PeachParts Mercedes-Benz Forum - View Single Post

rcounts · #10 04-22-2009, 12:50 AM

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...