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Dyno stuff (long)
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Posted by Vigge [Email] (more from Vigge) on Mon, 11 Apr 2005 12:26:11 Share Post by Email
In Reply to: Viggen dyno, Drew in Houston, Mon, 11 Apr 2005 10:32:52
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"Vigge, I'm not too sure why 3rd or 4th gear would make a drastic difference in overall measurement accuracy? Yeah 3rd would multiply the torque as compared to a 4th gear run, but the difference would be accounted-for in total speed. Considering the opposite, would it be unreasonable to suggest that a 3rd gear run could actually be expected to develop lower overall wheel numbers when compared to a 4th gear dyno run because when you consider the difference between crank rpm and final wheel rpm it would be reasonable to assume that the tranny itself would loose MORE energy to heat due to comparatively higher torque loading in 3rd as opposed to 4th when converting the final wheel rpm to a lower value than a higher one. What am I not considering? "

Some "nice to know" type information from few web sites

Unfortunately, seeing a dyno sheet of a particular modification on a vehicle is not necessarily a representation of what it does in reality. Dyno data can be manipulated for sales purposes in many ways.

Temperature: engine temp *greatly* affects power output. Dyno runs done during different levels of coolant, engine and oil temps will have drastically different results, with the cooler runs producing more power. Some engines, such as the forced induction vehicles, can lose up to 15% of total hp and torque as temps rise. Thus a colder run done after a particular mod will exaggerate gains, if any. Redirecting external cooling fan air can have tangible effects on data, especially when air is concentrated on intercoolers, open air filters, etc.

Gearing: a chassis dyno is susceptible to gearing bias. More power can be produced on a dyno graph by simply running the test car in a lower gear. Dyno operators should pick the trans gear that is closest to a 1:1 ratio to avoid gearing bias. This ratio is 4th gear in most cars.

Make sure that when a test is being done that the same gear is always used. This is probably the most important factor to remember, especially if you are having a before and after conversion test. The reason this is so important is down to the basic physics of gear ratios. Why does a car accelerate faster in first gear than top gear, after all the engine is still the delivering the same power? The answer lies in the gear ratio. If your first gear has ratio of 4 to 1 and top gear is 1 to 1 it will accelerate (theoretically, but not in practice due to extra friction in lower gears) 4 times as quickly because the effect is to give your engine 4 times the power. This ratio difference has the same effect on the horsepower registered on the dyno. It is normal to a test a car on a dyno in as near as a 1 to 1 ratio as possible so you get the truest horsepower reading you can. This is often difficult with modern cars as no gear is a straight 1 to 1. Most 5 speed gearboxes have a 4th gear at say 1.3 to 1 and 5th gear at .93 to 1, meaning that in 4th gear it will overstate the horsepower and 5th understate it. If we take this to it's logical conclusion a car first tested in 5th gear and giving a 100 horsepower at the wheels with a .93 to ratio is really 107 BHP whereas if the vehicle is then tested in 4th gear with a 1.3 to 1 ratio the horsepower reading will be 100x1.3 = 130 BHP. These are hypothetical figures, as the friction, and hence horsepower losses, in the gearbox are greater the further you go way from 1 to 1, but the principle is still the same. So be quite sure that every time your car is on the dyno the same gear is always used.

Using the above formula to calculate out "hypothetical" fourth gear dyno result for drews will result to 170WHP =200bhp (15% loss used for calculation)
and for Rico we will get 158WHP = 200bhp (20% loss used for calculation)
Like already stated above, these figures are nothing more than hypothetical calculations!

Atmospheric conditions: all dyno runs should be corrected for atmospheric conditions, typically humidity, barometric pressure, and ambient temperature. Neglecting to correct for these variables can skew data, sometimes purposely.

Manual graphing: in reality, dyno data can be totally fabricated. There are many software programs that will generate graphs with any data inputted, such as Microsoft Excel. Try to request actual dyno sheets, bearing some sort of external run data.

Up until a few years ago, all the load type dynos out there had primitive operating systems, if at all. They worked by running the car on the dyno, loading up the rollers (usually via a water brake system), and reading the horse power number when the load prevented the engine from accelerating past a predetermined rpm.

This gave you the max hp the engine could produce at that particular rpm, since it couldn't overcome the force being thrown at it. Take readings at several rpm points (usually manually recorded), and a power graph can be made, extrapolating torque numbers mathematically. Thus, the load type dyno graphs are not "smooth" because not every point along the curve is actually plotted, but just the predetermined (by the operator) rpm points. The more points tested the more complete and smooth the curve.

However, that is quite time consuming, and hard on the car's engine and tires. Plus, operator skill is essential in preventing tire slip from corrupting the data gathered. Loading up an engine is how a manufacturer determines and engine's power output, by the way.

This is all in contrast to the Dynojet inertia type testing, which simply requires the car to run on the rollers from standstill or low rpms, to redline or anywhere in between (in one or several gears). The rate of acceleration is measured, and power is mathematically calculated using the constant of roller weight. The time it takes to accelerate a given mass to a particular speed can be used to calculate hp. It is relatively painless to the car, and quickly measured. Plus, the Dynojet software allows automatic graphing, which produces an attractive consumer product.

However, the downside to this measuring form is that, in the real world, the vehicle is working harder than what is required in accelerating the rollers on a Dynojet. Between the weight of the car itself, elevation changes, and surface friction, the actual environment the engine works in during use is quite different from its experience on an inertia type dyno. This is an important factor to consider when interpreting the hp gain of a particular modification or performance product. For example, lots of ignition timing can be thrown at an engine to produce big hp numbers on an inertia dyno, but the same ignition setting can result in less hp when the engine is actually required to work on the street. Under load, (on the street or on a load dyno) pre-ignition or detonation can occur, tripping the knock sensors and causing the ECU to roll back the timing to less than stock. This is not good for power. Another example is fueling. Leaner fuel maps may work well numbers-wise when subjected to the relatively gentle inertia rollers, but can be disastrous when under load on the street.

The Dynojet is a great marketing tool, but is not an accurate, real-world measuring device. Dynojet now has a load type device that can be added to their existing models, but it currently is fixed, and cannot be altered. It suffers from the same drawback as the older type load dynos. The Dynojet 248H is a version of their standard dyno with a "high weight" inertial roller but is simply a heavier weight roller and is still inertial in design.





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