Post-Turbo EGT Responsiveness

[Cooker]

I finally got around to wiring my post turbo EGT probe today. I have a few questions for those of you running a post turbo probe, or both.



The post probe seems to be less responsive than the pre, is this normal?



I used to wait for my pre probe to hit 350 before shutdown, ususally 10-15 seconds depending on my driving , but my post probe takes at least a 30-45 seconds to hit 300, and in that time my pre gets to around 275. How long should I expect to wait for the post probe to hit 300 under unloaded, easy driving conditions?



Thanks for the info,

Ryan

 

[work2muchplay2little]

Ryan, don't have a direct answer for you as I won't have a post-turbo EGT in addition to my preturbo EGT until my SPA gauges arrive. But, the turbo itself represents about a 35 pound thermal mass that has to cool off before the EGT can drop as well. Do your gauges read about the same before you start it in the morning (both reading stable ambient temperature)? I would believe it and just wait until the post-turbo temp hits 300, IMO.

 

[EMDDIESEL]

Post turbo gauges will be less responsive then Pre Turbo, thats normal, not to worry. I have seen dual probes in a truck and watched the gauges rise at different speeds.

 

[HC]

Some may think this is splitting hairs, but, the post-turbo gauge responds just as quickly as a pre-turbo gauge. The "lag time" that you see, is real. Work2MuchPlay2little said it well - the thermal mass of the turbo causes the temperatures to lag. If you're getting on it, the turbo is absorbing heat, so the exhaust is cooler for a few seconds until that mass "catches up". If you are cooling down, the turbo is still giving off heat even after you have backed off and even begun idling.



For the purpose of "when to shut down", post-turbo is the better reading, so believe your gauge. If you have pre-turbo too, then continue to use the pre-turbo gauge for deciding when to back off the throttle.



The times you quote sound about right; I've found that with careful driving for the last mile I can significantly shorten my cool-down.

 

[Cooker]

Thanks for the info fellas. This puts my mind at ease about something not being quite right.



What would I do without the knowledgeable brotherhood of the TDR



Thanks,

-Ryan

 

[Josparkz]

Would someone with pre and post turbo pyros say what the difference in the temperatures on your vehicle are, please.

Thank you.

 

[Cooker]

The other day while pulling my trailer (~3500 lbs) down the highway pre was around 850 and post was 600-650.



-Ryan

 

[Sky]

I have post and It runs around 600.

 

[Vaman]

One thing that a couple of you have mentioned that is important to keep in mind is that the pre and post turbo readings do not directly correlate all the time. On normal driving, steady state, pre will be higher than post by an amount proportional to load. On acceleration, the difference will be greater. At idle, depending on driving history before the idle, post turbo temps can be HIGHER then pre turbo temps. Those of you with both pre and post have posted this before. If you heat your turbo up to 1000 degrees under heavy load and then go to idle, your pre turbo 300 degrees will be higher on the post turbo side simply because of the heated mass of the turbo. Having both pyros is the ideal situation... pre to protect your engine and post to protect your turbo.