The two chassis dynos my car has been on have both made me get it into top gear before they would allow full throttle, so there were no "shift points" on those runs. I thought they were all like that. Shows what I know...
Bill
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| Author: | billdedman [ Sat Dec 07, 2013 8:19 am ] |
| Post subject: | RPM vs. MPH on chassis dyno reasults |
I never could understand that MPH business either because the rear axle ratio and tire size is critical to the MPH figure.... How can you compute the RPM if you don't know those for that particular test? Surely, the dyno has that information (RPM.) I have chassis dyno results for my own car that is (also) given in MPH and was never given the RPM... Always wondered why. Any info on that score??? Bill |
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| Author: | CNC-Dude [ Sat Dec 07, 2013 9:04 am ] |
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That's because a chassis dyno is comparable to a road test and uses the whole drivetrain to spin the big drum where MPH is the standard it is measured against. An engine dyno is stationary and only uses the engine and uses RPM as its standard of measure. It would be pointless for an engine dyno to use MPH as its unit of measure since there is no known gear ratio or tranny being tested with it to determine that, the same is true why a chassis dyno doesn't use RPM as its standard when it does use a rear end and a tranny, an RPM standard of measure would be pointless, since that's no the purpose they were designed for. |
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| Author: | Lobster1 [ Sat Dec 07, 2013 9:31 am ] |
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I'm breaking the engine in @ 6 psi, but certainly hope to ramp it up. It is inter-cooled and meth injected (has not been tested yet). We think the turbo is an '87 Buick GN. I'm relieved to hear that 220 is good, it is still not well tuned. The powerjection system is leaving me frustrated. However I found some PJ threads and most people seem to eventually resolve their glitches. My best guess on the rpm is it is close to peak power at 2800 rpm. The certainly have the rpm. They were using the dyno for tuning purposes as well. The tuner was quite impressed with the distributor curve, thanks to Duster Idiot... B |
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| Author: | billdedman [ Sat Dec 07, 2013 11:55 am ] |
| Post subject: | Final drives and chassis dynos... |
Can someone explain to me why a different rear axle ratio would not SERIOUSLY affect the chassis dyno horsepower numbers in a diagnostic test? It seems to me that a 5.38:1 ratio ring and pinion would accelerate the inertia-drum a lot faster than a 2.76:1, and wouldn't that show up as an increase in horsepower? There's obviously something I don't understand here... Can you help me out? Please? Thanks for any info! Bill |
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| Author: | Dart270 [ Sat Dec 07, 2013 2:06 pm ] |
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I believe the dyno is looking at the rate of change of drum RPM for a given load, and you input RPM and gear ratio to get the RWHP calculated. Lou |
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| Author: | Joshie225 [ Sat Dec 07, 2013 2:26 pm ] |
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Bill, This is exactly why the dyno is operated in a certain way. Take note of the "road speed" on the graph. You'll see that is doesn't even begin until almost 50 MPH. Inertial dynos like this have a fixed load. Rate of acceleration therefore correlates directly to horsepower. Without an RPM reading that's all you get. With RPM you can derive torque. It's backwards from an engine dyno where you know RPM and torque and then derive horsepower. |
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| Author: | billdedman [ Sat Dec 07, 2013 3:46 pm ] |
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Josh, That is all well and good, but since it IS a fixed load, a 5.38 axle ratio will effect a longer lever against that load and accelerate it faster... right? So, why doesn't that "longer lever" (a 5.38 vs.a 2.76 rear axle ratio) affect the HP reading? It WILL accelerate the mass faster.... There's no argument there. You can't make horsepower with gears. I really don't understand. Bill |
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| Author: | emsvitil [ Sat Dec 07, 2013 5:25 pm ] |
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I've had a motorcycle chassis dyno run, and the graph was in RPM..... It had a hook up for a spark plug wire (just like a timing light) to get the rpm. The car dyno should have the same thing to get the RPM. |
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| Author: | Joshie225 [ Sat Dec 07, 2013 9:02 pm ] |
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Bill, The gears will only help at low speed. As you say the gears don't change the horsepower, just the rotational speed at which the power is delivered. |
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| Author: | billdedman [ Sat Dec 07, 2013 10:17 pm ] |
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Josh, The (deep) gears will cause the drum to increase ITS speed a lot quicker. That should indicate a horsepower increase if it computes horsepower based on how quickly the drum can be accelerated. That is what has me baffled. I have a thick head; explain to me how the quicker-accelerating drum will not show a horsepower increase, as opposed to a slower-accelerating drum (as you'd have with less gear ratio.) It's all about how quickly the drum can be accelerated, as I understand it. A 5.38 rear gear will surely accelerate that drum in less time than a 2.76 will... That's not too hard to understand. What IS hard (for ME) to understand, is how the dyno deals with this, somehow negating different rear gears and the effect they have on the horsepower results. Sorry to belabor a point, but it's problematic... Bill |
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| Author: | Lobster1 [ Sat Dec 07, 2013 11:53 pm ] |
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My speculation is that the gears do not affect horsepower or torque, but only where the the power curve falls. If I had taller gears the car would produce the same power, but it would reach its peak at higher mph, sacrificing low end acceleration in favor of top end speed. Same engine output. I think the results would be more clear if they would have given the numbers in rpm, and I'm not sure why they didn't? Their system monitored rpm. The gears need to be matched to the engine, weight of the car and the purpose of the car. I think of gears as a calculator that determines where the power falls. There is always a plus and a minus. I run 3.21 gears and if I ran 2.45 my power would come on at higher speeds, but the potential power is just the same. I think? B |
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| Author: | emsvitil [ Sun Dec 08, 2013 2:47 am ] |
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Actually, bigger gears (4.56 vs 3.23) will show a decrease in HP, TQ with the inertial dynos. I know that with motorcycles, you get slightly higher numbers with 5th gear than you do with 4th under the same circumstances. The gears may have a leverage (linear) advantage to accelerate the drum faster, but accelerating the drum faster is a square function. Accelerating the drum twice as fast needs four times the energy..... |
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| Author: | billdedman [ Sun Dec 08, 2013 4:26 am ] |
| Post subject: | dyno calibration... |
Well, if you can alter the test results with different gearing, how accurate can this test be? I would think, not very... There needs to be some sort of a modifying factor... some kind of calibration that would defeat this gear-ratio change syndrome. I would think that it's all mathematics and should be a part of the dyno software, to enable accuracy. But, come to think of it, they never asked me for gearing information when my car was tested. ??????????????????????????????? The mystery deepens... Bill |
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| Author: | Charrlie_S [ Sun Dec 08, 2013 12:22 pm ] |
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I had my Cuda on a chassis dyno at Pitsburg Raceway in 2006. IT gave the Hp, Tq, and RPM, not MPH. But thinking about it now, the curve did not show the shift points, as I would think it should have. |
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| Author: | billdedman [ Sun Dec 08, 2013 3:22 pm ] |
| Post subject: | shift points... |
The two chassis dynos my car has been on have both made me get it into top gear before they would allow full throttle, so there were no "shift points" on those runs. I thought they were all like that. Shows what I know... Bill |
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