With gears now available for these cars, I decided it was time to create this dicussion. I will start this off with some good reading. A Dynojet (or any other inertia type dyno) is not reliant on mathematical estimates of gearing or measurements of the "brake type" dynos that measure the amount of power absorbed by the water, oil or eddy current brake system. Assuming it was an inertia type dyno, the dyno is measuring the work being produced by the wheels of the cars acting directly on the two large drums. The computer is detecting how fast the drums accelerate from a full stop. This type of dyno uses the simple Force = Mass x Acceleration formula. It then renders a horsepower number based upon the old HP = RPM x Torque/5252 formula. In reviewing this, there's not doubt that gearing plays a critical part in producing whatever comes out the rear tires (gears do multiply torque), but I don't see where the gearing would "fool" the dyno - the dyno simply detects how fast the drums rotate. If the idea is to take the dyno results and create a facsimile of a quarter mile run or estimate what that power will produce in terms of elapsed times, then accurate gear numbers are CRITICAL for those computations. But the dyno will tell you the power output regardless. Now, a brake type dyno often utilizes a lot of variable math inputs to arrive at the result (its reading power in a more indirect fashion), but I still believe it would also not be fooled unless part of the data input included gearing (which again, might be the case if mph or even gas mileage estimates were being generated). I've got a grok ache from all this, but I think this makes sense. Some other handy formulas to use in this process: Work = Force (or torque) x Distance Power = Work / Time Torque is a rotational force. The inertia dyno works on the Force = Mass x Acceleration formula. (So do the brake dynos, but more indirectly). Force (torque) is created when the 2700 pound drums are rotated, and the computation would be F = 5400 x Acceleration (2 drums of course). As the drums move from rest, the sensors measure the speed of the rotation, which plugs into the formula as "acceleration", ie, so many rotations per minute (rpm). The dyno neither asks nor needs to be told what gears are in the car spinning the drums (or how much they are multiplying torque, or how much mechanical inefficiency is created by this gear set or that type of gear lube, etc.) It just reads what speed its sensors tell it the big round drums are spinning at, over what period of time. Given this result, it can compute how much torque has been created (directly - it doesn't need to know anything more about the car) - and with other formulas tell you how much horsepower has been created. NOW, to match the RPM's of the drums to the rpms being turned by the gear set in the car (which of course, due to mechanical losses, will never be the number of RPM's actually being turned by the crankshaft unless you're using an engine dyno, which is not what we've been discussing here), THAT is where the dyno would need to be told the correct numbers. This would distort the chart on one side (the rpm side), but would not create wrong top numbers, just wrong numbers at a given rpm. The shape of the curves would also be distorted by this (as pointed out earlier). You would know the maximum numbers generated during the run - but the chart would be useless for much else. If they saved the run and you later went back and had them plug in the correct numbers, you should be able to get a good chart. Regardless of what method of transmitting power to the wheels, the dyno reads the results AT the wheels. It doesn't care what method (a580 with a torque converter, 3.55 gears, 3.06 gears, etc) is used in the drivetrain, it just measures what happens at the wheels, compares that result to how many RPMs the engine is turning, and voila. Now, if someone is trying to discover how much power is lost between the flywheel and the rear wheels, the dyno can't tell you that. Manufacturers do the bulk of their testing on engine dynos, and they usually advertise using that higher number, vs the real world number which comes out at the wheels. Gears CAN change everything, just as you say, and less efficient gear sets (poorer quality, for instance) will transform more power into heat and less into torque. A slipping clutch will do the same - a bad torque converter or worn clutch packs in an a580 - slippery TIRES that don't grip on the dynos rollers... Air pressure, humidity, timing, ad nauseum, can all affect engine performance, and anything that affects engine performance (or parasitic losses, another can of worms) can affect what happens in terms of power production. But Dynos don't care about all this stuff. They just tell you what is happening at the wheels. Engine diagnostic equipment which is sometimes used in association with a Dyno can be used to figure out fuel/air, spark advance, troubleshoot, etc. Final drive ratio and gear ratios throughout the drivetrain affect power output (via efficiency if nothing else), as does the clamping force of a given clutch, the condition of the surface of the flywheel, the balance of the driveshaft, etc, etc. The stronger the gear set, the taller the resulting power curve (and the shorter the amount of time the drivetrain requires to reach operating peaks). However, all other things being equal (efficiency, quality of install, all the other variables), no particular gear ratio (in and of itself) will generate better horsepower or torque readings than another UNLESS the ratios are so high as to affect the operation of the engine, or so low as to prevent the engine from achieving maximum volumetric efficiency (this last might occur - the other would be very difficult to pull off). In the real world where gear ratios are sane and well-matched to the cars, neither problem is likely. Its easy to get bogged down in the logic, while the dyno is busy cutting the gordian knot and reading it like it is. Its really just a very fancy gauge, designed to yield some numbers. Sure, like any gauge it can be off - have a defect or need calibration - require adjustment to compensate for unusual variables like weather - or be mis-operated. Dynos are like all machines, no better than the human operator. But what the chassis dyno (good or bad) is always limited to the reality as it comes off the wheels.
Great topic Andy! I got most of it but got confused with the above. It seems that gears would effect the outcome. (hopefully you're talking about gear ratio's and not quality/efficiency of a particular set of gears). I'm assuming that's the reason for doing the measurement in a gear that results in a final drive ratio as close as possible to 1:1? If you were in first gear you would be able to accelerate the drum (from whatever starting reference point) much faster than if you were in 4th gear. I don't think it matters that the drum is initially rotating at a given speed to start (assuming they don't add resistence proportional to drum speed to simulate air resistance, etc. since you're measuring power and not simulating a 1/4 mile), it only matters how fast you increase the velocity (acceleration). For example let's assume that in first gear you can accelerate the drum from 10 rpm to 50 rpm (40 rpm difference) in 1 second. In 4th gear it might take 3 seconds to accelerate the drum from 100 rpm to 140 rpm (same 40 rpm difference). So it would seem that the dyno needs to take the drum velocity into account? How does that work?
I have no idea how exactly it works, but as an example, we can dyno the same car in both 4th and 3rd gear and while it will take longer to conduct the pull in 4th gear, the numbers are near identical. I believe there is a calculation based on time/mph/rpm to determine a theoretical 1:1 gear.
When I dynoed mine (Mustang dyno) the 4th gear #'s were nearly 30 HP/TQ higher than 3rd. We only did it once for the last pull. It's hard watching that engine scream for 20 seconds and your just waiting for parts to fly everywhere. I've seen many other charts that reflect the same result. Not arguing with you just stating what I've seen and I'm just as curious to learn all the variables as the next guy. Thanks for the info
The first time I went to a chassis dyno I was told gear ratio didn't affect the result. I had a hard time understanding it. Because the dyno is recieving a rpm signal. I understand it as time vs engine rpm vs drum acceleration speed. So in reality the dyno does know the final gear ratio,,, so to speak. IMO this is why non 1-1 gear ratio pulls offer up a different # One would think a 4.56 gear(example) would skew the results because of the faster acceleration of the drum. But at the same time the engine rpms are climbing fast also. It's just a tool,,,(toy) to work (play) with. Nice right up Andy.
Thats really unusual. Usually if anything 4th gear pulls are lower in hp than 3rd gear, but its not because of gear ratios it because of load usually. The 4th gear pulls put more load on the car and can cause KR depending on how lean the tune it. Also, most cars are setup to run richer with each gear which will slow slightly lower numbers as well.
Dyno's are for TUNING. If someone is using a Dyno for HP #'s only, then they are being a Dyno QUEEN! :ronlove:
Next thing ya know, Ron's gonna post that the sky is blue. :ronlove: LOL Great post, PPP. I'm one of the 11 people online that will actually read the entire thing (next time, put a picture of boobs after the first line....that will keep their interest). I've been fortunate with my vehicle in that the 2 dynos that I've been on (Mustang & DynoJet) have produced numbers I'm pleased with, and should be expected considering the conditions the runs were done under (heat soak, outside temp, mods, etc.). With the new SCT tuner (Roe tunes), I'll be making a few more runs on a dyno to check that all is well. While I'd love to have more HP, I'm more interested in a strong engine. What I learned from this thread is that the LX's don't have a 1:1 gear in their transmissions.
