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  #1  
Old 05-12-2009, 08:23 AM
wizard wizard is offline
 
Join Date: Sep 2006
Location: Eagle, ID
Posts: 61
Default Hobbs vs Tach

What is your typical Hobbs:Tach ratio? I just put my first 100 hours on and my ratio is 100:75. That indicates that I spend 1/4 of my time on the ground taxiing, warming up, or in the air practicing stalls. That doesn't seem right to me. Admittedly, my first 100 hours began in November and I probably saw more warm up time than I will in the next 6 months. But still...makes me wonder if clocks are working right.
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RV-7A N887BH - SOLD
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  #2  
Old 05-12-2009, 09:13 AM
Rockyjs Rockyjs is offline
 
Join Date: Aug 2006
Location: Gulf Breeze, FL
Posts: 408
Default

I'm not sure how your TACH time is set to record. On my Dynon FD180 EFIS/EMS here is an excerpt:

"The Tach Timer keeps track of engine time (normalized to the user-configured cruise RPM). The Hobbs Timer records the duration of time engine oil pressure is at 15 PSI or higher"

My Dynon originally defaulted with a higher number than I presently have set. I also had a large difference initially between the Hobbs and Tach times. My hobbs/tach are within about 5% after I adjusted the RPM setting for cruise (lowering the RPM). When the adjustment is made in the Dynon it actually changes the TACH time currently displayed using the RPM memory data residing in the EMS.

Check your configuration and you can probably narrow the ratio
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  #3  
Old 05-12-2009, 09:41 AM
wizard wizard is offline
 
Join Date: Sep 2006
Location: Eagle, ID
Posts: 61
Default AFS system

I have an AFS system. The manual states that Tach Time is that time the engine is above 1250 RPM while Hobbs Time is that time the engine is above 0 RPM.
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  #4  
Old 05-12-2009, 10:43 AM
breister breister is offline
 
Join Date: Dec 2006
Location: Atlanta, GA
Posts: 1,231
Default

What is your typical cruise RPM vs. redline RPM? If you typically cruise at 2400 rpm using an everyday Lycoming, then your tach time during flight is less than 90% of your Hobbs. Now add in the time spent taxiing at 30%, and it is easy to see how you could end up with 75%.

Hobbs time is in many ways totally artificial - it gives no inkling as to how much work the engine has performed.

This (the Tach time being lower) is a GOOD thing, because your maintenance generally comes based on Tach time (or, how much work has the engine actually done).
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  #5  
Old 05-12-2009, 11:35 AM
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John Myers John Myers is offline
 
Join Date: Feb 2006
Location: Newalla, OK
Posts: 24
Default Hobbs vs Tach

Another data point for you, I am also using an AFS system and have about a 100:78 ratio (145h to 113t). That is pretty close to your ratio and may have more to do with the length of flight times than anything else. If the temp is colder than about 40 degrees F, I preheat and don't take off until oil temp comes up to about 115 degrees F. My ground run time is not normally very long that way and I live on a farm strip, so no long taxi times either. Hope that helps.

John Myers RV 8 N851JM
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  #6  
Old 05-12-2009, 11:53 AM
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strahler13 strahler13 is offline
 
Join Date: Jan 2005
Posts: 369
Default Vision?

Anybody know how the Vision hour meter would compare with a tach or Hobbs? Does it start "counting" at a certain RPM?

Thanks, Mark
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  #7  
Old 05-12-2009, 02:47 PM
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Mark Albery Mark Albery is offline
 
Join Date: Jul 2007
Location: Warwickshire UK
Posts: 703
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Depending on the exact model of tacho, it will run true at a set RPM - typically 2400RPM. The Hobbs will record true time from master switch on to off unless it is fitted with an oil pressure or airspeed switch.

I usually cruise at 2200 RPM at low altitudes so my tacho is only recording 92% of true time in the cruise, less during descent and landing.

My Hobbs reads about 25% higher than the tacho.

Have a look at this to see how variable tach time can be.

Most renters charge Hobbs time because it gives the biggest number.
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  #8  
Old 05-12-2009, 11:04 PM
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Greg Arehart Greg Arehart is offline
 
Join Date: Jul 2007
Location: Delta, CO/Atlin, BC
Posts: 2,391
Default

I may be wrong, but my understanding was that Hobbs time is from engine on to engine off - basically any time there is oil pressure in the system. Tach time on the Dynon actually counts RPMs and then calculates time based on the user preset RPM. So if I set the "tach RPM" to 2700, I should get a much lower tach time than Hobbs time, since I rarely run at those RPMs! However, if I set the "tach RPM" to, say, 1800, then I might actually get a tach time that is LARGER than the Hobbs time, because I would be (normally) cruising at 22-2400 RPM which would compensate for the time I spend on the ground at 7-900 RPM.

I currently have my tach timer set for 2200 RPM, which is probably a bit lower than I normally cruise.

greg
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  #9  
Old 05-12-2009, 11:11 PM
rwhittier rwhittier is offline
 
Join Date: Oct 2006
Location: Glendale, AZ
Posts: 256
Default 80-20

My ratio has held very consistently at flight = 80% of hobbs. It been steady at that for about 8 years.

Quote:
Originally Posted by wizard View Post
What is your typical Hobbs:Tach ratio? I just put my first 100 hours on and my ratio is 100:75. That indicates that I spend 1/4 of my time on the ground taxiing, warming up, or in the air practicing stalls. That doesn't seem right to me. Admittedly, my first 100 hours began in November and I probably saw more warm up time than I will in the next 6 months. But still...makes me wonder if clocks are working right.
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RV7A Quick Build, Tip Up
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  #10  
Old 05-13-2009, 08:28 AM
Jim Percy Jim Percy is offline
 
Join Date: Mar 2005
Posts: 179
Default

The EIS has a user settable RPM, above which time is counted. It doesn't average or normalize anything, it's either on or off based on an engine RPM "switch". I have mine set at 1400 RPM and it is amazingly close (within just a few minutes over almost 200 hours) in total time to my CNX-80, which is set to kick in at 40 knots ground speed. I consider my tach to be a pretty accurate representation of actual off-the-ground time.

To add a data point to the original poster, my tach time seems to run about 75% of the hobbs- same as yours.
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