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Old 08-31-2015, 11:22 PM
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Incomplete dyno run?

Introduction
Decided to dyno my DD for ****s and giggles. However, the data seems to be incomplete. Granted, there's no tach in my car, and the shop just got the dyno. Figured I'll have someone else check my reasoning before telling the guy how to do his job.


Physical Observations
* DynoJet 224x never showed higher than 2600RPMS during the pulls, which was clearly wrong (likely incorrect crank-degrees-per-spark setting).
* Supposedly blue and red pulls are in 3rd gear, green in 2nd.
* The last pull audibly went to a higher RPM.


Known Variables
Tire size 205/70R15 = 26.3 in diameter
2nd gear 1.57
3rd gear 1.00
Final drive 3.05
Rated HP 175 @ 5200
Rated TQ 195 @ 4000


Graphical Analysis
* Dynos measure engine speed and either HP or TQ, and calculate the other.
. . . Stock motor does not pump 100 ft-lbs over rating.
. . . Therefore, HP is the measured value; torque was calculated using (incorrect) RPM.
* 3rd gear pulls ~70 to 100 MPH == ~2700 to 3900 RPM, according to the math.
* 2rd gear pull ~67 to 77 MPH == ~4100 to 4700 RPM, according to the math.


Conclusion
I have only a hodgepodge of data, and should tell the shop to do it over. The first two pulls ended really early, and the third still didn't go to redline. I can't even stitch the lines together because their start/end RPMs don't overlap, and the different gears will have different drivetrain losses anyway. Right?
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Old 09-01-2015, 09:18 AM
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Quote:
Originally Posted by dunc2027 View Post
Introduction
Decided to dyno my DD for ****s and giggles. However, the data seems to be incomplete. Granted, there's no tach in my car, and the shop just got the dyno. Figured I'll have someone else check my reasoning before telling the guy how to do his job.


Physical Observations
* DynoJet 224x never showed higher than 2600RPMS during the pulls, which was clearly wrong (likely incorrect crank-degrees-per-spark setting).
* Supposedly blue and red pulls are in 3rd gear, green in 2nd.
* The last pull audibly went to a higher RPM.


Known Variables
Tire size 205/70R15 = 26.3 in diameter
2nd gear 1.57
3rd gear 1.00
Final drive 3.05
Rated HP 175 @ 5200
Rated TQ 195 @ 4000


Graphical Analysis
* Dynos measure engine speed and either HP or TQ, and calculate the other.
. . . Stock motor does not pump 100 ft-lbs over rating.
. . . Therefore, HP is the measured value; torque was calculated using (incorrect) RPM.
* 3rd gear pulls ~70 to 100 MPH == ~2700 to 3900 RPM, according to the math.
* 2rd gear pull ~67 to 77 MPH == ~4100 to 4700 RPM, according to the math.


Conclusion
I have only a hodgepodge of data, and should tell the shop to do it over. The first two pulls ended really early, and the third still didn't go to redline. I can't even stitch the lines together because their start/end RPMs don't overlap, and the different gears will have different drivetrain losses anyway. Right?
The only power measurement is torque, horsepower is a calculated value of torque times RPM. Torque is a force that can be measured, horsepower is a computation of the work that torque can do in a period of time of its application.

Yeah chassis dynos do a lot of arithmetic; there are a lot of assumptions about driveline, accessory and exhaust system losses between the tire rollers and the crankshaft flange.

Looks like you're paying for them to lean how to use this thing.

Bogie
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Old 09-01-2015, 12:16 PM
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The pulls are usually done in a direct 1:1 gear from 3000- redline or predetermined max.The dyno jet is an inertia dyno Vs a magnetic like a "Mustang" You have zero information on the sheets,did you pay? You do not want someone learning on your car,too expensive even if they do it for free. Also if they make a mistake or a driveline part breaks,,,what will save your engine?
hope they used a big cooling fan in front of car and FOUR hold down straps
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Old 09-01-2015, 12:42 PM
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Hmmm...Painted Jester was looking for a dyno education thread
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Old 09-01-2015, 02:01 PM
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I did pay, $100 for 3 pulls. He used two straps in front, crossed over another, and chocked the rear wheels. There was a small but high-output fan directed at the engine.

Actually, according to the DynoJet manual, the only thing it physically measures is time between revolutions of the roller. From this it calculates angular velocity, acceleration, force, and finally horsepower. It can then back-calculate the engine torque if you elected to use the spark sensor.
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Old 09-01-2015, 02:50 PM
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. I assume this is something like a GM/Olds Tech 4 DOHC 4-banger engine... FWD vehicle...


. Would need the dyno to reach about 130 MPH for engine to reach rated 5200 RPMs HP with an auto trans... 133 MPH with a stick...

http://www.f-body.org/gears


. Like the other guys said, he prolly shouldn't have charged you over $20 for the day while experimenting with your vehicle and setting up his new dyno...


. The bottom red and blue lines look about right for the FW HP... other lines look like garbage and/or 1st and 2nd gear gross torques... the torque scale looks way off...


. If taken out to 130 MPH, looks like engine graph line could have been 140 - 150 FWD HP... about right for that engine in good tune... prolly more than the 351"M in my '78 truck currently produces... LOL!


. He should use 4 straps... chocks will do next to nothing if something goes wrong...
.
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Old 09-01-2015, 04:38 PM
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Close. It's a 60 V6. GM 3100 on a 4T65-E.
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Old 09-01-2015, 04:41 PM
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And yes, he can't do it in 3rd. Governor will kick in before redline. It'll have to be in 2nd, from 45 to 85 MPH. Approximately 2700 to 5200 RPM.
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Old 09-01-2015, 04:48 PM
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. Didn't think it was powerful enough to need a governor... Dyno will need to know the 2nd gear overall gearing...
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Old 09-01-2015, 06:18 PM
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Governors don't really have to do with power... most passenger cars have them, ostensibly for safety reasons. This one cuts off fuel at ~105 MPH.

Also, the dyno doesn't care what gear you're in. Most people use 3rd so the tires won't break loose.
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Old 09-01-2015, 06:22 PM
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Just got back from the shop. Owner was gone, but the tech from yesterday looked at me like I was crazy and said the pulls were fine. Don't know if I should return to make my point, or just let it go. I did pay (waste?) $100, after all.

Perhaps I should clarify: the dyno was installed in December, and they claim to use it all the time. Not sure if they really are clueless, or were just being lazy since my car wasn't "nice."
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Old 09-01-2015, 06:33 PM
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Quote:
Originally Posted by dunc2027 View Post
Governors don't really have to do with power... most passenger cars have them, ostensibly for safety reasons. This one cuts off fuel at ~105 MPH.

Also, the dyno doesn't care what gear you're in. Most people use 3rd so the tires won't break loose.

. If top speed of a car is low, then no need of a governor... or sometimes they govern to avoid using more expensive speed rated tires...


. The X and Y scales on the dyno graph won't be right if dyno computer doesn't know the gearing and/or RPMs...
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Old 09-01-2015, 06:39 PM
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Quote:
Originally Posted by dunc2027 View Post
Just got back from the shop. Owner was gone, but the tech from yesterday looked at me like I was crazy and said the pulls were fine. Don't know if I should return to make my point, or just let it go. I did pay (waste?) $100, after all.

Perhaps I should clarify: the dyno was installed in December, and they claim to use it all the time. Not sure if they really are clueless, or were just being lazy since my car wasn't "nice."
where is all the other info? bsfc? water temp? oil pressure? Air fuel mixture? ambient air temp? barometer reading?

NOTE: direct gear ,,1:1 is usually selected because other gears have more parasitic loss of power through transmission
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Old 09-01-2015, 07:20 PM
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Buzz --
1) Fair enough.
2) That's why the dyno uses either a spark sensor or OBDII port. Otherwise you'd have to enter in not only transmission gear, but also differential gear and tire size. Have you ever had to enter all that information in a dyno? You actually can pull without measuring RPM, if the software will let you; the results will be simply show HP vs MPH, with no torque curve.

Vinnie --
1) There were no other measurements taken. Just strap on, clip the spark sensor, rev three times, drive off. In retrospect, seems like a crappy deal.
2) Good to know.
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Old 09-01-2015, 09:41 PM
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Quote:
Originally Posted by vinniekq2 View Post
NOTE: direct gear ,,1:1 is usually selected because other gears have more parasitic loss of power through transmission

. Of course, some of these FWD cars have no 1:1 gear... especially the manual tranny ones...


. HP vs MPH doesn't tell me much easily... I like to see torque vs RPMs...
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