Rammers,
There are very few people that are aware of the fact that thermocouple / gauge combos are not 100% accurate as they may believe. We discovered this when doing research when we designed the Turbo Lifesaver.
It turns out that a thermocouple / gauge combo is only "spot on" at the manufacturer's calibration temperature - the first variable, as how many people know what the calibration temperature is for a given combo? If your cold junction (CJ) temp of your themocouple is LOWER than calibration, the gauge reads EGT as HIGHER than actual. If CJ temp is higher than calibration, then the EGT reading is LOW. To confirm what I just wrote as fact, please see Westach's catalog page at this link:
http://www.frostalarm.com/cat/p53.html (don't know why it has frostalarm on the page name, but the actual document explains the operation of a thermocouple/gauge combo).
Here is where the problem arises with a turbo cooling unit that reads EGT . . . The page listed above says that Westach calibrates their EGT gauge at either 32 or 70 degrees F, depending on hot or cold weather usage. We have all experienced the blast of hot air when we open our hoods after operating our trucks for any length of time. How hot is the air? I guess at least 200 degrees most of the time, often hotter, especially if ambient temps are above 90. Using the 200 degree figure, with a gauge calibration temp of 70, your gauge is reading at least 130 degrees low! A turbo cooling device that reads EGT that is set to idle the engine until the gauge reads 350 is actually shutting the turbo off at 470 degrees. Some guarantee that the device will prevent turbo coking. Some may say, "well I'll just set my engine shut off point artifically low" to compensate for this error (as far as I know ALL EGT reading devices have a trimpot that allows the installer to set the engine shut off temp). If this is done, often times the EGT readout will never get low enough to reach the installer's set point and the engine will run for hours on end. Setting the trimpot artificially low once for a moving target is not the answer (CJ temps under calibration will result in high gauge readings).
With a time based device, you can always set an extra minute or two of insurance time to make sure your turbo has plenty of time to cool. For more info on this subject, visit this page on our website:
http://thebritebox.com/DTLS.faq.html#Anchor-Why-37516.
Another issue with any device (timer or EGT reader) that automatically keeps the igintion on after the key is removed is safety with stick shift vehicles as runaway trucks are a definite possibility, with major property damage likely and maybe even injuring or killing someone (I am aware of the original poster's sig that he has an auto) - just another issue we (and our attorneys) considered in the design phase. FYI - with Turbo Lifesaver you progam a run time into the unit right before you shut off the key. If you don't do this the engine shuts off when the key is turned off. We call this semi-automatic operation.
Dodge & Cummins as well as Ford & Navistar (who we talked to during our design period) did not like the idea of automatically keeping the ignition on when the key was removed due to the stick shift safety issue, and the warranty issue. Warranty you may ask? Imagine the following TRUE scenario that happened to a customer who HAD and EGT reading device and then switched to ours . . . He had a shop install the EGT reader. He was climbing a grade when he blew a radiator hose, pulled over and removed key. Engine continues to run. Without coolant, EGT's will never get down to set point where engine will shut off. Luckily he was able to find the unit (installed under hood) and YANK power wire out to shut off engine. If he was not able to do this, total ebgine destruction would have occured, maybe truck even going up in flames. Shorty after he replaced that unit with one of ours.
The makers of the EGT reader may or may not supply you with a master toggle switch to interupt power to their units should something like the above happen (one of the makers of an EGT reader who I will not name shows an OPTIONAL switch in their schematic, but they don't supply you with the actual switch). Please consider that even if you put that switch in a very visible location, not everyone who drives the truck will have the presence of mind to flip it to shut off the engine should a radiator hose or other disaster occur.
Lastly, we sell our units to dealers who install them into new trucks. Good indicator that we are both warranty and user friendly.
I'm sure this post will create some lively traffic.
Regards,
Joel Toy
Baker Auto Accessories
970. 879. 4201
bakerautoaccessories.com
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