Thermocouple reaction times and max EGT

[JR2]

I have an SPA Boost EGT gauge on my truck and last month the TC failed. I got a new one (from SPA), only the size of the TC was different. It was much larger than my old one (old one was 1/8th new one is much bigger). After installing the replacement I noticed right away that it was took a long time (30 sec’s) for the gauge to show the correct values at idle (350 deg). Then I went for a drive and did some WOT runs and I was not seeing the same max readings on my gauge as I had before. Prior to the new TC I would see 1300 at WOT +/-, with the new TC I see 850 deg +/-. I remember reading the article in the TDR mag about response times of TC’s but had never thought about it it actual terms of use of the measurement. The reaction time of the TC actually hides the high temps I was seeing before. I guess the moral of the story is that if you have a big thermocouple and think you are only getting to 800 degrees with those big injectors and fuel boxes you are sadly mistaken. It would be well worth the investment to swap to a smaller faster reacting thermocouple to see what is really happening. I have another new one on the way and I am going to use the fat one for post turbo egt.



I am also going to go back and read the article again, now that I have some idea of what it means.

 

[Cliffman]

not surprised

This doesn't surprise me one bit. I have had nothing but problems with my SPA gauges. from what I understand there under new management and things are supposed to get better. We'll see!

 

[LSMITH]

I have the old style TC that came with my SPA and it is very quick to react. I have not had any trouble with mine either. It surprises me that they would send you a TC like they did.

 

[JR2]

I was not meaning to bad mouth SPA here, the new guy has been nothing but helpfull. He sent me everything I asked for and more. I was just wanting to let guys out there know that if you have a big thermocouple you are NOT seeing the real temps your engine is making.

 

[Bigsaint]

John, just curious and sure that you probably checked it... display refresh on the SPA. I know they come set at a refresh rate of 0. 04 seconds ( at least that's what my SPA pressure gauge was set at). Would lowering the display refresh rate help, if your's is set higher ???? I had to move my refresh rate up to once every 1. 5 seconds to keep the readings from jumping around every 1/2 second. Just a thought. I don't have my SPA manual in front of me, but I believe that they can be set to refresh at as long as every 20 seconds, maybe yours is set too long ???



Scott W.

 

[Cliffman]

Peak temp

John, did you check the peak? If your tc is just slow it should still regester the hi temps one would think?

 

[Strick-9]

My SPA thermocouple failed as well. I bought a replacement online (can't remember the company) 1/8" with the quick connectors to plug right in to the SPA wiring.



It was about $26 and I've seen 1700° in 4 seconds with it.



I believe a lot of people are running beyond the response time of their thermocouple and don't even know it.



-Chris

 

[Brandon]

good point big saint.....



we have had good luck with the new pyro's



ps the spa is maxed out at 1750... . I talked with them about making her go to 2000 but it would take about 2-3 bills to make it happen



so far nothing but good with the new pyro

 

[JR2]

Oh ya I am sure, the refresh is set to less than 1 second.



Strick-9 - that is exactly my point. It may not be revelant for most guys, but the guys with big BOMB's need to realize that the fat 5/16ths thermocouples are slow to react, as in it takes time for them to reach the temp thats there motors are seeing.



Case in point, on the same road with the same set up and same driving style I am seeing 4-600 less in EGT. I know my engine and I know what the egts did in the past at WOT and this TC just does not reflect the real temps because it can not react fast enough to the temps. The writeup in in the TDR mag confirms this. Various TC's took a long time to reflect the true temp. If you make a WOT run that last less than the reaction time of your TC you are not going to see the correct temp reading in your truck. I wanted to test this by making a wot run and holding the high temps for a whilel, but there is just no safe place in the winter to go 100 mph for 2 minutes.



If anyone knows who has the SPA 1/8th TC's for sale please PM me so I can get an extra.