pyrometer controversy... before or after the turbo? - Diesel Forum

centerline · 02-15-2006, 11:31 AM

I saw the site that shows how to install the thermocouple in the drivers side exhaust manifold, but I have talked to a couple of different PSD mechanics in my area and they said it dosent make much difference where it is installed as long as it is there if I am towing with a chip. they had pros and cons about both ways but none that could convince me or them that one way is any better than the other. although there was some speculation on weather a person could get in trouble if, on the manifold where the probe was installed, you were to lose an injector for some reason, the EGT's would not register as high on that side. then you could actually overheat due to the increase in fuel being delivered to the remaining injectors trying to make up for the lost power.(one cylinder would be passing unheated air thru). Having it installed after the turbo would have a more balanced reading. it would be so easy to install it in the downpipe behind the turbo. Can someone convince me why the exhaust minifold is a better place to install the probe, and what length of probe would that require?. I have actually seen only 4 of the PSD's with pyros installed and all of them had the probes installed in the downpipe. the max operating temp is 1300, the max "safe" operating temp is 1250 for a probe installed in the manifold according to what I have read. does anyone know what the temps would be if the probe is installed in the downpipe right behind the turbo? Thanks for the help...

PSDfreak · 02-15-2006, 12:10 PM

I'll share my expirience and what I have heard...take it for what you want....

I installed mine in the Manifold. As I understand it, the reading you want is a reading that is AS CLOSE to the valves as possible. This way it is much more accurate. If you are reading further down the line, say in the DP, you are giving that exhaust a lot of time to cool down. Now some say you can just add 300* to the reading and you'll be fine, but I dont like that method because exhaust temperature is not ALWAYS going to be that 300* difference exactly...there could be some variable affecting it one day and not the next...I feel much more comfortable looking at my gauge and knowing that, THAT is the temperature I want to know without adding/guessing what it probably is....The Manifold, IMO, is the way to go hands down...and its REALLY not hard at all...just get the right size bit & tap and you're set...Not sure on the size of the probe wire...mine just came with the autometer kit.

Good Luck...Hope this helps a bit.

-Casey

scottginfla · 02-15-2006, 12:12 PM

as u know the reason for having a pyrometer is to measure the temps in the cyclinder, so the closer to the cyclinder you get the more accurate the reading, mounting in the down pipe is imo the lazy way out.
in edit- i have had a probe in both locations one time or another and have seen as much as 500 degree differencs in temps.

hheynow · 02-15-2006, 12:23 PM

Centerline, THIS write up by Dale Isley will give you some more info.

96F3504X4 · 02-15-2006, 12:56 PM

I mounted mine in the downpipe and the highest reading I would ever see is 800 deg, chip in extreme setting. After seeing on this site that it should be mounted in the manifold, I moved it there. Now in extreme, it reads 1300. I did find that if you stay steady on the throttle, it reads about the same as before or after the turbo. Any change in load or acceleration is where you see the difference. I like having it in the manifold because it seems to be alot more accurate to what's going on at the moment.

Brian Cloke · 02-15-2006, 01:38 PM

On large marine engines you will find the pyrometer probe mounted
at the exhaust elbow at the cylinder head. On a sixteen cylinder
engine you have sixteen probes led back to the control room to a
switchable control for readout.
However on my truck I installed an autometer pyro in the down pipe.
This position was the recommended position by autometer.Due to the
possibility of probe meltdown and consequent turbo damage. On marine
engines I have experienced probe meltdowns. I have not heard of this
happening on our trucks. Better be safe than sorry.
Cheers.
Brian. [img]/ubbthreads/images/graemlins/warmsmile.gif[/img]

brahamfireman · 02-15-2006, 06:20 PM

Pre Turbo. You will have to be over 1800º to melt these pyro probes, at least thats what mine from Isspro is rated for.

FWIW the Exhaust vanes are tungsten steel, they could possibly survive a melted probe going through it, I'm realy am not worried, a toasted piston and a rebuild is alot more then a toasted turbo. [img]/ubbthreads/images/graemlins/wink.gif[/img]

A 2.5" probe should be long enough. Here is a pic for ya.Pre-turbo Probe picture.

Diesel Rod

97Strokerman · 02-15-2006, 06:51 PM

I have a dual gauge with both pre and post turbo mounted, and I can get my temps to 1400-1600 in the extreme setting pre-turbo before my post mount will even read 1000. now after a short period the post turbo mount will get closer in temps, but always seems to be around 300 deg less..................Dave

richmondfarms · 02-15-2006, 07:17 PM

You must install the probe pre-turbo to know your engine exhaust temperature. The downpipe location tells you the temperature of the turbo. Just like most everyone is telling you, it's the melted piston that will do the most damage to your engine and your wallet. The driver's side exhaust manifold is easily accessed from under the truck, actually easier to me than the downpipe. You can run the wire right through a grommet in the floor under the dash.

Old_Machinist · 02-15-2006, 11:05 PM

As you have seen in the above information unless you just want to know how hot your turbo is you really need to go pre turbo. You will not belive how FAST these units can get hot depending on what you have for injectors and the tuning on your chip. I want to know before I have to get out the dust pan to pick up the pieces not that I need to start sweeping!!

Patrick Feeley · 02-16-2006, 12:37 AM

Just my two cents: I've been on this board (and others) for a little while now, and I have yet to hear of a pre-turbo probe breaking off. Ultimately, its up to the individual to assess the risk associated with each method and decide where to put it. Since I perceive the benefit of pre-turbo mounting to give quicker response and indicate more accurate readings of the engine exhaust temps (and the risk of possible probe breakage to be quie small), I chose pre-turbo. Cheers!

radarman · 02-16-2006, 01:21 AM

Just to add to this, on this board, I have heard of two different cases where someone ended up smoking his engine because he was relying on a POST turbo pyro, and running it at a bit over 1100* on a long grade.

So.. two smoked motors for POST turbo, and no smoked moters for PRE turbo pyros.

I vote for keeping the gauge as accurate as possible and as close to the heat source as possible, ie PRE turbo.

Gosh, for that matter, I will go out on a limb here and say:
What is the controversy here? Sounds like simple physics to me.

PSDfreak · 02-16-2006, 01:14 PM

Yeah i remember a guy not too terribly long ago that had a problem pulling a 5th wheel? up a decent grade and was keeping his EGT's @ 1100 as he thought was safe at the advice of his mechnic...well all a sudden the truck power went in the hole and it started bucking and smoking. Pulls over, pops the hood and the turbo is GLOWING RED. The probe was POST turbo so 1100 was way too hot...

The previous post just reminded me of that story.

-Casey

radarman · 02-16-2006, 01:22 PM

yep that was one of em..

Swamp Donkey · 02-18-2006, 12:12 AM

Here is #8 piston out of my truck. Melted Piston
I have dual probes. When this piston melted, post-turbo EGT's never went over 1000* F , but pre-turbo temps were 1600* ++ (Like pegged hard at 1600* for a long, long time).

Any other questions?

-------------------------------------------------------------

So no one panics if they see 1600*, this was not just bumping 1600* getting up to speed pulling a heavy load and then backing off. This was sustained.
16 hours with an extremely big and heavy load through mountains in brutal head and cross winds....boost never under 22 psi, EGT's never under 1200*, at one point I went through two tanks of fuel (18 + 19 gallons) from full to bone dry in 200 miles. (It was in the name of research as to what EGT's were safe. [img]/ubbthreads/images/graemlins/biggrin.gif[/img] )