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Author Topic: EGT temps?  (Read 13653 times)

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Offline ddahlgren

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Re: EGT temps?
« Reply #15 on: July 19, 2007, 05:05:18 AM »
If your egt is high and you retard the timing it will probably go higher.. retarded timing and way rich two very easy ways to hurt an engine..the more the rpm goes up the worse it gets..think about the burn time per unit of fuel and where you are in the cycle vs how many milliseconds left to finish you can end up with combustion in the pipes..way rich has issues with end gases and all that jazz just remeber an engine has both a rich and lean knock limit.. high combustion pressure and lots of unburned fuel is a grenade.
egt is a tool to be used along with other ones in the end. I put the probes about an inch from the port and see 1500 all  the time on a sustained run. In a drag race or typical dyno pull they are too slow to get up to temp before the run is over.
Dave

Offline Dakin Engineering

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Re: EGT temps?
« Reply #16 on: November 12, 2007, 07:12:40 PM »
What if your pipe is only say, 4 inches long? (turbo)
What thermocouple should I use?

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Sam
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Offline Harold Bettes

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Re: EGT temps?
« Reply #17 on: November 12, 2007, 08:22:32 PM »
The whole issue of EGT references is somewhat cloudy and mostly neurotic from folks carrying tales of magic solutions. The reading on an EGT is dependent upon many variables. Open or closed types, size of the wire of the thermocouple, location in the exhaust pipe, and many others. Using the EGT to tune with is a sure recipe for disaster! Unless you have a reference temperature for the chemically correct ratio, you can "see" the same temperature at two points on a bell shaped curve, with each one referencing a different A/F ratio. When you are looking at an EGT reference you are always looking at history because they are fairly slow to react to changes.

As an example: What is the air temperature where you are? Is that rich or lean? There ya have it. It is an indication and must be related to other issues to really mean anything important.

BTW, exposed chromel-alumel wire (type K thermocouple) will only react to a temp change in about 800ms or so. Pretty slow.

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HB2
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Offline interested bystander

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Re: EGT temps?
« Reply #18 on: November 12, 2007, 08:40:49 PM »
Wise statement, Mr. Bettes, EGT understanding varies like the opinions of the blind men and the elephant.

I especially liked the neurotic reference.
5 mph in pit area (clothed)

Offline Dean Los Angeles

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Re: EGT temps?
« Reply #19 on: November 12, 2007, 09:35:43 PM »
Horsepower is a product of heat. The EGT gives you an indication of where you are in the heat curve. As Harold mentioned it is a bell curve. As you lean out the engine the temperature will increase, peak, and start down the other side. If you are on the wrong side of the curve you are in trouble.

Like all tools, you need a full tool box to do the job. EGT is only one of the tools that you need. I think they are worthwhile for long pulls like Bonneville. Even at El Mirage they provide decent information. On a short run like El Mirage you can manage 1500 and get away with it.

The aluminum piston is going to melt in the 1200 range depending on alloy. But the piston actually sees much higher than the EGT indicates on the power stroke, like 4800 degrees. Then it gets cooled by the incoming charge. You get the average at the EGT.

The distance, type of thermocouple, open or closed tip, and other things keeps it from being an absolute tool. As a relative tool, being able to compare run-to-run helps.

At 1200 you are giving away HP. At 1300-1400 you are close. Over 1500 it better be short!

An oxygen sensor coupled with the EGT gives you a really good tool. The amount of oxygen exiting the engine should be zero. In a perfect world that would be the case. In the dynamic environment of the cylinder, you can't get a perfect mix of fuel and oxygen because of the short time for that to happen. Excess fuel is required to make sure that you get peak HP.

And that doesn't even begin to cover fun subjects like detonation, the effect of compression ratios on flame temperature, incoming charge temperature, humidity . . .

On the other hand, with a single thumper, run it till the valves float, then SHIFT.
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Offline hombre

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Re: EGT temps?
« Reply #20 on: July 25, 2008, 02:08:51 PM »
Related question: How far from the exhaust port (or back of valve) should the EGT probe(s) be placed?   Engine is a supercharged Triumph Rocket 3.

MCR

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Re: EGT temps?
« Reply #21 on: July 25, 2008, 02:48:08 PM »
At very high EGT's nitrous will actually ignite aluminum pistons, at least in a diesel.  Nitrous is kinda like a catalyst, and aluminum is flammable.

Your turbocharger has a limit to how much heat it can run.  1500 deg sustained will damage most turbines.  But the key is sustained.  For 10 seconds you can go past 2000.  Drive pressure comes into play as well.  I've melted inconel turbines, cracked turbo hsg's, and turned 400 stainless thick wall tubing into plastic.  Yeah, and ignited pistons.

I mount pyrometers near the turbine hsg, away from the exhaust port, but that is only because I do just one or two pyros.  If you mount it too close to the valve, seems they don't last as long as if you go several inches downstream, if you are doing one-per-header-pipe.

GM has a new design pyro that I'm playing with.  It reads in ohms instead of microvolts.  Not a thermocouple, a "thermistor" that appears to read very accurate from 0 deg out past 1850 deg.

Offline RichFox

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Re: EGT temps?
« Reply #22 on: July 25, 2008, 02:54:43 PM »
Jack Connelly and Al Holoway ran a partial streamlined BSA triple with a small roots blower at Bonneville about 35 years ago. It was thought to be some sort of cabin compresser.  Jack still has a Triumph triple in a Rickman frame. Very pretty. PM me if you would like Jacks email.

Offline hombre

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Re: EGT temps?
« Reply #23 on: July 25, 2008, 03:56:15 PM »
MCR... thanks for your response, but I run a Rotrex blower, not a turbo.

MCR

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Re: EGT temps?
« Reply #24 on: July 25, 2008, 04:00:33 PM »
DOH!!!  Sorry!  :-D

Offline ddahlgren

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Re: EGT temps?
« Reply #25 on: July 27, 2008, 04:35:29 PM »
I would only consider the dyno numbers if you are willing to hang with them until they stabilize on a value. Doing a 5 second pull and calling it the target is mindless when you have to run for 2 minutes.. I woory more about and egt that will not settle down and keeps going up that is a failure waiting to happen.
Dave

Offline Richard Thomason

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Re: EGT temps?
« Reply #26 on: July 27, 2008, 07:13:32 PM »
I always ask our engine builder before each run, 'what temp should I click off at?" He always says "if it noses over then you went too high".