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 Post subject: exhaust gas temperature
 Post Posted: Sun May 13, 2012 7:40 am 
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Location: southern,ohio
hey guys,
I am wondering at what temperature is the combustion process getting to hot?
i will give you a little history of what im doing,i bought some innovate stuff(dl-32,tc-4,lc-1,etc) and i wanted to monitor egt and be able to shut the nitrous off if the egt's got to high,well then i found out that the innovate stuff does not have an output for that so i am monitoring 4egt's with the innovate stuff (#1,2,3,4) and i will monitor the other 4 with an altronics "red alert" system (#5,6,7,8) that does have a programmable output to shut the nitrous off....but im not sure what that to high egt is?


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 Post Posted: Wed May 16, 2012 11:00 pm 
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Well, Since no one has stepped up I'll share a little hard earned info with you. Don't let EGT's be what you base your tune up on. Watch and track them as you make changes. Use them to develop patterns of the changes you make..It is my opinion that with all the combination variables there is no perfect EGT number to use on every engine.
I have had some 2.5 HP Per cu in engines make max power @ 1050 degrees while being 1200 at peak TQ. That is due, I believe at having a fuel curve/timing curve that favors richer @ Peak Tq and leaner at Peak HP. I have had some melted pistons with 945 deg EGT's. When the EGT's start getting cooler and cooler and power is going up the temps in the cylinder are going out of this world..."Something is burning!!!

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 Post Posted: Thu May 17, 2012 6:08 pm 
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Melted pistons at 945 deg?,i would not expect that from 2618 JE's,i really thought the number was going to be much higher

what would be considered an "emergency lean" egt number i guess is what im looking for?

thanks


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 Post Posted: Thu May 17, 2012 8:53 pm 
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As said, the combustion problems of destroying parts has no relationship to EGT. You can have identical EGT, one engine will be detonating and destroying the pistons the other will be making the most power you've ever seen. The EGT is only a measure of the unused energy from the fuel. It has to be taken in context with a lot of other information. Also by the time an EGT probe reacts to a dangerous lean condition with Nitrous, you already have a hole through the piston. You'd be better off looking in the mirror waiting for the smoke trail than using EGT.
The best way to develop Nitrous is to learn slowly, use small amounts to start with and slowly increment changes. The sparkplug will show you far quicker whats happening. People must read plugs. Always go rich on the fuel side and retard on the timing side, then slowly adjust fuel and timing for best performance. Use a nitrous system that atomizes the fuel well, as the nitrous cools the charge so much that fuel has trouble vaporizing, so the droplets of fuel have to be very small if your using extreme amounts of nitrous. Working on cylinder distribution is very worthwhile. All engines are limited by the weakest link. Unfortunately EGT isnt always an indicator of cylinder AFR distribution issues either. People use it for that but if they actually installed WB sensors or took gas bench readings per cylinder they would discover how wrong they are for some situations.

But your looking for a general rule so heres one. For gasolines dont exceed 1500f EGT. I personally wouldnt go that high without consideration of a lot of other factors and a well founded knowledge of the particular engine. For alcohol fuels dont exceed 1350 or so. You will find a well known rule of optimum methanol EGT at 1280F but thats BS. Some people say that rule and then add " because thats the melting point of aluminum" WTF is that all about!!!
The important thing to observe on race engines is the sparkplugs and the piston tops. All fuels have carbon in the molecules and all fuels make maximum power when around 15% richer than stoich so all fuels will deposit carbon upon surfaces. The carbon is deposited because of a lack of oxygen to completely burn all the carbon. Therefor all fuels will eventually Tan a plug or piston etc. Anytime you dont see Tan or dont see colors ranging from tans to blacks you are in a dangerous place combustion wise because the combustion in that area is either LEAN or nonexistant. If you see Green or the less dangerous Yellow deposits stop and retune. Greens and Yellows are from lean.
Thats why you have to look at sparkplugs. If an engine has a richer than stoich mixture and the plug is white or clean (which is not uncommon) the mixture preparation is poor. Lean burns are hot and destructive. Adding Nitrous to an engine with that problem is stupid. The first thing to do with building a nitrous engine is to create a really good normal one. Get all the bugs out of it first, then add nitrous.


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 Post Posted: Fri May 18, 2012 5:01 am 
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thanks shrinker,that is what i was looking for,the egt shutoff is a safety no different than a low fuel pressure cutoff would be but also as you said it will not be my primary tuning tool but it is more information to use to understand the state of the tune as the engine is running


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 Post Posted: Fri May 18, 2012 5:35 am 
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I wouldnt go as high as 1500 in a Nitrous gasoline full on race motor. 1380 would be it for me, if it went over that I'd be looking closely at other issues. Its hard to set a point because its so interactive with other things. Undoubtedly numerous people will offer advice and offer concrete temp figures with conviction they are correct, but thats not me. Id just pull plugs etc. The value of EGT is in detecting if something major is changing, for instance it can pick up a roller lifter starting to collapse part way through a run, stuff like that. You have to really know what your engine looks like in a condition that your happy with it, then you can check if somethings gone wrong.


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