EGT "theory" and cool down time

[Martin White]

This is a new thread, because I can't find anywhere on this forum that this specific question has been addressed. I've noticed that the recommendation on this forum is that, more or less, as soon as the EGT reads around 375 (ish) F, that you are ok to shut down (I'm pre turbo).

Now my question. IF you get the turbo housing really hot, and assuming the point of letting it cool is to prevent coking / cooking oil inside the turbo passages, is it a valid assumption that just because your EGT's have dropped to that 375 ish range, that the housing itself has also dropped to that same temperature? It seems to me that the housing is a HEAVY chunk of metal that will not cool as fast as the EGT's drop while idling (I know it's heavy, last time at Ateb I looked at one pulled apart, and trust me, it's a LOT of steel). Or am I missing something here? Please elucidate, if I am.

 

[crushers]

the main reason to let the turbo spin down, the blades can be turning at 20,000 rpm and if you do a quick shut down then the blades (and related shaft, bearings etc) are spinning with no oil (lubrication) getting to them. read: bad news.
i use the temp as a direct relationship between the rotating mass and the ioling needed.
the higher the heat, the harder the turbo has been working the more the turbines have been spinning...
i am sure that shutting down the turbo could also lead to burnign of the oil and plugging the passage ways.

for me it is easier to sit and watch the EGTs than try and figure out anopther method of timing the shut down...

this is all just my theory, not based on any facts...

<seems to work well though>

cheers

 

[Towpack]

You are 100% correct in your assumption that the turbo will take MUCH longer to cool down than the EGT.Having just fitted a pyro to my truck (pre-turbo) the first thing I noticed was how quickly the EGT goes up and down.Even when nudging 1200F towing a load at highway speeds,in the time it takes to slow down and pull over the temp is back down to around 300F so fitting a pyro (especially pre turbo) just to monitor turbo temp or judge cool down time is a waste time and money.

Tubo spool-down takes a relatively short time,15-20 secs maybe.Cool down time can take several minutes.

 

[crushers]

the turbo housing will be in direct relation to the exhaust gasses flowing through it. if you take the heat away the housing will cool quickly just like a cast frying pan when cooking, remove the pan from the heat source and it cools to the touch quite quickly... for fun, take a infra red heat meter and do a test to see how fast the turbo casting will cool down to acceptable levels...if i had one i would give it a try.

 

[Martin White]

Now that is an interesting idea and sometime, I might do that. I think you know me well enough to know that I would actually do something like that Wayne.

Ok, so my read on this thread so far is that the main purpose of an EGT is to monitor the effects of your driving on EGT's and a smaller secondary purpose is to give a guideline on shut down. An interesting aside. Nick was a bit trashed for his suggestion on long idle times. But if there is a delayed cooling effect perhaps he was closer than we thought.

Another notable fact: the toyota suggested cool down times pretty much coincide with the time it takes for the EGT's to cool down to the 375 range, from my experience so far. And it's hard to believe Toyota would engineer something that would suffer significant damage by sticking to their recommendations.

 

[cruiser_guy]

... for fun, take a infra red heat meter and do a test to see how fast the turbo casting will cool down to acceptable levels...

I've got one in storage from the business I was in. Next time I'm by my buddies house where my junk is I'll pick it up and try it out!

 

[wussyPup]

Martin,

Maybe you could put a temp sender on the turbo oil outlet? The outlet oil temp would still be cooler than the turbo housing, and after shutdown the oil remaining in the turbo would absorb some additional heat.

Or, isn't there a lower-temp EGT-like thermocouple that is designed to mount to a solid casting and measure its temp instead of exhaust gas temps?

You need a regular pyro to measure pre-turbo EGT for safe running, and a turbo body temp gauge for safe shutdowns.

So far, there's not been any additional support for the turbo-blanket idea - reduce the cooling rate so that the turbo body is less stressed and less likely to crack?

 

[cruiser_guy]

So far, there's not been any additional support for the turbo-blanket idea - reduce the cooling rate so that the turbo body is less stressed and less likely to crack?

The turbo blanket is primarily for race applications to keep the exhaust gases hot (ie: expanded) for the maximum possible boost. It was not designed, as far as I know, to help cool the turbo housing.

 

[lowenbrau]

Coking is supposedly a thing of the past...

snip from banks list of myths

"MYTH #7
You have to let a turbo-diesel idle for two minutes before you shut it off.

FACT
This is a current myth that has a basis of fact stemming from many years ago. It also has a kernel of truth regarding today’s turbocharged gasoline engines that operate at higher peak exhaust temperatures than turbo-diesels. In the early days of turbochargers, the turbo shaft was supported by a babbitt bearing that could seize, or even melt, if the engine was shut off immediately after sustained boost conditions where the turbocharger would “heat soak”. A two minute cool down at idle allowed the turbocharger to dissipate any remaining spinning inertia, and the oil circulation cooled the bearing and prevented oil “coking” in the bearing area. Turbochargers haven’t used babbitt bearings for over 30 years, and today’s oils resist coking. Synthetic oils won’t coke, period. With a turbocharged gas engine, it’s still good insurance to let the engine idle for 30 seconds to a minute to allow the turbo or turbos to dissipate any inertia and to cool the bearing area to prevent oil coking, especially if the engine has been worked hard just prior to shut-down. Of course, using quality synthetic oil eliminates this potential coking problem.

Today’s turbo-diesels are a different story. There is really no reason to “cool down” a turbo-diesel these days, but you won’t hurt anything by doing it either. You can still find people who swear you have to do it, but the myth is fading. Maybe they just like to sit and listen to the radio."

 

[lowenbrau]

The turbo blanket is primarily for race applications to keep the exhaust gases hot (ie: expanded) for the maximum possible boost. It was not designed, as far as I know, to help cool the turbo housing.

A turbo blanket also reduces underhood temps and heat transfer from the exhaust side to the compressor side. I plan to use one the reduce how often the turbo gets splashed with cold water when off road.

 

[Gold Boy]

http://bankspower.com/tech_TD-fact-Fiction.cfm

 

[Gold Boy]

so what is the difference in egt from post to pre turbo?
does a turbo cool the egt?

 

[astr]

I don't know about the Toyota turbos, but on some engines there is a water jacket in the turbo bearing casting through which coolant circulates. Seems that this would limit temps in the bearing area to around 240* F (temp around which coolant boils under typical cooling systems pressures). If this is the case, it is unlikely that the oil temp in the bearing area would exceed 300* F, well within the specs of most motor oils. It seems to me that the primary reason to delay shutting the engine down is that, once the engine is stopped, there is no oil pressure, and if the turbo is still spinning due to inertia, it would no longer have oil pressure to the bearings. i have no clue how long a turbo will continue to spin without exhaust passing throug it.

 

[wussyPup]

A turbo blanket also reduces underhood temps and heat transfer from the exhaust side to the compressor side. I plan to use one the reduce how often the turbo gets splashed with cold water when off road.

I understand how a turbo blanket reduces heat transfer to the engine bay, and I understand why cold water on a hot turbo body is a bad thing.

But I don't understand how a turbo blanket reduces heat transfer from the turbine side to the compressor side. Doesn't the blanket cover the entire housing, i.e. both the turbine and compressor sides? Wouldn't that trap the heat in the turbo body and facilitate increased transfer to the compressor side and to the charged air?

Thanks for the explanations, -Steve

 

[Greg_B]

I understand how a turbo blanket reduces heat transfer to the engine bay, and I understand why cold water on a hot turbo body is a bad thing.

But I don't understand how a turbo blanket reduces heat transfer from the turbine side to the compressor side. Doesn't the blanket cover the entire housing, i.e. both the turbine and compressor sides? Wouldn't that trap the heat in the turbo body and facilitate increased transfer to the compressor side and to the charged air?

Thanks for the explanations, -Steve

http://www.forcedinductions.com/product7.htm

I do not know much about turbo blankets, but looks like over the hot side only. Seems to me some other benefits (over the less heat in engine bay and cold water crossing protection) would be faster turbo response. Heat is energy and if you are loosing heat to atmosphere (heat sink) it is lost to the turbine. I wonder the efficiency of ceramic coat vs a turbo blanket. Would the best of all worlds be to ceramic coat the exhaust manifold, both the hot and cold side of the turbo, and also use a blanket for the cold dunk issue?

gb

 

[lowenbrau]

I understand how a turbo blanket reduces heat transfer to the engine bay, and I understand why cold water on a hot turbo body is a bad thing.

But I don't understand how a turbo blanket reduces heat transfer from the turbine side to the compressor side. Doesn't the blanket cover the entire housing, i.e. both the turbine and compressor sides? Wouldn't that trap the heat in the turbo body and facilitate increased transfer to the compressor side and to the charged air?

Thanks for the explanations, -Steve

The ones I've seen cover only the exhaust side but as I understand it the concern is keeping the charge air as cool as possible and that happens best by reducing under hood temps. I've gone to great effort to install an aftercooler, if installing a blanket helps keep that air cool all the way to the engine, I'll be happy. I should probably insulate the the post aftercoolercooler plumbing as well.

 

[crushers]

I don't know about the Toyota turbos, but on some engines there is a water jacket in the turbo bearing casting through which coolant circulates. Seems that this would limit temps in the bearing area to around 240* F (temp around which coolant boils under typical cooling systems pressures). If this is the case, it is unlikely that the oil temp in the bearing area would exceed 300* F, well within the specs of most motor oils. It seems to me that the primary reason to delay shutting the engine down is that, once the engine is stopped, there is no oil pressure, and if the turbo is still spinning due to inertia, it would no longer have oil pressure to the bearings. i have no clue how long a turbo will continue to spin without exhaust passing throug it.

have you ever seen how much pressure the coolant runs at through those small cooling hoses? hardly any pressure at all, i find the cooling idea a joke after having one of the hoses come off.
teh turbo shop that builds my custom turbos supply the turbos with the ports for water cooling but told me not to bother hooking them up unless i am really going to push the engine... they seem to really know their stuff so i didn't bother to question it.
as for under hood heat, the factory turbos come with heat sheilds already in place blocking the exhaust side...

 

[cruiser_guy]

does a turbo cool the egt?

Simon, the only reason it's cooler on the outlet of the turbo is because the pressure is lower. There is a slight amount of temperature decrease because of radiation from the housing but I think most of it is due to expansion of the exhaust gases.

 

[Tapage]

teh turbo shop that builds my custom turbos supply the turbos with the ports for water cooling but told me not to bother hooking them up unless i am really going to push the engine... they seem to really know their stuff so i didn't bother to question it.

So thinking in water cooled turbo .. electric water pump, independ rad and eletrci fan would be nice stuff for a really hard worker turbo setup ..

 

[cruiser_guy]

So thinking in water cooled turbo .. electric water pump, independ rad and eletrci fan would be nice stuff for a really hard worker turbo setup ..

I firmly believe in water cooled turbos and I think what you're describing would be a waste of money!