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Author Topic: EGT and Fueling  (Read 1663 times)

February 27, 2011, 11:04:31 pm


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EGT and Fueling
« on: February 27, 2011, 11:04:31 pm »
I'm pretty new to diesels and I'm trying to understand how these things work.

Recently a VW diesel mechanic who was trained in VW's Wolfsberg facility in Germany told me that a "richer tune is cooler than a lean tune."

At first I thought it fit in with what I thought I understood about diesels but after thinking about it I decided it was indeed a head scratcher.

Here is what I thought was true about EGT:

-  EGT is a function of air to fuel ratio.

- Too much fuel and not enough air (over fueling) leads to higher EGT.

- If you have an over fueling condition if you can do the following to reduce high EGTs:

1) Decrease the fuel.

2) Add more air.

Typical ways to add more air:
- A Turbo.
- A Bigger Turbo
- A Charge Cooler (intercooler)
- A Liquid to Air Charge Cooler
- Larger Exhaust
- Less Restrictive Intake
- Clean Air Filter

It my understanding that all of the above items work to a point and then they don't help most likely because of other restrictions on air flow or such things such as turbo lag, etc.

Searching the interwebs I've seen people say it both ways; that richer tunes lead to higher EGTs and that richer tunes produce lower EGTs.

So please help me understand what is wrong in my thinking. Why would a richer tune (black smoke) lead to a cooler EGT?

And are there other factors to consider when trying to lower EGTs?


David R.
Oakland CA

Reply #1February 27, 2011, 11:54:53 pm


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Re: EGT and Fueling
« Reply #1 on: February 27, 2011, 11:54:53 pm »
You are right, too much fuel and too little flow and air will cause EGT's to rocket. You can decrease the fuel, but what fun is that?? lol

Turbo an Intercooler are the easiest ways to get a lot of cool air to the engine. that coupled with a free flowing intake, and exhaust.. you've got a mighty setup for good efficiency.

Black smoke ultimately means HIGH EGT's. That black smoke is unburnt fuel that is hot coming out the exhaust. The best performance with as little smoke as possible is ultimate. Smoke is just wasted fuel.

Reply #2February 28, 2011, 12:57:17 am


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Re: EGT and Fueling
« Reply #2 on: February 28, 2011, 12:57:17 am »
When he said that a rich tune is cooler than a lean tune, he might be referring to a gasoline engine. With a gasser, stuff starts melting if you run lean. A diesel melts when you run rich.

Everything you said in your post is correct, you definitely are on the right track.

One thing I found interesting, and I'm not sure if it's true or not, but apparently diesel's start to smoke at about 80% stoich. Meaning, puking some black smoke can make more power than just a light grey haze. You'll hit EGT melting territories quickly if you're blowing thick black smoke though!

enough boost is when you have 3 dimple marks in the hood from the valve cover nuts..  ;D