allowed for uncontrolled owner adjustments.
Just as sitting your bike on a dyno won’t increase its power, neither will fitting a device that allows you to adjust the air/fuel ratio. If you don’t modify your bike, the stock air/fuel ratio is probably fine.
We like to tinker, though, and modern electronics is making it easier to get the best from the changes we make. The Power Commander V is a good example (but certainly not the only one – check the internet) of what’s currently possible. If they make one for your particular model bike, fitting it will be a breeze.
In the old days (10 years ago) most riders who fitted a sports can to their bikes found it went slower at anything less than full throttle. A dyno test usually revealed the can was leaning out the fuel mixture and to get any benefit from the change, the mixture had to be made richer at certain revs.
Dynojet (maker of the Power Commander V) has now done most of this work for you. If, for example, you own a Suzuki GSX-R600 and you’ve invested in a Yoshi can, you can use your own computer to download the air/fuel ratio changes necessary to get the best results from the new can. You don’t have to use a dyno.
Experienced dyno operators rightly point out the Dynojet mapping is done in the US and doesn’t translate perfectly to Australia’s fuels and atmospheric conditions. If you have a Power Commander fitted, though, I’d try the Dynojet mapping first. It’s possible that advances in this kind of technology will reduce the need for fine-tuning on a dyno but they’ll still be a useful workshop device for many years to come.
Having your bike dyno-tested isn’t that expensive. Many workshops with dynos will be happy to do it from between $100 and $200. What you’ll end up with is a chart that shows the maximum power your bike’s engine produces and the maximum torque. It also shows you how the power and torque build through the rev range.
On the same chart you’ll also see the air/fuel ratios at various revs. This computer read-out is the starting point for any tuning work required. It may confirm your belief that yours is a good engine in fine tune and good condition. It may also explain why the engine feels breathless when it’s climbing hills in certain rev ranges.
Experienced dyno operators are the best at interpreting the data displayed. They know what tiny fluctuations mean and can get the most from the information acquired. Respect them.
The potentially expensive part of dyno-testing comes when you decide what you want to do with the results. Minor changes to the air/fuel ratio at certain revs may make your bike easier to ride but will come with a labour/technology cost. Your bike can spend hours on the dyno just to make it smoother rather than more powerful.
If you’re interested in more power from your existing engine, a dyno is very useful in charting the results of the changes you make. Get a dyno report before you start modifying so you have something to compare your new results with. Fitting flat-slide carbs, increasing the compression, gas-flowing the head, fitting race cams, boring and/or stroking, retuning for higher octane fuels etc will potentially give you more power and a dyno is the best way to check this. Otherwise you can confuse loud with fast.