EGT TEMPS ON 4.8 PETROL PATROL

Engines and Engine Systems
Mystical_Beast
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EGT TEMPS ON 4.8 PETROL PATROL

Post by Mystical_Beast » 15 Oct 2018 09:34

Good morning all

I am looking for a couple of pointers here..

I installed a VDO Egt Gauge when I had my Patrol super-charged, but even though I thought I would pay proper money for a decent gauge, there was no indication as to where one should actually fit the probe.
We ended up fitting it at the accumulation point where the two downpipes come from the manifold.

Now its obvious that wherever you mount the probe will affect the reading.
It was determined by RGM that the accumulation point would be the hottest point.

In all reading on various forums, EGT probes are mostly fitted to diesels and it seems the bench mark safe temp is around 650 degrees.
Any Idea what it should be for the 4.8 Petrol motor.

In general driving, without really pushing at all I can easily see 750 degrees, if I push it I can go to 880 degrees. The gauge itself only reads to 900!!

So it seems to make the whole gauge installation superfluous. This on a petrol motor.

So lets say I push on and sit at a steady 150 km/hr, the EGT Gauge will sit at around 750 to 800 degrees.
I also have an oil temp gauge and that shows oil as being just on operating temp.
The third line after zero, so oil remains cool.

Is it possible the thermo-couple is faulty, or are the temps I am seeing pretty much what I should expect?

It may be worth mentioning that taking the foot off the pedal sees an immediate drop in EGT readings.

My issue is I have no bench-mark parameter to interpret what I am seeing on the gauge and naturally I want to make sure that I’m not running too hot etc.

The issue then is the temps are way up on the chart regardless, even a gentle cruise will see me at around 700 degrees.

Any advice, pointers or opinions would be appreciated.
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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by graham1 » 15 Oct 2018 10:11

A quick search on EGTs, I found a guy talking about them on his 4.0 l Prado. He says the max he got while towing was 690.

Dont even know what thats worth, but on first judgement it does seem yours run quite high. is that a bad thing for the 4,8 though? Who knows?

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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by Mystical_Beast » 15 Oct 2018 10:37

Thanks Graham..

I still have to wonder about the whole measurement of EGT in an aftermarket scenario.

I can move the probe 10 cm away and now say Temps are fine?

I have a heat gun, think I must test the probe..

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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by hugejp » 15 Oct 2018 11:34

Petrol EGT figures will be higher than diesel, as petrol is less efficient & the unspent energy escapes as heat.
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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by Mystical_Beast » 15 Oct 2018 16:43

Thanks JP

That's the kind of info I need to know.

Anyone know what normal operating EGT's on a large straight 6 would be?
I could test the thermocouple with a heat gun except that at prsent the distance between Heat Gun and Patrol is 300 km :biggrin:

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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by David M » 15 Oct 2018 17:18

I am assuming that the 4.8 also has aluminium pistons which are the weakest part of the whole setup as aluminium is not keen on temps above (some sources say 660 and others 750).
If the purpose of your EGT gauge is to protect the pistons you want the probe as close to the pistons as possible.
If your exhaust manifold is the 6 into 2 setup I would put it as close to no 6 as possible as this is the cylinder that gets the most toasty on any straight 6.
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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by Peter Connan » 15 Oct 2018 17:37

Yeah this is a bit tricky.

As David points out, the whole point of the exercise is to protect the engine, and this is done by keeping the temperatures of the cylinder head and piston crown below the temperature where they go through a change in cristaline structure.

The materials in petrol and diesel engines are the same, thus around 750 is about the maximum safe limit.

But it could be that you are getting some combustion still happening after exhaust, and who knows where that goes. On F1 engines the exhausts glow white hot, probably in excess of 1200 degrees...

So I'm not really helping much here. Sorry.
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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by Peter Connan » 16 Oct 2018 08:16

Some interesting reading matter, which indicates that the temperatures you are seeing may not be out of the ordinary:

The article also gives some interesting insights into turbocharger design and some of the pitfalls associated with smaller, more highly-stressed engines...

http://www.turbos.bwauto.com/tools/down ... =105&d=327
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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by Michael » 16 Oct 2018 08:31

EGT's on a petrol engine is not as usefull as on a diesel.

On a diesel the EGT's are probably the most important number you want to see where on a petrol I would say the AFR is what will be the killer if it runs too lean........ soecially on forced induction.

Julian, I dont think your readings are out of the ordinary. From what I have read on the net the petrols read higher EGT's than diesels.
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Re: EGT TEMPS ON 4.8 PETROL PATROL

Post by Peter Connan » 16 Oct 2018 08:42

And here is a slightly conflicting viewpoint:

https://www.team-bhp.com/forum/technica ... t-gas.html

According to that, 900 should probably be considered as a maximum. More importantly, high EGT's may point to a lean-running engine. If you are not already measuring AFR, then you probably should. At the very least, get it back on the dyno with exhaust gas anyliser attached, as running lean will kill your engine.
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