Guest bastaad525 Posted November 10, 2004 Share Posted November 10, 2004 Okay I'm just wondering again. Whenever I dyno test, they run my car in 4th gear for the test. I already figured this is because 4th has the 1:1 ratio, so takes the hp multiplication out of the equation (well, not counting the differential). So here's what I"m wondering, if you dyno test and put down X hp to the wheels in a 1:1 ratio gear, but lets say you were to retest in 2nd gear (1.858:1 ratio), would that increase the hp to the wheels by 86%? Just wondering exactly how gear ratios and torque multiplication work exactly. Also, if you tested with the 3.54 diff (which I did) and then retested the exact same setup with a 3.9 diff, would it read a higher hp to the wheels? Quote Link to comment Share on other sites More sharing options...
GabeRoc Posted November 10, 2004 Share Posted November 10, 2004 no and no. energy (power) cannot be created or destroyed, your engine is producing a certian amount of power and gearing can only alter the how much torque (or conversely what rotating speed) is being applied to the wheels at the expense of speed (or in the converse case, torque). the dyno would record a higher torque number in 2nd gear from the drum and a lower drum speed. net power is the same. (assuming that the transmission has the same effeciency in 2nd as it does in 4th). in short, NO MATTER WHAT you do torque*rotating_speed = constant. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 10, 2004 Share Posted November 10, 2004 Gabe is right. My friend dynoed his 510 recently and they didn't calculate out the rear gears, so his power was 115 or whatever and torque was 470 or something like that. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted November 10, 2004 Share Posted November 10, 2004 Power is the constant. Torque varies with the gear ratio. There might be differences in drive train losses across the different gears. So any power differences measured would equal the difference in the drivetrain losses. Quote Link to comment Share on other sites More sharing options...
blueovalz Posted November 11, 2004 Share Posted November 11, 2004 So this still does not answer the question in "mine own eyes." If my car is run on a chassis dyno, two separate runs, in different gears (or with different final drive ratios), then how does the dyno (or operator) bring together the two torque measurements (is rpm used, tire size, gearing, etc fiqured into the calculation?) to reflect that the same motor is used in both runs? Gabe is right. My friend dynoed his 510 recently and they didn't calculate out the rear gears, so his power was 115 or whatever and torque was 470 or something like that A fiqure given in the quote above would make me want my money back being it does not appear to reflect the motor's parameters. Be kind, I know nothing about how a chassis dyno differs from the engine dyno other than the phyically different properties. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted November 11, 2004 Share Posted November 11, 2004 He basically has to compute the HP being generated at the rear wheels then plot that vs. engine RPM. The dyno operator is measuring engine RPM directly. He is also measuring rear wheel RPM and rear wheel torque. Run the engine up to some RPM, compute HP as the rear wheel RPM times rear wheel torque then make a data point on a plot (HP at that engine RPM). After he runs revs the engine thropugh the power band, he takes the plot and divides HP by engine RPM to get the torque curve. Actually the dyno probably measures the RPM of the dyno drum and not the rear wheels. So he doesn't need to know anything about the cars gear ratio or tire diameter to get the engine curves. And 115 HP with 470 ft-lbs of torque would be an odd engine. Sounds like they were giving the measured torque value at the rear wheels? Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 11, 2004 Share Posted November 11, 2004 And 115 HP with 470 ft-lbs of torque would be an odd engine. Sounds like they were giving the measured torque value at the rear wheels? At the rear hubs. This was one of those dynos where you take the wheels off and bolt the 2 stands to the rear hubs. The car has a 3.90 rear end so when we factored that in everything looked fine. Those numbers (115 and 470) aren't the actual numbers, but it was something really close to that. I guess it was a brand new dyno and it was actually the first time they had a car on it, so the guy wasn't quite sure what he was doing. It really didn't matter to my friend, because he just wanted the wideband O2 readings to get his carbs jetted correctly, which he was able to do. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted November 11, 2004 Share Posted November 11, 2004 And 115 HP with 470 ft-lbs of torque would be an odd engine. Sounds like they were giving the measured torque value at the rear wheels? At the rear hubs. This was one of those dynos where you take the wheels off and bolt the 2 stands to the rear hubs. The car has a 3.90 rear end so when we factored that in everything looked fine. Those numbers (115 and 470) aren't the actual numbers, but it was something really close to that. I guess it was a brand new dyno and it was actually the first time they had a car on it, so the guy wasn't quite sure what he was doing. It really didn't matter to my friend, because he just wanted the wideband O2 readings to get his carbs jetted correctly, which he was able to do. thats the Dynapack I was bitching about a couple weeks ago, where I was also saying the numbers I got didn't jive at all. Those don't measure the RPM of the engine directly, like a dynojet does by hooking up a sensor to the spark plug wire. The dynapack assumes engine rpm by figuring in the gearing of the trans and diff... but I think the guy either entered my info wrong or something, giving me numbers that just didn't match up. Maybe Dynapack owners should pass some kind of mandatory test before being allowed to operate them because it seems they seem to have some problems figuring them out?? Jon, once they got his gearing right, what did the 510's numbers actually end up being? And can someone please clarify this dyno thing??? Does a dyno directly measure power or torque??? The dyno owner I went to last says it measures torque and then computes hp from RPM. The magazines I've read also say the same thing. Yet I keep seeing people say the opposite here. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 11, 2004 Share Posted November 11, 2004 It was like 110 hp and 120 ft lbs. If you want I can dig for it, but that is pretty close. In the example, take the 470/3.9 = 120.51 ft lbs. I think a lot of the problem he had was that it was literally the first time the operator had used this dyno. He was a friend of the engine builder, and when the engine was done my friend called the machinist and he said, "I know a guy who just got a dyno" and it kinda went from there. Quote Link to comment Share on other sites More sharing options...
GabeRoc Posted November 11, 2004 Share Posted November 11, 2004 a dyno measures the force being applied to the drum and the rotational speed of the drum. this gives the torque applied to the drum and the horsepower at the drum. this horsepower number *must* be equal to the value the engine is producing at that instant (if transmission losses due to friction ect. is taken into account) if the dyno power number (and hence the engine power number) is divided by the engine speed, engine torque is found. for those more mathmaticaly wired power is conserved: engine_torque*engine_speed=wheel_torque*wheel_speed=power wheel_torque*wheel_speed/engine_speed=engine_torque hope this is clearer Quote Link to comment Share on other sites More sharing options...
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