EGT's - gas versus diesel engines

[Ncostello]

We're all pretty much aware of what EGT's do in a diesel engine. A diesel (in general) can idle all day long and maintain low EGT's. But what happens in a gasoline (non-turbo, non-supercharged) engine during extended idling? I've never really thought too much about EGT's at idle in any engine until I got the Dodge.



The local Dodge dealer shop once told me idling their V10 for long periods will eventually burn up the catalytic converter. We got into a discussion about high idlers for diesels and eventually started talking about gas engines.



Do gassers have higher egt's at idle than while under load? I know you can buy high idlers for gassers as well as diesels.



If they are higher at idle... would a high idler reduce the egt's since you will not be at idle? If true... I would suspect the throttle plates in a gasser would reduce airflow thus causing the higher egt's at idle.

 

[WadePatton]

Dunno, but I've often wondered why gasser performance folks don't use EGT's for tuning engines--especially carburetted ones.



I suspect that the idle temps are high because gassers run hotter when lean and they've got the fuel cut back for emissions/economy at idle.

 

[Wheaties]

Don't quote me, but I have heard that gassers will idle at 1200F. Compared to a diesel, the fuel/air ratio is constant on a gasser, so the exhaust temps are more or less the same.



However, the catalytic converter is a different story. It's job is to catalyze combustion of unburned hydrocarbons. So when the mixture is made richer to maintain idle, there's more unburned fuel to deal with, and the temp goes up, way up.



Matt

 

[mikel]

If I had to guess, I'd say that the gasser probably runs rich at idle and the extra fuel makes the catalytic converter heat up as it burns off the excess. Done long enough this will lead to meltdown.



Mike

 

[Ncostello]

I guess another question may be how long will the converter last... or what happens to it when it finally gives up? Does it clog up or simply quit doing its job as far as emission control goes?



I know I drove a 97 Chevy 1500 for work and let it idle for long periods all the time. I got it new and put 50,000 miles on it without any trouble. It was emission tested right before we replaced it and passed fine.

 

[Joe G.]

I use a piece of four inch flexible sewer hose to put my diesel exhaust out of my garage. I used the hose on my '82 Imperial and burnt a hole in it just idleing.

 

[Blakers]

Gasoline engine exhaust runs hotter than diesel exhaust (less efficient so more heat lost), but they run hotter under load than they do at idle.

 

[HoleshotHolset]

Gassers do use EGT gauges for tuning, but the average Joe's of the world are more likely to rely on feedback from an oxygen sensor.



With the advent of wideband oxygen sensors - the data the sensor picks up is a lot more reliable and useful in tuning than older single wire sensors. You only really find wideband oxygen sensors on cars with aftermarket programmable ECM's or some shops have them in a standalone environment so that they can offer dyno tuning services. Note that OEM multiple wire sensors do not necessarily mean that they are wideband... . more likely scenario is that they have heating elements in them that require extra wires.



Gassers operate on a variable air/fuel ratio. 14. 7:1 is stoichiometric or the optimum efficiency ratio... . turbo or supercharged cars are advised to stay on the rich side of this ratio to avoid running lean.



My own tuning experience with programmable ECM's is that a car runs the best and makes the most power right on the ragged edge of being 'lean'... .



When EGT's are high in a gasser... that means you are running lean... . Diesels are a whole 'nother ball game... . high EGT's indicate a rich condition.



Matt

 

[Ncostello]

Wouldn't computer controlled fuel injection would eliminate any rich/lean conditions that a gasser might encounter... especially at idle. My buddy told me tonight his V10 E350 van just turned 110,000 miles. He starts it at 6am and it does not shut off until 7pm. About 4 hours of that time is spent idling and he has yet to suffer any trouble such as converter meltdown. So that kinda tells me its not that hard on converters to extend idling... at least on a stock vehicle.

 

[Wheaties]

Ncostello,



You may very well be right. I thought that injected or not, gassers need to run a little richer at idle. Another factor could be a bad oxygen sensor, which would fool the computer into compensating for a condition that didn't exist--not sure if this would make it run lean or rich.



Matt

 

[Forrest Nearing]

lots of my buddies use EGT's on their cars, but these days, you can get into a wide band O2 setup for not much more than an EGT, and it's much easier to dial it in... with an EGT you need to put the car on the dyno and see what EGT's yield the most power (or use the WBO2 at the dyno to see what EGT's correspond to what A:F)



and you also run into the fact that EGT can actually head in the other direction as you reach the extremes of rich/lean.



but the WBO2 is the way to go for a gasser IMHO... you don't even really need a dyno... just dial the car in for the right A:F and you're in business.



Forrest

 

[HoleshotHolset]

Depending on the car/truck... .



If you remove the oxygen sensor... or if it fails... the ECM looks up the correct A/F ratio in a table and doesn't have the oxygen sensor for 'trim'... . this will most likely end up in a rich condition under all circumstances. The oxygen sensor information (A/F ratio... ) in most vehicles is constantly going from slightly rich to slightly lean... .



With most gassers... you can only run lean once. You can run a diesel as lean as you want, for as long as you want... . until it stalls.



I remember when wideband oxygen sensors were a lot of money... mass production helped that quite a bit... . as did the ever increasing movement towards stand-alone/aftermarket programmable ECMs... .



Matt