Joe_Dezod

I'd like to give credit to the shop that does our dyno tuning on our Scion and D17 Honda platform. www.trickengineering.com, which is local to me, has one of the most advanced dyno's out there. This is why I always say our numbers would technically be higher, but that would be less accurate. So without further delay, here's something to do/read on a lonely Sunday afternoon.

Dynamometer Results - Fact or Fiction?

This discussion revolves around chassis dynamometer's and is intended to be informative and thought provoking. There are two types of chassis dynamometers on the market, inertia and loading. An inertia dynamometer (such as DynoJet) does not measure torque, but measures acceleration. A loading dynamometer applies resistance that is measured (using some type of strain gauge.)

The most often heard discussion is that what factor can be applied to rear wheel horsepower to reflect crankshaft horsepower. This is where we need to understand how the rear wheel horsepower number was derived. Since the DynoJet seems to be widely used and numbers quoted are those from a DynoJet, we are going to use them as our inertia dynamometer example.

First it is important to have an understanding of how DynoJet gets their horsepower numbers. Power in mechanical terms is the ability to accomplish a specified amount of work in a given amount of time. By definition, one horsepower is equal to applying a 550 pound force through a distance of 1 foot in one second. In real terms, it would take 1 HP to raise a 550 pound weight up 1 foot in 1 second. So to measure horsepower, we need to know force (in pounds) and velocity (in feet per second). Dynojet's inertial dynamometer measures power according to the terms just described. It measures velocity directly by measuring the time it takes to rotate two heavy steel drums one turn. It measures force at the surface of the drum by indirectly measuring it's acceleration. Acceleration is simply the difference in velocity at the surface of the drums from one revolution to the next. The force applied to the drums is calculated from acceleration using Newton's 2nd law, Force = Mass * Acceleration. Since the mass of the drums is know and acceleration has been measured, Power (horsepower) can now be calculated. Torque is then calculated using the horsepower number: Torque = Horsepower * 5252 / RPM.

Once they have these numbers a series of correction factors are applied, some made public, some hidden as proprietary secrets. The public correction factor is the SAE correction factor. This formula assumes a mechanical efficiency of 85%. The formula used is: Where: CF= 1.18 * (29.22/Bdo) * ((Square Root(To+460)/537)) – 0.18. To = Intake air temperature in degrees F, Bdo = Dry ambient absolute barometric pressure. This correction factor is meant to predict output in varying atmospheric conditions and is a +/- 7%. The proprietary correction factor is supposed to reflect the loss of power from the crankshaft to the rear wheels.

A Loading Dynamometer applies resistance to the dyne's roller(s) , typically using either a water brake or a current eddy brake. In either case, the amount of force is measure using a strain gauge. The measured force is torque which is a real, indisputable measurement of the actual output at the wheel. Horsepower than can be calculated: Hp = Trq * 5252 / RPM.

A Dynamometer can only measure actual power at the output location. Actual power produced AND delivered by an engine will be highest if measured at the crankshaft, lower at the transmission output shaft and even lower, but more meaningful, still, at the rear wheels. The power that you use is the power at the rear wheels. Some Dynamometer companies add to measured rear wheel power readings a factor that is based on ESTIMATED rear wheel power losses (under what power conditions? 3.0 ltr.? 5.0 ltr.? Under coasting conditions? with a 185/70/15 radial tire? a 335/35/18 radial tire? New heavy radial tire vs. worn old, light, racing tire? Who knows?) In short, there is NO meaningful "average" tire to get a correct rear tire power transmission loss measurement for all cars - so obviously, unless they actually measure the power lost in the rear tires, under driven load conditions, NO dyno company should BE ADDING incorrect power figures into the measured power . It's simply wrong. The fact that they add varying amounts of power to the actual, "true" amount of power delivered and measured to the surface of the drive roller creates a situation that makes it an onerous task to compare power figures from different brands of dynamometer systems. On simple inertial dynamometers, some (most) companies use an average for the inertial mass value of the engine, transmission, driveshaft, axles and rear wheels. This is saying that a 4 cylinder, 1.4 ltr. Geo Metro has the same rotating mass and same rear wheels as a 8 cylinder, 5.0 ltr. Porsche 928 S+4. This simply is not so and wrong .

It's expensive to measure frictional losses in the engine and drivetrain, requiring the dyno to be able to drive the vehicle with engine off. Add the cost of a 50+hp electric motor, controlled power supply, etc. It's just not likely that $20,000 dyno will be equipped with that equipment. It is also common for dynamometer companies to add to the power readings by adding transmission and driveshaft losses back into the measured power readings. Some companies make a concerted effort try to measure frictional losses and, optionally, add the power to the measured readings. Other companies - some that would surprise you - say that it's not important and give a blanket, single factor for frictional losses in every engine. Some simply say that there is a meaningful "average" for every car,( 4 stroke/ 4 cylinder/ 4 speed transmission, 4 stroke/ 8 cylinder/ automatic transmission) and apply it to every car and that it is not a significant difference. Blanket estimates of "average" losses and corrections are, quite simply, incorrect. At the upper levels of the industry, (we are talking about $150,000 - $500,000 AC or DC 4 quadrant dynamometers) it is not tolerated - shouldn't be - and needn't be. There is a dyno company that actually has different versions of software that displays their own identical data files as different amounts of power depending on whether you use the DOS version or the Windows version of their software!!

True, rear wheel horsepower is the standard of measuring the power that is actually delivered to the rear wheels. It is honest, true, fair and duplicable. It is the ONLY standard that can be duplicated by the entire industry - regardless of the dyno manufacturer. When comparing True, rear wheel horsepower to DJHP you must apply a factor. It appears that this is a sliding scale based on horsepower but the best estimate is 1.05 to 1.21 (maybe higher). What this means is that for those of you trying to calculate what your crankshaft horsepower is based on DJHP, and are adding 15%, the most common number I hear, you are actually doubling (at least) the factor. Why? Because DJHP already has a puff number added into their DJHP. Lets say DJHP shows 200 hp and you add 15%, you get 230 hp crankshaft horsepower. In reality DJ has already added in 15 or 20% to their 200 DJHP number. How does this help us.? It does not, and is fact harmful to the many dynamometer test facilities that report only what the dyno actually measured. I can not tell you of the many discussions that we have had as to why the horsepower numbers we recorded lower than that of DJ. For those manufacturers that use DJHP as proof of their claims, can you imagine the shock your customers get when the horsepower number of a vehicle tested on a load bearing dyno do not come close to their claim.

Proper tuning, especially on highly modified engines greatly affect the power difference. Due to the fact that the DJ dyno's sweep so quickly on sweep hp tests, there is no way to properly tune a fuel map. What you get is the acceleration and full throttle maps both triggered during the test, ending up over-rich, affecting the horsepower. The other factor that needs to be taken into account is that DJ dynos assume that every vehicle has the same rotating mass - they don't - and that disregard is another reason why the hp conversion figures are different. The most accurate measurement of rear wheel horsepower is in Steady State Mode (inertia is not a factor in power equation.) The inertial mass changes on each car affects the DJ power, but not the true, rear wheel horsepower. There's another message in the above example, besides the average true, rear wheel horsepower to DJHP conversion factor - It's up to the more experienced reader to figure it out.

Chassis dyne HP, What is it? What to call it? DynoJet = "DJHP". It's not really proper to call "DJHP" "rwhp", as neither the Mustang, Fuchs, Superflow or Land and Sea will necessarily produce the same numbers as a DJ dyno, except by luck - and the whole idea of true, rear wheel horsepower is that EVERY dyno manufacturer HAS the capability to provide those numbers! The Superflow chassis dynes, the Mustang, Land and Sea are all capable of measuring power in steady state mode and producing the same numbers - they all measure torque. Torque x rpm / 5252 = horsepower. We've not diddled with physics! The only factor that is added to the measured reading, in true, rear wheel horsepower, is the additional energy (dyne parasitics) required to spin the dyno(s) roller to whatever speed the roller is turning at - logical, proper and required for any measuring instrument, torque x rpm / 5252 = horsepower + parasitic power = true, rear wheel horsepower.

Chassis dyne HP, What can inflate HP readings on a dyno, but not really make more engine power in the real world? A few things can affect HP when using inertia dynos (not a dyne in Steady State Mode) to measure power (what else would you do??:-): Changing to light, worn race rear tires will improve power output on an inertia dyno, but, not improve real world top speed. A heavier (brand new street) tire that replaced the above, light, worn tire, will decrease measured power on an inertia dyno, but not decrease real world top speed. Lighter wheels are a good thing! Better acceleration in lower gears, especially 1st and 2nd (accelerating less inertial mass!). Better handling is possible, too! Driving hard on worn, light tires is foolish and is not being recommended.

Problems with Inertia dyno test procedure and fuel injected vehicles: A Sweep Test (hold throttle wide open and sweep from low rpm to high rpm) will often trigger the Acceleration Fuel Map, along with the Main Fuel Map, causing the fuel mixture readings to indicate dyno operator that the motor is overly rich. This would cause the tuner to lean out the main fuel map. Of course, in the real world, upper gears, the acceleration rate of the engine is much slower than what they tested, doesn't trigger the Acceleration Fuel Map, and the engine ends up a lot leaner in reality in top gear. It's not that common of a problem, since most people never drive that fast for that long to cause engine damage. Work around: Tune full throttle fueling in real world usage at dragstrip (to best trap speed) or in Steady State Mode on different dyno.

You can optimize tuning for a DJ dyno and make big numbers - and you can tune the engine to make the best power under load on a load bearing dyno and blow off the big DJ dyno numbers. Can a tuner cheat and make a load bearing dyno read higher? The only way that could happen is in a Sweep Test - Sweep Tests are the least reliable of all tests, period. There is NO question about that. Since the Rotating Mass is a variable in a Sweep Test (NOT a Steady State Test!), the actual inertia factor entered affects the final HP figure - Tell the software that the vehicle has a lot of rotating mass to accelerate, and the HP number increases. (torque, rpm, acceleration rate and mass are the factors) - just like DJ dyno ignoring the difference in mass of all cars - So - true HP, again - Steady State Test - No acceleration, mass makes no difference, anymore. Torque, RPM and dyne parasitics. Period. True. Can you make a Steady State Test read higher? Really hard to do - The software will NOT take data unless speed and load are completely stable - eliminating cheating. As far as atmospheric conditions making a +/- 10% difference? Unless you REALLY mess with the barometric pressure (and you can look at every atmospheric factor on the test report sheet - it's hard coded to display - and not an option), it is simply, absolutely impossible to do without obvious evidence. Are final tuning optimal dyno settings different on an Inertia dyno vs. a load bearing dyno? For many reasons, final tune settings are different - and, since most load bearing dyno's will do both , there is a choice of tests - from a DJ style Sweep Test to Steady State. Having a choice of those types of tests to do and seeing what the results on the track are, most tuners will choose the Steady State Test over a Sweep Test. Without a doubt - the Steady State test Mode is the most consistently superior method of tuning - anybody who has the capability to do it will echo that sentiment - it's only an arguable point with those who can't do it properly. One of the reasons why the load bearing dyno will provide settings that work better in the real world is that combustion chamber temperatures are more in line with the actual operating temperatures that the engine.

Does altitude make any difference at all in horsepower? The engine couldn't give 2 hoots at what altitude it is tested at - it only cares what the air pressure, temperature and humidity is. Sea level at 28.02 inches baro is exactly the same as 4000 ft at 28.02 inches, as far as the engine is concerned. When tested at 5000 ft, we get virtually exactly the same power (corrected to atmospheric conditions, of course) as we do at sea level - It's just about 24%-25% less on the track! I am confused why some dyno operators insist on putting altitude on their charts and swear that it's a factor.

Crankshaft horsepower vs. true rear wheel horsepower. That's a tough one. As each vehicle is different, the best way is to dyno the engine and then dyno the vehicle to see exactly what the loss is. The best estimate I can give you based on experience and research is take crankshaft horsepower, subtract 14.5% ( search SAE ), take that, and subtract around 10% to 15% and you'll get about true horsepower at the rear wheels. The actual formula contains a curve for power loss through gears and there's another curve for power lost in a tire. Remember, too - that unless you dyno your engine you are only likely to get a crankshaft number from the manufacturer and that's probably a "good" one that the marketing department is providing.

Does that help clear up any concerns for you guys? Let me know!

tcholic

nice article....very good info everybody should read and know before spitting out numbers that sometimes arnt correct

PlayinWithFire

a lot of this seems like physics and i currently have a D in that class lol..

with the dynopack dynos (the ones that hook right up to the wheel hubs) what reading am i getting "at the wheels" "at the flywheel" I really am dumbfounded.

Nick06tC

Awesome article. It doe not adress one thing tho: Dynos can be calibrated wrong. I was at an NHRA event and had my car dynoed on a portable dyno jet. I wasnt happy with my numbers so I had a friend with a totally stock supercharged tC. NOTHING AT ALL CHANGED TO IT.
He dynoed 177WHP. People say dyno jets dyno high. This obviously was a bad dyno number.

What are the thoughts on calibration of different dynos? And how can it affect outcome?

toyotaracer9

Does altitude make any difference at all in horsepower? The engine couldn't give 2 hoots at what altitude it is tested at - it only cares what the air pressure, temperature and humidity is. Sea level at 28.02 inches baro is exactly the same as 4000 ft at 28.02 inches, as far as the engine is concerned. When tested at 5000 ft, we get virtually exactly the same power (corrected to atmospheric conditions, of course) as we do at sea level - It's just about 24%-25% less on the track! I am confused why some dyno operators insist on putting altitude on their charts and swear that it's a factor.



so if altitude doesnt affect hp then why does it effect track times?? . Its because the air is thinner up at high altitude , and if you tune a car at sea level with a standard fuel management system " AFR/AFC " and then go up to high altitude and make the car go into open loop " WOT " it will run very rich , due to less oxygen . I am going to have to disagree with this paragraph .

Joe_Dezod

You can calibrate a dyno wrong but it's not too common. Most dyno owners are familiar enough with the equipment to know what they're doing. Basically, if they're competent enough to tune an engine, the should know how to calibrate the dyno. On the Dynomite that we use, the initial calibration is easy. You need to enter in the correct factors such as the motor type, climate, etc. You then need to do an RPM calibration so it matches up and plots accurately. We do the the RPM calibrations at 3k rpms. If the company was lazy and didn't calibrate it to read your RPMS correctly and/or the environment factors, I would agree that the tune could be off.

Joe_Dezod

I see where you're coming from. It states that it doesn't care what the altitude is but it DOES care what the barometric pressure is. By being higher up, the barometric pressure naturally changes. The pressure change is what corralates to the change in fuel curves and power. It's just usally safe to assume that when you go higher up, the pressure changes. That's why most people think that higher altitude alone is what changes your power, when in all reality it's directly related to more specific factors. Hopefully that clears it up. But I can definately agree with you.

paul_dezod

Good write up Joe.

BlkSandPrlTurbotC

tikbhoy

just want to add some more info

I use chassis dynapack (loading) on all my runs.

http://www.dynapack.com

Joe_Dezod

Another great alternative to the traditional inertial dynos.

Infamous_Scion

Joe_Dezod

Yeah we know it's good info.

Freez

Hi Joe.

I need to check a few things with you, as you seem to know a thing or two about dynos. I own my own DJ dyno, but the reason I am here is to look for answers, and not to pick on anyone, or their comments.

My main reason for posting here is to check your HP findings especially at high altitude. You mentioned that once correction factors are applied, the numbers should be similar, no matter what altitude.

I am questioning how accurate the SAE correction factors are, for my altitude, 5000 ft or higher. I read somewhere that the SAE model is only accurate till a1.07 correction factor. Anything higher, is not accurate. Again, I might be completely wrong here, but something like that rings a bell.

Here is my problem. The same motor at sea level will produce "X" power, but at 5000ft it shows around 20 to 25% less on the dyno. Both runs where made using a stock motor, and SAE correction factors, and similar dynos. Why would that happen?

According to Dynojet, this should not happen, yet, I cannot get the same numbers on my dyno as what others get at sea level. I can understand a small difference, here or there, but a 25% difference is massive.

In fact my dyno numbers are extremely low, and I am wondering why that is.

Any ideas?

Joe_Dezod

I would say that regardless of the correction factor the numbers will obviously be lower. Too much of an adjustment will say the numbers are higher than what they actually are at those altitudes. The engine is making less power at a point that high, but I didn't expect it to be 25%. Have you spoken with other dyno shops in your area to see if the results are similar? I'd also trying contacting other dyno shops with a dyno jet to see what there experiences have been, whether it be high altitude or not, to see what the problem could be. A lot of times the guys actually using the equipment figure out more things than the manufacturer, or anything you can read about online.

Sorry I couldn't help you more though.

engifineer

This is exactly why many of us try to explain that removing rotating mass does not add ONE single hp to the system. However, I have a feeling that those with pulleys and even flywheels will completely discard what you just stated. Thanks for posting it though. I read that article a while back at http://www.germanmotorcars.com/Dyno_...0inertia_1.htm and had flashbacks of trying to explain to people that removing rotating mass adds zero power to the system (it only allows the engine to rev quicker) and only "tricks" intertial dynos. Besides simple understanding of physics, it does a great job of explaining the differences to those not familiar with how different dynos measure torque and power. It is a very well written article for the most part and does a good job of explaining it. Plus, it is nice to see some physics-relevant, intelligent info posted every now and then :D

Joe_Dezod

I'm glad I'm able to please everyone here on the forums lol...

Thanks man!