Sluggish Pyrometer Response - PeachParts Mercedes-Benz Forum

biobenz617 · #1 03-03-2007, 07:34 PM

I finally installed my thermocouple and wired up the pyrometer gauge that's been in the car for almost 1/2 a year now. Since I might do a valve job sometime down the road and don't want to remove the manifolds twice, I opted to install the thermocouple temporarily on the downpipe and welded a bung right after the downpipe joint. It is a VDO pyrometer kit.

What is the typical response time for a pyrometer gauge? I warmed up the engine for about 10 minutes at idle and it didn't move from the minimum reading of 250F. Out on the open road, it was very slow to adjust to throttle positions. It took around 30 seconds to get to the max of 750F at WOT running 3200rpm and 12psi boost (I modulated the speed in "S" with my left foot

). After the run, I continued on at about 10% throttle with the temp dropping very slowly (30 sec) to about 450F.

What's the deal? Is this because it is post-turbo? What are typical idle and WOT temps pre/post-turbo?

Thanks for the help!

#2 03-03-2007, 07:41 PM

Quote:

Originally Posted by biobenz617

Is this because it is post-turbo?

Yes. The turbo absorbs a significant amount of energy.

biobenz617 · #3 03-03-2007, 07:50 PM

Quote:

Originally Posted by ForcedInduction

Yes. The turbo absorbs a significant amount of energy.

I realize my readings will be consistently lower post-turbo because of the enthalpy change through the turbine. Are you referring to added thermal mass upstream of the thermocouple that will significantly slow response? I'd think the exhaust manifold alone would make pre-turbo applications also sluggish.

Forced, you have an EGT, right? What is your typical response time from 0% throttle to instant WOT at, say 3000rpm? (~95% reading)

sailor15015 · #4 03-03-2007, 07:51 PM

Quote:

Originally Posted by biobenz617

I finally installed my thermocouple and wired up the pyrometer gauge that's been in the car for almost 1/2 a year now. Since I might do a valve job sometime down the road and don't want to remove the manifolds twice, I opted to install the thermocouple temporarily on the downpipe and welded a bung right after the downpipe joint. It is a VDO pyrometer kit.

What is the typical response time for a pyrometer gauge? I warmed up the engine for about 10 minutes at idle and it didn't move from the minimum reading of 250F. Out on the open road, it was very slow to adjust to throttle positions. It took around 30 seconds to get to the max of 750F at WOT running 3200rpm and 12psi boost (I modulated the speed in "S" with my left foot

). After the run, I continued on at about 10% throttle with the temp dropping very slowly (30 sec) to about 450F.

What's the deal? Is this because it is post-turbo? What are typical idle and WOT temps pre/post-turbo?

Thanks for the help!

I have my pyro installed pre-turbo, in the manifold. If I go from a cold start and let it idle for ten minutes it'll get up to around 300F, 350 after a run and I let it cool down. I can get it up to 1100F on a long steady grade by my house but usually don't see more than 1000F under WOT acceleration. The response of the needle is within a second or two of my foot, i.e. if I'm cruising at 65 with a reading of about 650F and I romp on the throttle, it'll be pushing 800 in about three seconds, same for coming off the throttle.

#5 03-03-2007, 07:54 PM

Mine reacts pretty much the same as above except that I can get into the 1400*f range.

sailor15015 · #6 03-03-2007, 07:58 PM

Quote:

Originally Posted by ForcedInduction

Mine reacts pretty much the same as above except that I can get into the 1400*f range.

Saw 1400F one time, climbing mountains for a long time in Colorado with the gf driving. I happened to look up from my reading and glance at the gauges. Needless to say she jumped pretty good when I suddenly starting hollering about blowing up my engine.

biobenz617 · #7 03-03-2007, 08:05 PM

Hmmm, so I've probably doubled or tripled the amount of metal that is absorbing the exhaust heat, meaning probably doubling or tripling the response time from what you guys gave. Still, mine seems much slower than expected.

What brand gauges to you both have? I wonder if i need a longer tc. I'll test it with a blow torch and see what it does with extreme and verifiable changes.

Anyone else have post-turbo installation?

#8 03-03-2007, 08:06 PM

It might have something to do with altitude. I'm going to Tulsa in June to see the exumation of the Belvedere so I'll be able to compare the EGT's between CO and OK.

http://www.forwardlook.net/19571958Plymouth/countdown.asp

EDIT: THIS is the gauge I use.

catmandoo62 · #9 03-03-2007, 08:06 PM

Quote:

Originally Posted by ForcedInduction

Mine reacts pretty much the same as above except that I can get into the 1400*f range.

my pickup is pre-turbo and is about the same but i have never seen anything close to your highs.one time mine acted really slow and had low readings.it ended up being bad connections on the thermocouple.

biobenz617 · #10 03-03-2007, 08:13 PM

How did you guys install on the manifold? Did you just drill and tap the collector portion for 1/4" NPT?

So is 1400F pre-turbo pretty much the max you can go safely or should the target be more around 1200F?

sailor15015 · #11 03-03-2007, 08:16 PM

Forced has adjusted the max travel screw on his ip so I doubt you'll see his numbers, more like mine. I ordered mine a long time ago so I don't have the link but it was a VDO kit. I think most people accept 1250*F as the max for any appreciable time. You can do more, just not for very long, as in seconds.

#12 03-03-2007, 08:17 PM

I removed, and tap/drilled the underside of the manifold about an inch from the turbo flange.

1250*f is pretty much the consensus for the safe limit for pre-turbo and 950*f post turbo.

#13 03-03-2007, 08:18 PM

Quote:

Originally Posted by sailor15015

Forced has adjusted the max travel screw on his ip so I doubt you'll see his numbers

My injection pump still has all it's stock internal settings.

ajrn · #14 05-11-2007, 07:55 PM

I just installed one in the downpipe, about 2" from the manifold on my '81 240D (uh I guess that's preturbo! sigh..) I had a port welded to a new downpipe-- then I drilled thru to put the sensor in.. I also bought a 'plug' so I can remove the sensor/gauge if I sell the car..

My thoughts were of adding propane fogging, for merging onto the highway. A manually controlled button, that would maybe fog, with a relay timed for 30 seconds or so-- then shut off.. Once she's 'to speed' she's okay, for the most part.. Getting there can get spooky, sometimes..

Today was the first I drove with it.. Idle is about 300F, and it briefly touched 1200F, when at 70mph on the highway, I decided to see if I could make her go 'faster...'

The consensus is that 1200F is still a safe max (prolonged) and a smidge more for a few seconds is okay???

If she can do that (1200F) without propane-- not sure if it even merits trying..

Anybody got thoughts on this??