EGT

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t8racer
Posts: 2
Joined: May 8th, 2008, 12:28 am

EGT

Post by t8racer »

I am fimiliar with a EGT guage in piston powered aircraft. The technique to adjust the mixture was to lean to highest EGT then enrich it. How will I use a EGT in my car for the proper jetting? If you qualify in the morning and race in the afternoon the conditions will change. Is that enough to change the mixture?
Frank
Posts: 179
Joined: December 15th, 2006, 1:26 pm

Re: EGT

Post by Frank »

You don't need to change jetting morning vs. afternoon. Basically the engine builder sets the jetting on the dyno and it usually works for almost all conditions.
24b4Jeff
Posts: 29
Joined: October 24th, 2007, 10:25 am

Re: EGT

Post by 24b4Jeff »

The previous reply, while usually correct, is probably not helpful in all cases. Conditions can change quickly enough to require a change of jetting over the course of a single day, but unless you have calibrated your car you won't know it until you get that afternoon EGT reading.

What you basically need to do is to measure the air density, which depends on absolute temperature, barometric pressure and relative humidity. It is then possible to compute air density. (Alternatively, one can find air density meters which do this for you, but I am not convinced of their accuracy.) Before going out, compute the air density. Then, while on a straightaway where your car will attain and hold 6000 PM or more, check the EGT. Based on your engine builder's instructions, you will then know whether you are running rich or lean. Do this repeatedly, and over the course of a season or less you will be able to correlate air density to running conditions, that is to say, establish an air density threshold for changing from the lean to rich jet, or vice-versa. (High density => "big" jet; Low density => "small" jet.

A further complication is that while you can sort your jets based on venturi diameter, that really does not equate to flow, because chamfering, scratches, and other deviations affect flow. So If you, say, lost your #50 jet, don't expect that one of the same venturi diameter borrowed from a competitor will have the same flow characteristics.

If you really want to be obsessive about this, there are computational fluid dynamics codes available that you could use, assuming you've got the requisite background in physics and math :-)

On the other hand, I ran for more than 10 years without an EGT sensor and all the computations, and don't notice any measurable improvement in my performance. But I have a good engine builder. If you are doing your own, it might make a big difference, though.
pillowmeto
Posts: 103
Joined: January 5th, 2008, 12:54 am

Re: EGT

Post by pillowmeto »

http://www.srh.noaa.gov/elp/wxcalc/dens ... tude.shtml

This is the air density calculator I normally use.
t8racer
Posts: 2
Joined: May 8th, 2008, 12:28 am

Re: EGT

Post by t8racer »

I normally use air density to change up or down one jet from my baseline that my engine builder used when it was rebuild. I went to a track that had a long straight. It was a normal day that required no change. I was running very lean by looking at the plugs. I ask another racer what jet he is using and he was running a very large jet and other guys were using a big jet also. After hearing my life story, would the time you are at a high RPM reqiure running a larger jet?
SR Racing
Posts: 1205
Joined: June 24th, 2006, 1:58 pm

Re: EGT

Post by SR Racing »

t8racer wrote:I am fimiliar with a EGT guage in piston powered aircraft. The technique to adjust the mixture was to lean to highest EGT then enrich it. How will I use a EGT in my car for the proper jetting? If you qualify in the morning and race in the afternoon the conditions will change. Is that enough to change the mixture?
The same principles are involved, but the conditions in racing will seldom change as much as those in an aircraft. Baro Pressure, Humidity, and Temperature will all affect air density and thus the jetting requirement, but the altitudes effect on baro pressure has far more impact than the others tytpically. If you are always racing at say an alitude of 600 to 1000 feet, a jet change won't be mandatory. (assuming your engine was tuned for that altitude on a typical day.)

It might be helpful to have an air density meter reading of the condition your engine was dyno'ed/tuned in, and then you can take a reading at any track you go to and have an idea of whether to go up or down a jet size. It would be very unlikely for the air density to change enough over the day to make a jet change mandatory, but it could be helpful with big changes.
SR Racing
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Joined: June 24th, 2006, 1:58 pm

Re: EGT

Post by SR Racing »

t8racer wrote:I normally use air density to change up or down one jet from my baseline that my engine builder used when it was rebuild. I went to a track that had a long straight. It was a normal day that required no change. I was running very lean by looking at the plugs. I ask another racer what jet he is using and he was running a very large jet and other guys were using a big jet also.
If your engine was tuned correctly on the dyno at (or adjusted to) conditions typical in your environment,... PAY NO ATTENTION to the jet sizes that other have.
Over the years the carbs have been cobbled up, emulsion tube holder eylets filed away, all kinds of float levels, venturies, etc. The main jet on a race prepped FV might be anything from 1.50 to 2.20 and a/c jets 1.80 to 2.60. Some for acceptable reasons and some because the builder/tuner had no idea what he was doing. :lol: So if you know your carb is tuned correctly it is fine to go up or down a jet size for conditions, but don't compare to another carb. It's usually apples to oranges. (Whereas with a 650 Holley for example, you can swap jets and expect to see almost exactly comparable conditions)
After hearing my life story, would the time you are at a high RPM reqiure running a larger jet?
Possibly.... but normally no.

Some builders use EGT as a primary tool in jetting. After a hard WOT run they hope to see EGTs at 1275 to 1325F. This is USUALLY indicative of an a/f ratio of about 13:1. You might be able to get a bit more peak HP by running slightly leaner. However this is harder on pistons and valves, etc. So they might have errored on the lean side and you could be running 13.8 to 1. Now you go to a track with a long straight and you might get quite a bit hotter (EGTs, CHT's and Oil temps) . So the real problem was that you were on the lean side.

Since the wide band 02 sensor technology is so much better than EGT's, we use it exclusively for jetting. EGT's are a great diagnostic tool (4 of them) on the dyno, but finding the sweet spot with 02 is the only way to do it with current technology.

Jim
lbaconll
Posts: 55
Joined: January 12th, 2007, 9:54 pm

Re: EGT

Post by lbaconll »

Jim, what ratio are you seeking with the broad band sensor?
L Bacon
SR Racing
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Joined: June 24th, 2006, 1:58 pm

Re: EGT

Post by SR Racing »

lbaconll wrote:Jim, what ratio are you seeking with the broad band sensor?
L Bacon
That's a secret. :lol: About 13.2 is pretty good. You won't lose much at even 12.8, and it will run a bit cooler. 13.6 or so is as lean it should get normally. With the correct a/c and main jets you should be able to keep it in range from 3500 to 7000 RPM. Ignore any RPM less than that. The race prepped carbs meter fuel very poorly much under that and you don't really care much since you will aways be racing above 3500rpm hopefully.

Stoich is 14.7 and that is fine for idle or cruise. (Street cars set for that.) But at WOT, you need much more fuel. Most of the factory street ECM settings go to as rich as 11 at WOT to protect the engine. But that is way to rich so as to keep warantee work down and have a margin of error for bad fuel, etc.

Also, since the induction system is pretty poorly designed (and modified), you may want to compromise the a/f a bit based upon EGT's on all four cylinders. I.E. If you have 2 running at 1200 and 2 running at 1300, you will probably want to lean it up slightly.

Jim
lbaconll
Posts: 55
Joined: January 12th, 2007, 9:54 pm

Re: EGT

Post by lbaconll »

Jim, thanks for the info!
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