guys, recall that we've been through the theoretical discussion with great enthusiasm and pretty much made the case for post-turbo thermocouple being superior for shutdown purposes. This is due to the fact that, during quick cooldowns, the thermal mass of the turbo will allow it to stay warmer then the actual temperature of the exhaust temperatures entering it. <br><br>Well, I'd like to know if any of you guys with both pre and post thermocouples have actually done any testing to see how pronouced this effect is. In theory, the pre-turbo exhaust temp could be 300, and your turbo bearings could be near the flash point of the oil that cools them (about 450). <br><br>I'm just wondering what the cooldown behavior of post-turbo gasses are versus pre-turbo gasses. Certainly, as I say, for rapid cooldown there will be a difference. but in actual practice I wonder how pronounced this really is. <br><br>Anyone done any experiments? I'm out to understand the effectiveness of shutting down at 300 degrees pre-turbo when the post-turbo temp may actually be higher.

 

Just had a very informative conversation with Cummins NW Seattle, on this and other related issues. As most of you may know, Cummins/Holset specs the ISB for 950 degrees post-turbo. they do not spec a pre-turbo temperature limit. <br><br>Interestingly enough, the reason they did this is to protect the turbo itself from thermocouple breakage, and so a lot of work has gone into correlationg the pre and post temps, in ISB engines. Apparently, thermocouple breakage is an issue that they continue to see.<br><br>But the limiting factor is the pistons, not the turbo, and the corrleation work performed leading to the 950 degree post-turbo (6&quot; down stream) was based on just that. So what we have is a considerable amount of work correlating piston temperature to and post-turbo temperatures, such that the post-turbo temperature of 950 is sufficient to protect the pistons in ISB applications up to 370 HP. up to 370 HP is the important part.<br><br>Because of the robustness of their work in correlating post-turbo temperatures to piston temperatures, the use of a post-turbo thermocouple carries the advantage. But, given the correlation work and the reliability of the 300 degree differential (this figure also has a lot of work behind it, comes indeed from cummins, and again is good for engines up to 370 HP), the pre versus post question is apparently moot for this purpose. In other words, under normal operating conditions for ISB's up to 370 HP, you can measure pre (1250)or post (950, it doesn't matter) because the post side cools down very rapidly anyway, and you are under no danger to determine the shutdown temp by measureing pre-turbo. funny thing is, cummins doesn't specify a shutdown temperature <br><br>The interesting thing for us, in my opinion, is this: It appears to me that for bombs up to 370 BHP, a post-turbo themocouple is really best, because thats where the body of cummins knowledge is and where they spec their production engines at. Bombing over 370 brake HP puts us outside of the engineering work already accomplished by cummins (no 5.9L runs in production work over 370BHP). <br><br>Once we go above 370 BHP, then we're on our own, and must measure pre-turbo using the 1250 degrees ceiling. But the interesting thing to me regarding the 1250 number is this: once we are outside the 370 HP mark, there will be a greater temp differential (piston to manifold) and I would think the 1250 degrees number should be tempered a bit. Short, higher-than-370-HP bursts are especially susceptable to this because there is no relavent cummins engineering work on the subject. <br><br>The interesting thing to me about shutdown is this: once you begin to back off, say after a hard pull and assuming you're bombed over 370 HP, you are no longer running the engine above 370 HP, and so the cummins correlation work now begins to apply very nicely -- which means to me that clearly the pre-turbo thermocouple will do very nicely for two reasons:<br><br>1. protect your pistons because you are running the ISB at a power level that cummins has not tested nor correlated with post-turbo EGT measurements. The 300 degree differential will probably not apply in this case, and so post-turbo measurements should not be trusted. get a pre-turbo measurement. period.<br><br>2. give you the visibility you need for shutdown purposes. this is because, once you start backing off, you begin to operate the engine in the region understood by cummins -- where pre and post track very well together . Shutdown based on temperature is just not an issue in production engines up to 370 HP.<br><br>The wierd part is that even for the larger (9-15 liter) cummins engines, they still don't specify a shutdown temperature -- they specify a time! (I just don't know what it is, but the ECMs on these trucks can be set to auto shut down at various time intervals). Cummins just didn't seem to think that shutdown timers were even necessary on our trucks, but the situation still remains: when you run the cummins ISB beyond 370 BHP you're on your own. To me, that means if you're running a 400 RWHP truck, you better have a pre-turbo thermocouple and, after a hard pull, you better run it for a few minutes to watch EGTs come down gradually before you shutdown. Once the engine has stabalized at a power level that is below 370 HP, then you can start to trust the EGT and kick in the 300 degrees rule of thumb for shutdown (although this did not come from cummins). <br><br>So my conclusions are:<br><br>1. post-turbo thermocouple is not necessary as a safer way to determine a shutdown temperature. the pre-turbo method at 300 degrees will do just fine, as long as you have been operating the engine below 315 RWHP. If you've been running harder than that, then you better stabalize a few minutes first before you trust the 300 degree shutdown figure. <br><br>2. pre-turbo EGT is absolutely a necessity for trucks over about 315 RWHP (370 minus 15%). below that, the post-turbo method has the advantage of protecting the turbo against thermocouple failure and direct correlation with cummins' own work on the subject.<br><br>3. the higher you are above 315 RWHP, the greater the temperature difference will be between piston and manifold and the less accurate your pre-turbo EGT reading will be for the purpose of protecting your pistons. This of course emphasizes the importance of air and no doubt is a conservative approach. me, I wouldn't want to run 1100 for very long at 400 HP. but the nature of high HP bombs that are used as daily drivers is that you probably never sustain high levels for very long. So you're pulling a sled and you want to pass a duramax at the same time -- after a 30 second hard pull, you can settle the engine back into the arena better understood by Cummins.

 

Doug, this is awesome info. I didn't realize that so much work had gone into this. I guess you learn something new every day.

 

Thanks Doug for taking time to spell that out for us. I think you just answered half a dozen or more questions I had in my mind on the subject.<br> <br> I am wondering if you would mind if we quoted you by adding that information in our Faq area?<br><br> I am certain our faq manager would love to have it in there as would I.

 

no problem on the faq thing. sounds good to me. cool thing about forums like this -- the body of knowledge increases as various members contribute their findings. <br><br>The guy at cummins was very helpful and knowledgable. What started it was that I had emailed him from the contacts section of the cummins web site -- asking about wiring in a shutdown override based on EGT. He wrote back suggesting a phone conversation which I gladly accepted of course. It cleared up a lot in my mind too!

 

There is a little more info required for you to decide on pre or post install.<br><br>The post installs up to 370 hp may NOT be suitable for our applications.. Why?? Because there are many that are not matching all components up when getting higher HP numbers on their engines.. The CPL numbers on the engines specify the fuel flow rate, turbocharger size, intercooler size, etc. for producing the power ranges of that particular package. If you order the ISB 350 engine out of the fire trucks, the turbocharger, fuel map, injectors and cooling system requirements are way different that those that have just slapped on an Edge Comp and some injectors.<br><br>For those that only change the injectors, or up the fuel rates by changing fuel plates cannot rely on post turbo temperature as the turbocharger is not sized for the application and, the air/fuel ratio is not within the boundries of the Cummins Testing.<br><br>Therfore, IMO for those that are BOMBing there engines, the safest place to install their pyrometer is pre turbo to remove any doubt about actual internal temperatures.<br><br>J-eh<br>

 

thats a good point, especially about the turbo component itself. Indeed, the pre-turbo figure of post-plus-300 would seem to work well in this case. <br><br>But an Edge box is not likely to significantly change the temp differential from piston to manifold. But clearly, the turbocharger itself could. So you're right, to mitigate risk, you're best bet is always pre-turbo. Indeed, the only argument in favor of post-turbo is the safety thing.<br><br>As I say, though, there is no problem relying on your pre-turbo thermocouple as a means of determining a safe shutdown point.