pyrometer location question
 

i believe to have asked this before, but upon inspection it looks to me like a good spot to drill for the pyrometer, i got my egr blocking plate kit and where the tube used to come out of the exhaust manifold for the egr; right where that little circular manifold block off cap goes looks to me like a great spot as its right by one of the cylinder exhaust ports and the pyrometer probe i have is about 3 inches long so it would be right by the #1? exhaust port which should be a good spot i would think to grab the exhaust temps... please let me kno if u guys think this would work, i really want to install this thing and i dont want to have to take off the exhaust manifold and turbo in order to do it, this sounds like a much better spot to me, thanks- J

That spot which might look to be an easy place to connect the probe. You wll not get the true reading of the heat going through the Turbo charger.
You are only getting the temp from that one cylinder.

To get the true temp, you will have to remove the manifolds, and drill & tap into the exhaust manifold where the exhaust goes into the turbo.

I have seen some drilled into the header pipe (down pipe) post turbo, but the temp will be a couple hunderd degrees lower.

this was just discussed on a thread a week or two ago. FORCEDINDUCTION might have a thread on it. I do remember BGKAST showing some pictures doing it when I first registered a couple yrs ago.

Charlie

Right after the turbo is a good enough spot., but you can certainly put it before as long as like charmalu says, all the cylinders exhaust needs to be present.

how much lower are the readings right after the turbo tho? or are they about the same?

charmalu's right

here is a picture of my install.

http://i131.photobucket.com/albums/p...g?t=1240355735

http://i131.photobucket.com/albums/p...g?t=1240355897

It needs to be pre-turbo to get an accurate reading. I know a few people that did experiments and the post-turbo temps would vary over 200 F, not to mention read lower.
Just before the turbo is best.

The EGT gauge is there to help prevent melting your turbo.

You absolutely cannot put the probe at this location. There is no exhaust flow at that point and the temperature is meaningless.

The probe must go between the head and the turbo............as close to the head as possible.

The EGR port is a separate tube about 8" long. It has no flow and the temperature readings would be grossly inaccurate.

The area where all the exhaust collect to go into the turbo is the best area. You can also install it into the turbo housing if you don't want to remove the manifolds.

Don't try to drill/tap with it still on the engine, the metal shavings will damage the turbo.

It should look something like the attached picture when installed.

I'm sure others will disagree (someone always does) but the real purpose of an EGT probe is to protect your pistons from melting down. Generally I believe the blades of the turbo (impeller and compressor) are made from steel. Not very likely that you will produce exhaust gasses hot enough to melt steel.

The pistons on the other hand are made of aluminum and the point at which the aluminum alloy used in manufacturing pistons begins to melt is right around 1275*F (690*C). That temperature is easily reached, especially if your running a little heavy on the fuel.

Ideally you want to measure the exhaust gas temperatures as close as possible to where they exit the exhaust port - to have a little time and opportunity as possible for the gasses to cool. The perfect thing would be to acutally measure the temperature inside the combustion chambers at the top of the piston. The problem with measuring the temps there is that you'd need a separate probe for each cylinder since even minor variations in air flow and fuel injected will cause the combustion temps to vary somewhat. The same thing applys to measuring the temps at one of the exhaust ports. That might be your coolest running cylinder and while it is running at 1250*F, one of the others might be closer to 1350*F - resulting in the hotter piston getting slagged - even though your gauge showed the temps being below the critical 1275*F.

So you definitely want to mount the probe where it can/will be exposed to the combined flow of gasses from all the cylinders. It still isn't going to be measuring the exact temp of the hottest one, but it will be measuring an average of all of them - which is better than measuring the temp of the coolest one (which you could end up doing measuring the temp right at one port - if you're unlucky enough to pick the wrong one).

When you move the probe to a post turbo location the variation in temperature becomes even larger. Depending on design, flow, RPMs, etc. the temp of the gasses exiting the turbo will generally be in the range of 200*F-300*F lower than they were when they went into the turbo. The turbo housing itself absorbs and dissapates a lot of heat and the gasses expanding inside the turbo causes them to cool as well. The good news is that the EGT temp drop for a given turbo is a pretty consistent number, or more precisely a consistent percentage.

So, as Forced stated, measuring the temps where the exhaust exits the manifold (into the turbo) is about the best you can do to get the most accurate measurement of the average exhaust temp. It works just fine to install the probe post turbo BUT you have to find out what the temp drop is across your turbo is at max EGT (assume around 1250*F to use a "safe" number with a little margin for error), and then set your "upper limit" on your EGT gauge that much lower.

For example, if the EGT drop across your turbo is 250*F when the incoming gasses are at 1250*F (a 20% heat loss), and you have your EGT probe installed post-turbo, you have to remember that the safe limit on your gauge will be 1000*F. This works just fine and as long as you make the proper compensation for your application it is plenty accurate enough to provide the needed protection from slagging pistions. Just about all piston-engined aircraft have their EGT probes installed post-turbo, and that is definintely an application where accuracy is critically important. A slagged piston at 20,000 feet has a little more serious repercussions than having one slagged in the engine of your car beside the road...

Like most things in life, having one probe in the collector area is a compromise. Its not 100% accurate but its close enough to be acceptable for daily driver people like us. "Driving by temperature" is a great way to maintain maximum efficiency.
If you're serious and willing to spend the dough, $400-600, there are several models of aviation pyrometers that have 4-6 inputs in a single gauge. Most can be set to automatically display the hottest cylinder and even have a high limit alert.

The exhaust will be cooler after the turbo, but I wouldnt risk drilling into any manafold. Like others have said, it will be accurate enough for what you are trying to do. Here is what I did, and I know I've posted this before.

That looks pretty good to me. One question: how much temp drop are you allowing for across the turbo (to determine max temp limit on your gauge) , and how did you figure out what the temp drop is?

I was thinking of using one of those non-contact infrared thermometers to measure the temp of the manifold pre-turbo and the downpipe post-turbo and use that to caculate the (approximate) percentage of temp drop across the turbo and then multiply that percentage by 1250 to determine my max EGT number for a post-turbo installation.

The only problem is figuring how to measure the two temps while under load & boost. I don't have ready access to a dyno...

The commonly allowed post-turbo temperature is 950*F.

If you are squeamish about drilling and tapping your manifold, go to the junk yard and get one there, if you some how destroy it you are out $20 or whatever they charge these days for a manifold.

pulling it at the yard will give you valuable experience, making it easier to install it once you have drilled and tapped it. If you can find a 78 SD you will find an exhaust manifold without the EGR. so you will never have to worry about that leaking.

Drilling and Tapping the manifold is safe, its cast iron. Cast iron is used in making engines because it is easy to machine, softer then steel.

It sounds like a lot of work to remove then manifold but trust me its an after noons work, and even faster if you practice on a car at the junk yard. and your pyrometer is in the correct location.