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Dustin Gust
Joined: 12 Nov 2008
Posts: 27
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 Posted: Wed Nov 02, 2011 11:18 am Post subject: EGT analysis |
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Hi all, I've just installed a EGT sensor to help me jet the Rotax DD2.
A simple question: the point is to look for the maximum EGT value while choosing a jet configuration, isn't it?
Thanks. |
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al nunley
Joined: 13 Nov 2006
Posts: 3035
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| Posted: Wed Nov 02, 2011 12:20 pm Post subject: Re: EGT analysis |
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| Dustin Gust wrote: | Hi all, I've just installed a EGT sensor to help me jet the Rotax DD2.
A simple question: the point is to look for the maximum EGT value while choosing a jet configuration, isn't it?
Thanks. |
Yes, and that applys to the low speed as well as the high speed. Detoantion is caused by excessive heat and/or pressure, so, on the low speed, where the pipe is doing it best at rasing the cylinder pressure, the EGT will be lower.
In both cases, if the temp goes down, the first thing to check for is a lean condition.
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If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting |
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Sean Cross
Joined: 27 Oct 2011
Posts: 4
Location: United States, Illinois, Lake County
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| Posted: Wed Nov 02, 2011 12:38 pm Post subject: |
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| Al you have it backwards a rich condition will cause a cooler egt and lean a hotter egt. This is a property of the cooling effect of the fuel vaporization. |
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Nick Weil
Joined: 18 Jul 2001
Posts: 1795
Location: United States, Florida, Orlando, FL US of A
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| Posted: Wed Nov 02, 2011 7:58 pm Post subject: |
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LOL!
Sean just told Al Nunley he is wrong about a jetting diagnosis. This should be good! Pulling up a chair and getting some popcorn out. 
_________________
Nick Weil
Authorized Rotax Service Center and Dealer since 2002
2004 Team USA Rotax Worlds Finals DD-2 Driver
Want to save money AND grow the economy? www.fairtax.org |
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Sean Cross
Joined: 27 Oct 2011
Posts: 4
Location: United States, Illinois, Lake County
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| Posted: Wed Nov 02, 2011 10:04 pm Post subject: |
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| Nick Weil wrote: | LOL!
Sean just told Al Nunley he is wrong about a jetting diagnosis. This should be good! Pulling up a chair and getting some popcorn out. |
I wasn't disagreeing on the first step just that a leaner condition wont always be cooler.
So you disagree that a mixture that is lean from peak power burns hotter than a richer mixture. If so you would be mistaken. When tuning for power lean/rich is a relative term, obviously there is a point when EGT will start to fall as you get leaner and leaner (>14.7:1) but that is really irrelevant since you will be flirting with detonation at that point.
Regardless EGT is pretty much useless if you don't have a baseline from an a/f gauge and/or a dyno run. There are many things that will effect EGT much more than a/f ratio the most significant one being timing. Reading the plug or piston is far better.
If you want to tune with EGT alone it is best to richen until EGT starts to fall if it falls immediately go leaner to find peak and then richen to achieve ~50f drop. That is what I was taught years ago before a/f gauges were around. |
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Jerry Lee
Joined: 06 Feb 2008
Posts: 16
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| Posted: Wed Nov 02, 2011 11:01 pm Post subject: |
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***Edit: just read Sean's last post, the below doesn't apply to the discussion but I thought I'd leave it up
This probably isn't a technically complete or correct way to explain it, but here is an easy, intuitive way to remember.
More power = More energy = higher temp. So the EGT will rise and rise until optimum mixture (i.e. more power) and past that, EGT will start dropping again.
Another easy way to remember is that after optimum mixture, more air that isn't combusted is being mixed into the exhaust. Or in simpler terms, more "not hot" air mixes in with "hot air" resulting from the combustion. Of course, on the flip side richer air results in lower EGT because of the excess fuel (also "not hot" substance) + fuel's cooling effects.
Not sure if this is right at theoretical stoic mixture, but it rarely is due to many different variables.
Not a technically clean way to explain it but intuitive. |
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Thom Andresen
Joined: 01 Feb 2006
Posts: 4358
Location: Kazakhstan, not USA, Rainbow Bridge
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| Posted: Thu Nov 03, 2011 4:44 am Post subject: |
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| Nick Weil wrote: | LOL!
Sean just told Al Nunley he is wrong about a jetting diagnosis. This should be good! Pulling up a chair and getting some popcorn out. |
I just popped a bag of Orville Redenbacher's Simply Salted Natural, the microwave EGT never went over 350*F  
Technically speaking, does anyone know why my Rotax DD2 has over 50 hours of high speed top end road racing on it and I don't let the EGT get over melting temperature of aluminum and it still works great and has never been unsealed? 
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I spend my money on hunting, racing, women, and whiskey - the rest I waste. |
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